Zenith C845 - loud white noise on FM, ground buzz on AM

[Jeffhs]

Hi all -

My Zenith C845 AM/FM hi-fi radio quit on me yesterday. I hear very loud white noise on FM, no stations, and can only get one local station, very weakly, on AM. (There is a loud "ground buzz" sound all across the AM dial as well.) The radio was working perfectly well until yesterday evening. The FM and AM antennas are connected. I tried substituting a couple of tubes; no luck. I don't have replacements for the RF amp (6BJ6) or the FM converter (12AT7), so these tubes are still the originals. Are there readily-available substitutes for these two tubes? Took the bottom cover off the chassis, didn't see anything out of the ordinary. This set has about a half-dozen wax paper capacitors in it, one of which is behind a couple of other parts, so replacing it could be a problem. I am aware that this radio, being 51 years old (made in 1960), badly needs a recap; I'm willing to do that, but before I take that step I'd like to know if there is anything less drastic I can do to get the FM working again. This is one of the best sounding radios I own (second only to my Zenith K731), so I'd like to get it singing again as it was before last night.

Thanks in advance.

[Reece]

Sounds like the audio section is OK. If you get white noise on FM but no hum, power supply and filters should be OK. Certainly it needs the paper caps replaced soon but first you could do some testing.

I'm wondering if the converter is working. You could do the test with another operating radio. Put another operating radio set to 1455 next to your Zenith and tune your Zenith through 1000 Kc. AM with its volume down. Should hear a squeal or rushing on the other radio. If not, suspect the 12AT7. If that's bad neither AM nor FM will work. You may get a little "bleed through" of a strong station on AM, as you seem to be getting. Try tapping the tube while set is on, may have an intermittent connection. If no squeal stop here and try another 12AT7.

Just for the heck of it, check the audio section. Set the phono switch to phono, turn up the volume, put one hand in your back pocket and with your other hand touch your finger to a screwdriver shaft and touch the screwdriver to the center of the phono jack. Should get a loud hum. Switch back to radio.

If audio's OK, look at the schematic and check the voltages around the IF, converter, AFC, and RF tubes. You tried new IF tubes?

[jr_tech]

Is the RF amp even used for AM reception or is it for FM only?
It might be wise to order both the 12AT7 and 6BJ6 while they are still available at somewhat reasonable prices, if you already have spares for everything else.

jr

[Reece]

The Tech Preservation Schematic Bank website shows the AM loop antenna does go through the RF stage. Bandswitch is shown in AM position.
http://techpreservation.dyndns.org/s...C845M_175.djvu

[wa2ise]

You could try a 12BA6 in place of the 6BJ6. pins 2 and 7 are swapped, but as one is the cathode and the other G3 (which is usually at ground) it should work. The tuned circuits around this tube would be a little off, but if you don't get any better results than what you currently have with a 12BA6, the odds are it's not the 6BJ6 tube and you can put it back in.

The 12BA6 and the 6BJ6 have the same heater current, and in a series string set that's what you want.

It's possible (and likely with wax caps still in the set) a B+ or screen grid bypass cap has gone shorted. That could drag down the set's B+ enough to disable the FM local oscillator, and make hum on AM. And you might as well replace the other wax caps while you have it on the bench, the rest of the wax caps will soon fail as well.

[Jeffhs]

Reece, wa2ise, jr_tech:

Thank you all for your help. I am happy to report that my Zenith C845 high-fidelity radio is once again working as it should. All I did was take the two tubes I mentioned (6BJ6, 12AT7) out of their sockets and reseat them. I also tried substituting a 12AX7 for the 12AT7, with no luck; according to a substitution table I found on the Web this afternoon (I did a Bing search for "12at7 tube" and found the table among about 10000 other search results), this sub should have worked -- the two tubes are identical in every way except for gain figures -- but it didn't, still the same white noise on FM and ground buzz on AM, so I put the 12AT7 back in the set. As soon as I did that and plugged it in again, the radio began to play just as well as it always did. I don't know what I did or didn't do to correct the problem, but in any case the set now works every bit as well as it did before it fell silent the other evening. I left it on one of the local Cleveland FM stations for awhile this afternoon to make absolutely certain I had found the problem. The set worked, but it drifted quite a bit until it was good and warmed up, as these older AM/FM sets had AFC circuits that were probably as good as the state of the art allowed in 1960 (the C845 uses a separate tube, a 6AB4, in this position). Once it warmed up, however, it was rock-stable and played very nicely, bringing in stations from one end of the dial to the other on FM including several from Erie, Pennsylvania -- I'm only 50-60 miles from that city, 35 miles from Cleveland, and about 60 miles from Youngstown and Pittsburgh, so when the FM band opens up I often can hear one station after another as I tune the dial, and it works passably on AM. I have a noise problem here in my apartment (mostly harmonics from TV horizontal oscillators) that makes daytime AM radio all but unlistenable on my older radios, including my C845, but my 1970s-vintage battery-powered portables work perfectly well, bringing in all local stations and even one daytime-only nostalgia station about 80-90 miles from here; of course, I get more stations than I can shake a stick at on AM after dark on every radio I own, including my C845.

BTW, I believe it was wa2ise who mentioned some time ago (in response to a question I had posted here regarding AM RF stages in older radios, particularly in the 800 series of Zenith table models) that the 6BJ6 RF amplifier in the C845 series of Zenith hi-fi radios is used both on AM and FM; a look at the schematic will verify that fact.

[old_coot88]

Congrads. Sounds like removing/re-inserting those tubes cleaned some connections that had opened due to oxidation/corrosion. Bill(oc)