zenith: Ch= 7G01; G725 FM Performance

I have three of these radios. They share the same problem: No FM reception.

I have:

1. Replaced all ceramic and electrolytic capacitors

2. Swapped out tubes

3. Cleaned all switch contacts

4. Checked or repaced all resistors

5. Replaced selenium rectifiers with diodes.

AM reception is excellent. No FM at all.

Ideas?

Thanks!

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Dear Steve,

I'm very sorry, but you made a very big mistake. Now it is too late, and I can give you only some general advice for future work.

1.       Do not replace, especially not in pre-stages like e. g. IF-amplifiers and FM-tuners

·         polystyrene capacitors;

·         ceramic capacitors;

·         inductors/coils and filters.

These are very critical parts and normally in working order, even if they are or look very old.

2.       Replacement of other parts, e. g. resistors, should be made only in case that you are sure that the suspicious part is really faulty by taking measurements of Ohm-values or voltages.

3.       Be very cautious regarding the original shape of wiring if you replace parts in pre-stages.

Why did you try to “repair” three identical radios all at once ? It would have been better to begin with one of them and then to finish or to give up or to ask here for help. Now you have no possibility to compare with the original condition unless you made photos before beginning with restauration.

Last question: What did you do with the removed parts?

Rolf

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Without specialist test gear and experience it's possible the FM part will never work if you change all the capacitors on it. You can try refitting the original, but even moving the wiring can be a disaster.

Never replace capacitors unless they are known to be faulty, especially in the RF stages, absolutely never in the FM RF  unless it's 100% definately short or open circuit.

See Replacing old Capacitors. Ceramic, Mica and Polystrene types rarely ever need replaced (usuually obvious physical damage). RF section capacitors are often critcal value and temperature co-efficient. On older sets the coils are adjusted with the radio in a test fixture to match the capacitors on VHF-FM.

The most commonly faulty capacitor is the Waxed paper type (Philips coated them in black tar stuff) and only critical on driver anode to output stage grid. You can test for leakage by unpluging the output valve and see if grid voltage is 0V.

Also the paper capacitor across mains if fitted and "tone correction" / snubber capacitor on Anode of O/P to HT or 0V.

Next is the Screen grid decouplers (low gain if leaking).

On "Live" sets the aerial, earth, gram, chassis capacitors need replaced with safe 1kV or 2kV rated parts .

Electrolytic capacitors can often be "reformed" unless they have leaked or dried out. They don't need replaced as often as wax  / paper kind.

I only remember having to replace ceramic capacitors on high voltage parts of circuits in TV sets, I don't beleive ever on a radio in 40 years.

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Rolf,

I worked on these radios over a period of several months. Being relatively new to this sport, I tried to go from obvious (paper electrolytics) to long shots. (ceramic capacitors). Since my posting I've had some luck by replacing open low-value ceramics with mica.

My only instrument is a multimeter, but I'm getting better with it.

Thanks for sharing your experience. Your thoughts on being especially careful with the pre-stages are appreciated.

Steve

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Michael,

Thanks for your specific suggestions about treading lightly on RF stages and replacing old chassis capacitors with 1 or 2 kv parts. Replacing some low-value tubular ceramics has helped in some situations. I'm learning quickly that FM is a lot less forgiving than AM!

Steve

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I can't imagine why you had to replace any ceramic capacitors.

Note that the actual value may not match schematic. You need to take into account tolerance and function. For example a resistor from grid to chassis can be 470K to 2M with little difference.

An Anode resistor only needs about 30% tolerance.

Output tube cathode resistor is the most critical, maybe 10% tolerance.

Audio, RF coupling and low pass capacitors on RF and Screen grid decoupling or HT are not critical in value at all (+/-30% or more) (e.g. series capacitor to grid or 100pf IF filter at detector).  The RF oscillator capacitors are very critical, but coils will have been adjusted to suit the actual value. Feedback and tone control capacitors are bit more critical in value.

Similarly IF tuning capacitors are critical, but the actual value may not match schematic any better than 10%, the IF coils will have been adjusted to suit the actual capacitors.

See here Very few electrolytic capacitors replaced. Many waxed paper (including the RCA model) and no ceramic or mica needed replaced. Maybe two resistors across all the models. I did replace out of spec resistors on an Ever Ready Sky Casket. I may refit the original. The fault was someone had attacked an oscillator coil.

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