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Looking for some troubleshooting advice. My 2215 scope trace intermittently flashes bright and the displayed waveform jitters. Occasionally a minute or two will go by with no transients, then they start up again. Occurrences correlate with the neon lamps DS868 and DS867 flashing in the DC restorer circuit. The service manual states that these two neon lamps "protect the CRT from excessive grid-to-cathode voltage if the potential on either the control grid of the cathode is lost for any reason." At first, I thought the CRT might have an intermittent grid to cathode short, but completely removing the CRT still results in intermittent flashing of DS868 and DS867. I noticed the anode voltage (CRT removed) is about 7.5 KV but jumps up towards 8.5 to 9 KV when the tow neon lamps flash. The cathode voltage appears to stay solid at -2 KV per spec.

Anyone seen this before and have some suggestions?

Thanks in advance.

I do not see how the +7.5kV PDA (post deflection acceleration) voltage
could be affected without the -2kV cathode voltage also being
affected.

Try inspecting the circuits around the DC restorer (C864 and C865) and
the -2kV cathode filter (C990, C992, and C995) in the dark to see if
any spark or corona discharge is visible.

Which version of the 2215 is it? Does it have the original TRIAC
preregulator or the switching preregulator?

OK, an H-K short would not do what you have, pins 1, 2, 3 and 4 are at nearly the same potential so that isn't likely to be it. The next grid is at a quite different potential but with the CRT disconnected a short between those two elements is eliminated.

Through process of elimination I think most likely that the CRT socket itself it arcing between pins 4 and 5. This would produce the symptom you have and still not make DS 870 flash.

Unless it is the type of socket you can tell right away is not arcing, try subdued lighting, and if you still don't see anything most of them come apart so you can. Many of them have spark gaps which upon inspection might reveal evidence of arcs.

The anode voltage jumping is probably because it is loading the cathode voltage and the regulation loop compensates for it. The sense it not direct from the HV sources, it is from the primary of the HV transformer. That means that it senses all sources simultaneously sort of via mutual inductance, so that behavior can be expected.

Another simple test would be just to find a way to disconnect pin 5, but that would not eliminate arcing through the spark gap so visual inspection is still best. I may be wrong :-(

Actually, though I say pin 5, ANY of the pins at that potential could be arcing through the internal spark gaps if it has them, which is likely. When you inspect it, don't just look for black and brown, look for green. Green would indicate that is took on moisture.

Dave,

The HV meter I used for the -2KV cathode measurement has a 40KV full range, so very possible I didn't see the -2KV move the needle. I'll have to see which type of preregulator it has. I noticed the possibility of either in the service manual.

I'll take a look at the DC restorer in a dark room to see if anything is arcing or glowing. Thanks for the tips.

Jeff,

Thanks for the tips. It's easy to see the back of the CRT socket in the 2215; not so easy to see the CRT side of the socket unless I pull the CRT shield off.

Perhaps I should also look at the potential across DS 870. It would just be my luck that the neon bulb is out so I'm missing the full story.

Thanks for the help.