I have had hands on experience with that plunger type trimmer over a number of decades. I will have to find one to double check it.

Thanks for that. I appreciate it.

Have not spotted one, however, you could lift the connection of one & check it with an ohm meter. If its a cap it will be infinite resistance.

It may have a value on it.

I'll give that a go.

I have now replaced all the electrolytic and wax/paper caps. I went to the trouble of identifying the foil side of the poly caps and installed them with the same orientation as the originals. I have powered the set up and measured the power consumption, adding an additional valve with each test. I measured the following power consumption for each tube:

6V4 6W
6AN7 5.8W
6N8 1.2W
6BD7 1.9W
6M5 5.1W

TOTAL 20W

Do these numbers seem about right?

The power supply voltage was 260VDC before I replaced the caps. It's now 285VDC.

I'm only getting 'snap, crackle and pop' right across the dial, with a couple of quiet spots. I have a long length of wire attached to the antenna input.

Where should I look next?

The audio stage is obviously working, so next step is to check IF and oscillator stages.
Firstly a quick check of voltages.
6AN7 anodes, pins 7 and 8 expect HT between 80 and 200 volts.
6N8 anodes, pins 1 and 6 expect HT between 60 and 200 volts.
6BD7, probably OK but measure anode pin 1, expect 70 volts.
Other valves would all be working, previous wattage test is of no real value.
Above measurements very approximate as we have not a circuit, and depending on meter used.
Analogue or digital will give different results.
If all anode voltages present, then next step is signal generator.
Inject modulated IF into the aerial circuit, and if that is heard OK, it would appear the local oscillator is not working.
Open circuit coil or faulty component in the oscillator section.
JJ

Hi Jacksprat,

I not sure if you got answer to the question you asked in your initial post about the component you highlighted in the red box.

That would be a low pF Capacitor, but I could be wrong so someone else please chime in.

George

Johnny,
Thanks, I'll check the voltages you suggested and report back.
I'm looking at making a Micronta style signal injector, but I'm having trouble sourcing a germanium transistor.

https://antiqueradios.com/forums/viewtopic.php?t=304022&start=0

I'm also working on a circuit diagram of the radio.

George,
someone posted that is was a 4pF mica cap made by AWA. Very robust apparently.

The cap is a 1930's style one; Its Mica and I have found them virtually indestructible and I test them for leakage at 500VDC, do be careful as to how you clean it.


Now, you can use a transistor radio to pick up the oscillator, it will operate 455kHz above the radio station, you may pick up the harmonic at 910kHz.

Secondly; If the oscillator or IF is not working it is very common to get a weak un-tunable signal from a strong radio station via the AGC.

With the snap crackle, I have had 6N8's do that. They are reported by some in the radio club to be a bit of a horror tube. It also pays to remove & replace tubes as the socket an pins can become corrosive & lose contact. You can tighten up some of the miniature valve sockets jaws.

I usually,(as stated many times) run an IF calibration early. That tells me if the IF & Audio are working. I use an Oscilloscope as the measuring device, as that will show distortion and it makes a signal tracer when the signal is lost, or distorted. That procedure also helps confirm a dead oscillator.

Thanks Marc.

Tubes have been removed and their sockets cleaned with contact cleaner and a "Pikster" inter-dental brush.

I posted my partially completed circuit diagram and some DC voltage readings on the UK Vintage Radio forum. Someone there suggested I check the 200R 3W resistor on the cathode of the 6M5. Sure enough it was open circuit. I'll grab a 180R 5W wire wound at Altronics today as a temporary replacement until I can get a 'proper' one. If that brings the radio alive I'll come back here for some guidance on aligning the set.

Fisk Radiola Circuit Diagram

When doing voltage checks, if there is a cathode resistor, all element voltages will approach B+ if its open. If it goes open and there is a cathode bypass cap, usually in the order of 40V, that will be destroyed and the cathode voltage, will normally be wrong.

Thanks. The cathode cap is a new 22μF 450V electro. Cathode voltage was 50VDC (measuring with a DMM).

I replaced the 200R 3W resistor with a 220R 5W wirewound. I'm now reading 6.4VDC on the cathode of the 6M5 and the radio is working again. I can get a few stations indoors with a 60cm length of wire as an antenna. The sound isn't great though. Way too much treble. Possibly due to my repair of the speaker cone stiffening it up.

One thing I noticed is the data sheet for the 6M5 recommends a 7000 ohm primary for the output transformer. I have measured mine and it's 580 ohms. It's possibly the original output transformer from the model 86Z. Could that effect the bass response?

One thing I noticed is the data sheet for the 6M5 recommends a 7000 ohm primary for the output transformer. I have measured mine and it's 580 ohms.

The 7000 ohms spec is the load of the speaker reflected through the transformer and I suspect your 580 ohms is the DC resistance of the transformer primary. You cannot measure the load impedance directly, you need to know the speaker resistance, the turns ratio of the transformer and some maths.

Harold

Ah yes, impedance vs resistance. I should have realised that!