Zenith 10S669
 

I just finished recapping this set, and redoing the wiring. Boy does this set use that old rubber wire. Anyways, to use the station push buttons, does the band selector have to be set to the "automatic" position? Also, should the inductor/resistor on the plate of the 7G7 be tested? If so, how should I do that? I'm not getting a signal through yet, though I've confirmed that the audio stage is working up to the back of the 2nd IF transformer.

Cheers,
Zach

Zeniths typically had an electronic preset switch so yes you have to switch to automatic to use the presets... not familiar with that resistor, but at the least make sure it is not open.

It could be an inductor that uses a resistor for a form... that type places a sub 5 ohm inductor in parallel with a more than 10k resistor.... the resistor is so high a value comparatively that the circuit don't notice it exists. This was done because resistors were the cheapest forms available.

:thmbsp: I looked the schematic and the parts list states that is as you say. It's about 5 ohms. I never thought of the concept of the coil form. :thmbsp:

Permeability tuned presets. Think individual tuning coils with ferrite cores. Some later Zenith AM-FM radios used this kind of tuner on FM.

Yeah, it's a resistor with a small coil, probably about 20-30 turns wrapped around the center of it. I wasn't sure if the resistor value was critical, but it makes sense. As for the tuning, I wanted to make sure I didn't have to push one of the buttons to enable dial tuning. It didn't appear that way on the schematic, but I just wanted to make sure. A few of the other sets require a button to be pushed before you can use dial tuning.

I was able to get the IF sections working, but I can't seem to get a station. I can get a signal through the grid of the mixer, but any further back and I get nothing. Oscillator troubles prehaps?

Definitely sounds like the osc. Or an open in the signal chain.

So what's the best course of action to see if it's running?And then what's the remedy to the problem if it's not functioning? Is it possible that the tube is bad? I checked all of the tubes and it tested good, though I've heard of tubes that test good but refuse to oscillate (philco model 89 comes to mind)

Got scope?... if so scope around for the osc.

An osc is basically a tuned positive feedback loop. If any part is bad it will die or go off frequency. Check everything.

I pulled both of the mica caps that are directly in the circuit and tested them for capacitance and leakage. All of the resistors in the circuit test within their range. I do not believe the oscillator itself is running, as the oscilloscope did not show anything on either the grid or plate of the tube. I suppose the variable caps in the circuit need to be adjusted to get it functioning again? I also swapped out the 6J5 for a spare one to no avail.

Check that the tuning caps and trimmers are not shorting. They would either short from bent plates or bad insulators, or you'd see a tunable RF sinewave on your scope.

Another thing to watch for is open or shorted tube sockets...When all else if right that can cause seasoned techs to chase their tails.

Well, none of the trimmers nor the main tuning caps are shorting. I noticed that the voltages on the grid and plate are way off, is that just because it's not running, or could that be the reason it's not running? I'm not sure about shorted tube sockets though

Hard to tell without the spec and deviation......Could be either.

Don't discount open socket issues either...The metal can break inside the insulator (hiding the issue) or the connection can get loose or dirty.

What's the best way to proceed from here? Replace the socket? Or is there some other troubleshooting I should try before doing that?

Sockets are not fun to change. Best to check for shorts between adjacent pins as well as pin to chassis. If you have a dud tube that fits the socket use the base of that tube as an adaptor and measure base to socket continuity for each pin. If you don't have a base try probing the pin contact and measuring from that to the solder tab.

Gentle wiggling of the solder tab may reveal intermittent opens

Well, I'm happy to note that I didn't have to replace the socket, it turned out to be a cold solder joint from the B+ line. After checking the socket for loose connections, I double checked the oscillator loop, and then the surrounding connections around the RF and mixer sections.

All of the B+ feeds a central point on one half of the radio and a single wire that feeds the RF and Oscillator. Because of the 6 wires feeding one post, one of the wires had an intermittent connection.

Thanks for all of the help thus far, does anyone have the alignment instructions for this set? I don't see it on the riders schematic

Thanks,
Zach