Even after replacing all electro's, waxed paper caps and every resistor could not get the Ferris to receive anything. Plenty of noises, atmospherics came through but nothing else.
Suspected something to do with the oscillator of the 6AN7 convertor stage. No H.T on the osc anode yet about 100v at the other end of the 47K supply resistor. As well this resistor was getting a bit warm for some reason.
A poke with the CRO revealed no oscillator. Found a 100pf mica coupling capacitor was almost a dead short.

Put in a new 100pf disc ceramic (all I had on hand) and the '94 burst into full life.

Excellent signals all over the band, with 6 valves these old Ferris's are a top performer for sure.

You've earned yourself a dancing banana!

Mica caps don't like a lot of DC across them. And the ceramic cap should be fine, as it's not tuning the frequency. As a test, see if the radio drifts during warmup when tuned to a station at the high end of the band. Of course, some other part of the local oscillator might do that.

Thanks for your comments and must admit did dance a bit of a jig when the radio burst into life.

It's the first time I have come across one of these old micas go bad, usually they appear to be quite reliable. But after 58 or so years guess things can happen.

Noted the remarks about drift and after a lot of running the receiver is happily very stable.

A few tidy up jobs, new dial lamps and sundry items which will finish the rebuild of.

Cheers

In the main the Mica's here are far more reliable than those in the US, especially ones marked "Micamold" which are bad news, even Micamold resistors are a worry.

The open construction type Mica's of the early twenties,especially on leaky grid detector resistor cap combinations were not good and the resistors equally liable to be out of spec.

The open type Mica of the later 30's were extremely reliable and I can't recal replacing any of those. I did put a set back into service recently, with even the ones on the OP valve were tested, but none replaced.

The fully encapsulated ones (like Simplex SM) do have the occasional failure. DC is usually on them, but that is not a "hard & fast rule", others fail.

Marc

Thanks Marcc for your comments, the one that shorted was a brown sealed Simplex MS.

As mentioned this is the first one that tripped me up for a while. Certainly something to keep in mind for the future.

As for the resistors nearly all had gone high in value and a number of the waxed paper caps were rather iffy as well.

Sometimes its better to just go through the whole set and replace the lot. If nothing else safeguards from future failures.

Cheers

I do quite a bit of commercial fixing and have over a span of over 40 years seen a lot of radios.

I have an AWA car radio, which being transistor, is a type I am only just starting to get. It needs to be repaired as it is the Ford's original factory fit..

It is quite obvious that caps in these, the electrolytics in particular are failing. The current one has 4μF coupling caps & they have all failed as has the 640μF main filter. This is consistent with the faults in the last one I had.

It is my recommendation that you rid the set of all paper caps and old electrolytics..All of them. It is good policy to check the resistors as you change the caps.

Some resistors will not give a correct reading in circuit, but if you have it out of circuit, there really is no excuse for not checking it.

The only reliable way to catch leaky Micas, is with some form of HV insulation tester, or High voltage Megohm meter, operating on, or near their rated voltage. One end out of circuit..

That from experience, will save a lot of grief & rework later as, by that point you should have eliminated the highest proportion of duds, that could likely bring the set down.

Marc

I agree, wax caps get replaced on sight. No need to test them, they are guaranteed to be bad. And old electrolytic caps are probably bad as well, replace them too.

Thanks and agree with comments made re old components.

Resistors and capacitors are not that expensive in this day and age and far better quality than what the set was originally built with.

Am often surprised how clean and excellent quality audio an old valve radio can reproduce with a full suite of new parts.

All voltages and biasing are correct, with a bit of luck the radio will give many years of good reliable service.

Cheers