Author Topic: Hameg 408-1A  (Read 39973 times)

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Offline tautech

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Re: Hameg 408-1A
« Reply #50 on: May 28, 2017, 08:34:39 am »
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.
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Offline SpecmasterTopic starter

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Re: Hameg 408-1A
« Reply #51 on: May 28, 2017, 10:36:05 am »
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.
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Offline tronde

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Re: Hameg 408-1A
« Reply #52 on: May 28, 2017, 03:35:42 pm »
-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.
 

Offline SpecmasterTopic starter

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Re: Hameg 408-1A
« Reply #53 on: May 28, 2017, 03:54:39 pm »
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?
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Offline tronde

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Re: Hameg 408-1A
« Reply #54 on: May 28, 2017, 05:28:24 pm »
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.
 

Offline tautech

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Re: Hameg 408-1A
« Reply #55 on: May 28, 2017, 06:25:07 pm »
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.

Quote
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.

Quote
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.

Quote
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.

Quote
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.
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Offline SpecmasterTopic starter

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Re: Hameg 408-1A
« Reply #56 on: May 28, 2017, 07:20:51 pm »
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.
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Offline tautech

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Re: Hameg 408-1A
« Reply #57 on: May 28, 2017, 08:26:12 pm »
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.  :)
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Offline SpecmasterTopic starter

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Re: Hameg 408-1A
« Reply #58 on: May 28, 2017, 08:59:48 pm »
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.
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Offline SpecmasterTopic starter

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Re: Hameg 408-1A
« Reply #59 on: May 29, 2017, 11:46:05 am »
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  :palm:.

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.  :-+
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Offline SpecmasterTopic starter

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Re: Hameg 408-1A
« Reply #60 on: May 29, 2017, 03:04:21 pm »
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. :-//
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Offline tautech

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Re: Hameg 408-1A
« Reply #61 on: May 29, 2017, 08:27:12 pm »
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.
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Offline tautech

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Re: Hameg 408-1A
« Reply #62 on: May 29, 2017, 08:28:54 pm »
BTW, for the power up do use a dim bulb tester, say with a 100W incandescent bulb.
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Re: Hameg 408-1A
« Reply #63 on: May 29, 2017, 09:37:47 pm »
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.
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Offline tautech

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Re: Hameg 408-1A
« Reply #64 on: May 29, 2017, 10:03:10 pm »
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%.

Quote
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.

Quote
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
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Re: Hameg 408-1A
« Reply #65 on: May 29, 2017, 11:13:20 pm »

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.
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Re: Hameg 408-1A
« Reply #66 on: June 06, 2017, 11:51:14 pm »
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?
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Offline tautech

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Re: Hameg 408-1A
« Reply #67 on: June 07, 2017, 05:56:50 am »
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.  :scared:
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.  :)

Quote
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.
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Re: Hameg 408-1A
« Reply #68 on: June 07, 2017, 06:38:56 am »
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.
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Offline 9a4wy

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Re: Hameg 408-1A
« Reply #69 on: June 07, 2017, 07:07:19 am »
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
 
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Offline tautech

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Re: Hameg 408-1A
« Reply #70 on: June 07, 2017, 07:37:42 am »
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.  ;)
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Offline tronde

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Re: Hameg 408-1A
« Reply #71 on: June 07, 2017, 04:49:07 pm »

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.
 

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Re: Hameg 408-1A
« Reply #72 on: June 07, 2017, 11:12:57 pm »
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.
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Offline SpecmasterTopic starter

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Re: Hameg 408-1A
« Reply #73 on: June 07, 2017, 11:32:27 pm »
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.
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Offline tautech

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Re: Hameg 408-1A
« Reply #74 on: June 07, 2017, 11:43:23 pm »
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.
Quote
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.
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