Admiral 20B1 Combo restoration

[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.

[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.

[SwizzyMan]

Here is a video of the issue I would advise muting the sound since the washing machine was on and very loud. https://www.youtube.com/watch?v=Mj-cJ4nwUeM

[EdKozk2]

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

[SwizzyMan]

You are right Ed. Once I unplugged the washing machine I did notice that the horizontal was a lot easier to lock but the vertical is still an issue. Anyone know what else could be wrong here?

[Eric H]

I would guess, since both horiz and vertical lack sync, there's something wrong with the Sync Separator circuit.
I would start by looking at everything from Coil L16 to pin 1 of V12.
Also everything else involved with that tube, particularly resistors.

[SwizzyMan]

The thing is that I think I replaced all resistors in the sync sep circuit. But I will check voltages tomorrow.

[EdKozk2]

If you replaced all the C3(a-c) and C4(a-c) electrolytic caps, then try spraying the vertical hold control to see if it slows or stops the vertical roll. It still looks like you are not getting the proper sync pulse at C44. Out of spec resistors in the vertical circuit will cause rolling issues also.
Ed

[SwizzyMan]

Should I replace C44 the 4700pf cap since the vertical circuitry has had almost all of its resistors replaced?

[SwizzyMan]

Yes?

[EdKozk2]

If your horizontal Sync is staying locked then replacing C44 may help. A better video may help us diagnose better. As many of the other members have mentioned it looks like AGC/ SYNC troubles. Do you have Scope to check the wave forms? Your AGC may even be overloading which can cause sync troubles.
Ed

[SwizzyMan]

Replacing c44 did help out with the horizontal hold but it still has a bit of jitter on the horizontal. The vertical still wont lock but I can get it to almost stop rolling. I am really starting to think this is not agc but a sync issue. I would hook up the wo-33a now but my bench is a mess.