Horizontal help for a 20Y1 Admiral - Videokarma.org TV - Video

6GH8cowboy · #1 05-10-2011, 05:12 PM

Had the recapped chassis running for a couple days and it quit. Scope says no horiz osc. The original coil had a black bubble on the side of one of the windings. Replaced the osc/ shape coil and osc is running way high in frequency, maybe 10 times, thus no HV. (I can tease a small rf arc off the plate of the rect with an insulated screwdriver). I have rechecked all resistors and verified all caps and swapped some as well. The 75k osc plate resistor runs hot. Is there any way to run the osc circuit in what I call "stand alone" mode to rule out things like bad or missing pulse, voltage from HV section. (Is the HV section pulse help determine gross frequency or just strictly sync?) The cathode current is low at the HO tube and I guess thats good for now testing. I have tried a good one and an osc tube as well. The replacement coil was a miller 6183.

Thanks for any insight.

wa2ise · #2 05-11-2011, 12:38 AM

I have the Beitman's (1950, page 20) for the Admiral 20Y1 chassis, and I don't see a "osc/ shape coil". Inside a metal can there's a couple of coils for horiz frequency and horiz lock, on the plate of a 6SN7 horiz osc tube. Elsewhere there's horiz width and linearity, electronically near the flyback.

I may have the wrong diagram, though most Admirals of the time used similar circuits.

6GH8cowboy · #3 05-11-2011, 06:03 AM

My mis-speak. That is the schematic. The second section aka "lock" coil shapes the waveform for the pulse width type controll of the horizontal oscilator.

6GH8cowboy · #4 05-11-2011, 06:06 AM

As to Komamjutt: I guess it depends wether the sprinter is running East to West or West to East and being followed by a bear.

6GH8cowboy · #5 05-11-2011, 12:18 PM

I have checked and experimented slightly with component values and observed little change. I have even connected a negative bias supply at pin-3 of the osc control and still little change. Is it possible that a Miller 6183 is not a suitable replacement? When I was in the biz most coil needs were peaking coils that went open so I have little recall what these replacements did in sets like these. It seems that the native frequency of the circuit using this coil is wrong about 4 times what it should be after extreme adjustments. Component values have little effect on the gross frequency. My original coil was bad and the new coil does bring the circuit back to life, just the wrong freq. I am presently testing without a HO Tube. so no pulse from the FBT. I dont think this would swing the osc frequency this much if missing but like I said I artificialy ran the negative voltage up and down at pin 3 of 6sn7 control and the freq didn't change much.

Penthode · #6 05-11-2011, 06:03 PM

Quote:

Originally Posted by 6GH8cowboy

I have checked and experimented slightly with component values and observed little change. I have even connected a negative bias supply at pin-3 of the osc control and still little change. Is it possible that a Miller 6183 is not a suitable replacement? When I was in the biz most coil needs were peaking coils that went open so I have little recall what these replacements did in sets like these. It seems that the native frequency of the circuit using this coil is wrong about 4 times what it should be after extreme adjustments. Component values have little effect on the gross frequency. My original coil was bad and the new coil does bring the circuit back to life, just the wrong freq. I am presently testing without a HO Tube. so no pulse from the FBT. I dont think this would swing the osc frequency this much if missing but like I said I artificialy ran the negative voltage up and down at pin 3 of 6sn7 control and the freq didn't change much.

Ah, yes. The classic RCA synchroguide horizontal AFC circuit! Even the component values are the same in the RCA sets. The lock coil is used for shaping the waveform to make the circuit more noise immune. You should be able to short out the lock coil and adjust the oscillator coil for horizontal sync. The alignment procedure should be readily available. Let us know if you do not have it.

John Folsom · #7 05-11-2011, 06:48 PM

6183 is the correct Miller replacement part number.

6GH8cowboy · #8 05-11-2011, 07:57 PM

Yeah I have the write up. Short C and D, count the bars, yada, yada. Un fortunatly the basic freq is so far off that the tuning procedure not posible yet. I spoke to another member who shared with me about a Dumont set that had a similar problem and only an original coil would do the trick. This may be as well. I'd like to think that this is a correct coil but you may know how it sometimes goes. When the set was operating that couple days the Horizontal was textbook responsive. Then the fail (no osc). All other related components checked OK so the coil was replaced. This seemed to be the only failed item. Presently the native frequency is way off after the replacement of the coil. I am assuming that the HO tube being removed would not cause such a large swing in the base frequency. I would not expect it to be locked though. I have just considered the posibility that the replacement coil could be labeled wrong so I will investigate that next I suppose, but it seems remote.

Electronic M · #9 05-11-2011, 08:04 PM

perhaps there is a slug in the coil that needs adjusting, I know I'm going out on a limb on that one.

Tom C.

Big Dave · #10 05-11-2011, 11:35 PM

Recheck any micas in the circuit, particularly the red ones. I have been seeing them fail more recently. Verify voltage readings on the osc and AFC tubes.

6GH8cowboy · #11 05-12-2011, 06:24 AM

Supposed to be a direct replacement for this and came with an application sheet whith component values scary close to the chassis and schematic. This morning I checked the resistance of the Miller and its not even half of the original (which I suspect has shorted turn or two) or the value on the schematic. I smell a rat.