Looking for a Zenith varactor tuning module

[drh4683]

Im looking for a zenith varactor tuner p/n 175-1503 for a 1973 zenith console with the 25DC56 chassis (horizontal, flat solid state). This is the 2nd generation varactor tuner zenith used. I have an apparent AGC fault with the tuner, and after troubleshooting the set, I had proper voltages at the chassis, so I suspected the tuner as the troublemaker. I have an indentical set in a different cabinet and pulled the tuner out as a sub to prove if the tuner was truly at fault. The set works fine with the other tuner, so Im hoping to find a replacement to bring this set back to life. I can't service the 1503 as the board and components are coated in epoxy. Any help is greatly appriciated. Thanks!

[vintagecollect]

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[bgadow]

Check with Quality Tuner Service down in Texas; if he can't rebuild it, maybe he has a spare. If you can't find his info on the net let me know, I have it written down at home.

[Findm-Keepm]

Doug,

Can you get to the bottom of the board in the tuner? With one good tuner and one bad, you could troubleshoot perhaps to the bad component. Sams 1312, Folder 3A has the tuner with all of the voltages and resistance checks, and even a picture of the board - apparently the epoxy conformal coating came with a later production.

Looking at the schematic for the tuner, AGC voltage is applied to the RF Amp (a 121-787 dual gate MOSFET) via a 100K resistor, and tied via a voltage divider to the source lead of the same MOSFET. There are only 4-5 components total in the RF AGC circuit in the 175-1503, all located in the smaller of the two cavities.

Hope this helps - I have only three Zenith tuners - two VCR tuners and one mechanical UHF tuner.

Cheers,