Admiral combo - need some help!

[gary k.]

Hi All –

I’m in the process of restoring a 1949 Admiral combo (Model 4H18) for myself. It’s got a funky green lacquer finish with Chinese drawings on the front. I got it from the estate of the 95 year old original owner and it was very well taken care of. The cabinet sold me.

I’ve restored a few TV sets prior to this set but this is more complex (and about 10 years older) than the other sets that I’ve restored.

I’ve recapped all 3 chassis (tv, radio and power chassis). I’m having trouble with the video detector circuit/dc restoration. The scan of the schematic attached has 3 points noted (A, B and C). Additionally, the box to the left of AL5 references the IF can.

When a signal is injected into Pin 1 of V7 (6AL5), the signal does not make it across to Pin 7 and to the video amplifier. When the ground (B) is removed, and the signal is injected, we get a signal to go through the tube (A). I have never been able to get a signal from (C) to (A).

I’ve also replaced the 5600 ohm resistor in the IF can as well as the 300 mmf cap. This made no change to the signal, so I got a NOS replacement can from Moyer Electronics. Substituting this new can did not fix this problem.

FYI - for testing, we are using a B&K tv analyst 1077 for injecting a video signal.

I appreciate any direction the group could provide!

Thanks, Gary

[Don Lindsly]

What are you using to look at the outputs?

Can you inject a 26 meg, AM modulated signal into pin 1 of 6AU6 4th VIF and see something at Photofact test point marked diamond A with a TV grade scope?

All the points you've noted are pretty low impedence and no gain so I would not expect to see much without a good scope. The trouble may be elsewhere.

Measure the AGC at the first and second VIFs. Too much AGC voltage will cut off the IF strip.

Don

[wa2ise]

Be aware that video IF frequencies they used back then were from about 20 to 25MHz. Your generator is probably expecting an IF strip of 41 to 46MHz, which TVs newer than 1955 or so all use.