Phonoland

Hello, folks - this is my first post here. I've been reading the posts for some time, and I always learn something. Now I have something specific to ask about that doesn't seem to have an easy answer.

I've got a 1960 Magnavox stereo here, 1SR281F to be exact, that was performing fine until yesterday morning. Suddenly one channel (the one marked "internal") is dead. That's for radio and phono both. The internal speaker sounds when the selector switch is on "int," so the speakers themselves are fine, but in the "stereo" position there is no sound from those speakers. And, yes, I did check the selector switch with an ohm-meter and it works OK.

After doing some elementary troubleshooting, swapping input wires from live channel to dead channel and changing tubes around, I've narrowed it down to the main amplifier section. None of the tubes are at fault, since symptoms did not follow the tubes when they were changed out, and the windings in the output transformers still "read" on the ohm-meter. The only possible source of the problem remaining are the various resistors and capacitors that couple the amp stages together.

The circuit is actually not very complicated underneath, but a schematic would be helpful. SAMS does not have one for that model (I phoned them up to double-check) and the other various sites such as "nostalgiaair" and "stevenjohnson" don't handle anything that recent.

Anyone know where I can lay my hands on the schematic? Thanks in advance.

Sounds like a case of yet another vintage stereo used, but not properly restored.... then it failed, kaput.

It happens all the time, people neglect to have it upgraded, thinking "oh, it works fine, let's use it."

Not knowing the specific information about the amp in question (you never provided the chassis number) it's difficult to pinpoint the problem.

My guess would be an open OT primary, or an open cathode resistor, and quite possibly those old, leaky couplng capacitors doing their mischief as well.

Your best bet would be to have that amp re-capped, and the resistors checked for value, along with checking the OT primaries.
You don't need a schematic for that stuff, as long as you know the resistor color code.
And check values against the working channel.

I wouldn't rule out a problem in the preamp/tuner either.
Those nasty old caps have a way of creating havoc unless you upgrade them to newer ones.

Progress report: of the four 470K bias resistors for the 12AX7 plates, two of them were dead and another was 50% out of tolerance. When I saw that, I knew this puppy would need a component-level rebuild. I'm doing it one stage at a time, as the work-day and chores allow.

Thanks for the input, MattTech; your good suggestions are noted.