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| Tektronix 7633 Scope, Can anyone please help Old64goat!!! |
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| finom1:
--- Quote from: james_s on May 03, 2017, 11:53:26 pm ---Can anyone confirm whether the HV is directly generated or has a cockroft-walton multiplier? I ask because I've worked on HeNe laser power supplies where the capacitors in the multiplier looked fine at lower voltages but would arc internally at full voltage and short. It might be worth measuring the HT connection while ramping up the HV circuit on a variable bench supply and see if you get any HV up to a certain input voltage. --- End quote --- Thank you for helping out!!! |
| finom1:
--- Quote from: David Hess on May 04, 2017, 12:22:44 am ---So with a higher supply voltage, the frequency was lower and the AC signal was smaller? Here is what I think: 1. Aluminum electrolytic capacitor C1198 is worn out. At higher frequencies, the reactance of L1198 will be higher so the AC voltage seen at the collector of Q1195 will be higher. The high voltage inverter will not operate if decoupling capacitor C1198 is worn out. 2. Usually the control loop will drive Q1195 hard if the output voltage (-1475) is low blowing fuse F814. In this case, I suspect the fuse was replaced later if it was blown and driver Q1190 transistor was damaged or the gradual degradation of C1198 allowed excess drive current to damage driver Q1190. In that case, Q1187 could supply enough current through the 1k resistor and Q1190's shorted emitter-base junction to raise the current through output transistor Q1195 to the level you measured but not high enough to blow fuse F814 again. 3. There is no voltage on the output because of the low impedance of the heater winding, the high impedance of L1198 when C1198 is bad, and low drive current from Q1190. 4. Possibly there is a short on the output side like at the high voltage multiplier but that cannot be the only problem. If the problem was only on the secondary side, then fuse F814 would blow. The above is somewhere to start anyway but access to the high voltage board will be necessary to replace C1198 and test driver Q1190. I would just replace C1198 and test the old capacitor. You said that you lack a transistor tester but do you have an LCR meter or impedance bridge for testing capacitors? Just measuring the capacitance should be sufficient. --- End quote --- David, thank you for this valuable information and sharing your wisdom with us!!! |
| David Hess:
--- Quote from: james_s on May 03, 2017, 11:53:26 pm ---Can anyone confirm whether the HV is directly generated or has a cockroft-walton multiplier? I ask because I've worked on HeNe laser power supplies where the capacitors in the multiplier looked fine at lower voltages but would arc internally at full voltage and short. It might be worth measuring the HT connection while ramping up the HV circuit on a variable bench supply and see if you get any HV up to a certain input voltage. --- End quote --- -1475 volts for the cathode is generated by a half wave rectifier (CR1215) which has a relay which cleverly converts it into a half wave voltage doubler (CR1215 and CR1210) to produce -3000 volts for reduced scan mode. The +7000 volt PDA (post deflection acceleration) is produced by a half wave voltage doubler (one stage of a Cockcroft–Walton multiplier, CR1202 and CR1205) from a higher voltage extended winding on the transformer. It is difficult to control the output because there is closed loop feedback from the -1475 or -3000 volt cathode supply which controls the inverter drive. Just messing with the power to the primary side of the inverter will not control the output well and can damage the oscillator do to excessive current and heat if the input voltage is low. The 7623A is a 7633 without reduced scan mode. The 7623B is effectively a renamed 7633 and replaced both. |
| finom1:
I just fond this information: 7633 Storage Oscilloscope http://www.amplifier.cd/Test_Equipment/Tektronix/Tektronix_7000_series_mainframe/7633.htm |
| finom1:
David, Bill just sent me this message for you: "old64goat I am printing all this information out, David has been a great help to me, please thank him for me!" |
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