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| Tektronix 7633 Scope, Can anyone please help Old64goat!!! |
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| oldway:
When you say there is little or no history of faults in the HV transformers or in the voltage doublers of this oscilloscope, you make use of the rules of statistics. But the statistical rules tell us also that there is more risk of faults in a circuit with high voltage or with high current than in another circuit. NB: I don't know if Q1195 (TO 2N3055) has been checked for low Hfe ? A Q1195 with very low Hfe could explain why it is oscilating at very low level without burning the fuse. |
| finom1:
--- Quote from: oldway on May 12, 2017, 06:48:49 am ---I have not intervened until now because I have repaired many analogue oscilloscopes of different brands, including Tektronix, but never a 7633. For me, there are only two possible faults: faulty transformer or faulty voltage doubler. You worry about things you do not have to worry about: desolder the wire of the voltage doubler that goes to the transformer, whatever the insulation there is on it. You don't have to worry about this insulation ! Power on the scope with the HV board without the voltage doubler and check if you have the - 1475V on the cathode of the tube. There is no problem to let this auto-oscillator working without load because it is not a flyback and it has a regulation circuit of the -1475V voltage. If you have the correct -1475V voltage, it is the voltage doubler that is defective, if not, it is the transformer that is dead. Since there are many connectors on this HV board and that there is a risk of error in reassembling the scope, David Hess's proposal is also a good option. Again, desolder the wire of the voltage doubler that goes to the transformer. It seems to me that 40Khz is a bit high, I believe that these auto - oscillators work rather at a frequency close to 25 Khz. Connect the probe (x10 or x100) of your oscilloscope between the ground and the terminal where you have desoldered the voltage doubler wire and feed the transformer primary as you do with 25Khz, but starting from 0V ... (filament of CRT NOT connected !) Slowly increase the output voltage of your function generator and check the voltage at the secondary with your oscilloscope. If you have a very low voltage or nothing, the transformer is faulty. Nb: some scopes as Philips PM3264 have well known issues of corona discharges, but their HV voltage is 17KV's, not 7KV's as this Tektronix 7633. --- End quote --- Thank you Oldway, good to hear from you:) |
| finom1:
Thanks everyone for trying help The Old4goat out!!! |
| finom1:
New update from The Old64goat: Here is what I found out: I removed the HV multiplier from the HV transformer and I fed in 100 volts AC 60cly from my varac, I got 60 volts out of the multiplier. Next I put in 40 KC then lowered it to 25 KC, I had my scope probe on the secondary of the HV transformer where the HV multiplier was connected, NOTHING no signal, I turned up my function generator to max and even set my scope probe to RZ-1, again nothing. I put my scope probe on pin 3 of the 2N3055 plug and got my signal. Then I took the resistance readings of the HV transformer, here is what I got: From the HV transformer terminal where the two HV diodes connect to the reed relay to where the HV multiplier was connected I got 100 ohms. Next I took a reading from where the HV multiplier was connected on the HV transformer to the terminal where R1217 (10K) is connected and got 163 ohms. Finally I took a reading from where R1217 (10K) to the HV diodes and got 62 ohms. I think I got a bad HV multiplier AND a bad HV transformer, replacing the HV board may be the only way but I am not going to dump big money that I do not have into this. THANKS to all who have helped me on this!!! |
| oldway:
The voltage multiplier seems to be OK, low output voltage is normal because frequency of 60Hz is far too low for normal working. But HV transformer seems questionable. |
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