#1
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Partial horizontal scan on RCA 8t241, 9t240, etc. chassis
I've been working on a RCA 9t240 which was rather badly butchered by someone in the past. The power supply was a mess and there was a 180 ohm, 25 watt resistor that wasn't supposed to be there. I have taken the power supply back to what the schematic indicates and checked every connection from the power supply busses to the various circuits. Now it is somewhat working but the horizontal sweep only covers the left quarter of the screen. I've checked the yoke coils and every component and connection in the horizontal sweep circuit and haven't found anything wrong. Does anyone have any ideas? I'm afraid I have been at this problem to the point I may be missing something stupid. Thanks!
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#2
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Do you have a picture on the screen? Is it just squished narrow, or is it distorted and/or part of it missing?
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#3
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Thanks for your reply. I finally worked out the issues with the sweep frequencies. Apparently, among other things, resistor values do matter, lol. Someone had replaced a 1.5M with a 4.7M. After some more work I now have a in sync, horribly out of focus picture. The picture only extends from the left edge to about 1/3 of the way across the screen. It appears I am seeing a fairly normal picture that is shifted way to the left, as if a centering control (which doesn't exist in this circuit) were open. I believe the focus problem may be caused by low second anode voltage. I'm going to try feeding a drive signal to the HO tube from the VA62 tommorow and see what I get.
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#4
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Even with a drive signal to the horizontal output tube I have the same partial sweep and terrible focus, so the problem must be in the horizontal output stage, the flyback, yoke or some associated component.
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#5
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With the horizontal osc. tube in place the 2nd anode voltage is 6.5 kv.
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Audiokarma |
#6
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A high DC current through the horizontal sweep yoke windings could effect the centering. Also the boost voltage is probably low just like the HV.
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#7
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Looking into this further, it seems the only way the scan would be off center is if the DC through the yoke is incorrect, or if one coil of the yoke is bad (or possibly the 56pf cap across one coil is breaking down under voltage. Ringing the flyback with the VA62 yielded a best reading of 6 whereas 10 or better would indicate good. It appears I have a bad flyback, maybe, or most likely. I suppose this could cause DC current through the yoke to be off and certainly the low HV and boost voltages. I have to admit a certain amount of scepticism this is true. Every bad flyback I have encountered to date resulted in virtually HV and generally no raster. I guess this may be something I haven't encountered before. What say thee? Does this sound like I need to search for a new flyback?
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#8
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Virtually no HV is what it should read above, sorry
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#9
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I'm not really familiar with this chassis, and it's hard to trace things from page to page in the sams schematic, but it looks to me like the horizontal output winding of the flyback goes back to a 10 ohm resistor in the power supply (R115 in Sams). Could that resistor be open and sending DC through the yoke?
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#10
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Thanks for the suggestion. I checked the 10 ohm and it is good. I pulled a flyback from another chassis that uses the same one and with it I get nothing at all. It also rings bad. Apparently these are prone to failure. I just hope I can find a good one eventually.
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Audiokarma |
#11
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What number flyback does it use ?
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