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I am working on a 72-320 Tenma scope. It was supposed to have worked before it was put on a shelf, and after being taken off the shelf it has no trace. I'm pretty sure it's the power supply, but I'm not able to find the problem.

I have a schematic, but no operating voltages are marked except for what is supposed to be on the connectors. I see a 4 Vac voltage across the heater, and on the high voltage a I see a whole lot of around -1800 Vdc referenced to chassis ground voltages on the different tube pins, but can't find a trace to save my life.

I unplugged the vertical amp and horizontal time base from the tube to try and find the spot and just can't find it. No flash when I switch power off from the tube either. When powered on, even though I saw an ac heater voltage, I never saw a glow from the heater. So I removed the tube and put a 5 Vdc on the heater and got a glow from the heater, so that is good. Any ideas?

schematic

These are the voltages I read with the negative lead of my DMM attached to the chassis/pc board ground plane of the scope. In the process of trying to find the voltages correctly based off the cathode voltage as ground, I managed to slip a probe and probably zapped a diode in the HV side....aaggggghhhh dang it! So, does anyone know what a good equivalent is to the 1SS83 that is in the dc restorer circuit?

voltages to CRT referenced to chassis

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    \$\begingroup\$ Assuming you have an adequate probe, did you check the 2nd anode voltage? And how about checking the AC heater current with a clamp-on? Maybe (in the latter case) the tube socket is flaky. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Dec 19, 2021 at 3:33
  • \$\begingroup\$ I see there's no way to add pics. These are the voltages I found on the socket with the CRT disconnected. -1886 at heater pin 1,-1886 at heater pin 14, -1890 at cathode pin 3, -1931 at grid 1 pin 2, +99.63 at grid 2 pin 6, -1529 at plate 1 pin 4? , +22.35 at plate 2 pin 5. I really wish I could add pics. I'm curious based on the tube diagram which shows something that looks like four or five grids/screens, yet on the tube manufacturer diagram there are two anodes/plates, and one of those plates has a negative voltage? \$\endgroup\$
    – JasonB0020
    Commented Dec 20, 2021 at 16:57

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Alright, I guess this counts as an answer. I'm an idiot. I checked so many things, checked that CRT connector til I was blue in the face. Missed a not so obvious check though. The connector going to the board had a bad contact in the female side that I'd never have seen if I didn't try the separate transformer trick and still got nothing. I then checked the continuity from the transformer contacts with nothing connected to them except for the CRT connector and found no resistance, no continuity through the board so naturally the heater couldn't work. I used a 1n4004 to sub for the 1SS83 I fried , bent the pin where it could make temporary contact and now I have a trace. Not sure I'm 100% yet though, I see ripple in the trace, I guess that could be the vertical amp? Anyhow, that stands as a reminder, sometimes the simple stuff will make you lose your mind if you miss it. Thanks for entertaining my temporary insanity y'all.

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