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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Cold cathode
Hi;
Was working on a Zenith RPTV, a 9-1302 based set. Starts bad and gets worse like some of these do. Bad waveform, bad flyback. I did check it with the HVB disconnected and it was stil bad. With the new flyback now the waveform is good diconnected. I don't know how it happened but it did need both. Changed the HVB and finally got somwhat of a green raster with intermittent vertical sweep, which was enough for us to see the phosphor burn. Nothing from the red and blue though. The HVB was used so I tested the G2 output, it was there. It hit about 500V and with the meter loading that's about right. After the flyback didn't fix the set I checked everything off it and found nothing shorted, so I decided to try it with the yokes disconnected. Of course I disabled the filament supply first for obvious reasons. I had fogotten doing that and that's why it had no filament. The boss himself saw the weak green raster and what he calls buggers, referring to the phosphor burn. He stopped short of saying I was high (at the time :-) ) but apparently has never seen this. I have. It is VERY rare but it does happen. My nightmare is over with that set, it might get a used green tube or not, but that is not the point. The main questions: 1. Just how many of you have seen cathodes emit without heat ? I don't mean when they are designed to do so. I can remember a few RCSa doing it, but still working. I'm not sure of what other brands I've seen do this but they are out there. 2. Does anyone know what the physics of this phenomenon are ? Is it a CERTAIN contaminant, or is it somehow the current of the leakage in the tube is actually heating the cathode. The plug to the standup board on the 9-1302 probably supplies the other voltages to the CRT boards. The 12V or something might come from the signal board, I'm not sure right now. Also, the raster, such as it was, was there immediately. I doubt a leakage path could get something hot enough to emit that fast. I mean immediately. Also, with the filament supply working, the green tube is leaky/shorted obviously and that's probably the cause of the burnt phosphor. Now you know I'm not stupid, but I am a bit befuddled by the multiple problems. I could see if the customer literally pushed the power button 1000 times a shorted HVB might burn up a marginal flyback that may have had a month or two of life in it. The burn indicates they watched the set screwed up, all green. The intermittent sweep might clear up when hot. Other ideas are indeed welcome. Any valid responses are bound to be interesting and as such are greatly appreciated. JURB |
#2
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RCAs not RCSes. Sorry.
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