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Hi everyone, I’m starting with oscilloscopes and electronics. I got a Tektronix 475 that I would like to repair for my lab. It turns on but unfortunately I have no trace...I tested the voltages. Everything seems to be ok (+15v, +8v , +115v) except for the 50v, I get +65v...all components in this sector tend to heat up after 10 minutes. For the rest, I do not know where to start? Thanks in advance for your help, PS

Check the three transistors and IC associated with the 50V supply. One or more could be open/leaky/shorted. The series pass would be the first to check. Check to see if you have the 9V across VR1416 (if bad, BZX85B9V1 is the replacement). The 5620 ohm 1/2W should be 2W. A 5600 ohm will work. Make sure the physical size is correct for 2W. A 3,68 x 8,74mm resistor is NOT 2W.

Check voltages in the 50V supply to see where the problem is. The schematic and manual is available online for free.

It is advised to recap the power supply and some other areas due to age. The 1,5A bridge rectifiers should be 4A.

The heat is part of the series pass and some resistors. Some resistors should be increased in wattage. The originals could be out of tolerance or open.

Mark

Is there hv on the tube?
Do you see the heater glowing?

Measure the DC supplies with a scope, should be flat (-ish), no noticeable ripple.

Hello Marc, Thank you very much for your response. I tested all the Zener diodes and transistors following the schematic but so far I have not found any faults. What is the max/min voltage accepted on the 50 v unreg? Thanks a lot, Renaud

Hello David, thank you for your response. I did all the tests that Mark advised me below but without success. I think that the problem is with the HV module. Unfortunately, I do not have the equipment to measure voltages above 1000 v. Is there another way to check the HV module and the tube ? Thanks in advance for your reponse, PS

A quick go - NO go testing of the high voltage sections would be to run the back of your hand near front of the tube. Of the hairs on the back of your hand stand up due to the static then you can assume HV is present. No static on the front of the CRT it can be assumed that the HV is missing.

Power needs to be on of course but the case covers can be in pace for this test. To increase the sensitivity of the test the implosion shield can be temporarily removed to expose the front glass.

Zen

Renaud,

The 50V supply should be within ,25V, 49,75 to 50,25V. The 475A schematic has more voltages listed at points in the power supply. Pins 2 and 3 of the IC should be 9V. Pin 1 should be 10,4V. Pin 8 should be 23V. Check the voltage across VR4122 to be 43V. The B of Q1424 and C of Q1432 should be 52V. The B of Q1426 should be 51,5V. See if there is a 20V drop across VR1423. Check (TP)105/160V to see what voltage it is. Check TP 50V unreg. If that and the 50V out are the same, CR1426 is suspect of being shorted. Cr1424 and CR1432 could be bad. I do not know if you tested the transistors in the 50V supply out of circuit or not. The series pass can be left in with Q1424 being removed to test the series pass. If C-E is shorted, the diode mentioned above is suspect. Get the 50V working first then see about the high voltage section.

David and Zen are correct. Normally a crackle sound is heard when the high voltage comes up at turn-on.

Mark