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Tektronix 7633 Scope, Can anyone please help Old64goat!!!

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finom1:

--- Quote from: tautech on May 03, 2017, 09:44:40 pm ---A quick test to see if these inverters are working is to look for the EMI they produce when running. David has mentioned 40 KHz and this can be identified with another scope without connecting but just using a probe close by, ahla near field probe style.

--- End quote ---

tautech, thank you for this information, Bill does have another scope to use to check it.

Thank you for helping!!!

finom1:
Here is another update from Bill:
"MORE INFORMATION:
I removed the fuse  F814 and inserted +15 volts, I did this on the end of the fuse holder that goes to the HV supply.

My power supply showed about 500ma draw at +15 volts.

Then I took my scope and I saw a nice clean sine wave but I could not read the frequency.

I then connected my frequency counter using my scope probe set on the RX1 setting because I could not get any reading on the RX 10 setting, it read 41KC

NOW hear is the "kicker", I replaced F814 and of course I got the +22.3 volts on the collector, the frequency counter was around 12KC and it fluctuated around that frequency.

Very strange it seemed to give me 41KC at +15 volts but not with +22.3 volts.

The AC on the collector is low voltage, I had to set my scope to almost the max sensitvity so that is why I set my scope probe to X1."

Any ideas on this new information?

 Thank you for helping out solving this problem!!!

Electro Detective:
It appears Old64goat may be in the Tektronix tech trap merry go round I mentioned earlier,
working through test points and voltages whilst being hampered by dicky intermittent contact switches, dials, connectors,
all working together and at random to prevent triggering and displays on the screen, and come and go not HV readings, and so forth.  :-//

Assuming of course no half @ssed repairs were performed on this 7633, on a previous owner/s troubleshoot rodeo ride.

Too bad I haven't got that 7633 in front of me   :-BROKE

Mr. Carlsons Lab time, would make an awesome tear down and repair video!   :popcorn:   :popcorn:   :popcorn:

james_s:
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.

David Hess:
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.

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