Zenith AM-FM G725 - A825

[DavGoodlin]

These two radios have been collecting dust in my shop for a good reason; both are in great physical condition but have me completely baffled. This is the season when I'm dealing with these handsome stragglers.
G725 A825.jpg
G725/chassis 7G01 plays great on AM, FM hiss only. No oscillator operation, yet 10.7 MC will pass from 12AT7 grid thru IF strip with plenty of gain.

A825/chassis 8T01 has dead AM no noise, hiss FM no oscillation either band.

I have checked voltages and continuity thru band switch and coils. touched up solder joints on slug-tuned osc coil both sets.

I'm sure a component that rarely fails has done so, maybe a pf cap. Normally, these radios respond well to a recap and replacing the selenium rectifier is a must. The worst issue I have seen is the dreaded silver mica disease that plagues some of these big cans more than others. One of the cans was replaced in the lower one, A825.

Am I assuming SMD is not causing osc issues ?

[DavGoodlin]

The 1950 version of this classic, G725 has the same slug tuned FM local oscillator circuit. So does the 1955 A825 (T/Y825), which has no AM or FM but has an aux input. A third set I'm working on is a 1960 model C730 (marked G730), which has no audio. This set also has an aux input.

Here is what I found after tracing out the FM circuit (in red on Zenith schematic) and replacing the drifted 10 K resistor and 50 pf cap on 12AT7 pin 2, got the oscillator going. Trouble is, no matter how many pf caps I tried, the FM will not tune below 99 MC, with slug fully in. 108 MC is mid scale, so osc runs too fast. The 50 pf was the tiny tube style cap, a different style mylar film and disc was tried, no improvement. Tried replacing the other little tube, a 22 pf across the coil. Also swapped in larger values than 50; 68 & 70pf. 24 and 30pf for 22. No real change in frequency, disc caps aren't same as tubular ones? RF sure is wacky.
Zenith G725 FM osc.docx.jpg
I initially tried a metal film resistor for the 10K and that was not good, with overload on strong local, appearing all over the range. Shango has advised against using metal films in RF and IF circuits, I agree that carbon is better in this application.

The AM works fantastic, a great DX set but the FM is only good between 99-108. The set is in great physical condition, so when do you give up on trying to perfect FM to add an auxiliary Jack to make it sellable?

[DavGoodlin]

No sign of SMD in this set, not so for my H845 (1962 Z), which I performed my first IFT surgeries on.

The best the G725 will do is FM 101-108, a commercial end of band around here! Full in osc slug gets 101.something and top of scale is 113 MC on the frequency counter.

AM band while incredibly sensitive BUT maxes out at 1550 KC. Trimmer cap screw almost falling out just to get that high. Again to schemo, some parts of FM oscillator slug coil are low-pass for AM and thus, part of the circuit.

Both new caps seen below 50pF and 22pF, experimentally varying those two +30 /- 10% doesn't help for extending ranges. Moving stuff around doesn't change much either. WTF?
G725 12AT7 mix-osc.jpg
Some older caps remain, .001 dogbones and .68pF across pins 1-7. Also wondering if varying resistances of 10K and 1K changes osc freq?

[Electronic M]

Just a dumb thought: have you tried subbing the osc tubes in case there's an internal tube defect skewing frequency?

[DavGoodlin]

Originally Posted by Electronic M

Just a dumb thought: have you tried subbing the osc tubes in case there's an internal tube defect skewing frequency?

Good idea though Im used to not having it easy with "just a tube", I swapped a 12DT8 and several 12AT7, no real difference. I even took unused pin 9 out of 12AT7 socket, got one MC closer from that. No ears are on socket for shield, adding one actually made things worse, with image rejection. Now down to 98.5 with slug bottomed out using a Tung-Sol 12AT7, best of show.

"Padding" the osc by subbing in values just seemed to make it hotter, with one or two strong FM stations taking over the range.

The string-spring loaded-slug German 12DT8 tuners used in RCA and Motorola were similarly hard to tune down. In theory, increasing capacitance and/or inductance lowers the resonant frequency.

I suspect the oscillator coil has deformed, I honed out the forms so slug goes all the way in, lots of action when I unwind some of it. I have a great selection of fractional pf caps, disc up to 470pf. "Padding" the osc by subbing in values just seemed to make it hotter, with one or two strong FM stations taking over the range.

[DavGoodlin]

Putting down 1950 G725, I had a few ideas on the 1955 T825. Without changing parts other than wax caps and electrolytics, the 12AT7 socket ended up being the issue.

Numerous applications of Deoxit and tube swaps did not make any difference. I started pushing on tube socket pins with a chopstick, a useful tool for tuning coils and prodding suspected bad connections. Once I pushed the grid pins 2 and 7, AM and FM began to work. I replaced the 9 pin socket with a black molded NOS type. All bands work great now as I had done all but the RF alignments.

What I cannot understand is how and otherwise quality brown molded socket can fail? Is this an issue of corroded pins, not responding to chemicals, springing or reaming out with fine wire? Is it carbon-tracked?