Motorola 21T34BA has raster/no pic/no sound

[Robert Grant]

(formerly posted to Audiokarma early B&W forum in 2/2008)

Hello to everybody,

I am trying to repair an old Motorola 21T34BA Television set (Chassis TS-537, a 21" CRT B&W set, with voltage-doubler B+ supply and series-string filaments, it is featured in Sams Photofact Set 345, Folder 9).

All of the tubes light, and the B+ supply has full voltage with no sign of ripple. As the set "warms up", one can hear audio when the RF, IF and audio tubes are warm enough to work.

The problem: The very instant the sweep circuits start, the audio is lost, and the set warms up to a white raster and no audio (only a very weak white noise).

Probing revealed that the instant the sweep circuits start, the AGC bus falls to -40 volts below common (per the Photofact, it should be around 0 volts). Of course, dragging all the grids in the RF and IF sections 40 volts below zero shuts down the show, leaving me with a raster, but no sound and no picture.

The keyed AGC circuit is the triode section of a 6AU8. Replacing the 6AU8 did nothing. Suspicion then fell on one or both of two ceramic disc capacitors that couple the plate and the grid of the triode with dedicated taps on the flyback (C57 and C58 for anyone who happens to have the Photofact). Had either of these shorted, it would have turned the 6AU8 triode into an unwanted C- supply, shutting down the RF/IF's.

As you may have guessed, I replaced C57 and C58 with no change in the result. While the set was open, I measured the DC resistance between those two taps on the flyback, and it showed the specified 5.8 ohms.

There is another curiosity concerning the flyback: the black substance that surrounds the coil seems to be seeping and falling off the coil, I don't know if it melted or dried out. The flyback still produces enough HV to deliver a smooth white raster, however.

Of interest is that the "Quick Troubleshooting" in the Photofact lists the CRT (21ATP4) as a possible cause of "has raster, no picture, no sound". I can't find any logical scenario in this receiver by which a failure in the CRT can cause loss of sound. Is there something they knew that I don't know?

Thanks in advance to anyone who can suggest where I go from here!

Rob

[El Predicta]

Earlier this year, I restored 2 of these sets. I don't have the Sam's in front of me, but check out the video detector diode and its associated components, especially the small chokes which are always a problem in Motorolas of this vintage.

Larry W.

[Don Lindsly]

Look for high video amp plate voltage. That will cause the AGC amp to over-conduct causing high AGC voltage. The flyback and agc taps are OK if there is a white screen and -40 volts bias. It could also be a shorted AGC amp or no screen voltage on the video amp.