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Vintage 500 Series Tektronix scopes

I’m now confident the G plug-in is working correctly. I’ve just run it for at least 20 minutes and given the front panel trim pots a nudge on the calibrator square wave (which I tested with the Hameg to make sure it was right). The unmodulated trace is centred after this period of warm-up (there is a ‘position’ trim pot for that inside) and with a paper clip stuck in the calibrator BNC and both probes hung off it I get positive 1CM on channel A, negative 1CM on -B, both bang on the graticule (I didn’t need to trim that, it was just right), and A-B nulls out to a perfectly flat centred line, which as I understand it is correct. I’ve got the DC balance bang on so the trace doesn’t move if I turn the ‘Variable Volts/CM’ knob (this was batshit out to start with). I’m pretty sure on DC at least this is working as it should. I’ve not tested AC. Great to have one good working plug-in!

Next step is to get the CA working properly. I’m pretty certain some of the valves in that one are totally shot. Certainly one of the 12AT7s was bad, and swapping the existing set from one channel to the other throws it right off in crazy ways, and the 12AU6 and EF95s didn’t work right in the G either. They have to be shot. I have a new set of everything bar the 12AT7s on order and I hope that fixes it. I just can’t adjust it in with the existing valves. I really hope I can get this one working.

The 1A1 is likely beyond me. It isn’t working properly and being solid state with few internal adjustments is likely a proper component fail somewhere. It is impossible to position both traces, B wants to be higher than A and neither like the bottom of the screen at all getting very distorted. On the calibrator the square wave flattens out into a distorted almost triangle as you move the trace down the screen. Something very not right there. I remember it working initially, but then something just failed. I have to admit I’m not too bothered. I’ll be perfectly happy if I can get the CA to work as well as the G. It has far more corrosion inside than the others too. A shame as the front panel is in very nice condition.
 
I do believe you are getting the bug:) Kinda makes any item of hi fi equipment you've ever seen look like a fisher price toy yeah?

Great stuff on the G plug in! Many scopes used to get switched on when power to the workbench was switched on and could have been switched on 8+ hours a day for a long time so yes sometimes genuinely worn valves are an issue.

Sounds like the 1A1 does have a real problem but likely something not too serious effecting a voltage rail. If there are any tants suspect a short circuit tant before anything else. Transistors in sockets IIRC so could be worth a waggle but sockets usually quite bizarrely reliable. Something having gone completely open or short after all these years and taking out a voltage rail is VERY likely.

I use old gear like this in my normal work and so when getting old gear going again I'll often leave it on for at least 24 hours once it appears to be OK to kinda goad it into failing... any of those ancient parts going to fail then do it now whilst I've got the panels off and have access to everything. Same for anything that is simply going to drift out over 24 hours. I want to know as far as possible that when I actually need to use it in 3 months time it will be OK.

Oh, air filter! Tek used to warn about this the way car makers warn to do regular oil change!
 
Oh, air filter! Tek used to warn about this the way car makers warn to do regular oil change!

I stole the one off the 533A as it was missing on the 543B (it just had a light mesh grille). The air filter is a bit bashed up and clearly old, but I gave it a very good hoovering on a high setting so anything loose in there will have gone and it isn’t blocked/impeding airflow. Obviously it doesn’t have to do much here as it is just typical household dust, not 24/7 industrial grime etc, but even so I’d buy a replacement if I could find one.
 
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Just given the underside a good scrub. Looking pretty good there now. I also checked the can caps with my Peak ESR meter and all gave the double-beep of happiness. I didn’t bother checking actual capacitance as I’d have had to deep-dive the schematics to find what the values should be, but the double-beep suggests the ESR is very low, i.e. they are likely good. Certainly nothing leaked, bulged, running hot or anything. I suspect if there are cap issues to be addressed it will be the smaller black ones like the two I replaced yesterday. There are a fair number of the 0.01uF Spragues lurking under there, but I’ll leave them be unless I detect any functional wrongness.
 
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Just got rid of that huge shonky brown cardboard cap. As mentioned upthread it had a label that said it was across C858 and after yoinking on the wires and seeing where they went I eventually found it and C858 looked to be present and correct (the mod was just tacked on top of the tag strip). Very awkward location to get the iron to, but it came off ok and it is now resplendent in the trash. I have a feeling who ever did it hadn’t found Tektronix’s nice solder as the joint was a bit brown and crusty. Nothing much I can do about that, but it is solid. The big brown cap was actually leaking, really quite sticky on the bottom, so good to see the back of it.

The scope still seems to work, and it appears on power-off it no longer goes to a bright dot on the CRT, so removing it was maybe more than a cosmetic gain. Hopefully this scope is now totally stock spec and I don’t need to think about any other hacks/botches etc. It is far beyond my ability just trying to work with the scope and schematics as-is without having to figure out what anyone else has done!

The only other ‘off’ thing I’ve noticed was the striped cap I replaced yesterday, but that just looked like a fix for a fault. It was actually still in capacitance spec as far as I can tell, unlike the other Sprague one (C740) which I can’t even get a reading off.
 
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Anyone got any strong views regarding the Sprague ‘Black Beauty’ caps? There are these three in the CRT high voltage supply (I’m counting the brown one as one) and I think another few still lurking down in the bottom of the chassis where I replaced two (one of which fixing the main voltages as it was leaking into one of the valves in the supply).

I did some research and there seem to be several versions of these things; the ‘bumble bee’ which are guaranteed to have failed by now, black with white text, and black with red text. The red text are allegedly by far the most stable/least electrically leaky, but by saying that one was obviously leaking as replacing it completely fixed the 500V and 350V supply rails.

I’m really in two minds here… should I just preemptively replace the whole lot of them with modern caps for peace of mind, or leave them be? I’m tempted to do the former as it may bring increased stability as some of the fluctuations I’m seeing could easily be leaky caps. Thankfully there aren’t many of them as most of the critical caps are ceramic and as far as I can tell the electrolytic can caps are fine. I’m assuming orange drops or modern film caps would be fine. My Peak ESR meter can’t cope with such tiny values so unfortunately I can’t test them. In the HT there is a .0047 400V, a .001 1000V, and a .1 100V. Quite a few .01 400V in the bottom, and three .1 400V. About 12 caps in total, so not a huge amount of work. What is the consensus, worth doing?

PS Vintage Gibson Les Paul owners will fight to the death for these caps, the older and leakier the better apparently! Sadly these have fairly short leads, otherwise I could make a killing on eBay!
 
Paper "and anything" caps are always dodgy IMHO. With vintage valve kit almost all faults will be leaky paper caps and resistors gone high and sometimes noisy (plus switches needing a spray etc of course). Polyprops are simply superior in every way. Ceramics and micas almost always fine and yep it's amazing how those ancient "huge for the uF's" electrolytics seem to last.

I had to replace several similar UK made caps and of course resistors marked say 390K but measuring 1.6M to get that bigger than a microwave Marconi Instruments sig gen I mentioned up thread working and bought in polyprops and mylars to replace every other paper based cap that's in a lower impedance area of the circuit and less critical on leakage but fresh out of round tuits...
 
My inclination with kit that is sixty years old is that if there is no sign of imminent failure of a component it will probably see me out - this is a bit of an age dependent decision of course.
 
Damn it, it is now dead! It is powering up, the relay clicking in, the CRT is certainly alive but no trace. No bright dot or anything, nothing beyond some random CRT glow if I crank the intensity control right the way up. Not a plug-in thing, it is a consistent fault with both the G and CA.

I have been cleaning (it will take months to really get the dirt out of this thing) so I guess I may have killed something. I was last cleaning around the area I fear the most which is the solid state stuff. I’ve certainly not “broken” anything, no wires off, shorts or anything, but I guess I may have static-killed a transistor despite earthing myself. I was cleaning the general area and replugged the transistors, which were filthy. This is in the vertical amplifier section so may well be relevant, though I don’t understand why there isn’t a bright dot if there is no deflection.

Any ideas what to look at first, what to measure to narrow things down a bit?

My instinct is just to replace all the transistors as they are all tarnished and pretty ugly, but that won’t be easy as they seem obsolete and getting hold of them will be tricky. There is a good Tektronix community though so it should be possible.

PS Obviously this may be coincidental and something unconnected to cleaning has failed. It is obviously old. I am crazy careful about things and I’ve certainly not physically broken anything, e.g. all transistors have the same number of legs they started with and are in the correct orientation in the socket they were removed from.
 
I say start by measuring as many voltage test points as you can and see what that gets you.

Fuses?
 
Definitely don`t replace the transistors just because they look dirty, many were specially selected types. If they are functional just make sure the leads are clean, most are gold plated so not too much of a problem.

If the vertical amplifier is faulty it may be deflecting the trace right off the screen, check the voltages going to the deflection plates, should be more or less equal with the trace centered.

Using another scope to check what is coming out of the vertical and sweep output sockets may be informative.
 
Have a really good look at the area you cleaned for snapped leads shorts etc, a magnifing glass will help.

Pete
 
My inclination with kit that is sixty years old is that if there is no sign of imminent failure of a component it will probably see me out - this is a bit of an age dependent decision of course.

I've usually found very different.... today's update on the Tek being more usual.
 
Using another scope to check what is coming out of the vertical and sweep output sockets may be informative.

I’m not reading anything at the front panel Vert Sig Out socket. Maybe a little residual noise if I really crank the gain on the Hameg, but no voltage with scope set to either DC or AC. I can’t find a sweep out unless you mean the Sawtooth out? I am getting that and it seems to be about the 130V expected. I assume that means the timebase is working - the sawtooth responds as one would expect to the timebase control.

The scope trace indicator lights above the CRT (which weren’t very accurate to start with) suggest the trace is off the bottom of the screen. No bringing it back. The horizontal indicator will light with the knob turned hard left, but I can’t get it to light to the right hand end.

Have a really good look at the area you cleaned for snapped leads shorts etc, a magnifing glass will help.

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I’m pretty paranoid/obsessive with this stuff, I was grounded to the scope with an anti-static wrist-band plugged into one of the 4mm grounding sockets on the front panel whilst I was working and I’ve been through and cleaned every transistor leg. They are all there, and all in the correct place in the correct socket. I did one at a time to ensure I didn’t screw that up. I’d already cleaned the tag-boards etc a couple of days ago so I don’t think it is anything there. I have a feeling one is now dead, but which one?!
 
Have you got all the transistors back in the right way round? and bent legs or loose sockets?
You can tighten the sockets up by pushing the contacts in with a needle/ponity thing.
Pete
 


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