OK so here is the 502A !
Sorry for not posting yesterday when I got it. Was too tired once back home.
So I now know a bit more about this "transformer" shit on his ad, where he claimed the transformer was used for tube amps and was selling for parts just for that...
Actually turns out he has not one but TWO 502A ! He gave me the best one and opened it up for me spontaneously to show what condition it was in inside, and that no part was missing.
So, the story goes : someone contacted him about his 502A (probably not knowing he had two of them), saying he MIGHT be interested in the transformer and a couple specific tubes in it, just two of them, don't know which ones. That guy said he was a professional who designs new tube amps himself and sells them for lots of money. Apparently he is well known in the audiophool business. Didn't say his name, and I don't care anyway.
But, this guy said he is not buying just now, he said he needs to study the specs of the transformer in detail to make sure it's actually suitable for what he wants to do. So basically he didn't do his homework before bothering the owner.
So now the owner knew the transformer interested audio people, so he updated his ad description accordingly !
So, I said to the owner... since that guy really knows he only wants the transformer and a couple tubes, and nothing else... just remove these parts only, cleanly please... then I will buy the dead corpse from you, for 20 Euros for parts...
So maybe I will get a third 502A scope, albeit probably (or not, we shall see...) transformer-less. So a whole bunch of parts to maintain the other ones I have
Guy is heavy into wood working, 99% of his huge workshop is devoted to that. His old Tek scopes where stacked in a corner in the middle of all that wood stuff. Well I happen to like fine woodworking so it was cool seeing all his stuff and chat with him about wood. But he also knows his stuff about electronics. He taught electronics for any years in various engineering schools, so we talked electrons as well as wood...
Hell he even used to have / drive a Renault 21, like I did for 20 years before I got my Safrane 3 years ago... so we talked R21 as well !
Was really cool meeting him, spent 2 hours there
Anyway so here are a few pics I just took of the thing a few minutes ago, just because I love you soi much. Well sadly the pics look crap, don't know what's wrong... or maybe they are OK and it's my new computer monitor that's crap... my last (had 3 of them) beloved Samsung Syncmaster 700p CRT 17" monitor gave up the ghost a week ago, had to replace it in hurry with a crappy cheap LCD screen I luckily had been given recently as a thank you for fixing some electronic stuff for a friend. Will buy a new hopefully better monitor when I can afford it. Not now. For now it displays a picture which is all I expect to do. Crappy picture might be, but a picture none the less, so it's functional...
Overall condition is good. Not a rust bucket and no physical damage to the cabinet unlike my other one.
Just a fine layer of wood dust, just like Papa Smurf 547, well, a 100 times less... just a thin layer not a kilogram of it...
Exterior : cabinet needs repainting and carry handles as often need redoing. They are so far gone that tehy are missing altogether, only the metal work is is still in place, and it's rusty as hell.
A couple screw missing, the two at the front of one of the two painting cross members at the bottom of the cabinet.
Also one of the two screws is missing on the air filter housing.
Front of the instrument of course is super filthy, you would expect that, sitting for decades in a damp wood shop...but should clean up just fine I think, not worried at all.
Inside :
- The dreaded transparent plastic piece that holds the neck of the CRT, the part that likes to disintegrate and vanish completely like it did on my 575 curve tracer... well it's still there and still holding up for now, but not for long I think ! As you can see it's on the verge of turning to dust. It has turned into a "granular" material, like a casting made of grains of sand, really what it looks like.
- Tubes are lie new, super clean. I mean the ones located on the top side of the lower deck of course are covered with dust, but if you look at the tubes on the UNDERside of the upper deck, their glass envelope looks like new, like it just got cleaned with IPA or something. A few of them carry a sticker showing they have been pulled to go consult a tube tester... some labels are hand written, others have been printed on a type machine, looks luxurious...
- The fan motor looks strangely "modern". I mean usually the windings are wrapped in thick yellow fabric of some kind... making them look very vintage... but this fan has no such wrapping. The windings are visible, anc coated in green enamel. It looks too modern for the vintage of a 502A to me ! Maybe got replaced at some point.
All contacts/tracks on the wafer switches are very black. Access is not great on most of them, so lots of painful and time consuming disassembly work ahead to be able to work on them comfortably on the bench.
OK now time to power it up :
Didn't want to at first... From now on, I want to get a variac to bring the voltage up slowly to give the filter caps a chance to reform a bit before applying full power. It's the last time I take the risk to power up one of my old Tek scopes without a variac..I will get one, I will, I will...
Anyway, powered it up anyway, with some anxiety...
Result :
- Fan starts on the button, great. It makes one hell of a rattling noise.. the blades are hitting the air filter (it's a metallic wire mesh not a foam filter). Loosened the screws of the grill, rattle gone. Still not 100% quiet, of course needs oiling...
- The low voltage stuff seems to work : the tube filaments light up. Bulbs for the graticule illumination work well too.
- Neon indicator bulbs work too on the front panel. These require higher voltage, so there is at least some of the power rails working to some degree....
- However... ZERO life on the CRT no matter what ! Not looking good
- I spotted a tube that was glowing PURPLE at its base ! IIRC that's indicative of a tube that's losing its vacuum, not vacuuming anymore it is
Looked beautiful that said ! Wanted to take a picture, so turned off the lab lights, grabbed the camera and.... too late, purple glow had already vanished, tube now completely dead ! No picture for me/you !
It was a 12AT7 tube, V474 on the upper beam sub-assembly. Looked it up in the schematics, it's the tube that drives the lower deflection plate. Tek scopes seldom use the 12AT7, have very few of them... ended up pulling one from my other 502A (which I know works). Need to buy a new tube now then, of course. Hopefully they are cheap and I can buy two or 3 of them to have spares.
Then after a few minutes running, I heard a brief " TSSSSsssss..... " sound. Like... a big electrolytic cap spewing its guts out.... probably what it is ! Scope didn't catch fire, no smoke. Just the brief noise but no noticeable change to the way the scope was running. No better, no worse.
OK so it's late at night but I could not help but do some quick trouble shooting... and it's promising.
Looks like CRT HV is not dead after all : there is a rotary switch on the left of the cabinet, accessible through a hole in the panel.
It lets you select the XY mode, or the swept mode. In XY mode the upper beam is used as the 'X' beam.
I tried the switch in both positions, still no joy. In either position the CRT is dead. However... however... WHILE you are transitioning the switch from one position to the other... THEN you get life on the CRT !!!! So I left the knob set half-way between its two stable positions, and now the CRT does display something ! A get a couple dots, one for each beam. I get no sweep no matter what I do. So I only get the two dots.
They are bright, so the HV is working fine it seems, PHEW that's a relief !!!
One of the dots is sharp and crisp, that's the lower beam, and I can move it up and down.
Dot for the upper beam however, a huge fat and fuzzy dot, can't focus it at all, and can't move it up and down either, it just stays there. However the upper beam is also the one where I replaced that gassy tube, so maybe there are other related tubes that need replacing...
So I gave up on the faulty upper beam for now, knowing it needs some work, and moved on to check the lower beam further, as it looked more promising. I fed a signal to its input. I just used the probe calibrator, which I knew worked as I quickly tested it with my DMM. There was signal and voltage levels were what they were supposed to be.
Lower beam does respond properly : It shows two immobile dots (since there is no sweep...), spread apart appropriately as I modify the voltage level of the calibrator, or modify the setting of the attenuator.
It's 1AM and super tired so that's it for now !
So to sum it up looks like CRT HV is fine, that sweep is dead, and upper beam is very sick as well.
Next time tomorrow maybe, will work some more on it. From all I said above, I now need to :
- Check service manual to locate test points for the power rails, and check for ripple of course to try to spot which can cap I heard spewing out.
- Clean the XY switch so I can get proper operation without having to keep it in between its two stable positions...
- Start troubleshooting sweep and upper beam, some more dead tubes ahead I guess...
- Give the front of the scope a good quick clean as it's really filthy !