HP 120B scope updateOh not yet another of his updates, he pisses us off, someone please tell him to shut the fuck up !
Don't worry... this is the very last update on this scope... I have just received my new 6AQ5 oscillator tubes, so I started working on the scope again.
After only 30 minutes it was declared dead. It is now officially unfixable, 100% certain, it's a door stop.
For the few who actually care about the details... well here is what happened.
So I put one these NOS 6AQ5. It improved things...input grid voltage not so much, still at 150V like yesterday, however output gird voltage was better at -90+ Volts following power up (was80 yesterday), so closer to the nominal -123V indicated on the schematic. Plus, as seconds were passing, it was getting more and more negative, progressively, steadily, getting closer and closer to the nominal voltage. After I don't know, 3 minutes or so, it did get to the -120V mark !
So... at that point I thought OK, HV should be in good shape now, I should have a trace on the screen ! So I bent my neck to have a glimpse of the CRT... no trace. Then realized that the what.... WHAT ?!
The cathode heater in the CRT was not glowing anymore ?!
Then I started hearing arcing ! So I stopped probing the grid voltages and instead tried to locate / identify the arcing. It was random so not easy. After a few minutes the verdict came in : the main (mains) transformer is sparking all over the place ! Between the insulating tape on the outside of the winding, and chassis it was sitting on. I even saw sparks, once, between two of the ' E - I ' metal sheets of the core of the transformer !
So... I probed all the voltage rails, they were all fine.
Since the heater was not glowing, I then turned my attention to the winding whose sole purpose is to supply 6Vac to the CRT heater. Bingo... was dead while arcing.. but fine when not arcing.
This winding is elevated to the potential of the cathode, i.e. 2500V. The markings on the transformer state that this winding's insulation is good for 5.5kV. Well looks like not any more is it...
Well I guess I could alwyas fit a little 6V supply of my own to replace that failed winding. This way it would not be tied to the cathode HV hence should not spark ant more.
It would not look stock, it would be an add-on.. but, if it can get the heater working again, why not. Function first hey...
However I was still worried because the heater would not glow even when the transformer was not arcing and the heater voltage was present. So I feared that when the transformer failed it might have led to a spurious, transient HV event that blew the crap out of the filament. So I measured it and yeah, it's open circuit
Then as I looked through the glass of the CRT neck to try and see the filament inside.. I noticed oh no... a huge hair crack in the glass at the neck !
No, I didn't bang the CRT, always took great care of it, believe me or not I don't care.. I know what I did and didn't do, and hitting the CRT neck was never part of it.
So.... end of story.
I still like this scope very much and would love to see it fixed and working, so I can play with it but.... bad transformer and dear CRT, that's too much for me
so I will button it back up and maybe in 10 years I will find a cheap parts mule to fix this one, or the other way around, use this one as a donour to fix a twin brother.
But I am not holding my breath. I am not going to find another of these ever, over here. It was an impulse buy because it looked cool, was HP, was free and in good nick... so I felt it deserved to be saved but... it does not want to be saved it seems
So if someone finds another one of these dirt cheap, in bad condition, so I can salvage a CRT and transformer as well hopefully... don't hesitate to contact me !
That's all for this scope.... I am well pissed.