HP 120B scope updateIt's dry now, so put it back on the bench.
Re-installed the CRT.
Went to re-install all the tubes then realized oh no... many of them have only the European style designation marked on them, so had to Google them all to figure out the American designation number.
Resoldered a couple wires that broke.
3...2....1... FIRE !
... no smoke, no bang... looks like tubes are lighting up... no tragedy ?! How can that be ?! After such an intense and hour long cleaning session, it's not smoking ?!
GOOD !!!!
First I checked all the power rails of course...
Negative 150V reference rail still there and spot on.
18V rail for the beam rotation still there.
300V still there, still a tad high as it was before, but well within spec.
Then looked at the 100V rail... 35V !
This rail is derived from the 300V. Just a triode pass element dropping it to 100V.
Checked voltage at the grid of the triode... was good. OK so the triode is dead.
It's a 6U8 (which contains both a triode and a pentode). This was the tube I didn't have in "stock" in my Tek collection, so had to buy a dirt cheap (15 Euros or something IIRC, shipped !) x16 lot of NOS ones from Russia. Well looks like this one didn't last very long did it... no worries, having so many of them now, and for so cheap, I don't mind if I have to reject half of them, still a good deal !
So I picked another one and that fixed it, 100V rail is back up, great.
Then I went straight at the CRT HV board of course, to see what the situation was. Still the same. Zero HV, not a sausage.
Then I replaced the 6AQ5 oscillator tube by a black one, now I know the black colour doesn't in any way mean it's bad.
Well guess what ? There is progress !
The input grid now measures at 150V, still not the 92V it should be, but way better than the 300V I got before !
Then measured the output grid (it's a pentode so lots of grids there...), and great improvement there as well : I used to have positive one volt or so, instead of negative 123V. Now I get negative 80 volts ! That's MUCH more like it !!!
So since we are in much better shape than before, though still way off, I thought hey maybe this thing is alive enough to try to cough "some" HV, oscillate a little bit. And it does ! We have HV !!!
Still no trace on the screen because it's not HV-ing enough. Missing about one kV. I measure about -1650V instead of -2500V.
So looks like the 6AQ5 was indeed dead, and this replacement used black one is now willing to oscillate and produce some HV... just enough.
So maybe it's just tired and the problem will be fixed once I replace it with a healthy NOS one.. we shall see, crossing fingers ! If not, well more trouble-shooting will have to follow !
But since I lost HV all of a sudden, and the tube was dead it now seems... it's statistically unlikely that there is another issue that declared itself at the exact same time (except cascaded failures scenario of course, though here I don't see what that could be).
So chances are high that a NOS tube will fix the HV. I am sooo impatient to receive these new tubes ! Tracking said they left early this morning, a hub in Lyon.. that's at the other end of the country.... so I am not expecting my tubes this week.. or maybe Saturday if I am lucky.
EDIT : just reloaded the tracking page, my tubes that were in Lyon at 2AM, now at midnight are in Nantes, my local hub ! That was quick !
So I might get them today ! Well... tomorrow most likely... but maybe today !.. or not... but maybe I will, who knows !
Crossing fingers and toes !