Zenith C-845: no reception on AM or FM

[Jeffhs]

My Zenith C845 AM/FM radio (the set in my avatar) is giving me trouble, and I don't know what may be causing it. I have no reception on either AM or FM (ground buzz on AM, white noise and no stations on FM), though I'd be happy just to get it working again on FM since there's little worth listening to on AM anymore, as in most areas of this country nowadays. I do not have a tube tester or any test equipment except a DMM, and very little room to work on the radio since I live in a very small apartment. I thought about swapping out tubes in the RF sections of the radio (I have a currently non-working Zenith MJ-1035 with many good tubes, most of which are also used in the 845), but I am not sure just replacing tubes will cure the problem. I'll try swapping tubes between the MJ-1035 and the C-845 first, to rule out defective tubes.

If swapping tubes doesn't get the 845 playing again as it once did, however, I don't know what my next step will be, as I do not know anyone in my area who would be willing to look at the set and, as I said, I am in no position to work on the radio myself beyond tube swapping. There is a TV repair shop in the next town south of here, but I'm sure they wouldn't touch a C-845 with a ten foot pole, or even know what one was. That shop deals in TVs, mostly flat screens these days, of course, and would probably turn me down flat if I came in asking them to look at a 53-year-old radio with vacuum tubes.

Thanks for reading this, and for any replies. I don't listen to radio much these days (almost every station in Cleveland is playing that doggone rock and roll or worse), but my C-845 did sound great when it was working (little hum, believe it or not, even after 53 years), so I'd like to get it going again -- if it isn't too far gone.

[jr_tech]

Did you try pulling the tubes out and re-seating them? That seemed to work before a couple of years ago:

http://www.videokarma.org/showthread.php?t=250333

Might try cleaning the pins of the tubes or using some contact cleaner on the pins and sockets.

jr

[dieseljeep]

Inquire at the TV shop in the next town, and see if they have a 12AT7 and a 6BJ6.
Replace them both, especially the 12AT7. The B+ might a little low, so a weak 12AT7 might not oscillate. Look up the 1960 Beitmans and put it in memory, for future reference. You could do a preliminary troubleshoot with just a DMM.
Also clean the band selector switch with some good contact cleaner.

[Jeffhs]

Originally Posted by dieseljeep

Inquire at the TV shop in the next town, and see if they have a 12AT7 and a 6BJ6.
Replace them both, especially the 12AT7. The B+ might a little low, so a weak 12AT7 might not oscillate. Look up the 1960 Beitmans and put it in memory, for future reference. You could do a preliminary troubleshoot with just a DMM.
Also clean the band selector switch with some good contact cleaner.

No one at that TV shop knows or cares anything about tubes or tube radios, so they wouldn't know what I was talking about if I mentioned any kind of vacuum tube. (I found this out when I asked them for help about 10 years ago with my Zenith H511-Y radio.) The shop was established in 1947, but these days all they sell (and repair under warranty) are flat screen televisions.

As to the schematic, I have one for my C845 but I cannot read it. To make matters worse, there is no one in my town who can help me with either reading the diagram or repairing the radio, even if I knew where the problem was. I would not know where to start with troubleshooting, either, even if I could; besides, I don't have the patience for troubleshooting unless it's something very simple, like an open fuse resistor or a bad tube.

[Reece]

Wow, sounds like you're SOL. Did you even TRY wiggling the tubes, cleaning the tube pins even with rubbing alcohol if you don't have proper cleaner? I wish I could test it through the internet but somehow I just can't.

[dieseljeep]

Going by, what you say about that shop, they're not technicians, but parts swappers.
Anyone 35 years or older has some exposure to vacuum tubes or theory.
Try to buy some tubes from an on-line tube seller such as ESRC. They're good people to deal with.

[Jeffhs]

I tried Reece's suggestion and wiggled two tubes in their sockets. Turned the radio back on, and it works. Sounds wonderful (like all Zenith radios of that era, late '50s-early 1960s), and brings in more stations than I can shake a stick at, just using the built-in line-cord antenna. (I often wonder how many FM stations this radio could receive if it were connected to an outdoor FM antenna. ) I live near Lake Erie (within one mile of the south shore) and can get stations on this thing from Cleveland, Akron (a small city 30 miles south of Cleveland), Ashtabula (a city on the Lake Erie shoreline some 50 miles from Cleveland), and quite possibly several out-of-town stations as well.

The 12AT7, 6BJ6 and probably, even likely, most of the other tubes are more than likely quite weak, as most of them look as if they are the originals from when the set was new 53 years ago. The radio likely quit, replacing all stations with white noise, because the 12AT7 was not doing its job; however, the tube itself, other than being weak, is probably OK since the radio played perfectly after I reseated the tube in its socket. And I mean the set sounded great! That great Zenith sound, no discernible hum -- I bet this thing sounds as good now as it probably did when it was new. The dual-speaker arrangement with a tweeter as well as the main speaker is a hallmark of Zenith, as they used this arrangement in many of their higher-end radios as well.

Lower than normal B+ voltages, as VK member dieseljeep suggested, may well be due to a weak selenium B+ rectifier. After all, if the one in the radio is the original selenium (I think it probably is in mine), it is over five decades old and could short or open at any time. I will be on the lookout for the telltale rotten-egg smell and/or smoke of a shorted selenium, if or when, but since the radio seems to be working very well at this time, I'm not going to worry about it. However, if it does short, open or otherwise become defective, I'll replace it with a 1N4007 diode and a dropping resistor in series with it, as is usually done when a selenium stack goes bad for any reason.

[dieseljeep]

I think I have about eight of these Zenith radios in different variations.
My radios span about 4 or 5 model years. The chassis are basically the same, but the cabinet styles are different. IIRC, some have the electrostatic tweeters.
That model was made to last forever.
Most are still running on the original selenium rectifier.
The best part about it was, those radios were so expensive, that owners kept them and took good care of them.

[Reece]

The radio works, so the tubes are good enough. The tube pins and sockets oxidize just from the air. Get yourself a little can of control cleaner or even WD-40. Take one tube out, put cleaner on all the pins using a cotton swab. Or you can put a little cleaner in a bottle cap and dip the pins in (but not up to the glass) and then plug the tube back in the socket while the pins are wet and unplug and plug back in a few times. Repeat with all tubes. Let the stuff dry for an hour before starting the radio. Should be good for a long time.

[Einar72]

I wouldn't use WD-40 unless you have a way of getting it off. Besidea being FLAMMABLE (around hot tubes and power resistors, no less) give it some charge, and it attracts dust and particles right out of the air onto yore sockets/board/switch wafers. Lastly, over time it turns to GUM. I once thought it was the cure-all for old car clocks, boy was I wrong!

I recommend Rid-Ox or De-Oxit. If you can find a can of Blue Shower, grab it, it has no silicone to leave behind and attrct dust. Be careful with automotive-brand contact cleaners, they can destroy many plastics and are very bad to breathe the vapors of.

[jr_tech]

For sure WD-40 will gum up over time... I gummed up a R-R tape deck really well with the stuff.

I use mostly De Oxit D-5 for general radio/tv work. If you can't find it locally, Amazon sells it.

http://www.amazon.com/DeOxit-Cleanin...eywords=deoxit

No affiliation,
jr

[Reece]

Of course there are job specific cleaners but for an occasional user, and used sparingly,old WD won't hurt.

[Chad Hauris]

There is also the problem of bad mica capacitors in the IF cans. I have been running into this problem more and more recently and it has been the problem nearly every time the power supply/audio/oscillator stages are working in a radio but no audio. Silver mica caps, 22 pf for FM and 100 PF for AM and it has fixed them every time.

Weak tubes I have almost never found are a problem when there is no audio unless the filament is burned out (not to say that can't be a problem but it has been rare to me.) It has always been the IF cans. The caps in there can degrade over time so even if the radio was once working it can degrade to where it won't.

[Jeffhs]

Originally Posted by Chad Hauris

There is also the problem of bad mica capacitors in the IF cans. I have been running into this problem more and more recently and it has been the problem nearly every time the power supply/audio/oscillator stages are working in a radio but no audio. Silver mica caps, 22 pf for FM and 100 PF for AM and it has fixed them every time.

Weak tubes I have almost never found are a problem when there is no audio unless the filament is burned out (not to say that can't be a problem but it has been rare to me.) It has always been the IF cans. The caps in there can degrade over time so even if the radio was once working it can degrade to where it won't.

I'm not sure I understand what you are saying about "silver mica caps" or those numbers you stated (22pF and 100 pF. (I know, if I have an amateur radio license I probably should know what that means, but these days that isn't necessarily true anymore.) My C-845 is over 50 years old and still works; as I said in my last post, all I needed to do was to reseat two tubes to get it going again. I see no reason why it should quit now; however, if it does, I will be out of luck since I won't be able to fix it myself (had to give up my workshop when I moved here 13 years ago), and there is no one in my area who can help me. I don't want to bother the TV shop in the next town with this, as they do not seem to know or care about any kind of radio or TV over about five years old or that has vacuum tubes; they sell and service only flat screens today, and cannot be bothered with "stone age" technology such as vacuum tubes or discrete transistor circuitry.

[Reece]

If a radio develops noises like continual lightning crashes, it's an indication that possibly the internal capacitors of the IF transformers have failed. These are remedied by removing the transformers, cutting out the bad caps, and adding new silver micas under the chassis. As long as your radio is not sounding like a lightning storm when there is no lightning, you don't have to worry about this. Just enjoy it.