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laluna |
Posted: Sun Mar 06, 2011 6:56 am Post subject: |
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Follow up: I did manage to track down the fault after replacing the cascade and HV transformer at great expense. It was the bridge rectifier in the main LV DC supply All the buzzing and dotted trace was due to half wave rectified DC on the main supply lines. Cost, about 2 pounds sterling. Oh well!
Thanks for all the help and sorry it's late.
Dan. |
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laluna |
Posted: Sat Jan 24, 2009 9:06 pm Post subject: Yippee Iv'e found the fault! |
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After I don't know how many hours of cursing and head scratching I have fixed my oscilloscope. It was the bridge rectifier for the +12 volt supply. All the other supply lines use the +12 volt line as a reference so if that goes faulty then all hell breaks loose. The lesson learned is don't suspect the worst. The buzzing in the high voltage CRT supply board was from the 50Hz signal going through the HV transformer. I changed the 10kV Cascade and the HV transformer unneccessarily at a cost of £65. I also suspected arcing in the CRT which would have meant the scrap heap.
Anyway, thanks to my two helpers on ESI, much appreciated.
Dan. |
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jts1957 |
Posted: Fri Jan 23, 2009 2:39 pm Post subject: |
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Much info in manual, but it will answer questions about voltages and feedback. |
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laluna |
Posted: Fri Jan 23, 2009 2:21 pm Post subject: |
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Forgot to give X,Y scales on the waveforms above. Top one with fault is X 200uS/div, Y 0.5V/div and lower one without fault is X 10uS/div, Y 0.5V/div. The oscillations that trail off to nothing on the upper pic are the 33kHz chopped DC signal. |
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laluna |
Posted: Fri Jan 23, 2009 2:13 pm Post subject: |
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I have checked every electrolytic in the scope with an ESR meter and replaced a couple of suspect ones in the X-Final circuits though I didn't think they were connected to this fault. |
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laluna |
Posted: Fri Jan 23, 2009 2:09 pm Post subject: |
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The signal on the emitter of the switching transistor T610 when the buzzing sound is heard and the trace becomes dotted is shown below.
Disconnecting the cathode voltage rectifier D616 removes the fault and the emitter signal changes to the one below.
I don't understand the cathode supply circuit and what the 2kv 10nF capacitors do. Why is this supply connected to the heater also. Doesn't that mean the heater winding in the main transformer is at 1.8kV? The focus pot feeds back to the non inverting I/P of the 741 op amp, could this be connected to the instability of the circuit?
Thanks for the useful contributions everyone.
Dan. |
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jts1957 |
Posted: Fri Jan 23, 2009 1:53 pm Post subject: |
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You are correct about HV area. I was thinking more along the lines of I.C.s, discrete semiconductors, resistors, etc. |
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torbjorn |
Posted: Fri Jan 23, 2009 1:42 pm Post subject: |
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It is no good idea to use freezing spray in such high voltage, high impedance circuits. When freezing, moisture will fall out on the surfaces causing creepage currents and possibly flashover. Of course, on the primary side of the DCDC converter, it would not be a large problem.
Although it is not uncommon with failing electrolytics, it is very seldom with an aged electrolytic that works well when cold and fails when warm. A bad electrolytic will usually perform better when warm. |
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jts1957 |
Posted: Fri Jan 23, 2009 11:39 am Post subject: |
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Two thoughts:
Noting that scope "drawing" is 20 years old, have you checked all electrolytics?
When it's acting up, have you tried freezing components, one at a time with freeze spray? |
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torbjorn |
Posted: Fri Jan 23, 2009 10:22 am Post subject: |
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Perhaps the trouble could be in any other of the circuits supplied from the HV transformer - either the cathode voltage rectifier D616 with following components or the doublers supplied from the center tap via R636-R637?
Try disconnecting one after another of those circuits when the scope is warm (remember to discharge any charged capacitors first) and check whether the buzzing sound disappears.
Regarding the DCDC converter, i must say that is it somewhat strange that there is no snubber (R-C network) at all across the transformers primary or across the switching transistor.
Maybe it would be a good idea to check some waveforms at the primary side (with another scope) and see if anything changes when the buzzing sound comes. For instance, look at the voltage drop across the switching transistor's emitter resistor R651. |
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