Zenith C-845: no reception on AM or FM

[dieseljeep]

Originally Posted by 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.

A lot of truth in that statement.
I've never had problems with the large IF transformers in Zenith AM-FM sets. Mostly in the AM only sets. The IF cans are simular. But then, again never say never.
The 845 series, was unique in the fact that it has two stages of IF, for the AM stage. Only found in the higher end communication receivers.

[Jeffhs]

Thanks for the replies. I will simply enjoy my C845 as long as it works as well as it does; since it's a Zenith, it should play for a good long time -- that's my hope, anyway. My set lives up to the logotype on the front panel -- it is "high fidelity" all the way! With two NPR stations in my area (one being a translator for an NPR affiliate some 60 miles south of here), I'm now ready to enjoy that network's programming to the fullest (in mono, of course).

As to the silver mica caps in the IF transformers, I don't expect them to break down or otherwise become defective any time soon. I don't know why they would break down under normal conditions in the first place, since there can't be that much voltage going through them at any time (just normal B+ to the plates of the IF amp tubes). The only thing I can think of that could damage these capacitors would be a heater-cathode short in one of the two IF amplifier tubes. If I remember these caps correctly, there is little or nothing that can go wrong with them, unless they are overloaded by excessive voltage from a circuit fault or the mica insulators crack or shatter. I am not saying these caps will last forever; I'm just saying that the failure rate of mica capacitors is probably lower than that of other types of capacitors, such as wax-paper or electrolytic.

BTW, one thing I did with my C-845 while troubleshooting it for its no reception problem was to remove the wax-paper capacitor between the radio-phono selector and the chassis, which is probably just a line-bypass cap for hum suppression and to minimize the risk of electric shock should the shell of the RCA phono jack and ground be contacted at the same time. With the capacitor removed, of course, there is no protection any longer, so the usual precautions must be taken to avoid contact with the jack and ground simultaneously if the AC plug is inserted in the wall socket such that the radio's chassis is "hot" with the line voltage. This capacitor should be replaced, for safety reasons, with a new one anyway, if the radio is being completely recapped.

[dieseljeep]

Regarding those mica caps in the base of the If tranformer.
Like most of the mica caps, they aren't sealed like the ones we're used to seeing.
They're built in the base of the IF transformer, by means of a mica sheet and silver deposit coating. There's contact fingers, contacting the silver, subject to problems of moisture, corrosion and dust.

[Jeffhs]

Originally Posted by dieseljeep

Regarding those mica caps in the base of the If tranformer.
Like most of the mica caps, they aren't sealed like the ones we're used to seeing.
They're built in the base of the IF transformer, by means of a mica sheet and silver deposit coating. There's contact fingers, contacting the silver, subject to problems of moisture, corrosion and dust.

My set has gone 53 years without those capacitors breaking down (knock on wood), so I'm not expecting any problems with them at this late date; I'm not saying they won't eventually break down (they very well might), but if the caps have gotten past the half-century mark and are still doing what they need to do, I'm not going to worry about it. The radio plays extremely well at this time, no lightning-like static crashes or other noises on FM (I don't use the AM side -- in fact, I disconnected the AM loop the last time I had the back off, since there are no more AM music stations left in my area or even within 80-90 miles of here, as there once were).

As I said in my last post, this radio is high fidelity all the way, as Zenith intended when the set was built. I particularly like the idea that the radio has a real tweeter, as opposed to an electrostatic one as my Zenith K-731 (1963) has. I wonder when Zenith stopped using real high-frequency speakers in their radios, replacing them with those dinky little blue plastic things fed directly from the plate of the 35C5 audio output. The radio sounds plenty good enough to my ears with the plastic electrostatic tweeter and 7" (approximately) main driver. The only problem it has, if you want to call it a problem, is that the tone control is extremely touchy -- it will go from full-range to bass or treble long before the control gets to either end of its rotation. I'm thinking this is due to a bad paper capacitor or even an off-value resistor in the tone compensation circuitry, but since the radio still works half decently, I won't mess with it. Maybe the next time it needs a new tube, etc. I might look into the tone problem, but for now I intend to enjoy the radio such as it is.

BTW, when did Zenith start using these silver-mica capacitors in their radios? I have an H511 table model (1951 vintage). This radio has two IF transformers, but I have no way of knowing if these transformers have the silver-mica caps in their bases or not (I haven't had the chassis out of the cabinet in some time). Since the H511 series were AM-only radios, and the silver-mica capacitor breakdown problem seems to have shown up more in this type of radio than in any other Zenith radio of this vintage or even later (such as the C845 series), I wonder if there ever was such a problem in the H511 series.

[Chad Hauris]

If the IF cans have 2 slotted screwdriver adjustments for trimmer caps they won't have the mica problem (or at least I have never seen it on this type). This type of IF transformer will be fairly large in size. I have a '46 Zenith console and I think it still uses this type (will double check though).

Then around the late 40's/early 50's Zenith went to an early type of ferrite core IF can which uses adjustable coil slugs and this can still get the mica cap problem. It has the mica wafers in the base but is larger in size than the later K-tran type IF cans.

[dieseljeep]

I have to have the AM working, as well as the FM.
The AM circuit is what puts this series in a class by itself.
I have a higher end RCA AM-FM table radio. Power transformer, 5Y3 rectifier and 6V6 output tube. Great setup, but doesn't perform like the Zenith.