OK, so now that the CRT is fixed, I started testing the vertical amplifier now, more seriously.
The goods news : as I assumed might be possible, the trace shifting upwards very quickly at the same time that it was getting dimmer and dimmer... must indeed have been a direct consequence of the HV dropping.... because it doesn't do it anymore !
![Cool 8)](https://www.eevblog.com/forum/Smileys/default/cool.gif)
So one less problem on the list.
So now I can safely assume that all the symptoms I see on the vertical side of things... are indeed related to the vertical amplifier, and not a consequence of some other part of the scope misbehaving.
So, what symptoms does the vertical amp really have, then ?
Well, to start with... when you power the scope up, with the vertical position control knob centered as it should be... you do NOT see any trace on the screen ! ... it's there... but way, waaaay up, out side of the visible area. If you turn the vertical knob allll the way CCW, you manage to lower the trace enough to bring it in sight, but the best you can lower it to, is the center line (that was yesterday) or +1 DIV (that's today), and as the scope warms up, it progressively moves up a bit more, one more full division or so. So basically, once warmed up, the best you can do is bring it 2 DIV above "ground" level. Not great eh ?!....
As luck would have it, I remembered a YT video I saw a few weeks back, of a guy troubleshooting his Tek 317, the very scope I have, what were the odds....
Did not think too much of it at the time, but then realized the chap had the exact same problem I am having !
So watched his video again to refresh my memory :
His scope has a also a severe positive offset, though not quite as severe as my scope : at least he can see a trace at power up... I can't.
To rule out any problem with the CRT, said a simple test is to simply short the two vertical deflection plates... did that, worked fine : the trace is centered... well almost, maybe a bit less than a small division off, but I won't cry, and it proved the point.
Then I looked at the tubes in the vertical amplifier. I don't have a tube tester of course, nor do I intend to buy one as I don't plan on buying more tube gear.
However I noticed that one of the tubes in the second stage of the amplifier (V224, lower part of the stage), was glowing very noticeably dimmer than all the others around him. Guy on YT had also a problem on this particular stage of the amp ! Unlike I don't have a tube tester. However, though again I am no expert but my reasoning was that if the glow comes from the heater, if it's dimmer it's colder, hence the cathode is less energized hence will "produce"/free less electrons, therefore less current can flow through the tube, therefore the amplification will be less on that side/half of that stage, hence create an imbalance, which I guess could well cause a big offset. I tried to take a picture of the tubes glowing, not easy to capture this kind of thing properly using a camera, but I think we can easily see the difference in brightness none the less.
Had nobody around to tell me if that cheap newbie theory was worth anything, but at least I knew that if I was wrong, then swapping the upper and lower tubes should not induce any major change in the offset I am seeing. However if I was right, then logically the offset should still be there, and also going downward not upward. So, I swapped V224 and its counterpart V214.. and hey presto, now the trace is way... DOWN, not up anymore !
![Cheesy :D](https://www.eevblog.com/forum/Smileys/default/cheesy.gif)
I posted on TekScope to get some good advice about tubes, they were most helpful, great bunch over there. I ordered a few of them (6AU6 type) NOS items of course, not used, for a very reasonable price and as luck would have it, local to me/in France, so postage was cheap as well. The guy sent them the same day, so hoping to receive them pretty soon !
Then I tested the vertical amp some more : tried to check if the gain/attenuators/calibration was good or not.
Result : it's not good at all : it reads 25/30% too low in amplitude. A 1kHz sine wave that's 4 DIV on my main / working scope, shows up as 2.5 to 3 DIV on the Tek 317...
Then I tried the variable gain red knob : the world collapsed at this point ! LOL Its does all kinds of weird and wonderful things ! Most of the time, it will shrink the signal down to less than one DIV .. then if you fiddle with the knob with enough persuasion.. it will display the signal again, thank you very much. So it probably needs a lot of contact cleaner in there. Not a problem I know.
What's more weird however, is that when you turn the knob, instead of decreasing the amplitude more and more.. it will move the trace vertically, just like it were the vertical control knob !
![Shocked :o](https://www.eevblog.com/forum/Smileys/default/shocked.gif)
Also, it you turn it to the left, hence the trace goes downward, at the same time the amplitude of the signal will decrease a little bit, the more you go down, the more it will "compress" the signal.
Really weird indeed !
![Confused :-//](https://www.eevblog.com/forum/Smileys/default/confused0024.gif)
However I am not overly worried about that.. because in the schematics it shows the the variable gain knob is part of the first amplifier stage, that very stage with a dying tube ! So my take is that all the weird things that the variable gain does, are mostly due to the interaction with that dead tube.
So I will first replace the tube, and once this offset problem is fixed and that stage is back in business, I am pretty sure the variable gain will behave more normally... it will probably, then, just need a good cleaning... though it feels so crap that I may have to replace the pot altogether. I think I will try and pull the entire attenuator assembly out of the scope, so I can work more easily on this pot, and get better access to clean all the contacts in the switch assembly.
So.. I am waiting for the NOS tubes to arrive, then will report back...