The PHILCO Phorum
Bad 35c5 in 60's Zenith K725 killed the FM? - Printable Version

+- The PHILCO Phorum (https://philcoradio.com/phorum)
+-- Forum: Various and Sundry Categories (https://philcoradio.com/phorum/forumdisplay.php?fid=37)
+--- Forum: Other Radios/Phonographs (Non-Philco) (https://philcoradio.com/phorum/forumdisplay.php?fid=12)
+--- Thread: Bad 35c5 in 60's Zenith K725 killed the FM? (/showthread.php?tid=26660)


Bad 35c5 in 60's Zenith K725 killed the FM? - Jayce - 08-13-2026

I just got my freshly recapped, 60's Zenith K725 home today and the FM started getting weak immediately. I checked the usual suspects, but there was no change and the FM band went virtually dead. I just got back in from sweating in the workshop after checking all the tubes and had my Sencore checker fail the 35c5 on the grid leakage test. So, I tested and popped in a good 35c5 along with a cleaning of all the tube sockets and... the radio works well now. The only issue now is that the band switches on this series of Zenith isn't the greatest. It works okay on this radio, but the one on my recapped K731 has gotten to the point where it will just stay on FM. It's so stiff and fussy that even cleaning doesn't help.

RE: Bad 35c5 in 60's Zenith K725 killed the FM? - David - 08-13-2026

Try WD40 on the stiff band switch I know some will not be happy with me saying WD40. David

RE: Bad 35c5 in 60's Zenith K725 killed the FM? - MrFixr55 - 08-13-2026

I am not looking at one of these right now, but generically, check the spring and ball bearing that forms the detent, and check the holes for the detent. If the holes have rusted or worn to the point the ball gets caught in them, try a bigger ball bearing. Clean the detent and lube with automotive wheel bearing grease. This may help.

The 35C5 killing FM but not AM is a funky issue. Looking at the schematic, if the grid leakage caused either excess screen or plate current, it could load down the supply to the limiter, but it would also load down the IF stages that involve both AM and FM.

RE: Bad 35c5 in 60's Zenith K725 killed the FM? - Arran - 08-13-2026

WD40 is fine to loosen the switch shaft, but it will evaporate leaving a dry bushing, so it's best to follow up with a drop or two of oil.
Regards
Arran

RE: Bad 35c5 in 60's Zenith K725 killed the FM? - radiorich - 08-14-2026

Hello Arran,
Yep, WD-40 has its place and purpose, but I prefer using Zoom turbine oil.
Those pesky band switches—I’ve seen the same symptom on a Zenith I have, and found that a weak 35C5 was causing the issues.
Sincerely,
Richard

RE: Bad 35c5 in 60's Zenith K725 killed the FM? - TV MAN - 08-14-2026

Hmmm... the 35C5 killed just the FM? This is interesting. Grid leakage. It would be interesting to pop that tube back in and compare the B+ voltages. Maybe lower B+ killed the FM oscillator only. Be good to know. Maybe the oscillator tube is a bit weak? I know some TVs use the audio out tube for some kind of voltage divider, so if it fails, something else in the set wont work. Don't think I've seen this in a radio, but maybe because I didn't have a need to look for it. Have a Zenith that has almost the same symptoms that you describe, so might be worth a check.

RE: Bad 35c5 in 60's Zenith K725 killed the FM? - Jayce - 08-14-2026

I figured that the faulty 35c5 might have been drawing enough to cause FM issues, but since Zeniths shut off the plates of the 12DT8 when in AM, there might have been enough power to still work on AM. The band switch is a slider type, not the earlier drum type. Actually, I wish it was the earlier style. In any case, the radio is still working fine and I plan on it being my nighttime radio as I don't need the power and punch of the C845 at night!

RE: Bad 35c5 in 60's Zenith K725 killed the FM? - Arran - 08-15-2026

I have the relative of one of these that some gave me maybe 10 years ago. I can't remember the model number but it has a wood cabinet, plastic front, vertical slide rule dial, and a chrome ring around the speaker grille, I think that this applies to more then one model though. It used to work well, then it went a bit deaf so ether the "Bad-all" caps inside are dying, or the selenium rectifier is tired, then the dial cord snapped so it went into storage until I have time to deal with it. Like many of you I'm not suffering from choice when it comes to radios to repair/restore, or other things with power cords.
Regards
Arran

RE: Bad 35c5 in 60's Zenith K725 killed the FM? - Jayce - 08-15-2026

That sounds like an 845 series there, Arran. It is definitely worth the restore when you get to it. I have two C845s sitting in my bedroom right now. A black one that popped a line capacitor last year and a walnut version. Both are recapped now and working well, though the tone control on the black one is a bit fussy. The selenium in the K725 popped when I was testing it roughly three years ago and it was sitting under a high-boy console until I had it recapped.

RE: Bad 35c5 in 60's Zenith K725 killed the FM? - Arran - 08-16-2026

Jayce;
The set is in very good shape cosmetically, except that was also when they started using what I assume was vacuum plated plastic for the knobs, which is worn off. I wondered why it was such a good sounding set before I took the back off, it has an 6 or 8'' mid range with a tweeter, so it will definitely be on my restore list.
Another deaf AM/FM set I have is a Morse/Electrophonic, from the late 60s or early 70s, I thought that the audio amp was bad, then I tried out the record changer, which works other then needing to be taken apart ,cleaned, and re-lubed, it plays good and loud, so the tuner chassis, with it's 50+ year old electrolytics, is likely the problem, that or one or more dodgy early silicon transistors. The domed silicon transistors that Fairchild used to make are notorious for failing, and I think that there are some in there, but it will be recapped anyhow.
Regards
Arran

RE: Bad 35c5 in 60's Zenith K725 killed the FM? - Jayce - 08-19-2026

The 845 series was really a 'sleeper' radio in the collecting community for a while. I even passed up several cheap ones years back before I was wised up. Their more plain looks tended to male people like myself pass them up. What you have is a good high fidelity AM-FM radio with a nearly 8 inch main speaker and a true tweeter that doesn't die like the electrostatic versions. I think I also read somewhere that the tone control has a tap going right to the output transformer, making it rather sensitive. Be warned, when you play it on FM, turn the tone control all the way to treble and then gently bring it back until you have just the right amount of bass. The modern FM causes these radios to distort horribly of you give it too much bass. This wasn't an issue when the radio was made.