Tektronix 475 repair assistance

[bd139]

This has appeared in various places around the Internet. Feel like I need to consolidate it all on the best resource for such things, which is EEVblog forum by the looks!

I scored an apparently dead Tektronix 475 with DM44 module the other day for a respectable £29.

I'm going to enumerate the identified problems with it and what has been done and resolved so far.

Problem 1 - 15v supply out. RESOLVED!

This was a duff tant on the interface board on the output of the 15v regulator. All rails are in spec now. I will do a wholesale tantalum swap out job over the next few days. There are quite a few to replace!

Problem 2 - Occasional HV drop out. RESOLVED! NOT RESOLVED AGAIN FINALLY RESOLVED!

Every 2-3 minutes the display jumps in brightness and then returns to normal as per the following video.



Cleaning it appeared to make the problem go away for a bit but it's back again suddenly. I've opted to replace all the film capacitors and diodes in the HV section as a couple of caps look slightly mottled/cracked (like shitty RIFAs about to explode). They are being replaced with ceramic equivalents.

Note: postponing HV refurb as the issue may have been cured by another tant on the 50v rail going short periodically and self-clearing.

Confirmed it was the 50v rail tant that was doing this. It doesn't do it now that has been replaced with a low ESR electrolytic.

This occurred again and I found another dead tant on the 15v rail that might have actually been doing it to start with. Grr. Replace them all!!!

Problem 3 - chop doesn't work. RESOLVED!

As yet undiagnosed. Shows channel 1 in chop. Hopefully not the channel switch IC gone phut and just a dirty switch or something! Not a big priority at the moment. Drifted resistor in chop oscillator bias circuit and duff ceramic decoupling capacitor causing VCC to be around 4v which broke the startup condition for the oscillator. 7400N's are possibly hooky so I'm ordering a couple as replacements anyway.

Problem 4 - fine horizontal pot broken - IGNORE

Just going to try and replace this if I can find one or live with it. There's enough resolution in the coarse pot for now.

Problem 5 - at fast sweep speeds, the trace starts moving to the right - RESOLVED

This one is a real PITA. I've had another scope on the timebase and that appears to be working fine. Horizontal amp seems fine as well as it has full deflection and bandwidth in X-Y mode. However as you increase sweep speed from 1us to 50ns/div, the trace compresses and moves right. Scoping the timebase shows that the sweep return is much shorter for these timebase settings than others and if you turn the brightness up fully you can see the retrace so I'm wondering if it's Z-axis related. Also looks a little blurry in delayed sweep mode when the trace is intensified. This is possibly CRT blooming or on it's way out (is it?). Any hints welcome!

After much arguing I spent until 3AM working this out. Unplugged the z-axis cable to poke a scope in the hole, plugged it back in again and gone. Ahha we have a problem. So I resoldered each end of the z-axis cable's sockets and hey presto it works. Looks like one of the terminations was iffy and had an accidental filter in it.

Problem 6 - dirt and corrosion - RESOLVED!

Slowly working on this. Corrosion is minor and appears to be limited to the front cast panel. Everything else is cleaning up nicely. All the controls are in really good condition and the bits that are cleaned leave it looking like it's brand new!

Any help or suggestions on any of these issues would be appreciated as this is a fine cosmetic specimen; would be nice to get it up to spec again.

State after I fixed the power supply:

[tggzzz]

Problem 4 - fine horizontal pot broken

Just going to try and replace this if I can find one or live with it. There's enough resolution in the coarse pot for now.

Classic; can be caused by resting the scope on it when removing the cover. The spindle is forced inwards, separating it from the clarostat's rotor.

You can try pulling on the knob while rotating, to see if that re-engages the spindle and rotor. If it does, you can consider putting a coilspring between the two knobs, to provide the equivalnt of a gentle "pull".

The clarostats are available at the usual places; if not, I have a 311-1192 that I might be prepared to part with.

Problem 5 - at fast sweep speeds, the trace starts moving to the right

This one is a real PITA. I've had another scope on the timebase and that appears to be working fine. Horizontal amp seems fine as well as it has full deflection and bandwidth in X-Y mode. However as you increase sweep speed from 1us to 50ns/div, the trace compresses and moves right. Scoping the timebase shows that the sweep return is much shorter for these timebase settings than others and if you turn the brightness up fully you can see the retrace so I'm wondering if it's Z-axis related. Also looks a little blurry in delayed sweep mode when the trace is intensified. This is possibly CRT blooming or on it's way out (is it?). Any hints welcome!

Might this be related to the DM44? See what happens if the DM44 is in volts/ohms/temperature mode.

[bd139]

Yeah same trouble with my last 475. Seems to be a big failure of the device putting that knob there. Or letting whichever knob did it out of prison

I can pull it towards me fine and it will engage. I might drill the rivets out and see if I can repair the pot first. If not, I’ll look for a replacement. I won’t steal yours unless you really want rid of it as I know how rare they are.

With respect to the horizontal and the DM44, that’s a good question! This appears in horizontal A only mode however so I suspect it is related to something else. Worth a look and trace the circuits though.

Edit: oooooh tempting https://www.ebay.co.uk/i/282694103060

[bd139]

I finished cleaning it:

Before:



After:

[bd139]

Debugging chop circuit now. After some poking around it appears that the chop oscillator, a simple SN7400 multivibrator doesn't start by itself when you put it in chop mode. The moment you touch pin 9 with the scope it springs into action!

Circuit in question here:



More debugging later. Will write the voltages down and then pull and check all the parts.

[tggzzz]

Debugging chop circuit now. After some poking around it appears that the chop oscillator, a simple SN7400 multivibrator doesn't start by itself when you put it in chop mode. The moment you touch pin 9 with the scope it springs into action!

Check the solder joints and that the resistors are in tolerance and wiggle them to see they are not cracked. Use some DeOxit or equivalent to ensure IC leads are making good contact.

[bd139]

Good plan. I’ve yanked the IC quick and checked the bias voltages and they are well out so there’s a dicky resistor or two by the looks. There’s nearly a volt difference between pins 9 and 12 which makes it slight more stable than it should be.

[SeanB]

It's a 7400, so if the rest checks out change it, but beware, use only a 7400, not HC, LS, ACT or anything other than bog standard TTL there, as this relies on the input bias current of the gate to also provide part of the bias.

[bd139]

Ok I found one drifted resistor which I have replaced, I have touched up all the joints, swapped the IC out for another new SN7400 (not LS) that I had and it still won’t bloody oscillate without a kick.

Grr!

[bd139]

Found t! Bloody ceramic decoupling cap!

[bd139]

FMH it just blew up again. No HT at all!

[bd139]

C1428 across the 50v line was short. Who the feck puts a 50v tant across a 50v line? No derating at all!

Have replaced it with a Panasonic 15uF 63v low ESR electrolytic. Wonder if that was causing the drop out as per problem 2.

Really can’t wait until my pile of replacements arrives next week!

[tautech]

C1428 across the 50v line was short. Who the feck puts a 50v tant across a 50v line?

Many, as manufacturers from way back then didn't know any better.

For a bit of gear that you intend to keep I'd go through the BOM and schematics with a view to rate higher any Tants where possible.....especially so if you have in all apart. When rail voltages are 30+ the prefered derating of 50% becomes challenged and while probably fine you might want to explore other options like electrolytics with a ceramic in parallel. Implementation is key to deciding what to substitute with, sometimes it's only remote bulk capacitance while in others you also need the speed of Tants so there you need to add a ceramic too.
Today electrolytics are in much smaller packages and often just a low ESR version will do the job in which a Tant was used.

[bd139]

Fair point!

I've ordered a whole bunch of replacement parts for this. Anything which is close to the signal chain is tantalum, suitably derated. Anything else is low ESR electrolytic. I'm not derating by 50%. I've picked 25v for the 15v rails etc. It's "close enough" without increasing the price too much. New units are high quality AVX part.

The replacement BOM cost more than the scope did so far, which I was expecting. Budget is set at £150 to get this up to spec and the expenditure is £65 so far so I'm winning pretty well so far.

A positive note: the tant that just went short appears to have been causing the HV to drop out. Been staring at it for 20 minutes and no problems so far.

[Specmaster]

Hmm, never realised that you were having so many issues with this. It is probably a result of it being left unused for so long, I think it harms electronics to leave them unused for long periods, they start to decay and corrode, but you're doing a good job so far.

Considering how it was when you first saw it, it now looks a million dollars, great job.

I don't like the way that they seem to have cut corners with running those tants at their rated voltage, nothing lasts long if it constantly being driven flat out, are all Teks like that?

[bd139]

There's always many issues with Tek kit that has been left sitting around. You don't usually find them until you've been using it for a few days

Agree entirely. I think this was left in the guy's garage for a number of years. Some green flakey corrosion around the bezel indicates some damp as well. The main filter caps are all really good though; they're all sprague branded ones which tend not to die miserably even after 40 odd years! All of the major issues so far are failed capacitors so all the tants are being replaced in the unit. Just waiting for them to arrive now. Sometime this week apparently.

As tautech said, I don't think they knew any better back then. They worked as they were rolling off the production line and to the end of their expected service life absolutely fine. It's quite spectacular that the things even work at after this amount of time. I couldn't see my Rigol lasting more than a decade or so.

Edit: just to add, inside this scope was completely untouched. No visible repairs. It's the ones with the repairs that are problematic I find. You have no idea what quality soldering iron jockey has been inside it

[bd139]

Large bag of capacitors just arrived. Starting to regret this refurb 

22 to replace as priority! Rest (x53) will be shelved until I can be bothered and I'll do them in batches

Z-axis problems appear to be tantalum related as well. The VCC drops rapidly in a couple of places just after supply decoupling, probably due to increased ESR. Hopefully replacing them will do the job.

[tggzzz]

Z-axis problems appear to be tantalum related as well. The VCC drops rapidly in a couple of places just after supply decoupling, probably due to increased ESR. Hopefully replacing them will do the job.

Quick test: tack-solder a new capacitor in parallel, and see if the problem is improved.

CPC sells cheap tant beads.

[bd139]

Not any more they don't; I just bought them all. I'm joking obviously but I did get them from there though

I decided to skip the AVX ones from Tayda because I'm impatient and they take 2-3 weeks to turn up

Good plan - will do that.

[bd139]

Oh the b*****d thing has started dropping out again.

[Specmaster]

Debugging chop circuit now. After some poking around it appears that the chop oscillator, a simple SN7400 multivibrator doesn't start by itself when you put it in chop mode. The moment you touch pin 9 with the scope it springs into action!

Circuit in question here:



More debugging later. Will write the voltages down and then pull and check all the parts.

Was it C336 by any chance?

[Specmaster]

Oh the b*****d thing has started dropping out again.

Did you replace that tant that you thought might have high ESR?

[bd139]

Was it C336 by any chance?

Nope. Miles away from that on the schematic...

Oh the b*****d thing has started dropping out again.

Did you replace that tant that you thought might have high ESR?

I've literally just replaced 22 of the things and it's not doing it now (yet). I'm setting my digital scope on single trigger mode to see if I can catch any transients on the 50v rail. Fingers crossed!

[Specmaster]

Your digital one can save the image automatically I take it? I might have to invest one of them later on then.

[bd139]

It can indeed. You set it to trigger on single events and it'll quite happily just sit there until something interesting happens. In this case I'm connecting it to the 50v rail and setting the trigger point to 48v on a falling edge. With a bit of luck that will catch a power supply transient at the same time as the display problem. If it doesn't and it still does it, I'll work my way around all the rails and see where I end up. If it's none of them, I'll look at the HT again and probably rip it all out. There is a chance it's the HV multiplier at which point it requires a slightly extensive rebuild, one I'm not up for if I'm honest.

I power cycled the scope quick to trigger this so you can see what it would record:



You can zoom into that and analyse it after the fact (and screenshot it onto a USB stick which is really handy!)

[Specmaster]

Thats really cool, so a digital scope can be a really useful diagnostic tool on the bench then as well making easier to measure periods, voltages and frequencies then. Hmm I wonder though if any of them will still be around and fully functional in 30 years time?? 

[bd139]

Yes it measures just about everything automatically for you. It even does protocol decoding of SPI/serial etc. The analogue scopes are just interesting toys that I couldn't have afforded until they got to 30 years old. They're engineering marvels

Will it be around in 30 years? Hell no it'll be in the WEEE skip in 5-8 years.

Then again if we look at the "2017" price of a Tektronix 475 (with DM44) - US inflation, currency exchange, add VAT:

Tektronix 475: £15,599
Rigol DS1054Z: £349

I could buy a new Rigol every year and still be up on cash over a 475 made in 2017!

TBH I am supposed to be getting rid of the Rigol but the person who was going to buy it did a runner. I thought for a bit that I couldn't justify the expense but I'm less bothered now I've used it to debug some complicated things.

[Specmaster]

Maybe I'll put it on my Christmas list and see if one turns up, I think hell would freeze first though 

[bd139]

I only got mine because someone died!

[bd139]

Well replacing a few more tants has definitely fixed the HV issue. There was one periodically pulling the 15v line down. Bonus. At least the debugging methodology worked!

However time's up on this one. I have decided that I can't be bothered to fix the z-axis at high sweep speed issue on it now as I'm getting behind on a few projects so I've chucked it on ebay for the next person to deal with. I may have sinned against TEA there!   

[Specmaster]

What, you mean that there's a 475 out there on Ebay after all the work and money you spend on it, WOW I'm flabbergasted  Expect you're thinking now that you should have kept your 1740A 

[bd139]

For me, it's the journey that and the acquired skills and knowledge that counts.

I give it a week before I have another scope!

[Specmaster]

Just had a look at it, she looks great, hope you get your money back on it at least after the effort you have put into her. I doubt it will long before someone snaps her up 

[bd139]

Time will tell.

I spent total £68 so some beer money on top of that would be good

[bd139]

You know what. I can't actually point the gun at this one. I've taken it down again

[Specmaster]

Haha, I wondered if such an event might happen, you was taking it to the grave with you. 

Seemed a shame doing all that work and then letting someone else have the benefit of it. 

[bd139]

Yeah. Mental battle ensued. Plus I’m a member of the church of TEA and that would be going against the good teachings.

[tautech]

Well replacing a few more tants has definitely fixed the HV issue. There was one periodically pulling the 15v line down. Bonus. At least the debugging methodology worked!

This ^ !
Did you grab a screenshot ?

Correct me if I'm wrong but isn't this the rarely observed tantalum repetitive breakdown ?
I've heard it referred to as sputtering but according to Wikipedia that's not the right term. 

[bd139]

I think it is indeed that. And no I didn’t get a screenshot. It was a relatively uneventful affair. There was a 5ms fall from 15v to 3v and then it cleared itself.

Finding the f*cking thing was a problem. I went round all of them on that rail with the bench meter on relative ohms mode and replaced the 5 with the lowest resistance. Then goto 0 until fault cleared. There was one suspiciously low at 39 ohms, which is enough to draw some current at 15v. Whatever it was it was in that batch!

[tautech]

I think it is indeed that. And no I didn’t get a screenshot. It was a relatively uneventful affair. There was a 5ms fall from 15v to 3v and then it cleared itself.

Finding the f*cking thing was a problem. I went round all of them on that rail with the bench meter on relative ohms mode and replaced the 5 with the lowest resistance. Then goto 0 until fault cleared. There was one suspiciously low at 39 ohms, which is enough to draw some current at 15v. Whatever it was it was in that batch!

For those that have not experienced it ( I haven't) could you rig up the faulty one across a bench PSU, maybe with a series resistor so it won't fry immediately, or current limit it and take some scope screenshots ....fast timebase, slow timebase.
It's so uncommon it's worthy of a separate thread............

[bd139]

I would but it’s in the bin already. Plus when I pulled it I snipped the leads off at the body.

[tautech]

I would but it’s in the bin already. Plus when I pulled it I snipped the leads off at the body.

Bugger !

Missed opportunity to share something so uncommon with others. 

[bd139]

I’ll have to buy more scopes to see if I can reproduce it again

[Specmaster]

Maybe time to get on the M4 then http://www.ebay.co.uk/itm/TEKTRONIX-475-ANALOG-OSCILLOSCOPE-ANALOGUE-2x-200MHz-D264/292298225053 Might be good as aprts mule if nothing else?

[bd139]

Already on that one

[tggzzz]

Maybe time to get on the M4 then http://www.ebay.co.uk/itm/TEKTRONIX-475-ANALOG-OSCILLOSCOPE-ANALOGUE-2x-200MHz-D264/292298225053 Might be good as aprts mule if nothing else?

Looks as if it has been damp, and one of the other traditional "exposed knob" problems is present; see the A-trigger knob. It is significant that an electronics recycling company hasn't even turned it on.

Sounds like you are going to be in the never-ending cycle of getting scope N+1 to repair scope N

[bd139]

Been there already. Just joining the cycle again.

About 3 years ago I ended up with 5x Tektronix 465B's dotted around the house. As long as I don't get that bad again, all is good

[Specmaster]

Maybe time to get on the M4 then http://www.ebay.co.uk/itm/TEKTRONIX-475-ANALOG-OSCILLOSCOPE-ANALOGUE-2x-200MHz-D264/292298225053 Might be good as aprts mule if nothing else?

Looks as if it has been damp, and one of the other traditional "exposed knob" problems is present; see the A-trigger knob. It is significant that an electronics recycling company hasn't even turned it on.

Sounds like you are going to be in the never-ending cycle of getting scope N+1 to repair scope N

I'm not sure what it is that you mean, is that one below the power button?

[tggzzz]

Maybe time to get on the M4 then http://www.ebay.co.uk/itm/TEKTRONIX-475-ANALOG-OSCILLOSCOPE-ANALOGUE-2x-200MHz-D264/292298225053 Might be good as aprts mule if nothing else?

Looks as if it has been damp, and one of the other traditional "exposed knob" problems is present; see the A-trigger knob. It is significant that an electronics recycling company hasn't even turned it on.

Sounds like you are going to be in the never-ending cycle of getting scope N+1 to repair scope N

I'm not sure what it is that you mean, is that one below the power button?

Yes. The B-trigger knob in the top right is even more exposed; I've had to replace those in the past.

[Specmaster]

What makes you think its been damp then?

[tggzzz]

What makes you think its been damp then?

Looking at the pictures, what else?

[bd139]

You can tell because of how the dirt is stuck to the front. If it's dry, the colouring is uniform with shadowing where there is protection from dust. If it's damp, the dirt clumps.

A little bit of moisture won't do much harm to these. It's "been in the shed and the mains cable has disintegrated" levels of decay you need to worry about.

[tggzzz]

You can tell because of how the dirt is stuck to the front. If it's dry, the colouring is uniform with shadowing where there is protection from dust. If it's damp, the dirt clumps.

A little bit of moisture won't do much harm to these. It's "been in the shed and the mains cable has disintegrated" levels of decay you need to worry about.

The more difficult case (both to diagnose from such a photo, and to cure) is if mould has been growing. Knowing the provenance would help a lot.

[bd139]

I've bought stuff from them before and all dirt was superficial

[Specmaster]

Well they claim it was found in a cupboard.

I never tweaked twigged about the damp would make dirt stick to certain spots, see it's true, you do learn something new each day 

[tggzzz]

Well they claim it was found in a cupboard.

I've seen dank and musty cupboards before

[tggzzz]

Edit: oooooh tempting https://www.ebay.co.uk/i/282694103060

You may have a delicious dilemma: whether to buy that or to save your pennies (and brownie points) for the Kempton Park hamfest. I've decided to do the latter.

At Kempton Park I've seen a non-functional HP1740a go for £5 (or was it £10), a claimed working 465 for £40, and I've bought a working 5.5 7.5 digit DMM for £40. At another hamfest I got a Dekapot 4 decade switched resistor for £10; the clicks as you rotate the contacts is almost pornographically tactile

[Edit: 5.5 -> 7.5]

[bd139]

There’s only one answer and that is both

I could do with a substitution box actually.

I’m going to be honest: I want a 465B without DM44 for the bench. Trace is much sharper than the 475 is and the delay sweep is nicer. The DM44 is a compromise but not terrible. Also need a decent counter. I had a 5315A with OCXO and channel C but flogged it when I got my rigol DG1022Z with the intent of building a GPSDO but the 1022 is stupidly unsensitive.

How early do you have to turn up to get the interesting stuff at Kempton?

Spent some time with my head in the 475 and the trade moving right is related to timebase rather than z-axis. Managed to inject signals right into the amps from my DG1022Z and it works fine.

[tggzzz]

There’s only one answer and that is both

I could do with a substitution box actually.

I’m going to be honest: I want a 465B without DM44 for the bench.

How early do you have to turn up to get the interesting stuff at Kempton?

Clearly you are a really hoopy frood

Timing depends on whether you want best choice or best price. Overall you should expect prices to be lower than fleabay, but you can also twiddle knobs.
 
I would expect to see a 465 class machine, but a 465B is rarer. Resistor/capacitor boxes depend on what people are throwing out, but they are expensive on fleabag.

[bd139]

Towel in hand

Thanks for the tips. I’m going to take £100 cash with me and my wife hopes I will come back with some. I don’t want to look like a crazy person loading tens of dead scopes into the car though so being quite objective.

[tggzzz]

Take more than £100; demonstrate your iron will.

More seriously, £35 for a 465 was very cheap.

[bd139]

Never paid more than £40 for a 465. Granted they’ve all been dead. Only one was completely shot and that was repairable (caps/rectifiers out) and only got sacrificed for parts as it was more broken than the other two!

Sold both working ones for £150 each.

[tggzzz]

How'd you sell the 465s? Fleabay?

One reason for taking more than £100 is that you may well find a lot more than you expect. There is usually some interesting test equipment, quite apart from affordable good SMA/BNC cables.

BTW, I've just seen (and will correct) my previous statement: it was a working 7.5 digit DMM for £40.

[bd139]

Fleabay. But it now £150. Chuck some wittig probes in. Take some nice photos. Ignore offers from idiots. Wait about 2-3 weeks and done.

7.5 digits for 40 is pretty amazing!

Going to be honest: I’m very limited with space which limits purchases more than the cash. This is what I have to work with currently which isn’t ideal. All the vintage stuff lives on the table opposite this pile

[Specmaster]

Fleabay. But it now £150. Chuck some wittig probes in. Take some nice photos. Ignore offers from idiots. Wait about 2-3 weeks and done.

7.5 digits for 40 is pretty amazing!

Going to be honest: I’m very limited with space which limits purchases more than the cash. This is what I have to work with currently which isn’t ideal. All the vintage stuff lives on the table opposite this pile

Wow, you need some shelving badley

[Specmaster]

How'd you sell the 465s? Fleabay?

One reason for taking more than £100 is that you may well find a lot more than you expect. There is usually some interesting test equipment, quite apart from affordable good SMA/BNC cables.

BTW, I've just seen (and will correct) my previous statement: it was a working 7.5 digit DMM for £40.

Hmm you really a TEA when you can walk away from a working 7.5 digit DMM for £40 

[bd139]

Yeah tell me about it. The whole corner is being ripped out and replaced with an ikea (office grade) desk with shelving in the next few months. Keep spending my budget on toys though

[tggzzz]

How'd you sell the 465s? Fleabay?

One reason for taking more than £100 is that you may well find a lot more than you expect. There is usually some interesting test equipment, quite apart from affordable good SMA/BNC cables.

BTW, I've just seen (and will correct) my previous statement: it was a working 7.5 digit DMM for £40.

Hmm you really a TEA when you can walk away from a working 7.5 digit DMM for £40 

Erm, I only walked away when it was £50 Sometimes later is better, at Kempton and other car boot sales.

But then I got an 8.5 digit for £75 but had to spend £25 on a new Fischer mains switch/selector/filter and make my own cable But that was offset by the battery powered transportable 10V sources and Guildline resistors for £14 each (not at Kempton)

Took the reference to Hannover maker faire,  and it is 9.999571V, and the 8.5 digit DMM is (now,eventually) ~25ppm different.

[bd139]

Ok I walked away from that other Tek 475 when it went past £36.

Final price for a dead, untested scope with suspect status .... £55

Mental 

[Specmaster]

Well it was worth a shot anyway.

[Specmaster]

Erm, thats £25 extra for a single digit, but still fair play, you ended up with something better so your membership is still valid

[tggzzz]

Erm, thats £25 extra for a single digit, but still fair play, you ended up with something better so your membership is still valid

I ought to work out whether the price follows the square or cube of the number of digits Other 8.5 digit meters have gone for 20* that at a recent auction!

[Specmaster]

Well you certainly got a bargain thats for sure, just wish they had similar sales around the Essex area so I can see if I could nab a bargain or two 

[bd139]

There used to be a ham fest out that way in the Harlow area. No idea if it’s still on now. Still got the large bag of resistors I got there for 50p. All Philips 1% metal films mixed values with leads formed. A whole kg!  Would be about 25 quid on eBay now.

[tggzzz]

There used to be a ham fest out that way in the Harlow area. No idea if it’s still on now

As the dates get announced, they are noted here: http://rsgb.org/main/news/rallies/ Expect more to appear in the new year.

[Specmaster]

Thanks for the link, I'll keep an eye on that. At the moment there's nothing near me scheduled but that can change of course. I see you're being spoilt with at least 2 a year so I guess it must the biggest one in the year?

[bd139]

No idea yet. Sounds good though. My local amateur radio club are running it which should be interesting.

[Specmaster]

Shame that its a real thrash to get to it and also the timing, is when SWMBO will be at home, been a weekday it would not have been so bad, I get any booty in without without the tongue wagging and disapproving looks 

[tggzzz]

Thanks for the link, I'll keep an eye on that. At the moment there's nothing near me scheduled but that can change of course. I see you're being spoilt with at least 2 a year so I guess it must the biggest one in the year?

There are many many more. Near me there are some in Frome, Chippenham, Newport, Newbury. There's a good one in a field in Luton/Dunstable in springtime. There's always the national hamfest, but I haven't trogged up there.

[bd139]

Dunstable and Newbury are doable for me as well.

[tggzzz]

Dunstable and Newbury are doable for me as well.

Dunstable is tolerable (for me) since I can fit in a visit to TNMoC in the afternoon.

[bd139]

That’s a reasonable day out. Will do that as well I think

Back to the 475, I’ve traced the problem with the trace fading out at high sweep speeds is proportional to the A gate output. I found this by connecting the A gate output to channel two. If you move the trace as far left as it will go you can actually see this on slow sweep speeds but obviously the sweep is much slower so it has a less pronounced effect.

Ergo timebase debugging time this evening. Plan is to check the input and output of the blanking amplifier on the timebase and then slowly bisect the system until I find a hard edge to slow edge transition.

It’s my youngest’s birthday today so no time for anything during the day.

[bd139]

Ok this one is a pain in the arse. The unblanking is taking too long to happen.

I've got two scope traces below.

TP588 (yellow) label is the output of the trigger generator.
BLK (light blue) is the z-axis output. As this increases it increases the intensity of the trace.
SWP (dark blue) is the horizontal sweep. As this decreases, the trace moves from left to right.
Q574 (pink) is the collector of the input transistor of the blanking circuit triggered by TP588.

The output at TP588 and Q574 are absolutely spot on and nice and sharp. BLK is however soggy and looks like it has an RC-time-constant function on it.

As you can see with a relatively slow trace, when the trace starts, there is enough time for the unblank to happen before the sweep gets very far.



On a relatively fast trace, it hasn't even finished unblanking before the trace is done



Aaaand I'm done for the day 

[bd139]

Spent some time on this and it looks like there were some problems with decoupling capacitors again. Replaced a bunch of tants and it's looking better already. Sweep doesn't completely disappear now. Looks like the gain of the transistors were being shot by the tants gone high ESR. Grrrrrrr.

I've run out of tants now so just put down another £10 worth. Decided to stock up this time so bought 60 of the damn things in 1uF, 4.7uF and 10uF, 20 each.

[Specmaster]

  Now your making me thing that I ought to pull mine and check them individually instead of doing a short circuit test, it might cure my flyback issues / blanking issues.

These tants are not very reliable then, your's have only been installed for 7 days at the most   

[tautech]

Now your making me thing that I ought to pull mine and check them individually instead of doing a short circuit test, it might cure my flyback issues / blanking issues.

These tants are not very reliable then, your's have only been installed for 7 days at the most 

I very much doubt any replacements failed, BD139 knows what he's about  , they'll only be the ones he was too lazy to replace earlier without having to dismantle the scope.

[bd139]

Exactly that. Especially the lazy bit 

I did the interface board but not the trigger board. There are about 70 of the damn things in this and I’ve replaced 35 of them so far as it’s hard work and I ran out of tants . I’m trying to do as little work on it as possible really as each time you dismantle it you risk causing another problem to contend with.

[tggzzz]

I’m trying to do as little work on it as possible really as each time you dismantle it you risk causing another problem to contend with.

I refuse to admit that I have sometimes replaced tants without removing the board. All I do is note that Tek didn't bend the leads to hold them in, so it can be possible to heat the leads next to the capacitor, use a solder pump/wick, then pull the lead out. (The possibility of solder drops elsewhere in the case? Puts hands over ears. Lah-la-lah-la-lah; did you say something? )

[bd139]

I haven't removed any boards at all yet in this. All the replacements are done from the top. If you do it with wick, the probability of solder splatter is minimal. I'm using multicore solder so it's no-clean flux as well. I'll clean any high impedance traces with IPA but that's about it. The holes are all plated so it makes it nice and easy

When I refer to dismantling it, it usually means taking the case off and rescrewing the feet onto the base so you can stand it up while working on it.

Yes I'm that lazy

[Specmaster]

I can't stand my 1740A up on its feet, it doesn't have any 

[bd139]

Ah yes. Was rather lucky with this one. At least on the 465/475 scopes you can stand them up on the BNC box that sticks out of the rear if your feet have disintegrated.

Came across this the other day: https://www.thingiverse.com/thing:1410827/

[tggzzz]

When I refer to dismantling it, it usually means taking the case off and rescrewing the feet onto the base so you can stand it up while working on it.

Yes I'm that lazy

You're not a very experienced Wally (as in Dilbert); be more assiduously lazy I've never rescrewed the feet, since Tek had the forethought to make the dismantled case proud enough that components weren't damaged when it was laid on any side.

[tggzzz]

I can't stand my 1740A up on its feet, it doesn't have any 

Rubber doorstops, hacked with a stanley knife, are adequate.

[tggzzz]

Came across this the other day: https://www.thingiverse.com/thing:1410827/

There's a chap on fleabay who manufactures new feet on his lathe. He claims to have gone to considerable trouble to get the right material, which is more important than the shape.

[tautech]

Came across this the other day: https://www.thingiverse.com/thing:1410827/

There's a chap on fleabay who manufactures new feet on his lathe. He claims to have gone to considerable trouble to get the right material, which is more important than the shape.

I thought about doing that with Nylon 6 rod for the 1740A I've got to get around to fixing but you need ribs on them to stop the mains cord falling off when it's in storage. Instead I'll just get a set of 4 of these for a lot less mucking around:
https://www.aliexpress.com/item/CSS-Home-Stainless-Steel-Round-Base-Wall-Mounting-Door-Stopper-Holder/32597769591.html?spm=2114.search0104.3.168.WXbL35&ws_ab_test=searchweb0_0,searchweb201602_1_10152_10065_10151_10068_10344_10342_10343_10340_10341_10307_10137_10060_10155_10154_10056_10055_10054_10059_100031_10099_10338_10339_10103_10102_440_10052_10053_10142_10107_10050_10051_10326_10084_10083_10080_10082_10081_10110_10175_10111_10112_10113_10114_143_10312_10313_10314_10078_10079_10073,searchweb201603_11,ppcSwitch_5&btsid=aa391694-c17f-465c-87cb-2094d7d9df06&algo_expid=976cb595-cd1b-4f8b-9f98-1348657eae7f-22&algo_pvid=976cb595-cd1b-4f8b-9f98-1348657eae7f

[bd139]

When I refer to dismantling it, it usually means taking the case off and rescrewing the feet onto the base so you can stand it up while working on it.

Yes I'm that lazy

You're not a very experienced Wally (as in Dilbert); be more assiduously lazy I've never rescrewed the feet, since Tek had the forethought to make the dismantled case proud enough that components weren't damaged when it was laid on any side.

I only rescrewed the feet because I needed access to the Z axis input and gate outputs while I was probing it from three sides.   They certainly made these nice to debug.

[Specmaster]

I can't stand my 1740A up on its feet, it doesn't have any 

Rubber doorstops, hacked with a stanley knife, are adequate.

I had already looked at doing that but discounted the idea as the rubber stops are only 20mm high and I need at least 30mm, ideally 65 to allow the power cord to left and so it can be powered in the upright position if required.

[tggzzz]

I can't stand my 1740A up on its feet, it doesn't have any 

Rubber doorstops, hacked with a stanley knife, are adequate.

I had already looked at doing that but discounted the idea as the rubber stops are only 20mm high and I need at least 30mm, ideally 65 to allow the power cord to left and so it can be powered in the upright position if required.

How irritating: I'm wrong, you're right. I forgot that I did that and merely rested it on the CRT's cover, at an angle.

Since then I've replaced them with 30mm bars of (IIRC) nylon. Drilling the holes for the screws was a right bast**d since that plastic has a low melting point; and I don't recommend it. A better choice of plastic would help.

[Specmaster]

I can't stand my 1740A up on its feet, it doesn't have any 

Rubber doorstops, hacked with a stanley knife, are adequate.

I had already looked at doing that but discounted the idea as the rubber stops are only 20mm high and I need at least 30mm, ideally 65 to allow the power cord to left and so it can be powered in the upright position if required.

How irritating: I'm wrong, you're right. I forgot that I did that and merely rested it on the CRT's cover, at an angle.

Since then I've replaced them with 30mm bars of (IIRC) nylon. Drilling the holes for the screws was a right bast**d since that plastic has a low melting point; and I don't recommend it. A better choice of plastic would help.

I did find some metal ones on Ebay that I thought about but discounted them as well because the price was something like £40 for 4, almost as much as the scope itself and at the end of the day they did nothing to towards curing the problems it had electronically so was considered to be a wast of money at this stage. If the scope comes out the other side of the restoration OK and is considered to be a keeper, that then becomes an option, but for now I just want a cheap solution.

[bd139]

I saw one a 1740A on ebay a few weeks back where someone had made wooden feet for it!

[bd139]

Think I just landed myself another scope. Watch this space.

Turning into 200 scopes guy on ebay suddenly.

[Specmaster]

Think I just landed myself another scope. Watch this space.

Turning into 200 scopes guy on ebay suddenly.

Well certainly 3 guy, need to get a move on to catch up me [emoji16]

Sent from my SM-J510FN using Tapatalk

[bd139]

Challenge accepted! 

Edit: incidentally had a serious conversation with SWMBO last night and we both decided we need to move so going to bail out of London next year and get somewhere larger. Hopefully enough space for a scope room

[Specmaster]

That would nice, a lovely bank of them all displaying various tracers all at the same time 

So what scope did you land today then another 475?

[bd139]

Indeed it would.

It's not a 475 but a pretty cool machine. Will stick it on the TEA thread later

[bd139]

Update on this one. found out that the z-axis problems originated on one of the actual sockets on the boards. I resoldered the centre pin and we're good. It's not 100% perfect and drifts 1 div to the right on fastest sweep but that's absolutely fine as you can fix it with horizontal. It's certainly good enough for an ancient bit of kit like this.

All done and working 

[Specmaster]

Great, fixed 2 scopes this week and power supply, you're cooking on GAS and having a TEA party 

[bd139]

Haha yes. Got another two things on the eBay watch list already

[Specmaster]

Oh yes, whats that then, a garden shed and a camp bed?

[bd139]

I should probably add them