Hameg 408-1A

[Specmaster]

I have just aquired a pair of scopes, one is a Philips PM3217 and the other is the Hameg 408-1A. Both have some issues and I need a manual, user and service for the Hameg in order to get on with trying to resolve the issues it has, the Philips I have downloaded manuals for.

I have searched high and low on the internet and the only manual I can find for the Hameg is one in German. Can anyone loan me copies or tell me where to look for them on line at all? 

[tronde]

Hameg gives you the English user manual for HM408 and the service manual for HM408-1A in German, but with English text in the schematics part.

https://www.rohde-schwarz.com/search/hameg-archive_229476.html?term=hm408

I don't know what the diiference between 408 and 408-1A is, but you can compare the schematics if you download the German manal for 408. You can send them an email and ask if they have a service manual in English. They will most likely send you one if it exist.

[Specmaster]

Thanks for the link, both English and German manuals downloaded, English is 408 and Geman is 408-1A. Interestingly, they both appear to be the same as far the control panel is laid out so perhaps the 1A is just a later modification to the circuit?

It is a pig to work on, I have struggled to remove the board containing the timebase controls as neither of the vernier controls where functioning so it was impossible to set them in the "CAL" setting. Reason for the failure was the nylon flexible shaft couplers have split in half possibly as a result of being over tightened or rough handling? This it seems has been a common failure on this particular scope.  All I know is that I find myself needing to replace all of them, 11 in total, anyone have any ideas where these could be sourced from at all?

I have also noticed  that on the digital output board there is a capacitor that has either blown or been smashed (looks like its been chopped), schematic shows it as being  a 100pf 100v (grey with a black cap on top), cant recall seeing a capacitor that looked this before. Nothing came out of the case when dismantling it and there is no sign of any magic smoke having been released either?                                                                                                                                         

[tautech]

Pics please.
They don't need to be bigger than 100kB for us to see enough detail.

If the couplers end up being too hard to manufacturer ask Rich how to go about sourcing them in this thread:
https://www.eevblog.com/forum/testgear/rohde-schwarz-oscilloscopes-questionscomments-let-me-know!/

Hunt out the part # so it makes it easy for him.

[Specmaster]

Pics please.
They don't need to be bigger than 100kB for us to see enough detail.

If the couplers end up being too hard to manufacturer ask Rich how to go about sourcing them in this thread:
https://www.eevblog.com/forum/testgear/rohde-schwarz-oscilloscopes-questionscomments-let-me-know!/

Hunt out the part # so it makes it easy for him.

Pics are on their way, however after careful examination its only 5 of these particular couplers I now require as the others are metal and are OK. Someone has tried to fix some of these before me by wrapping wire around them and they still fail so a replacement is required.

I have no idea how I'm going to hunt out the part number since the Hameg manual unlike many others, does not have a parts list along with the relevant part number contained within the manual. Perhaps this is available from the makers direct?

[tautech]

I have no idea how I'm going to hunt out the part number since the Hameg manual unlike many others, does not have a parts list along with the relevant part number contained within the manual. Perhaps this is available from the makers direct?

That should not matter, just ask Rich in the thread I linked and provide a link back to this thread, he'll sort it out for you. Pics and location, maybe with a short description should give him the info or at least enough to link to one of their staff at the factory to hunt them out for you.

[Specmaster]

Here are some pics of the progress so far, pic c1b shows 2 caps in the power supply which look like that they have suffered a leak, still working though and pic c2b also shows another cap in the psu which has been sucked inwards? These I will be replacing as a precaution once I have figured out to remove this board to access the solder side.

Pics c3b and c4b show some of these couplers and how someone has tried before to cure the problem.

Pics c5b and c6b show the "blown" cap on the digital output board from the side and above views.

Any thoughts?

[tronde]

Maybe you can use something like this as replacement for the couplers? You get them in a lot of different dimensions for input and output shafts.


https://www.banggood.com/3mm-x-6_35mm-Aluminum-Flexible-Shaft-Coupling-OD19mm-x-L25mm-CNC-Stepper-Motor-Coupler-Connector-p-993317.html?rmmds=search

[theatrus]

The first caps def leaked. The dented cap may have come that way from the factory.

That sliced cap though - maybe its just a decoupling cap and no one ever noticed?

[Specmaster]

The first caps def leaked. The dented cap may have come that way from the factory.

That sliced cap though - maybe its just a decoupling cap and no one ever noticed?

The dented cap, could have come from the factory like that I suppose, but what would that have done to the capacitor rating? The scope looks as if it was made late 1990 or early 1991 and given its age and the possibility that this cap was compromised  at least because the dielectric insulation must be weakened by the dent, I'm surprised that it has not blown already?

As to the sliced cap, yes I think it is a decoupling cap as it sits across the output of an IC to ground, would a 10pf 100v ceramic disc capacitor suffice as a replacement or not? 

[Specmaster]

The flexible shaft couplers that I need are way smaller then those on offer from Banggood, the input shaft is 2mm and the output 4mm, length is 15mm and overall dia is just 12mm and these are in some cases sitting very close to the solder side of PCB's so I would not like to much larger in overall dia than that, the banggood ones are 25mm dia.

[carl_lab]

The flexible shaft couplers that I need are way smaller then those...

Maybe you can use a short piece of PVC air hose, if the potentiometer or switch is not too rough-running.

[coromonadalix]

yeah  +1  exactly my tought,  with the correct inside diameter,  it would do nicely while having a flexible functionality, there's some tubing who wont dry out in time ...

you could check some Radio Controlled websites they offer lots of parts ... 

[tronde]

You get them smaller than those from banggood, but maybe still too large.

Here is one that can sell you 2x4mm D15 L20.

https://www.aliexpress.com/item/1pcs-5mm-x-8mm-D15-L20-shaft-coupler-flexible-coupling-stepper-motor-encode-for-5mm-shaft/32670333088.html

You can also try with adhesive lined heatshrink tube.

[theatrus]

The first caps def leaked. The dented cap may have come that way from the factory.

That sliced cap though - maybe its just a decoupling cap and no one ever noticed?

The dented cap, could have come from the factory like that I suppose, but what would that have done to the capacitor rating? The scope looks as if it was made late 1990 or early 1991 and given its age and the possibility that this cap was compromised  at least because the dielectric insulation must be weakened by the dent, I'm surprised that it has not blown already?

As to the sliced cap, yes I think it is a decoupling cap as it sits across the output of an IC to ground, would a 10pf 100v ceramic disc capacitor suffice as a replacement or not?

Decoupling is usually 10-1000nF. 10pF isn't going to do much.

[Specmaster]

The first caps def leaked. The dented cap may have come that way from the factory.

That sliced cap though - maybe its just a decoupling cap and no one ever noticed?

The dented cap, could have come from the factory like that I suppose, but what would that have done to the capacitor rating? The scope looks as if it was made late 1990 or early 1991 and given its age and the possibility that this cap was compromised  at least because the dielectric insulation must be weakened by the dent, I'm surprised that it has not blown already?

As to the sliced cap, yes I think it is a decoupling cap as it sits across the output of an IC to ground, would a 10pf 100v ceramic disc capacitor suffice as a replacement or not?

Decoupling is usually 10-1000nF. 10pF isn't going to do much.

Yes your right, my mistake, I made a typo, it is in fact 100pf 100v. What is the best type of cap for this application?

[Specmaster]

You get them smaller than those from banggood, but maybe still too large.

Here is one that can sell you 2x4mm D15 L20.

https://www.aliexpress.com/item/1pcs-5mm-x-8mm-D15-L20-shaft-coupler-flexible-coupling-stepper-motor-encode-for-5mm-shaft/32670333088.html

You can also try with adhesive lined heatshrink tube.

These are a possible replacement, many thanks for searching them out, it need to be custom made version though to provide d1 2mm and d2 4mm.

I have already messaged Rich to see if he could offer assistance in sourcing the correct items, but in the event that he cannot help, then these could well be ideal.

[carl_lab]

...it is in fact 100pf 100v. What is the best type of cap for this application?

The defective one is a ceramic disk type, so I'd look for same type for replacement.

[Specmaster]

You can also try with adhesive lined heatshrink tube.

I have experimented with this idea and where the control is just a pot, this idea may have some merit provided nobody was ham fisted with the controls. Where there is a switch involved, then the resistance is just too much.

[TurboTom]

Varicap?? 

[carl_lab]

You can also try with adhesive lined heatshrink tube.

I have experimented with this idea and where the control is just a pot, this idea may have some merit provided nobody was ham fisted with the controls. Where there is a switch involved, then the resistance is just too much.

You can try multiple layers of heatshrink tube...

[Specmaster]

You can also try with adhesive lined heatshrink tube.

I have experimented with this idea and where the control is just a pot, this idea may have some merit provided nobody was ham fisted with the controls. Where there is a switch involved, then the resistance is just too much.

You can try multiple layers of heatshrink tube...

I did, I used 4 layers and the weak link is in the small gap between the 2 shafts.

[Specmaster]

Varicap?? 

I'm not sure, there is no mention of its type in the schematic and unlike some service manuals, there is no list of parts, part numbers or descriptions, so on this occasion I'm flying blind to a large extent as I've never seen any cap quite like this before. I did try and find out kind of cap is likely to be used in scope and I discovered that my initial thoughts of it being a ceramic disc cap was also borne out by carl_lab.

I attach the respective page from the manual and drawn a red circle around the circuitry in the area of the cap. Do you or anyone still feel that its a ceramic or varicap?

[tautech]

Ceramic cap, 100pF 50-100V NPO type.

It's listed as 100V but not sure why, I don't see anything on the schematic that indicates it need be. Maybe 100V versions were just easier to source. 

[Specmaster]

The on going struggle to locate suitable means of connecting the 2mm input shafts to the 4mm shafts of pots and switches on this has been a real pain so far. As you will have seen, there are solutions but they are very large scale and in some cases run the risk of grounding out on component leads etc soldered in on the PCB as well as being rather expensive to the point where you seriously have to question if the project is worthy of such expenditure on such antiquated equipment by today's standards? 

How about the following possible solution? whats your thoughts on this idea for hopefully a neat solution to the issue which further expands on the idea first forward by Tronde?

Using the direct shaft couplers from Ali Express (see attached picture) I could cut the coupler in half through the slightly narrower section and fit each half to the respective shaft and then heat shrink the 2 sections together? In so doing, both shafts would be gripped equally by the heat shrink being as the OD on the larger sections is 9mm so the turning effort exerted by the 2mm shaft from the front control knob would more in step with that transferred to the 4mm shaft of the pot so the heatshrink would be less likely to become screwed up because of the differences in the respective shaft diameters. Your thoughts please?

[tronde]

I think that should work well. Adhesive lined heat shrink is quite robust. The cost is low, so give it a try.

[tautech]

Wait a few more days to see what Rich comes back with from R&S.
The OEM part will likely be the best solution unless prohibitively expensive.

What caused the originals to fail would be my concern.
Maybe there's an updated part ?

Too much loading as lubrication dried out for the originals ?
Excessive misalignment ?
Impact ?

For those from Ali, I'd be getting the 2 and 4mm models, parting them in a jobbers lathe and sweating them together again with some Easyflo (silver based brazing) for a permanent repair.

OR, use the 4mm models and sleeve one end with some fine metal tubing down to 2mm. Sourcing the tubing might be tricky but models shops might have it or get some 4mm rod and drill it with a 2mm bit in a lathe.
You might need to cut a nick out to allow the screw to contact the 2mm shaft.

There'll be members close that can help you with this that have lathes.

[Specmaster]

I have ordered the couplers as a backup incase Rich can't help me in this case, they are quite inexpensive from Ali Express so if they are not needed on this occasion they maybe on another for the next scope along as this method of control seems to common on old analogue scopes. Modern ones are all computer controlled so don't need this sort of barn door engineering.

As to what caused the old ones to fail. I think it was impact related damage as 3 of the facia stand off pillars were also snapped in half, now all safely glued back to together, along with a large dollop of ham fisted operators.

[tautech]

I have ordered the couplers as a backup incase Rich can't help me in this case, they are quite inexpensive from Ali Express so if they are not needed on this occasion they maybe on another for the next scope along as this method of control seems to common on old analogue scopes. Modern ones are all computer controlled so don't need this sort of barn door engineering.

As to what caused the old ones to fail. I think it was impact related damage as 3 of the facia stand off pillars were also snapped in half, now all safely glued back to together, along with a large dollop of ham fisted operators.

Sounds like you've got it sussed.
For the bushing down to 2mm you might get away with some 1mm sheet metal formed into a short length of tubing around a 2mm mandrel, again with a nick removed to allow the screw to contact the shaft but maybe not. As it will be a slotted tube the screw may be able to crush it enough to get a good grip.

[tronde]

For those from Ali, I'd be getting the 2 and 4mm models, parting them in a jobbers lathe and sweating them together again with some Easyflo (silver based brazing) for a permanent repair.

Why complicate it if you want a non-flexible coupler? They are also sold with one end for 2mm and the other for 4mm. No need for any work then. It is also easy to enlarge the 2mm hole on one end with a 4mm drill bit.

The problem is that the coupler is meant to be flexible because you will have some mis-alignment both angular and axis wise. The reason the original broke is most likely caused by the outward pressure from the screw when it is thightened. This is a very typical failure for anything plastic made and used this way.

[tautech]

For those from Ali, I'd be getting the 2 and 4mm models, parting them in a jobbers lathe and sweating them together again with some Easyflo (silver based brazing) for a permanent repair.

Why complicate it if you want a non-flexible coupler? They are also sold with one end for 2mm and the other for 4mm. No need for any work then. It is also easy to enlarge the 2mm hole on one end with a 4mm drill bit.

The problem is that the coupler is meant to be flexible because you will have some mis-alignment both angular and axis wise. The reason the original broke is most likely caused by the outward pressure from the screw when it is thightened. This is a very typical failure for anything plastic made and used this way.

Yep, you nailed it in a previous post. 
Only way to go.
https://www.eevblog.com/forum/testgear/hameg-408-1a/msg1203063/#msg1203063

Funny thing further down the Aliexpress listing is in the blue box:
RS flexible coupling 

Made for the job. 

[Specmaster]

For those from Ali, I'd be getting the 2 and 4mm models, parting them in a jobbers lathe and sweating them together again with some Easyflo (silver based brazing) for a permanent repair.

Why complicate it if you want a non-flexible coupler? They are also sold with one end for 2mm and the other for 4mm. No need for any work then. It is also easy to enlarge the 2mm hole on one end with a 4mm drill bit.

The problem is that the coupler is meant to be flexible because you will have some mis-alignment both angular and axis wise. The reason the original broke is most likely caused by the outward pressure from the screw when it is thightened. This is a very typical failure for anything plastic made and used this way.

Yep, you nailed it in a previous post. 
Only way to go.
https://www.eevblog.com/forum/testgear/hameg-408-1a/msg1203063/#msg1203063

Funny thing further down the Aliexpress listing is in the blue box:
RS flexible coupling 

Made for the job. 

I cant see this blue box that you mention. I can loads and loads of flexible couplers but I cannot see any that have 2mm at one end and 4mm at the other and most of the ones I have seen are around 20mm dia and 25mm long, which are too long for at least 3 locations and at 20mm dia, may be pushing the electrical clearance a bit others. If you have found something with 2 and 4mm ends of less overall size, I'd appreciate it if you could post a link. 

[tautech]

Clearly mentioned in the Product Description:
2mm ? d1 / d2 ?8mm.
For example d1xd2 (mm) : 2x2 , 2x3 , 2x3.17 , 2x4 , 2x5 ,2x6 , 2x6.35 , 2x7 .................

The mention of RS is just coincidental, down above the mechanical drawings.

Sure if they are a little large that's bad luck but could they be spun down or even shortened a mm ? 
As they're cheap as chips I'd give it a try.............but I've got the gear to do it. 

[Specmaster]

Clearly mentioned in the Product Description:
2mm ? d1 / d2 ?8mm.
For example d1xd2 (mm) : 2x2 , 2x3 , 2x3.17 , 2x4 , 2x5 ,2x6 , 2x6.35 , 2x7 .................

The mention of RS is just coincidental, down above the mechanical drawings.

Sure if they are a little large that's bad luck but could they be spun down or even shortened a mm ? 
As they're cheap as chips I'd give it a try.............but I've got the gear to do it. 

For the purposes of clarification, is it possible for you to post a link to the page where you saw this information please. here is a link that takes you to RS Flexible Couplings and I can find zero mention of a 2mm to 4mm version anywhere on the page?  https://www.aliexpress.com/premium/RS-flexible-coupling.html?ltype=wholesale&d=y&origin=y&isViewCP=y&catId=0&initiative_id=SB_20170511031920&SearchText=RS+flexible+coupling&blanktest=0&tc=ppc

[tronde]

This my first aliexpress link

https://www.aliexpress.com/item/1pcs-5mm-x-8mm-D15-L20-shaft-coupler-flexible-coupling-stepper-motor-encode-for-5mm-shaft/32670333088.html

The seller says under product description:

"   Please note before buying : Picture just as a reference.the actual delivered goods shall prevail ! inner hole d1 or d2 need greater than or equal to 2mm,and less than or equal to 8mm,please write the order leave message d1xd2 . if not instructions.we will send d1xd2=5mmx8mm . thanks !"

Just leave a message when you order and say you would like to get the 2x4mm version.


Fixed coupling 2x4mm. Maybe a little expensive for what it is?
https://www.aliexpress.com/item/2mm-4mm-Jaw-coupler-Motor-Copper-Shaft-Coupling-brass-Coupler-model-ship-model-airplane-Motor-connector/32690441472.html

Another option that can be made to fit. I guess the price is wrong, but you hve a name to search for.

http://accel.fr/produit/accouplements-en-acier-a-ressort-sorflex/

[Specmaster]

This my first aliexpress link

https://www.aliexpress.com/item/1pcs-5mm-x-8mm-D15-L20-shaft-coupler-flexible-coupling-stepper-motor-encode-for-5mm-shaft/32670333088.html

The seller says under product description:

"   Please note before buying : Picture just as a reference.the actual delivered goods shall prevail ! inner hole d1 or d2 need greater than or equal to 2mm,and less than or equal to 8mm,please write the order leave message d1xd2 . if not instructions.we will send d1xd2=5mmx8mm . thanks !"

Just leave a message when you order and say you would like to get the 2x4mm version.


Fixed coupling 2x4mm. Maybe a little expensive for what it is?
https://www.aliexpress.com/item/2mm-4mm-Jaw-coupler-Motor-Copper-Shaft-Coupling-brass-Coupler-model-ship-model-airplane-Motor-connector/32690441472.html

Another option that can be made to fit. I guess the price is wrong, but you hve a name to search for.

http://accel.fr/produit/accouplements-en-acier-a-ressort-sorflex/

Thank you so much for this gem. Strange thing is I do remember seeing this a couple of days ago but then could not find the page again. I personally find that Ali Express is a real little treasure trove of gems but almost impossible to navigate easily, as can be seen by my link, it took me to the right products but did not list this particular one. I wonder if the web site has different links / portal of entry that is country biased seeing as your in Norway and I'm in the UK. I think the last time I saw that page was if I remember rightly when I was following another link on another post? oh well, we got there in the end.

These couplers are much larger then the originals, so if Rich is unable to help me, these will fit but it will mean having unsolder many items like wiring harnesses and pots in order to slide these over the input shaft but I get the feel that these would outlive the rest of the scope once fitted.

Many thanks.

[Specmaster]

The first solid 2mm to 4mm couplers from Aliexpress arrived today so I tried a little experiment with the coupler used as it is, and it was causing slight flexing of the PCB as expected, so I cut the centre section out and mounted the bosses on their respective shafts as close to each other as possible and then applied heat shrink tubing.

The result can be seen in the attached photo, once it was allowed to cool down properly, the shaft was operated and it turned the pot perfectly and also operated the switch so that "CAL" mode can be entered and it did so with no visible flexing of the PCB or twisting of the heat shrink tubing. In fact it is impossible under normal usage to force rotation of the 2mm shaft once the limits set by the pot and switch are reached so I think that currently we can call this a fix so I shall be setting about repairing all the affected controls with with this bodge type of repair. If Rich does come back to me with the correct item and the cost is not to prohibitive then I shall probably revert back to the stock part, but so far I have heard nothing back from him other then to say that he was looking into it.

Just by way of a backup plan, I have also ordered some of the flexible type from Aliexpress as well but they are slightly larger in overall size then the current solution and may not be a suitable in some locations because of their larger diameter, as can be seen from the photo, there is not a lot of clearance between the shaft and the IC mounted below it, so fingers crossed that the current solution holds or Rich can deliver the correct items.

[Specmaster]

Murphy strikes again.

Just as I thought it was all going swimmingly, Murphy struck again, dam him.

I was reassembling the scope just fine and dandy until the question about how are the Y1 and Y2 position pots connect to the 2mm shafts? See attached pictures for details. It seems that there is or should I say, was, a couple of push in connectors / adaptors that fit the slots in the flat pots for the trace position controls. These were missing when I acquired the scope as both shafts just pulled out from the front panel, which I discovered when it was impossible to control the trace positions and these controls were a sloppy on the panel and tipping the scope forwards allowed the shafts and knob to fall away from the scope.

Anyone got any ideas on how I can overcome this hurdle?  I will ask Rich if he can help at all in case there is no easy fix.

The brass couplers from Aliexpress, cut into 2 and then sleeved with heat shrink work really well as a replacement for the original nylon couplers, only problem is the lack of adjustment on the variable input as the shaft has to be heat shrunk prior to reassembly as the pots are contained within the module that the flat pots for trace positions are located on. All others can be fine tuned prior to heat shrinking as they are fully accessible from the bottom of the chassis.

Hopefully once I have a solution for this latest issue, I can get on with the re-capping of the power supply and then I think this beauty will be fully functioning once again.

[tautech]

You're going to need two more of these 2mm couplers:



Cut them to leave the centre portion on one piece and just the boss with the setscrew on the other.
The piece with the centre portion will need to be reduced and flats added to fit the pots.
The other end needs to used somewhere on the shaft to act as a retainer to prevent the "drive" end escaping from engagement into the pot.

[Specmaster]

You're going to need two more of these 2mm couplers:



Cut them to leave the centre portion on one piece and just the boss with the setscrew on the other.
The piece with the centre portion will need to be reduced and flats added to fit the pots.
The other end needs to used somewhere on the shaft to act as a retainer to prevent the "drive" end escaping from engagement into the pot.

Yes, that exactly what I thought, so I tried it out, problem being that halfway or so through the coupling, the bore changes to 4mm  and when adding the flats, the brass just crumbles away  as there is not sufficient material left to add the flats too. I might have to order up some more with a 2mm hole right through it. That might just leave enough brass to accept the flats.

[tronde]

The potentiometers seems to be from Philips (CRC17 ?) . They sold separate shafts in different guises. It might be you can use a shaft for the Piher PT-15 trimmer.

http://www.ebay.co.uk/itm/2-PCS-Piher-Potentiometer-PT15-Shaft-12mm-YELLOW-/320573536187

See slso attached datasheet.

[Specmaster]

The potentiometers seems to be from Philips (CRC17 ?) . They sold separate shafts in different guises. It might be you can use a shaft for the Piher PT-15 trimmer.

http://www.ebay.co.uk/itm/2-PCS-Piher-Potentiometer-PT15-Shaft-12mm-YELLOW-/320573536187

See slso attached datasheet.

I think this might be the answer, I'll have get some more couplings that go from 2mm to 6mm and then cut down the original steel shafts once the assembly has been put back in the chassis and see if I can reach the far shaft to tighten the grub screws, access is very tight.

Food for thought, I'll keep you all posted on how it goes.

[tronde]

One more option.

One M6 unbrako screw (because of the rather large head). One file. One small drill bit. One half of the chinese coupler and some heat shrink tube.

[Specmaster]

One more option.

One M6 unbrako screw (because of the rather large head). One file. One small drill bit. One half of the chinese coupler and some heat shrink tube.

I presume that I use the brass coupler with heat shrink at head end, yes? The head of the coupler measures 9mm.  The dia of the push in shaft for the pot is 6mm and has splines on it, I could cut of the boss on the rim end of the coupler and the centre section is 7.4mm dia. I could perhaps cut some splines on the coupler then shrink wrap the two together and then trim the 2mm shaft  to the required length? The pots have very little resistance to movement that I reckon it would work well.

[tronde]

My idea is that you use the modified unbrako to turn the pot as an alternative to the plastic shafts for the PT-15 trimmer.

Then you use one half of the chinese brass coupler on the long(?) shaft from the front panel and connect the brass coupler to the unbrako's head with heat shrink just as you did with the other shaft.

I can't say for sure that the shaft for PT-15 will fit the Philips pot. I have seen both, but not on the same time. I remember them as similar, but my memory fades. It should be easy to flatten the unbrako so it will fit the Philips pot in the scope.

If you can't find an unbrako, you can try with a normal screw and one or two nuts just behind the screw head to increase the size. If it is difficult for you to flatten a metal screw, you find a lot of nylon screws in China.

[Specmaster]

That's just as I thought. I have actually ordered some of the plastic shafts for the pots now and plan to cut the 4mm boss of the Chinese coupler, cut splines, or grooves in the middle section of said coupler and connect them together with heatshrink. Thinking that the splines will provide sufficient bite to prevent slippage. Should anyone accidentally pull on the knob, the shaft may disengage but could be pushed back again hopefully without breaking the heatshrink coupling.

[Specmaster]

Postman delivered my flexible couplers from Aliexpress today and I'll be using 3 of these on the bottom of the scope, Intensity, Focus and Readout where there is reasonable clearance as there are no components mounted beneath them on the pcb. Started to reassemble the thing as far as I could go until the PT15 shafts arrive, just patting myself on the back when Murphy struck again, 2 of the 2mm shafts for the focus and intensity pots have gone walkabouts. Seemed that they planned an escape from the bits box I put them in, good job there's a plentiful supply of 2mm stainless steel rods on Ebay, oh well more waiting around.

Someone had been at this before and made a right mess of it, seems that 2 of the smaller knobs are not from this model at all as 2 of them are meant to be fitted to shafts with a flat on them, and the knobs just push on, does anyone know where these small knobs that fit a 2mm shaft can be sourced, all the ones I have seen so far start at 4mm? I suppose it is possible that they might have been the original knobs for the Y1 and Y2 position pots as may have in the first instance long plastic shafts because they snap into the pot at one end and the holes on the fascia are infact 4mm for both of them and also the AT-Level control which has a 4mm shaft as well?

To be honest the manual (service) for this model sucks, extremely limited information given to work with.

Be glad when this ones done to be honest, I've got a philips scope waiting for my attention in the corner of the workshop....

[Specmaster]

All the couplers are installed, just need to trim the shafts down a bit once suitable knobs have been sourced. All removed item have been re-installed and attention turned to the power supply and those blown caps.

Caps replaced, old ones tested once removed and surprisingly checked out as fine, but they were going to cause problems in the future so it was a good job done anyway.

In the HV section I discovered that the insulation shield had been subjected to extreme temperature as can be seen from the photo. Removal of the shield revealed the area was awash with SMD's, which did not look to bad to be honest but the section to the top left was of concern. The SMD's here had been replaced with discrete parts as per photo, the resistor should be 1.2R not the 4.5R as this one reads. This feeds the CRT heater filament, what rating (W) would you suggest I replace it with? The diode is fine.

Speaking of diodes, I did quick visual check of other parts within the blackened area and I discovered that that some other diodes had gone and new ones ordered, D444 was removed for checking out of circuit.

Bearing in mind that this scope is 27 years old would you think that the majority of the discolouration of the insulation shield is somewhat to be expected in the areas away from crt supply section and nothing to be overly concerned about or do you think that there may problems there as well? I have given the whole area a good visual check and checked random resisitors and caps etc and they seem to be ok?

[tautech]

That's a bit ugly. 
Try and find something to replace the insulation sheet, I've seen that stuff used in PC PSU's.
As you can see someone's been there before you and best advice would be to keep your eyes wide open and expect everything and anything.
Don't consider that any previous repair has been done right. Tripple check component values against the schematic.

Resistor in series with the CRT heaters is uncommon IME, but if it's specified do the calcs on what wattage to use based on current. Take care as heaters are sometimes common'ed with the CRT cathode and at elevated minus voltages.

[Specmaster]

I have checked the schematic, it shows that resistor as being 1.2 ohms but the schematic also shows that in area, all parts are SMD's as shown on the overlaid board. I could post a copy of the schematic for this board but it is on my desktop PC which currently is awaiting a new CPU cooler tomorrow.

SMD's are not usually any higher than about .125 watts so I was thinking about trying a 1.2 ohm 1/4 watt resistor here?

[tautech]

Spent a bit of time with the schematic:
https://cdn.rohde-schwarz.com/hameg-archive/HM408_1A.pdf
11 Mb

P55
Yes the heater windings are @ -1850V hence the slots in the PCB. That area needs a good clean and maybe some further attention to prevent HV tracking. Corona dope ?
A higher resistance for R480 will have caused higher dissipation and visible damage.
I don't see where caps C440, 441 and 449 are in your pic, are they present ?

In your last pic, where is D444 for the +12V and +UREF supplies ?

I read your OP and you say it has some issues, does that imply you've had it powered up and had something on the display ?
Have you checked the LV power rails are to spec ?
AFAICT regulated rails are +12, +5, +55, +115, -12, -5 and -2kV and all LV +/-2%, I guess that's the tolerance spec not ripple. (can't read German)

It has a SMPS supply generating all but the PDA voltages for those following along at home.

[Specmaster]

Yes, I did see the 1850v on the schematic but dismissed it as a misprint because the CRT Board on P54 shows those two connections from P5 (actually soldered direct to CRT socket) as being 6.3v also as C440 being rated at 22uf @ 35v? C441 is on that section with the HV slots and is surface mounted, I'm not sure where C449 actually is at present, still trying to locate it myself. C440 is mounted the other side adjacent of the PCB to the transformer on the small section that has the slots cut away round it. The component overlay pages P68 and P69 show the required detail, P68 is readable but P69 is extremely hard as the test is black over dark grey ground plains etc.

D444 I removed for better testing as access to the area is restricted and I was not sure of getting a good connection on my meter probes, it is open circuit, so new ones ordered, I'll replace all the diodes in that section once I have sourced the other two as well.

When I first clapped eyes on this the screen when booting up did display the Hameg usual display of the logo and some text along the bottom, once the logo disappeared it was replaced with random dots over the screen so I knew it had problems along with the control issues which were mechanical with faulty couplings, missing shafts and knobs which had 4mm holes somehow wedged onto 2mm shafts.

Hence why I have been concentrating on regaining control, I could see no point in tackling the electronics without being able to operate the controls?

Once Amazon arrive with my desktop PC cooler I could send you a copy of the manual in English but as you have already seen, it is not exactly the best manual in circulation.

I have not checked the voltages yet as I'm struggling to find suitable test points as the boards do not indicate what voltages should be where. Unlike other scopes I have worked on where the voltagess are printed on the PCB by the sockets. This one has loads of plain grey ribbon cables pushed into sockets everywhere without any indication as to what they are for etc.

[tronde]

-1850V on the heater is correct. It is tied to the cathode voltage of the tube to avoid internal flashover between the heater and the cathode. -1850V on the cathode and +12kV on the anode gives about 14kV accelerating voltage as specified. The heater voltage is still 6.3V.

[Specmaster]

OK,  thanks for clearing that up. I have cleaned the area with ipa already what else do you recommend I use to get rid of much contamination  as possible to prevent any flash over?

When I did have it powered there was no sign of arcing etc, no noise was being emitted which is what I would have expected had there been any leakage in that area.

Tautech  suggests replacing the plastic insulation with a new piece, is this sort of material available in small pieces and from where?

[tronde]

With experience from repairing old TVs I would try to remove as much as possible of the burnt PCB material, especially where you have more than 100V.

When it is as burnt as in this image I would use a Dremel-like tool and a small cutting bit and repair it with epoxy (Araldite).


Maybe you can use single sided copper clad PCB glass fibre (FR4) laminate as insulation sheet? I have seen them with thickness down to 0.4mm on ebay. Just leave the copper facing the enclosure. I have seen FR4 without copper as well sold to the RC-hobby people.

[tautech]

Yes, I did see the 1850v on the schematic but dismissed it as a misprint because the CRT Board on P54 shows those two connections from P5 (actually soldered direct to CRT socket) as being 6.3v also as C440 being rated at 22uf @ 35v? C441 is on that section with the HV slots and is surface mounted, I'm not sure where C449 actually is at present, still trying to locate it myself. C440 is mounted the other side adjacent of the PCB to the transformer on the small section that has the slots cut away round it. The component overlay pages P68 and P69 show the required detail, P68 is readable but P69 is extremely hard as the test is black over dark grey ground plains etc.

Cool, just checking. C440 has no GND reference and has the heater voltage only across it so a 35V rating is appropriate.

D444 I removed for better testing as access to the area is restricted and I was not sure of getting a good connection on my meter probes, it is open circuit, so new ones ordered, I'll replace all the diodes in that section once I have sourced the other two as well.

Good, in the pic one of the diodes has a suspect solder joint.

When I first clapped eyes on this the screen when booting up did display the Hameg usual display of the logo and some text along the bottom, once the logo disappeared it was replaced with random dots over the screen so I knew it had problems along with the control issues which were mechanical with faulty couplings, missing shafts and knobs which had 4mm holes somehow wedged onto 2mm shafts.

Hence why I have been concentrating on regaining control, I could see no point in tackling the electronics without being able to operate the controls?

Ok, so we were always heading here for HV repairs. It looked like mechanical repairs were all that was needed at the start.

Once Amazon arrive with my desktop PC cooler I could send you a copy of the manual in English but as you have already seen, it is not exactly the best manual in circulation.

A link is enough, but I think I can glean enough info from the German copy.

I have not checked the voltages yet as I'm struggling to find suitable test points as the boards do not indicate what voltages should be where. Unlike other scopes I have worked on where the voltagess are printed on the PCB by the sockets. This one has loads of plain grey ribbon cables pushed into sockets everywhere without any indication as to what they are for etc.

The headers will offer test points but you might have to trace out where each rail is and draw a legend.
This is where I've printed the appropriate pages from the manual to have on hand and make notes on. 

RS or Farnell will have the sheet material, trying to remember the name of it....maybe Mylar.

[Specmaster]

Spent a bit of time with the schematic:
https://cdn.rohde-schwarz.com/hameg-archive/HM408_1A.pdf
11 Mb

P55
Yes the heater windings are @ -1850V hence the slots in the PCB. That area needs a good clean and maybe some further attention to prevent HV tracking. Corona dope ?
A higher resistance for R480 will have caused higher dissipation and visible damage.
I don't see where caps C440, 441 and 449 are in your pic, are they present ?

Located C449, it is infact on the component side of the PCB, some distance away from you would expect to find it, just above the cutout for the CRT socket, just above C448 and to the left of C460.
It looks to be in fine order, later I'll de-solder it and do a check on its value, but externally it looks as good as the day it was installed. C441 is a SMD part and I can try to remove it and test this but in situ it is reading 4M in one direction and O/C in the other  which would, would it not suggest, that this is OK?

The PCB materiel in the area of C441, D440 and R480 I have been advised to clean up but I'm not even getting a glimmer of a reading even on a 40M resistance scale, likewise on the insulating shield, Mylar or what ever, I cannot get the DMM to flicker so would this also suggest that although the area is blackened, the insulation properties are still fine? At least for the testing purposes at this stage? Maybe if thats the case and I can get the scope to work again, it might be worthwhile giving this area a bit more attention, but I'm aware of the costs are beginning to escalate and at this stage, I'm not sure if what I have discovered and repaired will cure the problem or not. I don't want to get to point where it is a case of chucking good money after bad all the time.I don't really know what the current market value is for a 408 in working order as they seem to be extremely rare.

As to getting a copy of the manual in English, the link is https://www.rohde-schwarz.com/search/hameg-archive_229476.html?term=hm408 and its the first item that is required. the second item gives the manual completely in German but the only part that changes is the first 37 odd pages and the first link gives these pages in English.

[tautech]

Spent a bit of time with the schematic:
https://cdn.rohde-schwarz.com/hameg-archive/HM408_1A.pdf
11 Mb

P55
Yes the heater windings are @ -1850V hence the slots in the PCB. That area needs a good clean and maybe some further attention to prevent HV tracking. Corona dope ?
A higher resistance for R480 will have caused higher dissipation and visible damage.
I don't see where caps C440, 441 and 449 are in your pic, are they present ?

Located C449, it is infact on the component side of the PCB, some distance away from you would expect to find it, just above the cutout for the CRT socket, just above C448 and to the left of C460.
It looks to be in fine order, later I'll de-solder it and do a check on its value, but externally it looks as good as the day it was installed. C441 is a SMD part and I can try to remove it and test this but in situ it is reading 4M in one direction and O/C in the other  which would, would it not suggest, that this is OK?

Not for me, you need confirm it is with further checks out of circuit.
If you have trouble with SMD, a little trick is the 'razor blade technique', that's cutting a trace to get a measurement without removal and soldering over the cut later.

It's looks a nice scope and probably more capable than the Philips so IMO it's worth spending some time and effort on.
Costs if reasonable are immaterial, what you learn from an exercise like this are worth far more than the value of 3 or 4 CRO's and stand you in good stead for ANY further repairs later on.
This I can't stress enough, it's now I got into the game and gives one good understanding of the many design principles that are used in equipment.
It's shaping up to not be an easy repair but if you crack it, it will be very rewarding. 

I'm a bit suprised member Oldway hasn't chimed in.......maybe he thinks we're doing OK thus far. 

[Specmaster]

Trouble is that the traces are not visible so I'd need to mark it prior to removal in order to replace it the correct way round. So I'll have a look at this later, still getting my desktop running again. Got caught with that blasted ransom ware and had to do a fresh install of everything again, so time consuming.

[Specmaster]

Not for me, you need confirm it is with further checks out of circuit.
If you have trouble with SMD, a little trick is the 'razor blade technique', that's cutting a trace to get a measurement without removal and soldering over the cut later.

It's looks a nice scope and probably more capable than the Philips so IMO it's worth spending some time and effort on.
Costs if reasonable are immaterial, what you learn from an exercise like this are worth far more than the value of 3 or 4 CRO's and stand you in good stead for ANY further repairs later on.
This I can't stress enough, it's now I got into the game and gives one good understanding of the many design principles that are used in equipment.
It's shaping up to not be an easy repair but if you crack it, it will be very rewarding. 

I'm a bit suprised member Oldway hasn't chimed in.......maybe he thinks we're doing OK thus far. 

Ok, latest test results for you, I removed C440, C441 and C449 and pleased to say that both C441 and C449 are spot on their quoted ratings albeit C449 schematic shows it as being 2.2nf 3kv but the item is a 100nf @ 100v. The unit showed no signs of having been worked on before either so I poked around a bit more and discovered that C449 is actual listed twice on the schematic, the second  time is on the 57v rail and connected to the centre point of transformer winding 13 & 14 via D443 and L403and the value of it is 100nf @ 100v so that means that the one we have been looking for is missing from the PSU. Could this be on the CRT socket in the scope I wonder? I'll check this out later on.

However, C440 was actually reading as 22uf but with an ESR of .14k, so this has been replaced with a 33uf  50V which has an ESR of .7ohm.

D444 has been refitted as I discovered that it is infact perfectly OK, I was testing on OHMs range instead of Diode test and therefore I was getting a reading of my body resistance as I was holding the probes on the diode  .

So maybe just the elusive C449 2.2nf 3kv to resolve and clean up the PCB around the HT section, replace the Mylar and just maybe home and dry. I'm looking forward to the big switch on and finding out if we are indeed on the home run.

Still need to sort out some control knobs that will fit onto the 2mm shafts, which I need to to cut down to size once the knobs have been sourced, currently using brass couplings as temporary knobs for testing purposes. 

[Specmaster]

I have enlarged the board layout drawings to 500% and there is no sign of the elusive C449 on the HT supply to the CRT heater etc so is it likely then that this was either added or removed at some point in its production run as a modification but the schematic was never up dated to reflect this at all? It is after all is said and done only sitting on leg of the supply and ground, so is this critical to the operation or not? I really can't see as it is.

[tautech]

I have enlarged the board layout drawings to 500% and there is no sign of the elusive C449 on the HT supply to the CRT heater etc so is it likely then that this was either added or removed at some point in its production run as a modification but the schematic was never up dated to reflect this at all? It is after all is said and done only sitting on leg of the supply and ground, so is this critical to the operation or not? I really can't see as it is.

The CRT heater supply is unusual as it is essentially a DC supply whereas most heater supplies are AC.
The caps will be to get it as noise free as possible as not to impart anything onto the cathode supply.
The noise sources are the SMPS itself and the switching noise from the diode.
As the schematic shows it need be an HV device as it's between the HV cathode supply and Gnd.
2n2 @ 3kV shouldn't be too hard to source, they're normally 2 leaded dipped MLCC and when I can't find them locally I get them from Aliexpress.

You could leave it out for now IF there's easy access to add it later.

[tautech]

BTW, for the power up do use a dim bulb tester, say with a 100W incandescent bulb.

[Specmaster]

I have ordered some oi the 2.2nf 3kv caps, be with me later this week, also ordered some of the BAS21 diodes rather the one current fitted and although the original pads have long gone from the HT section on PCB, I can recreate them making solder pads of the part leads that protrude through the board from inside the screening box, it will then be back to as much original as possible. Do you think in place of the original 1.2 ohm SMD resistor, a .25w 1 ohm carbon one would suffice?

Regards, the 2n2 @ 3KV, I can find no sign of where this was ever fitted to that board as the schematic suggests. There is no ground connection within easy reach of that location which shows any signs of being interfered with, it all looks original and no un-populated holes anywhere on the board it could have occupied for the ground connection???

As to the dim bulb tester, I'll have to build myself one of those first.

[tautech]

I have ordered some oi the 2.2nf 3kv caps, be with me later this week, also ordered some of the BAS21 diodes rather the one current fitted and although the original pads have long gone from the HT section on PCB, I can recreate them making solder pads of the part leads that protrude through the board from inside the screening box, it will then be back to as much original as possible. Do you think in place of the original 1.2 ohm SMD resistor, a .25w 1 ohm carbon one would suffice?

Possibly.
If you wanted to check this out more thoroughly use a bench supply to power only the heater and measure the heater current and/or the drop across 1R2. Drop times current will give wattage required and derate by 50%.

Regards, the 2n2 @ 3KV, I can find no sign of where this was ever fitted to that board as the schematic suggests. There is no ground connection within easy reach of that location which shows any signs of being interfered with, it all looks original and no un-populated holes anywhere on the board it could have occupied for the ground connection???

Unless you can see a schematic addendum stating it's been omitted I'd be inclined to add it as per original design.

As to the dim bulb tester, I'll have to build myself one of those first.

Real easy if you have a junk pile, here's pics and explanation of mine:
https://www.eevblog.com/forum/repair/suggested-for-a-sticky-part-one-comments-or-additions-please/msg470686/#msg470686

[Specmaster]

Bearing in mind that that the original resistor was only a small surface mounted one and I think I'm correct (please advise me if not) that the wattage of these is .125w dissipation and I'm talking of doubling that. The diode is capable of passing 200ma then that gives a simple ohms law calc of approx 40mw dissipated across the resistor, well with in the capabilities of the 1/4 watt resistor.

The 2n2 @ 3kv when it arrives, will just have to be piggy backed onto the nearest ground point then.

As to the dim bulb tester, I do as it happens, have a dead PC PSU that I could just gut and modify , using the old input din connector and adding in a flying 13A socket from an extension lead.

[Specmaster]

Dim bulb tester built and tested OK, I'll add in a bypass switch at a later date to switch on full power.

Power supply is now rebuilt and ready for testing, hopefully tomorrow before re-installing in scope. Can anyone throw some light on the connection to be found on plugs P404 (pin 3) and P407 (pin 6) marked as FCV? what voltage (if any) should I expect to see there when testing?

[tautech]

Dim bulb tester built and tested OK, I'll add in a bypass switch at a later date to switch on full power.

That's now I had mine originally but changed it to just switching the series bulb on or off.
Didn't want to make a silly mistake by having the bulb bypassed or when plugged in the DUT power was always on, even through the bulb. 
Check DUT with tester and if all is well then remove and apply power directly.
It's not something you use everyday so I wanted mine idiot (me) proof. 

Can anyone throw some light on the connection to be found on plugs P404 (pin 3) and P407 (pin 6) marked as FCV? what voltage (if any) should I expect to see there when testing?

FCV must be an abbreviation of something German, check the 408-1 English manual and the circuit description.
I thought it might be Line trigger but AFAICT that's taken care of in P400.
The only other thing it might be is UREF, derived from the 12V supply. I guess it's just a signal line as it's supplied via 1k so it can't power bugger all other than some sensing.

[Specmaster]

Good point about the dim bulb tester, always a possible accident waiting if a bypass switch is fitted. So I guess I'll just fit a on/off switch. I tested it with the Philips scope and was pleasantly surprised as there seems to be very little wrong with it apart from from some distortion on a particular timebase setting which might just a bit calibration tweaking or termination with a 50 ohm input for calibrating instead of a direct connection.

Once the 408 is working again, I'll have to keep my eyes open for another none working 408 as a parts donor as those pesky knobs seem to be almost impossible to get. I found a supplier who can get them for me but I have to order a moq of 100 pieces in order for the manufacturer  (ELME) to make them even though they are in their catalogue. So it looks like I'll be using my home made knobs a bit longer yet.

As to the FCV I'll check the English version of circuit description and see if it is mentioned in it. I did think it might be Focus Control Volts but that's just a single control and it's highly unlikely to be connected to 2 plugs as these in general tend to fed a single board each.

Fingers crossed everything checks out with the voltage rails, then hopefully this scope might be breathing again.

[9a4wy]

Dim bulb tester built and tested OK, I'll add in a bypass switch at a later date to switch on full power.

Power supply is now rebuilt and ready for testing, hopefully tomorrow before re-installing in scope. Can anyone throw some light on the connection to be found on plugs P404 (pin 3) and P407 (pin 6) marked as FCV? what voltage (if any) should I expect to see there when testing?

FCV is "focus voltage"...goes to FOCUS POT.(see schematic page 41/77)
I do not remember exactly...but you can expect between -6 to 12 VDC...something like that.
My problem was in HV board...PITA to open it every time for change/measure.
Mine was restored and fully working condition now...
K
Kristian

[tautech]

Good point about the dim bulb tester, always a possible accident waiting if a bypass switch is fitted. So I guess I'll just fit a on/off switch. I tested it with the Philips scope and was pleasantly surprised as there seems to be very little wrong with it apart from from some distortion on a particular timebase setting which might just a bit calibration tweaking or termination with a 50 ohm input for calibrating instead of a direct connection.

I'd be suspicious of the PSU or some contacts if it's just one setting affected.
You've got another working scope, right ? Go looking. 

[tronde]

Once the 408 is working again, I'll have to keep my eyes open for another none working 408 as a parts donor as those pesky knobs seem to be almost impossible to get. I found a supplier who can get them for me but I have to order a moq of 100 pieces in order for the manufacturer  (ELME) to make them even though they are in their catalogue. So it looks like I'll be using my home made knobs a bit longer yet.

What kind of knobs are missing? Rotatating with double shafts?

A picture of what you have now and some mechanical dimensions would be useful.

[Specmaster]

What kind of knobs are missing? Rotatating with double shafts?

A picture of what you have now and some mechanical dimensions would be useful.

OK, picture in the morning, but basically they are the typical small knob that you would find on controls like focus, position etc, rotating, single, 2mm shaft and the base of the knob is approx 9 or 10mm dia and approx 6mm at the top. More precise dimenions in the morning.

[Specmaster]

Powered up the PSU tonight, no bangs but strange voltages, nothing is anywhere near what it should be, 115v is about 500mv, 5v is 3.5v and dropping all the time and this is open circuit, just the PSU on the bench. Started poking around in the SMD stuff now and what to naked eye looks to be OK, under a USB microscope is horrendous. Massive amount of gunge around everything and a few suspect joints to be reflowed and already found at least 1 diode that has become a resistor, and a few resistors that are way out of spec.

Before I go any further I need to clean everything up on the solderside and I'm wondering if there isn't a better way of doing this than what I'm doing.

I'm using cotten buds and Isoproyl Alcohol BP 70% but the cotton buds very quicky come undone and leave huge amounts of cotton fibre on eveything. I'm using dental picks to scrap away flux residue etc in order to get access to the joints, both to test the compenants and also remake the joints but takes a lifetime.

I'm also wondering if there is another more common Hameg that uses the same PSU that I might be able to whip the PSU out of and install and then I could do the old PSU up at my leisure.

[tautech]

Powered up the PSU tonight, no bangs but strange voltages, nothing is anywhere near what it should be, 115v is about 500mv, 5v is 3.5v and dropping all the time and this is open circuit, just the PSU on the bench.

+5V is the feedback reference for the SMPS and I think it should have some small load on it. Say a 6 or 12V auto bulb.

Started poking around in the SMD stuff now and what to naked eye looks to be OK, under a USB microscope is horrendous. Massive amount of gunge around everything and a few suspect joints to be reflowed and already found at least 1 diode that has become a resistor, and a few resistors that are way out of spec.

Before I go any further I need to clean everything up on the solderside and I'm wondering if there isn't a better way of doing this than what I'm doing.

I'm using cotten buds and Isoproyl Alcohol BP 70% but the cotton buds very quicky come undone and leave huge amounts of cotton fibre on eveything. I'm using dental picks to scrap away flux residue etc in order to get access to the joints, both to test the compenants and also remake the joints but takes a lifetime.

Tooth brush is what many use with IPA.
Have a look at repair vids by Defpom, he also uses some tough little wipes under the brush to mop up as his last cleaning action.

[tronde]

What kind of knobs are missing? Rotatating with double shafts?

A picture of what you have now and some mechanical dimensions would be useful.

OK, picture in the morning, but basically they are the typical small knob that you would find on controls like focus, position etc, rotating, single, 2mm shaft and the base of the knob is approx 9 or 10mm dia and approx 6mm at the top. More precise dimenions in the morning.

OK.
Something like this and a metal tube soldered or glued on the shaft should do.

http://no.farnell.com/elma/020-2220/knob-black/dp/1209749

http://www.ebay.com/itm/Brass-Pipe-Copper-Pipe-Copper-Tube2mm-3mm-4mm-5mm-Long-300mm-Wall-0-5mm-/222369005754

This knob is for 3.175mm shaft, but I think it sould clamp on 3mm as well. The tube with 2mm inside is 3mm outside.

The same knob is available in 4mm too, and if you can find 2x4mm tubes it might be a better choice.

[Specmaster]

+5V is the feedback reference for the SMPS and I think it should have some small load on it. Say a 6 or 12V auto bulb.

Tooth brush is what many use with IPA.
Have a look at repair vids by Defpom, he also uses some tough little wipes under the brush to mop up as his last cleaning action.

So the +5v rail controls the others then, without some form of a load, the others effectively shut down? I thought the +5 rail was the supply rail for all the IC's in it and there are plenty of those.

I tried the toothbrush idea, it does indeed make a far better job and much much quicker but also leaves the board with a white coating like a frosting over it. I'll give it another go today and see if a second cleaning removes the frosting effect.

As I work my way around this scope, I see there are a number of what seems like 2 pole testing points cropping up and I find no mention of these in the service manual so I'm guessing that either are test points used during the manuerfacturing process or there is somewhere in existance a proper workshop manual that their own service dept used to use when these machines were current, if so then I'd like to get my hands on a copy, if only to burrow it while I'm trying revive the beast.

[tautech]

+5V is the feedback reference for the SMPS and I think it should have some small load on it. Say a 6 or 12V auto bulb.

Tooth brush is what many use with IPA.
Have a look at repair vids by Defpom, he also uses some tough little wipes under the brush to mop up as his last cleaning action.

So the +5v rail controls the others then, without some form of a load, the others effectively shut down? I thought the +5 rail was the supply rail for all the IC's in it and there are plenty of those.

Yes but you said you had the PSU on the bench and without a load, SMPS don't normally like that.
You can see where the FB comes from the +5V rail to an op amp and then onto a circuit where all the other LV rails terminate too. This I believe is some sort of LV rail monitoring too and will affect the FB level in some way.
If they are all connected on the PSU board to this part and you apply some small load to +5V I think it will operate.

I see there are a number of what seems like 2 pole testing points cropping up

2 are marked on the schematic between the base of T454 and +UREF

[Specmaster]

Finally reassembled the scope and plugged it into the dim bulb unit, switched on, bulb glowed dimly for a second and went out, the power led and others on the front flashed about 5 times and went out then nothing. Switching off and on again, produced exactly the same results except that the 100w bulb now does not glow??

There was no arcing, bang or magic smoke at all.

As I had to rebuild the heater connections in the PSU it is possible that I have got the connections to the heater reversed bearing in mind that F14 connection to the heater also goes to the HV board?  Could this be the problem??

[tautech]

Finally reassembled the scope and plugged it into the dim bulb unit, switched on, bulb glowed dimly for a second and went out, the power led and others on the front flashed about 5 times and went out then nothing. Switching off and on again, produced exactly the same results except that the 100w bulb now does not glow??

There was no arcing, bang or magic smoke at all.

As I had to rebuild the heater connections in the PSU it is possible that I have got the connections to the heater reversed bearing in mind that F14 connection to the heater also goes to the HV board?  Could this be the problem??

Maybe, the heater supply is polarised and need be around the right way.

Otherwise sounds good. 
Try it with a lower wattage bulb and if all is well it should do similar but not go completely out.
With the tester in circuit you might like to check the SMPS is working with another scope......no connection ahla near field probe style.
Look for it oscillating in the 20 -40 KHz range.

[Specmaster]

Finally reassembled the scope and plugged it into the dim bulb unit, switched on, bulb glowed dimly for a second and went out, the power led and others on the front flashed about 5 times and went out then nothing. Switching off and on again, produced exactly the same results except that the 100w bulb now does not glow??

There was no arcing, bang or magic smoke at all.

As I had to rebuild the heater connections in the PSU it is possible that I have got the connections to the heater reversed bearing in mind that F14 connection to the heater also goes to the HV board?  Could this be the problem??

Maybe, the heater supply is polarised and need be around the right way.

Otherwise sounds good. 
Try it with a lower wattage bulb and if all is well it should do similar but not go completely out.
With the tester in circuit you might like to check the SMPS is working with another scope......no connection ahla near field probe style.
Look for it oscillating in the 20 -40 KHz range.

I would love to do that but as I don't have a near field probe, I can't.

I have also bypassed the bulb with same results and talking with 9a4wy he suggested that I measure the voltages each time I switch on and here the results
Measured at the test socket for voltages (P2700} and had to break out my analogue Avo for this as my DMM was unreadable with the constant voltage changes and I got the following results

Expected        Actual
-12v               -3v
-5v                 -2.5v
+5v                +2.5v
+12v              +5v
+115v            +60v
+57v              +20v

All of these were only present for a second and decayed to zero with in a couple of seconds.

So its common on all rails.

We have also checked the resistance to ground for each of the rails at the same location and the results are as follows:-
-12v     75.5ohms
-5v       257ohms
+5v      440ohms
+12v    202ohms
+115v   36.92k
+57v      32.93k

so we can rule out short circuits.

He also recommends recapping the whole PSU so that is my next task but I need to order up another couple 470uf 25v as I'm short.

So thats where I'm presently at.

Picked up a lovely Hitachi V525 scope this week almost mint condition apart from a (rusty handle, been in its carry bag all its life and the probves still in sealed bags), and a slight problem on channel 2, the volts per division are not changing on screen, timebase and both perfectly ok on channel 1, but that one can sit while I refurb this Hameg.

[tautech]

OK, cool.

Ahla near field is just using an ordinary scope probe close to some EMI source.......try it on a wallwart or something. 

[Specmaster]

I'll try that then, thought you was talking a higfh impedance probe for high voltages for a minute.

[tautech]

It might seem that the SMPS is going into UVLO. Check that it's VCC pin is getting full voltage as spec'ed in the datasheet as needed to continue running.
Check C422 on the VCC supply too, I've seen them leaky and their current source not strong enough to maintain voltage for the SMPS IC VCC.

[Specmaster]

OK, cool.

Ahla near field is just using an ordinary scope probe close to some EMI source.......try it on a wallwart or something. 

I got the scope back on the bench tonight and tried the probe as you suggested, it does indded seem to be oscillating ok, holding the probe near the transformer and switching the scope does indeed produce waveforms and then after a couple of seconds disappears as the smps shuts down.

Tomorrow I'll start the recapping process, albeit I'll be 1 cap short (470uf) until my order is delivered, but at least I'll be able to replace the C422 and see if that cures the problem.

[Specmaster]

OK.
Something like this and a metal tube soldered or glued on the shaft should do.

http://no.farnell.com/elma/020-2220/knob-black/dp/1209749

http://www.ebay.com/itm/Brass-Pipe-Copper-Pipe-Copper-Tube2mm-3mm-4mm-5mm-Long-300mm-Wall-0-5mm-/222369005754

This knob is for 3.175mm shaft, but I think it sould clamp on 3mm as well. The tube with 2mm inside is 3mm outside.

The same knob is available in 4mm too, and if you can find 2x4mm tubes it might be a better choice.

This looks like a good solution and one that I'll try out once I get the power supply working correctly, thank you.

[tronde]

2x4mm tubes

http://www.ebay.com/itm/Inner-Diameter-2-12mm-Brass-Round-Tubes-1mm-Wall-Project-Pipe-Tube-500mm-Long-/282447547848

The knobs are known as "collet knobs" on ebay.

[Specmaster]

It might seem that the SMPS is going into UVLO. Check that it's VCC pin is getting full voltage as spec'ed in the datasheet as needed to continue running.
Check C422 on the VCC supply too, I've seen them leaky and their current source not strong enough to maintain voltage for the SMPS IC VCC.

All electrolytics have now been replaced, did find one that was essentially a resistor, 139k to be precise. switched it on and its the same, power there for approx. 2 seconds and off.  SMPS is always bit of a mystery to me so I'm poking about in the dark really Is there a really fool proof method of deciding if the unit is oscillating etc without any need for specialist equipment?

Any idea on what to check on next?

[tautech]

It might seem that the SMPS is going into UVLO. Check that it's VCC pin is getting full voltage as spec'ed in the datasheet as needed to continue running.
Check C422 on the VCC supply too, I've seen them leaky and their current source not strong enough to maintain voltage for the SMPS IC VCC.

All electrolytics have now been replaced, did find one that was essentially a resistor, 139k to be precise. switched it on and its the same, power there for approx. 2 seconds and off.  SMPS is always bit of a mystery to me so I'm poking about in the dark really Is there a really fool proof method of deciding if the unit is oscillating etc without any need for specialist equipment?

Any idea on what to check on next?

Read and study this:
http://www.nxp.com/documents/data_sheet/NE_SE5560.pdf

Monitor VCC on startup, it must remain above UVLO.

[Specmaster]

I already have downloaded this data sheet but I have to say that it does not make a wonderful lot of sense to me. A lot has changed in the last 50 years since I was last active in the field of electonics so certain aspects leave me struggling a little. I have scoured the data sheet looking for any reference to UVLO which I know is Under Voltage Lock Out but not seen any.

What sort of voltage should I be expecting on Pin 1 and what other pin (pin 16?) would I be monitoring for UVLO?

[tautech]

I already have downloaded this data sheet but I have to say that it does not make a wonderful lot of sense to me. A lot has changed in the last 50 years since I was last active in the field of electonics so certain aspects leave me struggling a little. I have scoured the data sheet looking for any reference to UVLO which I know is Under Voltage Lock Out but not seen any.

What sort of voltage should I be expecting on Pin 1 and what other pin (pin 16?) would I be monitoring for UVLO?

The NE5560 is an old chip now, the datasheet says 1994 and some of the terminology is not the same as used today.
They refer to UVLO as: Low supply voltage protection and it means the same thing. On P3 it's mentioned as:  low supply voltage protection thresholds and the value to use is 10.5 V on pin 1 (VCC) relative to pin 12 (Gnd).
10.5V must be present for the chip to start and continue running and SMPS will sometimes start then fail as there are generally 2 sources of current to supply the SMPS IC and the second (run) supply might not take over. For the start supply there is generally some high value resistor or chain from rectified mains, in this case I believe via R422 and then another supply from a feedback winding on the SMPS transformer. This supply (run) I believe is via R421 and fed from winding 2.
The components in each supply chain need be checked as each must be OK for the IC to start and continue running.

Monitor pin 1 and if it falls below the UVLO after start, there is your problem. 
They (the IC) can be coaxed to run with a bench supply and output then checked with a scope but this should be done without mains connection.

[Specmaster]

The NE5560 is an old chip now, the datasheet says 1994 and some of the terminology is not the same as used today.
They refer to UVLO as: Low supply voltage protection and it means the same thing. On P3 it's mentioned as:  low supply voltage protection thresholds and the value to use is 10.5 V on pin 1 (VCC) relative to pin 12 (Gnd).
10.5V must be present for the chip to start and continue running and SMPS will sometimes start then fail as there are generally 2 sources of current to supply the SMPS IC and the second (run) supply might not take over. For the start supply there is generally some high value resistor or chain from rectified mains, in this case I believe via R422 and then another supply from a feedback winding on the SMPS transformer. This supply (run) I believe is via R421 and fed from winding 2.
The components in each supply chain need be checked as each must be OK for the IC to start and continue running.

Monitor pin 1 and if it falls below the UVLO after start, there is your problem. 
They (the IC) can be coaxed to run with a bench supply and output then checked with a scope but this should be done without mains connection.

Thank you, it makes sense now, I was looking for the term ULVO. I came accross the section on page 7 where it discuss the low voltage protection being active when V(1-12) is below 10.5v. This I thought was saying that the voltage between pins 1 and 12 must be above 10.5v for the psu to function.

The same section also mentions that the circuit can be fed from a DC supply between 10.5v and 18v maximum, so do I take it then that I could connect say 12v to this SMPS via Pin 1 and ground, leave the mains disconnected, and the scope should fire up?

If so, I could also do this, I have a bench supply capable of providing upto 30v @ 5A with built in current limiting. If I have understood this correctly, it would allow me to see if there are any other issues apart from the SMPS?

[tautech]

The same section also mentions that the circuit can be fed from a DC supply between 10.5v and 18v maximum, so do I take it then that I could connect say 12v to this SMPS via Pin 1 and ground, leave the mains disconnected, and the scope should fire up?

No it doesn't work like that.
12V on P1 and Gnd will only (hopefully) get the SMPS IC to start PWM output and at: When the supply voltage surpasses the 10.5V level, the IC starts delivering output pulses via the slow-start function. (soft start)

When powered appropriately from a bench supply you then check the output waveform.....this only confirms the IC is operating as it should.
When mains powered you need next confirm VCC on pin 1 maintains 10.5V +.

We need find a better datasheet, one that the typical application schematic diagram is legible. 

[Specmaster]

As I look round the schematic there are at least 3 different symbols being used to represent a ground connection. I always thought that ground was ground, but is that not the case here, is there like a circuit ground for the DC and an earthed ground for the AC mains on this chassis?

I just took a voltage reading from pin 1 on the NE5560N and the actual chassis and I actually got -115v. According to the schematic pins 14 and 12 are strapped together and grounded. I cannot get a path between them and the chassis??

[tautech]

As I look round the schematic there are at least 3 different symbols being used to represent a ground connection. I always thought that that ground was ground, but is that the the case here, is there like a circuit ground for the DC and an earthed ground for the AC mains on this chassis?

I just took a voltage reading from pin 1 on the NE5560N and the actual chassis and I actually got -115v. According to the schematic pins 14 and 12 are strapped together and grounded. I cannot get a path between them and the chassis??

Good catch.
Yes it appears there at least 2 circuit Gnd's, one of which seems exclusive to the PSU...........use it. 

[Specmaster]

OK, found a common point for the ground in the PSU primary and VCC is +12.5v ref pin 12 but is only there for the same period, approx 1 to 2 seconds and goes to zero.

Resistors R421 and 422 located and seem to be in spec at R421 = 6.8ohms and R422 = 2.4k ohms. D415 also checks out OK.

[tautech]

OK, found a common point for the ground in the PSU primary and VCC is +12.5v ref pin 12 but is only there for the same period, approx 1 to 2 seconds and goes to zero.

Resistors R421 and 422 located and seem to be in spec at R421 = 6.8ohms and R422 = 2.4k ohms. D415 also checks out OK.

Do you mean D416 ?

There's a few things I'm having trouble getting my head around, T403 for instance. It seems to be in the current path for IC VCC and the base is driven when the mains switch is closed. 


We need to study up on theory of operation from the English SM.
There's a bit of nutting out to do, eg I think I've got pin 10's circuit sorted, it's remote switching configured to ON from the output of winding 11.

Time to take a step back and understand how all these bits interrelate and might affect start and run.
You could just check all these bits (circuits) one by one but the more challenging is to diagnose the faulty part/area.

You need to be a bit careful sticking a scope in this SMPS while mains powered as any probe reference point is not mains ground referenced like a scope is.
Gotta go.

[Specmaster]

OK then, I started looking around and was not able to find anything around the area that was wrong, so I started to revisit everything that I had done previously and found that a pad had been disconnected from the trace on R434 3.3k 4w which one side is connected to pin 13 on NE5560N, DVR, the trace on the other side was the affected one before it connected to anything above the resistor. Would the fact this connection was open circuit cause the smps to shut down? I thought I'd ask for your thoughts before re-assembling the boards and psu again to test it as it is such a pain to do each time.

[tautech]

OK then, I started looking around and was not able to find anything around the area that was wrong, so I started to revisit everything that I had done previously and found that a pad had been disconnected from the trace on R434 3.3k 4w which one side is connected to pin 13 on NE5560N, DVR, the trace on the other side was the affected one before it connected to anything above the resistor. Would the fact this connection was open circuit cause the smps to shut down?

Yep, that might do it but I think you're reading the schematic wrong, it's R433 1M5.
R434 is part of a snubber that protects the power FET switcher.

I thought I'd ask for your thoughts before re-assembling the boards and psu again to test it as it is such a pain to do each time.

No problem. You've invested some time now so you need to be careful not to loose this time, do things methodically and don't work on it when fatigued.

[Specmaster]

.p
Yep, that might do it but I think you're reading the schematic wrong, it's R433 1M5.
R434 is part of a snubber that protects the power FET switcher.

I thought I'd ask for your thoughts before re-assembling the boards and psu again to test it as it is such a pain to do each time.

No problem. You've invested some time now so you need to be careful not to loose this time, do things methodically and don't work on it when fatigued.

Oops you're right, fatigue has set in in that case my information was all wrong, it was the connection to the 330pf cap that was broken and that wouldn't cause the problem I doubt?

I checked the components around T403, that is R432, D423, R417, T403 and R422, all checked out OK but while checking that leg, I also checked L401 which shows as a 270uH but measures 64uH on my LCR bridge. I checked R433 as well and it is 1M5 so that OK.

[tautech]

Oops you're right, fatigue has set in in that case my information was all wrong, it was the connection to the 330pf cap that was broken and that wouldn't cause the problem I doubt?

Correct, that shouldn't stop it.

I checked the components around T403, that is R432, D423, R417, T403 and R422, all checked out OK but while checking that leg, I also checked L401 which shows as a 270uH but measures 64uH on my LCR bridge. I checked R433 as well and it is 1M5 so that OK.

Test frequency for L401 could alter measured values.

We need to understand IC405 and how it affects the SMPS IC VCC, it's just a 4093 so cheap to replace.
Gotta go.....later.

[Specmaster]

Oops you're right, fatigue has set in in that case my information was all wrong, it was the connection to the 330pf cap that was broken and that wouldn't cause the problem I doubt?

Correct, that shouldn't stop it.

I checked the components around T403, that is R432, D423, R417, T403 and R422, all checked out OK but while checking that leg, I also checked L401 which shows as a 270uH but measures 64uH on my LCR bridge. I checked R433 as well and it is 1M5 so that OK.

Test frequency for L401 could alter measured values.

We need to understand IC405 and how it affects the SMPS IC VCC, it's just a 4093 so cheap to replace.
Gotta go.....later.

Thing that keeps bugging me is that before I took the psu out of the scope, I was getting dots on the screen and the LED's lit up, now, the LEDS are on for approx 2 secs and go off, nothing on the screen. I suppose it could just be a coincidence that it was going to fail anytime and it chose then to do so....

[tautech]

Thing that keeps bugging me is that before I took the psu out of the scope, I was getting dots on the screen and the LED's lit up, now, the LEDS are on for approx 2 secs and go off, nothing on the screen. I suppose it could just be a coincidence that it was going to fail anytime and it chose then to do so....

Maybe but you've done a bit of work on her so it's time to revisit that and double check you've not made any error/s.

Thinking about this a good part of today and I think we need use the inputs into the SMPS IC to help tell us what's going on so time to study both the schematic and IC datasheet, the parameters that get it going and keep it going.

As it's not wise to connect a scope (elevated voltages) we're stuck with using a DMM, hopefully yours has a Peak Hold function that might capture voltages on the various inputs at power up before it falls flat on its face.

Otherwise this is differential probe territory but you might be able to do something with Maths and 2 channels so that there's no probe reference connection needed and zero risk of ground loops.

Assuming the SMPS circuitry is faultless then the shutdown is normal behaviour for a fault in the circuitry it supplies as the current sense R457 in series with the transformer primary will throw an excessive voltage into the CS pin 11 input. Specs are on P4 of the datasheet. Small mention at foot of P7.
Further explanations and graph showing hysteresis on P12.

Yeah, yeah, I know SMPS is not your strength, not really mine either but that's where we need to go IMO.


Edit
BTW I looked at the EN 408 manual and there seems to be zero in the way of circuit description or theory of operation, these are just User manuals but at least they have schematics and PSU voltages.

[Specmaster]

I was revisiting what I have previously done and thats how I noticed the pad and trace were not joined. I will be tracing my steps back again shortly and also trying to fathom out IC401, 403 and 405 relationship.

What about the opto couplers, could they be playing a part in this?

As you said, the chips are relatively inexpensive so I also ordered up a replacement set while they are still available, the NE5560N is already obsolete.

[tautech]

What about the opto couplers, could they be playing a part in this?

I wondered that too but if they are now, why ?

One is to transfer the Line frequency to the timebase board so that won't have an influence on startup and the other is secondary side voltage feedback but if it's working correctly it should be only for regulation.

You could try feeding only the +5V from a bench supply and check the current drawn seems reasonable.
This would be the only rail I would check like this to minimise the chance of damage to componentry.

[Specmaster]

Thats sounds like a plan, I'll do that after lunch.

[Specmaster]

You could try feeding only the +5V from a bench supply and check the current drawn seems reasonable.
This would be the only rail I would check like this to minimise the chance of damage to componentry.

The results of the 5
+5 current drain test is without the psu being connected is 800mA and the single largest consumer of that is the digital board and not surprising given the large number of IC's on it, over 100 of them.

Testing the psu on the +5v draw is zero which I think is correct as the IC's on the secondary side of it are jfets and those on the primary side are connected to the 5v rail, instead getting their power directly from the incoming mains, via droppers and diodes and or the full wave bridge rectifier.

[tronde]

BTW I looked at the EN 408 manual and there seems to be zero in the way of circuit description or theory of operation, these are just User manuals but at least they have schematics and PSU voltages.

No description in the German manual either.

It says that all DC voltages are electronically stabilised and should be within +/- 2%. If outside it is something wrong in the circuit. It also says that the only adjustable voltage is the +12VDC. Adjustment and test-point for that is on the X/Y PCB.

+12V, +5V, -12V, -5V, +55V, +115V, -2000V and +12kV are listed as DC voltages and 6.3V and 12V are listed as AC-voltages. 6.3VAC is for CRT heater and 12VAC is for screen graticule light and mains triggering.

[Specmaster]

BTW I looked at the EN 408 manual and there seems to be zero in the way of circuit description or theory of operation, these are just User manuals but at least they have schematics and PSU voltages.

No description in the German manual either.

It says that all DC voltages are electronically stabilised and should be within +/- 2%. If outside it is something wrong in the circuit. It also says that the only adjustable voltage is the +12VDC. Adjustment and test-point for that is on the X/Y PCB.

+12V, +5V, -12V, -5V, +55V, +115V, -2000V and +12kV are listed as DC voltages and 6.3V and 12V are listed as AC-voltages. 6.3VAC is for CRT heater and 12VAC is for screen graticule light and mains triggering.

I know, it seems that there must be another manual somewhere for this, I downloaded what was supposed to the service manual, it doesn't give you the info for calibration either unlike the one I have for my Iwatsu scope which gives much more detailed information, about 200 pages and is called "Instruction Manual" and has the following sections within it 1) Specifications, 2) Operating Information, 3) Measuring Procedures, 4) Theory of Operation, 5) Maintenance, 6) Check and Adjustment, 7) Schematic Diagrams, Electrical Parts List and lastly 9) Mechanical Parts List and Illustration. It also is full of photos and screen shots etc. to assist.

For electrical parts it also provides the circuit reference, description and the Iwatsu parts number and it does that circuit by circuit and does the same for the mechanical parts which includes are the all the case parts as well, chalk and cheese eh?

There are other threads on the forum about Hameg scopes and it seems that Oldway has had cause to contact them before, maybe for his large number of scopes as more then a couple are Hamegs, and he recommends people to contact them as they did have in 2014 at least have spares still available. To that end I have also emailed them @ contact.uk@rhodes-schwarz.com to see if they can throw any light on this problem, and also how does this smps operate particularly the interplay between the 3 IC's in the control section. Hopefully I might hear something back soon, who knows, I thought is was worth a try at least. 

All the chips I ordered have been dispatched so should be with me in few days time, if we can find nothing else wrong, it may be an idea just to swop them out for the new chips and give it a go?

I had been toying with the idea of getting one of those cheap Chinese IC testers but I doubt they would be upto testing old obsolete chips as their internal libraries would be full with the more modern chips I think.

[tautech]

You could try feeding only the +5V from a bench supply and check the current drawn seems reasonable.
This would be the only rail I would check like this to minimise the chance of damage to componentry.

Testing the psu on the +5v draw is zero which I think is correct as the IC's on the secondary side of it are jfets and those on the primary side are connected to the 5v rail, instead getting their power directly from the incoming mains, via droppers and diodes and or the full wave bridge rectifier.

The secondary side opamps plainly get their supply from the unregulated +12 and/or -12V SMPS secondaries, nothing to do with the +5V rail.

Yes there will be another manual for the scope but I think with the datasheets and schematics it is viable to fix this.

To understand the operation of the logic gates you need to grab the datasheets and study the Truth tables.
4093 is a Nand Schmitt and 4049 is an inverting buffer.

This website can be handy to visualise the operation of many types of logic functions:
https://tams.informatik.uni-hamburg.de/applets/hades/webdemos/10-gates/00-gates/chapter.html

[Specmaster]

I'm poking around the Nand Schmitt and Inverting Buffer and have discovered that the T403 is infact drawn with its Emitter and Collector reversed. Also I have found that R407 is shown as 475k but measures 400k, would this make enough difference that it could shut down the psu?

[Specmaster]

Had a response from rhodes-schwarz today and they gave me the phone number of their technical support centre 01252 818 900 which goes straight through to the Munich office. I called it and got a recording saying today they have a public holiday, so I'll try them again tomorrow and see if they offer any guidance for us.

[Specmaster]

A quick update for those playing along at home, I forgot to phone their tech support line Friday, so I'll do that in the morning.

All the replacement chips have now been delivered so if all fails, I can try replacing the chips as it is possible that the previous damage could also have taken out a chip or two. I have slowly but surely working my through the schematic for the smps, locating and checking the parts against the specified values and so far nothing has actually lept out as a likely cause of the failure? The mystery rumbles on...

[tautech]

Also I have found that R407 is shown as 475k but measures 400k, would this make enough difference that it could shut down the psu?

Not sure but if it has drifted ~20% from original values it's on the way out.
Remove it and remeasure to be sure and replace if needed. Even crap SMD resistors will meet a 5% tolerance.

[Specmaster]

I took it out to test it, but because it was minimelf type it had been glued in and attempting to get it free from the glue, it pinged across the room and promptly became lost, oh well, I now have 50 on order so I'll have a few spares haha.

[Specmaster]

A quick update of the current position.

Still awaiting the arrival of the 475k minimelf resistors.

Spoken to the UK arm of Rohde-Schwarz who gave me a phone number for the technical support center in Munich, Germany. Had a long chat with a helpful person in Munich who explained that they only do the calibration there but emailed me a copy of their schematics and also gave me the name and contact info for a company who do the repairs for Hameg scopes in Germany.

I have now emailed this company giving them the information that we have arrived at, to see if they could direct us where to look for this particular problem and to check if the resistance values are good enough to continue or are we looking at other issues on the main boards?

I'm also informed that they carry spares so may be a source of replacement knobs, once the PSU is up and running. Worryingly though, they also have a download section on their web site for manuals they but don't list the HM408 in any of its guises??

So at the moment I'm in limbo awaiting resistors and hopefully a reply from the company (SKY Messtechnic GmbH) in Germany.

Here are the resistances to ground taken on the isolated PSU:
-12v     13k
-5v       11.5k
+5v      13M
+12v    5.3k
+115v   40k
+57v     66k

so no short circuits there although the +5v seems to rather high, especially in relation to the others, could this really be as high as 13M or doe we have an issue here?

[tautech]

IMHO there is something wrong with the high +5V rail resistance. Could it be an open joint or faulty connector......you need do some continuity checks.

Why ?
If there are sockets and looms, check the sockets pins for cracked and/or dry joints.

Let's say when it powers up the +5V rail presents little load for the SMPS.......the +5V rail is the rail that the
SMPS feedback is sensed from..........it then signals overvoltage to the feedback opto isolator.
Study the feedback circuit (PSU schematic, btm right) and its feedback path to the opto.

[Specmaster]

It seems that all the resistors connected to the op amps IC408, IC407 & IC406 are reading approx 33% lower (in circuit) so I pulled out the op amps and the readings are the same, tomorrow I'll lift one end of each and measure out of circuit. I didn't order any of the op amps (TL081 and TL802) but I did get just in case a set of IC's for the primary side inside of the screen can plus another opto for the timebase board interface. No sign of any cracked or broken connections at this stage, more might be revealed tomorrow when I start desoldering.

[tautech]

It seems that all the resistors connected to the op amps IC408, IC407 & IC406 are reading approx 33% lower (in circuit) so I pulled out the op amps and the readings are the same, tomorrow I'll lift one end of each and measure out of circuit. I didn't order any of the op amps (TL081 and TL802) but I did get just in case a set of IC's for the primary side inside of the screen can plus another opto for the timebase board interface. No sign of any cracked or broken connections at this stage, more might be revealed tomorrow when I start desoldering.

Let me restate my concerns as to the 13M resistance of the +5V rail.
I don't think this is anywhere normal, it should be MUCH lower. Find out why it is not.

[Specmaster]

Oops, looks like my DMM is misbehaving, possibly due to the caps on that rail? Sometimes it shows M instead of K it seems, I had to drag my AVO out and double check and the results are, depending on which way the probes, + or - to ground I get the following .75k or 22.5k, that seems to be better. I say due to the caps because the readings start low and increase slowly, which I guess is due to the 1000uf caps charging?

[tautech]

Oops, looks like my DMM is misbehaving, possibly due to the caps on that rail? Sometimes it shows M instead of K it seems, I had to drag my AVO out and double check and the results are, depending on which way the probes, + or - to ground I get the following .75k or 22.5k, that seems to be better. I say due to the caps because the readings start low and increase slowly, which I guess is due to the 1000uf caps charging?

It will be plus any others that aren't local to the PSU.

That's a more realistic figure. 

[Specmaster]

It will be plus any others that aren't local to the PSU.

The PSU is not attached to the scope so its just those on the PSU that will be having an effect.

DMM with auto ranging can be handy but they can also be a problem when the digits keep changing, whereas an analogue can be so much easier to read at times.

[Specmaster]

Double checked the resistors associated with the op-amps today which all checked out fine after lifting one leg from the pcb so I decided to remove the transistors for checking, T404 and 406 came out ok and passed testing, but the SMD ones are a different story, after using my hot air gun to remove T407 for testing, I promptly lost it when despite sticking it onto double sided tape to stop it escaping when probing with the test leads, it managed go flying into the ether, new ones on order...

Did not find any broken traces or bad solder joints anywhere as went round the board and checked traces point to point with my meter.

The +5v circuit all appears to be OK, diodes, caps and resistors, traces and sockets all checked OK.

I also tested for hotspots with an infra-red thermometer before removing the smps and all I found was a few IC's in places across the boards were warming up to mid 30C area, which is what I expect them to do anyway.

Had a response from Sky Messtechnic, they will respond with some schematics and tips / hints on what we need to check on Monday

[Specmaster]

The info from Sky is that the problem is likely to be contained within the red circled area which monitors if any circuit is taking to much power and shuts down the psu in that event. Given that they have seen the circuits resistance to ground and these passed with no comment, I think we can take it then there is likely to be a fault here somewhere. I have given everything as far as possible a quick check in circuit in that area and the biggest error so far found is that R462 on the +115v rail (221k) reads approx 78k. This could be partly due to the caps on that rail and the transformer windings? That would mean that D441 was suspect but that has been pulled from circuit and passes the diode test on the DMM?

As the transistors are small and difficult to hold down and test, I have ordered enough to replace them in this section and also of course T407.

They also have, should it be required, new psu's in stock at 102 Euros, this might be more than the complete scope is worth surely seeing as it about 20+ years old?

[tautech]

Great, good pointers from Sky that also point you to the SMPS FB circuitry that I mentioned in reply #77.
R462 out of spec will certainly bugger things up and all those divider resistors need be checked.

Good to know you can get another PSU if needed. 

[Specmaster]

Great, good pointers from Sky that also point you to the SMPS FB circuitry that I mentioned in reply #77.
R462 out of spec will certainly bugger things up and all those divider resistors need be checked.

Good to know you can get another PSU if needed. 
[/quote
Yes you did indeed, I thought the FB was the IC's, never gave that section of the circuit any great thought, silly I know when you look at it, all the voltages are present there (or rather they should be haha).

As to the new psu if required, do you really think the scope is worthy of that kind of money being spent on it? Or would it be better put towards something a little newer and more advanced?

[tautech]

Great, good pointers from Sky that also point you to the SMPS FB circuitry that I mentioned in reply #77.
R462 out of spec will certainly bugger things up and all those divider resistors need be checked.

Good to know you can get another PSU if needed. 

Yes you did indeed, I thought the FB was the IC's, never gave that section of the circuit any great thought, silly I know when you look at it, all the voltages are present there (or rather they should be haha).

As to the new psu if required, do you really think the scope is worthy of that kind of money being spent on it? Or would it be better put towards something a little newer and more advanced?

Proceed with this repair........remember the SMPS is shutting down as it should in a fault condition, the FB for this to happen is likely coming only from the part of the circuit currently being focussed on. A few dividers and a couple of transistors shouldn't be hard to get right now that the componentry around the SMPS IC has been carefully checked.
This scope looks like a very nice unit and if it is close to being fixed and proves it is indeed a top CRO then the PSU could be well worth the money.
Once you have this and the Phillips fixed you can make the choice which to keep and maybe fund the PSU from the sale of the Phillips.

[Specmaster]

Great minds think alike.. I already have the Philips on Ebay, while I was waiting around for the various parts to arrive for the Hameg, I looked at the philips and thought it was performing reasonably well. Checked the calibration with my tester and it seems to be so close to being right that it wasn't worth while cracking it open to adjust it. Run through all the various parts that I could with it and it seems to be working well so it's up for sale.

I have that sweet little Hitachi V525 (50Mhz) that I got a couple of weeks ago, also a basic Rapid Electronics 25Mhz one in mint condition only a couple of years old which I doubt has been used for more than a few hours, along with a lovely old 60Mhz Iwatsu SS-5710 which is a 4 tracer and a Goldstar OS-9100D (100Mhz) which is a 3 tracer so with the Hameg (I'd have to keep it after all the work that has gone into it now and does have that storage capability even if it is basic when compared to a DSO) I don't think I'd need more than those haha.

I know, never say never, I just enjoy working with them and putting them back into some sort of usable state rather than letting them going for landfill.

[tautech]

I know, never say never, I just enjoy working with them and putting them back into some sort of usable state rather than letting them going for landfill.

Me too.....in the past.
That's how I got into the industry and repairs of TE is very satisfying.
I used to put all the manuals I could find onto a CD, provide new cheapie probes and resell them.
I got enough back that I could buy more broken units, rinse and repeat.

Whatsmore on this forum we provide a valuable resource for those with similar problems and learn a lot along the way too.
Your detailed posts and regular updates make threads like this a good read.

I've still got a few old 'not right' CRO's tucked away for repair threads.......if I ever get around to them. 

[Specmaster]

Yes I agree, TE has a draw to it, especially CRO's many of which are some 30 years old and are still capable of being everyday tools, I wonder if the same could be said about the new breed of DSO's with their custom chips etc. That's one reason why I have so far avoided Tektronics scopes because of their heavy use of custom hybrids which almost certainly means that once they get to a certain age, Tektonics will pull the plug on supporting the old hybrids.

Is it me or do you find that DMM's suck at measuring resistance with the item still in circuit? I replaced that 475k minimelf one with a brand new one that I checked prior to soldering it back and it was bang on 475k, once soldering back, it read again around the 400k as the previous one did. The DMM was ever so slowly creeping upwards in its reading. I dragged out my old Avo Model 8 and tested it in circuit and it went straight to approx 475k. So is this because of the higher impedance of the DMM that there is far less current being drawn and therefore any caps in circuit can only charge up very slowly whereas my old Avo being a 20,000 ohms per volt instrument will allow more current to flow and therefore any caps in circuit become almost immediately charged and that 475k resistance in circuit now reads around 475k. Have I been chasing ghosts all time using the DMM for in circuit measurements?

[tronde]

That's one reason why I have so far avoided Tektronics scopes because of their heavy use of custom hybrids which almost certainly means that once they get to a certain age, Tektonics will pull the plug on supporting the old hybrids.

Some of the more advanced Philips scopes suffer from the same disease.

[tautech]

Yes I agree, TE has a draw to it, especially CRO's many of which are some 30 years old and are still capable of being everyday tools, I wonder if the same could be said about the new breed of DSO's with their custom chips etc. That's one reason why I have so far avoided Tektronics scopes because of their heavy use of custom hybrids which almost certainly means that once they get to a certain age, Tektonics will pull the plug on supporting the old hybrids.

The US seems to be the place to be as the supply of old gear to fix and donor units seems inexhaustible. 
IMO the future there will likely be mostly electrolytic replacement and SMPS repair as much SMD construction these days is very reliable. Other than rare infant mortality products I sell are proving very reliable.

Is it me or do you find that DMM's suck at measuring resistance with the item still in circuit? I replaced that 475k minimelf one with a brand new one that I checked prior to soldering it back and it was bang on 475k, once soldering back, it read again around the 400k as the previous one did. The DMM was ever so slowly creeping upwards in its reading. I dragged out my old Avo Model 8 and tested it in circuit and it went straight to approx 475k. So is this because of the higher impedance of the DMM that there is far less current being drawn and therefore any caps in circuit can only charge up very slowly whereas my old Avo being a 20,000 ohms per volt instrument will allow more current to flow and therefore any caps in circuit become almost immediately charged and that 475k resistance in circuit now reads around 475k. Have I been chasing ghosts all time using the DMM for in circuit measurements?

Some equipment is better than others at in-circuit measurement and one need be careful with stressing other componentry than what you are trying to measure. It is true that there is some interpretation required and mostly I just look for resistance that IS NOT greater than the marked value as there is often another current path to shunt the value lower than marked values. Resistors mostly drift high so this method works for me. Semiconductors and caps is where I focus my attention. Poor joints and connectors too.

Oh yeah, and I have an Avo 8 that gets used occasionally too. 
Most of my measurements are done with a ST-3 smart tweezer, it's so fast to use and quite accurate. Many think they're toys but I can assure you they're not. Convinced a few members to get them (now ST5) and I don't think any are disappointed. Trouble with them....they're not cheap. 
With a Fluke 15B and the above there's a lot I can do before having to drag out any of the bigger guns in my range of demos.

[WastelandTek]

Is it me or do you find that DMM's suck at measuring resistance with the item still in circuit? I replaced that 475k minimelf one with a brand new one that I checked prior to soldering it back and it was bang on 475k, once soldering back, it read again around the 400k as the previous one did. The DMM was ever so slowly creeping upwards in its reading. I dragged out my old Avo Model 8 and tested it in circuit and it went straight to approx 475k. So is this because of the higher impedance of the DMM that there is far less current being drawn and therefore any caps in circuit can only charge up very slowly whereas my old Avo being a 20,000 ohms per volt instrument will allow more current to flow and therefore any caps in circuit become almost immediately charged and that 475k resistance in circuit now reads around 475k. Have I been chasing ghosts all time using the DMM for in circuit measurements?

sounds like your meter was charging a cap to me

[Specmaster]

Yes I know, I did have a look at them myself and decided that most of what I get involved with is likely to be through hole stuff, I got my self a pair of tweezers that plug into any meter for now. Eventually I think I can see myself getting some along with an IC  tester as well.

[Specmaster]

Is it me or do you find that DMM's suck at measuring resistance with the item still in circuit? I replaced that 475k minimelf one with a brand new one that I checked prior to soldering it back and it was bang on 475k, once soldering back, it read again around the 400k as the previous one did. The DMM was ever so slowly creeping upwards in its reading. I dragged out my old Avo Model 8 and tested it in circuit and it went straight to approx 475k. So is this because of the higher impedance of the DMM that there is far less current being drawn and therefore any caps in circuit can only charge up very slowly whereas my old Avo being a 20,000 ohms per volt instrument will allow more current to flow and therefore any caps in circuit become almost immediately charged and that 475k resistance in circuit now reads around 475k. Have I been chasing ghosts all time using the DMM for in circuit measurements?

sounds like your meter was charging a cap to me

Yes that's what I thought, the DMM is about 20M per volt whereas the AVO is 20K per volt therefore more current with AVO to charge the cap considerably quicker to the point that the eye can't see it as much because the resolution is less with analogue meters.

[tautech]

Yes I know, I did have a look at them myself and decided that most of what I get involved with is likely to be through hole stuff, I got my self a pair of tweezers that plug into any meter for now. Eventually I think I can see myself getting some along with an IC  tester as well.

Never bothered with IC testers as mostly functionality can be checked with a scope and/or DMM. Knowing how to interpret datasheets is the key and checking the parameter measurements in the circuit are correct for the IC to do its job. IC's are fairly tough but with catastrophic failures close by they need be suspect.
The one tool I have on a wishlist is an infrared camera. 

[Specmaster]

Never bothered with IC testers as mostly functionality can be checked with a scope and/or DMM. Knowing how to interpret datasheets is the key and checking the parameter measurements in the circuit are correct for the IC to do its job. IC's are fairly tough but with catastrophic failures close by they need be suspect.
The one tool I have on a wishlist is an infrared camera. 

The reason for the IC tester was not just to check if they are running OK, but more importantly to help identify chips that either have poor printing on them or have been wiped by the equipment manufacturer so that you have to source spares from them, if you catch my drift?

What I use to spot anything that is getting hot is an infra-red temperature probe, works really well with a colour changing screen so that you can set safe temperatures to have say a green display and readout and anything that exceeds your chosen setting could set the screen to red? The screen also always reads out the actual temperature regardless of your chosen ranges so you can actually see what each part is sitting at.

The screen has a choice of 3 colours, Blue for temperatures that are below your set figure, Green for anything within your safe range and Red for anything that is over your safe range. You have a laser dot that you aim at the component and it transmits its temperature back to your handheld gun and by scanning this slowly over your test circuit you can quickly identify anything that is getting hot or cold and it only costs a few £ from your local Lidl store when they get them in. Maplin and other such electronics stores sell them but far more expensive then Lidl's price when they get them in.

[tautech]

Yep, used IR guns on machinery and they are useful for spot checks. IR camera is the ideal for PCB's but the cost. 

[Specmaster]

Camera is the best for all heat related issues if you can afford one, but IR gun is a good second choice, I run a infra-red scan over all my scopes looking for hot spots on the PCB's but as yet have not found anything that was running hot, not even on the Hameg so I don't think we have any overloading going on, just a dicky smps which as we already know, had been worked on before I got it so the damage was already done. Leaving me with a game of cluedo to solve now looking for that smoking gun...

[Specmaster]

Quick update, started to go round the board looking at testing everything I can either in circuit or by lifting one leg which is fine with the through hole stuff but proving to be a real pig on the smd stuff, still awaiting one or two transistors (smd's) as they tend to got lost once I can get them pried away from the dreadful glue that they are bedded down on. I say get lost, truth is that they require a bit of force and they go bloody flying and get lost. Give me through hole circuits everytime please. Its proving to be a bit of a pig TBH as there are so many possible paths to ground etc. when in circuit testing and even using a DMM or AMM can in many cases give conflicting results.

Anyway, so far not found anything that far out of spec that should cause the smps to shut down, so is the smoking gun one of the IC's on the board or an overload on one of the other boards causing the problem? Once the missing transistor arrives and is fitted I'll try it again and if no change, I'll swop out the IC's and see if it works then...

[tautech]

Spotted a neat and simple way to check optos on the forum a day or two back.

2 DMM's, one on diode test checking the LED (needs sufficient voltage to do so) and the other across the photo-transistor using continuity check.
Shit easy I thought. 

[Specmaster]

Spotted a neat and simple way to check optos on the forum a day or two back.

2 DMM's, one on diode test checking the LED (needs sufficient voltage to do so) and the other across the photo-transistor using continuity check.
Shit easy I thought. 

In or out of circuit? I suppose you could use a AA battery to power the LED and then only require a single meter, even an old analogue one on continuity mode?

[tautech]

Spotted a neat and simple way to check optos on the forum a day or two back.

2 DMM's, one on diode test checking the LED (needs sufficient voltage to do so) and the other across the photo-transistor using continuity check.
Shit easy I thought. 

In or out of circuit? I suppose you could use a AA battery to power the LED and then only require a single meter, even an old analogue one on continuity mode?

OUT, and you can't use external power without a current limiting resistor or you'll blow the opto LED.
Have a fiddle with some LED's and DMM's to find which will light them up on diode test mode.

[Specmaster]

Well thats easy, I only have the one DMM so its that one and I'll try it now. Well that was easy peasy, on my meter it didn't work at all, mind you I was using yellow, green and red LEDs, I don't have a white one to try. I thought my battery was flat so I tried a few normal diodes, they tested perfectly but with an LED I get nothing in both directions?

[Specmaster]

Just checked batteries and their fine, swapped them for new ones anyway and the same result so I guess that my meter gives an ultra low voltage which is not high enough make an LED conduct.

[tautech]

Well thats easy, I only have the one DMM so its that one and I'll try it now. Well that was easy peasy, on my meter it didn't work at all, mind you I was using yellow, green and red LEDs, I don't have a white one to try. I thought my battery was flat so I tried a few normal diodes, they tested perfectly but with an LED I get nothing in both directions?

Not unusual.
Not all DMM's have enough diode test Vf to check LED's, you need up to 3 V to check all colours.

Just checked batteries and their fine, swapped them for new ones anyway and the same result so I guess that my meter gives an ultra low voltage which is not high enough make an LED conduct.

Yep.
Grab the opto datasheet and check the Vf and max current for the diode and power it externally via a current limiting resistor.
If you look at the SMPS FB circuit, the opto LED is essentially fed +12V via 5k6 R473 assuming the opamp TL081 swings to full rail voltage. Further along the rail monitoring network will be pulling the opto LED voltage down in fault condition.

[Specmaster]

Yep, I just checked my LEDs using my bench supply and they start to conduct at around 2v.

[tronde]

I have some vague memories from the late 1980s when I used to repair TVs about trouble with optocouplers in the feedback loop. They appeared to be OK, but did not work in the circuit. I understand that they can change their transfer characteristic over time. We gave up using generic parts and got replacements from the manufacturer.

[Specmaster]

Well it has crossed my mind that it is possible that one or both of the opto's could be the cause of the problem so I got in some replacements but have not got the smps in a state to try swapping out the old ones yet. I'm still waiting for some smd transistors to replace one that flew across the room upon removal for testing.

[Specmaster]

Update.
All the suspect parts have been replaced etc, all solder joints have been re flowed etc, psu connected back and via dim bulb tester, bulb glowed dimly briefly as the capacitors charged up then went out but nothing on the screen or LED's lit or anything so for the time being until I can get confirmation from Sky or anyone else who actually has a 408 just what the resistance to ground should be at P2700 then I'm doing nothing more with this for the time being.

I think the general consensus is that it a scope worthy of restoring but at what cost? I can get a new replacement psu for this from Germany but is it worth it, especially until we know if that is all that is wrong with the scope and I feel that conformation of the ideal resistance to ground on the various rails should be at the check plug should greatly assist with that. The last thing I want to is go and order a new expensive replacement, plug it in and any prevailing fault elsewhere takes out the psu again.

I suppose it would be a long shot that another member here in the UK has a 408-1A that either they no longer want, that I might be able to make a really good one from the pair?

I thank all the members who have so readily given me no end of advice and assistance so, it really is appreciated.

[tautech]

Where's P2700 ?
Which PCB and schematic page ?

[Specmaster]

Quick update on those resistances (with the psu disconnected apart from the -5V and the connections to the timebase board.
With positive meter lead to ground we get.
-12v  75.5
-5v    610
+5v   414
+12v  199.6
+115v  O/L (in excess of 40M)
+57v   47k

and with the negative lead to ground
-12v  75.5
-5v    610
+5v   414
+12v  199.6
+115v  860k
+57v   929k

The last set of figures with total isolation was
-12v     75.5ohms
-5v       257ohms
+5v      440ohms
+12v    202ohms
+115v   36.92k
+57v      32.93k

The last 2 reading show a massive variance, could this be down to the fact that the boards caps having drained away during the period the psu has been removed? I cant see the timebase board being left connected would make any difference as it is connected via an opto coupler and the only other connection still made was the heater for the CRT.

[Specmaster]

Where's P2700 ?
Which PCB and schematic page ?

Its on page 41 (attached)

[Specmaster]

Today I located the required items to fix the control knobs and the shafts for the CH1 and CH2 trace positions so tonight I stripped the scope down, replaced the parts and reassembled again. While I was doing that I reconnected all the leads to the psu and then took fresh resistance measurements to ground from the check location P2700 and there was a variation depending on if I had the -ve lead as ground or +ve lead.

Resistance  -ve as ground
-12v     74.5
-5v       610
+5v      483
+12v    188
+115v  36.8k
+57v    32.8k

+ve as ground
-12v     74.5
-5v       610
+5v      411
+12v    188
+115v  29.8k
+57v    26.2k

Resistance across the incoming mains terminals is 800 ohms, so no short or blown circuit on the primary.

Tomorrow I'll do what I should have done the other day when it appeared to be dead, read the voltage rails again at P2700 location and see what if anything we get there.

No response as yet from Sky, hopefully I might get a reply tomorrow.  I'm really looking forward to getting this resolved now it has all come together nicely with the exception of the supply issue but surely the break through can only be just around the corner now??

[Specmaster]

Update for all "those playing along at home"

Finally got a reply back from Sky and apparently you cannot tell the circuit resistances from test port P2700 so that appears to be a rabbit hole.

To run / test / work on the PSU on the bench, out of the scope (its a real pain in the A*** to keep disassembling and reassembling the scope every time to do a test), I need to load up all the rails and they kindly gave me the resistor values that I need to do that and they are:-
+5V      6R8  5W
-5V       22R  3W
-12V     22R  7W
+12V    22R  7W
+57V    1K    5W
+115V  3.3K 5W

The 2 blue wires are only carrying 6.3v to the CRT heater not the -1850V as shown on the schematic, that is only true when connected up to the CRT because of the other external links from the CRT socket, so the biggest worry is working on live PCB with 230V in places.

This is really turning into a pig of a job this one. Oh well off to raise an order now for these resistors to enable battle to recommence again......

[Specmaster]

Update on the progress so far.

Ordered the dummy load resistors so that I can power the smps up without having to keep reconnecting to the scope which is a real pain to do. These have now arrived and so I'll be connecting them up to the unit and testing it out of circuit. I think its time to start rechipping this sucker now as I have been chasing down what I thought was a problem only to find it was one of Daves rabbit holes, bummer.

Keep you all posted how I do.

[Specmaster]

Ok then its update time again, load resistors arrived and connected, powered up and nothing happened, double checked all connections, all ok, so fitted new set of chips and, you guessed it, nothing again and I have other pieces of equipment arriving all the time that require my attention, so I decided to chuck the towel in on this, at least for the time being. I have ordered a new PSU from Germany which should be here in a couple of days or so.  Maybe I'll go back to the old PSU once the other bits have been looked at.

While I have been waiting for parts for this I looked at the Philips 3217 that I got at the same time and after a few tweaks to the calibration, some internal and external cleaning, it all came good so that particular unit has been sold to fund the new PSU.

Awaiting attention now is a Hewlett Packard 1740A scope, and in transit to me is a Philips PM 2521, 4.5 digit bench meter, condition unknown, which once working, I want to try and fit some form of lighting for the display and just maybe if I can get it at a good price, a Marconi 2019 signal generator, once again not sure of its overall condition other than it powers up and seems to be working. Not to mention as well I have a couple of lovely Hacker Sovereigns RP25's that I have started restoration of and got them working but I want to work on the AM side and improve its performance.

So I say to all of those that have contributed advice and comments towards this project, a big heart felt thank you for your efforts, I have learned a lot from you all.

Watch out for new updates on these as I progress and the saga of Hameg HM408-1A PSU may well rear its head again to make a backup unit or a replacement for another machine if required. There will be at least one more update on this thread once the new unit arrives and is fitted.

[Specmaster]

OK, new refurbed supply unit arrived and duly fitted and nothing, power is switched on a few milliseconds and promptly shuts down again. I have had to get one of my analogue meters out, (DMM's are just too slow for this job), and measure the supply rails before the systems shuts down again and all the voltages appear to there with exception of the +12v which only gets +3v before shut down, so -5, +5, -12, +57 and +115 are present. I've pulled power connections where possible from board to board and the same problem is there.

Power supply was bench tested prior to dispatching it to me, so it seems the problem is with the rest of the scope but darned if I can spot it. I checked the resistance to ground for +12v rail from the point where it exits the power supply to feed the scope, i.e., from the end of the ribbon cable and it reads a respectable 198 ohms which means using ohms law, that a current of 60ma is theoretically being drawn which is well within the capabilities of the power supply.

I have gone round the other boards looking for any caps that are on the +12v rail and there aren't that many and visually they seem fine and at this stage I don't feel inclined to dismantle the other boards so that I can pull them and check their ESR values as that should not cause the problem surely? I could understand it if one of them was shorting to ground but that would manifest itself when measuring the circuit resistance and give me a massively lower reading. I also checked them thoroughly for any signs of overheating tell tale marks, or cracked resistors, dodgy looking solder joints etc. and drew a complete blank.

Anyone got ideas?

[fossi]

I know the topic is old, but I think it' will help people on repairing Hameg 408
I had the same problem at an HM-408 and on a HM204.

No problem when the couplers are broken. You can order the original parts at sky-Meßtechnik in germany.
https://www.sky-messtechnik.de/reparaturdienst-service/hameg-ersatzteile

The people are very nice and helpful and have a lot of spareparts not listed on their website.
I got my parts within 2 days very well-priced

Installed and Hameg is ready to go again

regards
fossi