Re: 475 with no Display and no HV


On 2/27/2019 9:09 PM, bob.ripley@... wrote:
Hello all,
We had several Tektronix scopes donate to our museum (Museum of Radio and Technology) late last year. One is a Tek 475 with no display and no HV at the -2450 volt test point. All of the low voltage rails are within specs.
There is 20 vdc at TP1318 but the waveform is only about 700 mv p-p at 22.3Khz instead of 40 volts p-p at ~50Khz. The waveform at the base of Q1308 is 150 mv p-p also at 22.3Khz but the DC voltage measurement at that point is 3.4 volts instead of 4.4 volts. The rest of the voltage measurement compared to the manual are no where near correct.
I have check all the tantalum caps on the driver side of the HV oscillator, the solid state devices, and even the resistors. All appear to be good.
I have even taken a working 475 and did a side by resistance check of T1320 for open windings or shorts between windings of that transformer. A side by resistance check was done between the two scope for BCE to ground on transistors Q1306 thru Q1318. The only significant difference that I found was from pin 6 of transformer T1320 and the junction at the taped secondary of that transformer and C1320 (the .01 uf HV cap). One the scope that works I measure 9.6 Megohms. The 475 that doesn't, work shows a resistance of 1 Megohm.
I am sitting here scratching my head. Do I have a bad transformer or is there something else that should be check. Also know that I took the cussed thing apart to get to the high voltage multiple and the input to GND resistance measured infinite, if I remember correctly. It certainly did not read a low resistance or a short.
Anyway, I welcome any comments or tips
I've had those HV caps go leaky - maybe that accounts for the
resistance reading?


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