Admiral 20B1 Combo restoration

[SwizzyMan]

OK so should I start at the agc or the sync sep? I can barely get the horizontal to lock but the vertical is impossible to lock.

[Kamakiri]

Test the resistances at both tubes, compare them to the chart in the Sams and see what you get.

[SwizzyMan]

Found some off value resistors in the vertical section. Put some new resistors in although I am not finished and it almost locks just barely moving up in a slow speed. Now the horizontal is pretty weak I can get it to just barely lock but it loses sync quickly. Off value resistors in the horizontal circuit? I also noticed a broken, cut in half 68 ohm resistor going to the horizontal drive variable capacitor could that be a big issue?

[bandersen]

Yes - that 68 ohm resistor feeds the grid of the horizontal output tube. No way can it work without it so there must be another connection to the grid.

[SwizzyMan]

Thanks for the input Bob. Will replace that and see what I get!

[bandersen]

You should also check to see what is connected to that 6BG6 horizontal output tube grid first. There's no way for you to have any HV or raster with that resistor cut in half unless it's being bypassed somewhere.

[SwizzyMan]

replaced pretty much all the resistors in the vertical and sync sep circuits. Vertical still rolls but extremely slow but I still cant get it to lock completely. The horizontal is semi lockable but only at low brightness and contrast settings. This is after I replaced all the paper caps in the whole chassis and all the resistors pretty much in the vertical and sync sep section. Also replaced the 8500 ohm damper cathode resistor after it checked out at 1.5 MEGS! What to do now? I am somewhat confused why my vertical will not lock.

[kvflyer]

I am not an expert by any stretch of imagination. If you can get the vertical to stop rolling but not lock on, it seems as though you are not getting the sync pulse. Have you checked the sync circuit? If it providing proper sync, you should be able to turn the vertical almost from end to end and still have a stable picture.

[SwizzyMan]

Well the sync section which is half of the vertical osc 6sn7 and the 6au6 sync sep have had all of the resistors and capacitors replaced in the circuit. Only thing that might be an issue is that I burned a .1 cap too much with my soldering iron and I think that cap may be bad. But that cap goes from a pin on the vertical output tube to the damper and that cap doesnt seem to have much to do with any vertical or sync circuitry. Should I replace that cap? all of the tubes are new ones that I replaced the old bad ones with so I am not sure where to go from here!

[Kevin Kuehn]

Originally Posted by SwizzyMan

Well the sync section which is half of the vertical osc 6sn7 and the 6au6 sync sep have had all of the resistors and capacitors replaced in the circuit. Only thing that might be an issue is that I burned a .1 cap too much with my soldering iron and I think that cap may be bad. But that cap goes from a pin on the vertical output tube to the damper and that cap doesnt seem to have much to do with any vertical or sync circuitry. Should I replace that cap? all of the tubes are new ones that I replaced the old bad ones with so I am not sure where to go from here!

Are you talking about C61, which is connected from the cathode(pin 3) of the damper to ground? If you melted into it with your soldering iron, it should be replaced. That cap filters your boost voltage, which supply's voltage to the plate circuit of both the vertical and horizontal oscillators, as well as the screen of the ACG amp. It could also have something to do with you low high voltage. Sams says you should have 415 volts DC on pin 3 of the damper.

[Kamakiri]

Try swapping around a couple of your 6SN7s just for grins. How are your resistances at the tube pins?

[SwizzyMan]

OK I think my agc circuit is a bit off and this may be my sync issue. At the junction of the .5 uf agc filter and the 47k agc dropping resistor the voltage when a signal source is connected should be -5 volts but I read -4.17 volts instead. The problem is that I replaced that resistor and that capacitor. Also I am using only one lead of the antenna connection since the other one broke off yesterday but I had the same issue before the lead broke. What do I do? replace all the agc resistors? I know that the tubes involved in this circuit are in fact good.

[EdKozk2]

Hi Swizzyman,
Do you have a scope to check the waveform at pin 1 (grid) of the 6au6 sync sep?
It should look similar to the waveform at the output of L16 ( where the syn signal is removed from the 6AC7 video amp output). Make sure inductor L16 is not open. The output signal of your 6AU6 syn seperator pin 6 should only
be a pulse without any video image information. If you don't have the enough of a pulse signal going into the sync phase splitter then you will never get the
horizontal or vertical oscillator to lock in. The vertical pulse is the summation of all horizontal pulse after passing through the intergator network R59 thru R61 along with C41 thru C43.
If you don't have a scope, check L16 just in case. It should be a low resistance.
Sam's says to measure the 6AU6 AGC voltages from pin 7 (cathode) to other pins.
pin1 (-6.8vdc)
pin5 (-140vdc)
pin6 (+115vdc)

Ed

[SwizzyMan]

Replaced some of the resistors in the agc and the big 1 watt 2.2k filter resistor which checked at 113k. Now it is even harder to maintain vertical and horizontal hold! What is going on? I am out of ideas now.

[EdKozk2]

Looks like the washing machine is modulating the video, interference of the low frequency type. The interfence will over load the agc.
Ed