Responding to TV Collector's suggestions:

1. With another AM radio tuned to about 1440 (the nearest station to 1500), I tuned right across the range of my faulty Philips and did not observe any change in the other radio's reception at any point.

I had my DMM reading the Hz from the oscillator coil where connected to V1 (6AN7) during this test. It sat generally at around 17-1800kHz and did not appear to follow the tuning capacitor down the range. I'm wondering now whether having the DMM attached might have interfered with the tuning. I'll try again tomorrow without DMM. attached.

2. Standing voltages on the first two valves, measured to ground with a DMM (still no load) at each pin were

V1 6AN7

1 30vdc
2 1.4vdc
3 gnd
4 gnd
5 6vac
6 gnd
7 228vdc
8 92vdc
9 -8.9vdc

V2 6N8
1 132vdc
2 -1.4vdc
3 gnd
4 gnd
5 6vac
6 222vdc
7 -1.35vdc
8 -0.6vdc
9 gnd

I'm wondering whether there could be any indication of a break or significant crack in the ferrite slug inside the first IF transformer (the original topic of this thread). I know I have continuity and impedance within one ohm of spec. It is so slathered in beeswax it's hard to tell how it is constructed. There is in fact a crack in the beeswax that goes right around the core, just above the base plate where the bypass filter caps are mounted.

Should I try poking a screwdriver into the hollow core to see if I can feel/turn a threaded slug? I could make a wooden one by shaping the end of a (clean) kebab skewer. In which case, any ideas whether I should make a flat blade or some sort of hex key end?

Maven

Best to use a TV alignment screwdriver, they have a metal tip and a plastic shaft. Bamboo may not be strong enough. At any rate you may have to work the grubscrew/slug loose with a standard metal screwdriver and then tune with an alignment screwdriver. The wax these used to be sealed with when tuned goes hard over time

I wouldn't subject the IF transformer to any more poking and prodding unless you know for sure it has a problem. You risk damaging what may now be a perfectly serviceable part.

The voltages you have recorded are most revealing!
Take a look at the voltages on pin 2 of the 6AN7 and 6N8. These pins are the G1 connections on both valves and one has -1.4 volts on it the other 0.1v. This is a problem.

For a valve to work correctly as an amplifier the G1 grid must be negative with respect to the cathode. As both valves have grounded cathodes the grids must have a negative bias applied to them. This is supplied via R4 to both valves.

Since the 6N8 has bias but the 6AN7 doesn't you need to concentrate on looking at the circuit around pin 2 of the 6AN7. First check for continuity between pin 2 of the 6N8 and pin 2 of the 6AN7. If it is an open circuit you may have a problem with the aerial transformer. Check that L2 does not have damaged windings.

Another possibility is a short from pin 2 6AN7 to ground. You mentioned that you had a short on the tuning gang. Are you sure it is ok now?

TVC -

I rechecked the voltages and found that pin 2 on the 6AN7 was actually at -1.4vdc as it should be. Sorry, I must have made an error with the first measurement. I've edited the post to correct that.

Also, no short from pin 2 to ground.

I tried a couple of other spare 6AN7 valves in the socket, just in case. No improvement, but I noticed that V1 seems to take a long time to warm up, compared to the other valves. Voltage on the heater circuit is 6vac, but I am wondering whether the current may be low or leaking somewhere.

Giving the casing of IF1 a sharp tap, while on, has a "microphone" effect reproducing the tap (faintly but accurately) at the speaker. So I believe there is a signal path from IF1 through to speaker.

Tuning across the dial, I still find no tuned signal, but at around what should be 850kHz there is a band where a quite loud 50Hz buzz comes up - probably local device EMI.

Haven't found any evidence of shorts, but will keep looking.

Still wondering about some major alignment problem at the IF stage.

Maven

Now I have a confession, and a new symptom.

For the nth time I rechecked the schematic and discovered a couple of errors I had made through misreading the circuit around the oscillator coil. I had accidentally tied the two gangs of the tuning capacitor together, and had also connected a bypass capacitor across the coil instead of to earth.

With those corrections I am getting a tuned signal through to the audio, but at poor level and with some distortion.

The new symptom is this:

If I touch any part of the tuned signal path between V1, IF1, V2 and IF2 with an insulated screwdriver, the audio signal disappears. When I remove the screwdriver, the audio signal comes back at a noticeably higher quality and volume but sinks back to the reduced level over about a quarter second.

I don't fully understand how the Automatic Gain Control works, but could this be malfunctioning? Or maybe a capacitor somewhere absorbing too much signal? Or something wrong in the grid bias supply?

Lost again, but still plodding on...

Maven

Further update:

After replacing a resistor at V3, plus the last remaining paper cap at the volume potentiometer, the volume suddenly snapped to pretty much normal. Tuning seems normal, but the audio has a burbling low-frequency distortion over a low hum at low volume. Distortion increases with volume, sounds like typical amp overload.

What does that type of distortion indicate?

I read online somewhere that the AGC is driven from V3. I am pretty suspicious now that my remaining problem is in the V3 area. Pretty sure that IF1 is OK and alignment is at least within workable range, if not perfect.

Maven

Well done on the progress you have made so far. It is very easy to introduce wiring errors when you are replacing large numbers of parts so constant checking for errors even when you are positive there are none is always worthwhile.

AGC voltage is controlled by one of the diodes in V2 (the upper one on the diagram).

I agree that your remaining problems probably lie in the amplifier section. Has every paper capacitor now been replaced? Have you checked the resistance of all the resistors? Have the 3 electrolytic capacitors been tested or replaced?

If you can post all the voltage readings on V3 and V4 like you did with V1 and V2 that may reveal the problem. The amplifiers on these radios are more complex than usual with several feedback paths. These can introduce all sorts of strange problems if there is a component fault.

Working off memory (dangerous). I seem to recall 123L is back biased & has an electrolytic from H.V Centre tap to chassis across the back bias.

Double check where the caps are earthed. The firs HV one will likely go to the centre tap (not chassis) the one after the choke / dropping resistor: Chassis,

The one across back bias (the resistor/s between CT and chassis is positive to chassis.

Get that wrong & it will Hum

Marc

Marc
The cap I have between CT and chassis (C25 on schematic) is BIPOLAR electrolytic. Is that a problem and do I need a polarised one here? Backbias voltages seem OK.

TVC

I should say that after swapping a few more off-spec resistors around V3 things sound better, but there is still a base-level 50Hz hum.

Apart from that hum, fidelity is clear up to about 25% of volume pot rotation, then distortion increases as volume increases, till defitnitely overloading something at about 60% volume know rotation.

Voltages on audio valve pins:

V3 (6N8)

1 16vdc
2 -7vdc
3 gnd
4 gnd
5 6vac
6 87vdc
7 gnd
8 gnd
9 gnd

V4 (6M5)

1 225vdc
2 -5.7vdc
3 gnd
4 gnd
5 5.5vac
6 n/c
7 210vdc
8 n/c
9 n/c

Maven

It looks like your problem may be solved.

You need a 10uf/63v polarised electro. And make sure the positive side goes to the chassis (earth). See circuit diagram.

Cheers.

Can you confirm that the voltage on pin 5 of V4 is 5.5v AC.

If it is you need to work out where you are loosing voltage for the valve heater. With the heater being under run the valve will be low emission and will distort at higher volume levels.

Voltages across R16 & 17 are quoted as -6.4V an -1.7 for R17. Those would indicate if the current draw is correct.

So all of the voltages acording to AORSM's are low. Where is it going? Is the mains voltage down?

Low heater & B+ volts, with hum, could also indicate a heater cathode leak. Some 6X5's are notorious for going short.

Marc

I swapped in a polarised electro at C25, but that didn't change anything. So for the record, it seems that a bipolar might not be a problem there.

I checked again with an external signal from the Pick-Up terminals, and the audio amp stages worked fine with no distortion. The pickup signal taps in after the second IF transformer.

So the distortion is coming from the tuned signal area.

I swapped the last remaining old bypass cap from IF2 to ground, 100pf ceramic, just in case it was leaking though I understand highly unlikely. It was one of those glass bead types in a brass mount. Whether this was faulty or not, after a bit more prodding around looking for dry joints, I noticed that the hum/distortion was gone and radio signal back to optimum. I'll recheck voltages in this condition.

I'll look again and even harder at the heater circuit in case there is some partial or intermittent short to chassis. One suspect area is the dial lamp mounts, which are on that circuit and a bit loose.

I have some extra 6X5s so I'll try swaps. Maybe also swap R16 and R17 which are specified as wired-wound but look to me like all the other resistors. They both read within 10% of spec.

Maven

I would not bother with the back bias resistors & do not be tempted to "heavy" them up. The closer they sail with the wind the better.

If the 6X5, or something goes short it is preferable for them to burn, rather than the transformer.

You will often get a small amount of hum modulation from the heaters. Replacing parts and having them in slightly new positions can be an issue especially around the area of the det audio. I have on several aoccassions had to add sheilded wire & take other measures there.

Marc

It seems the major hum distortion has gone, but I am left with the original problem of unpredictable and significant volume changes that often follow any kind of spike in the B+. It can change with no intervention, or it can change if I touch a screwdriver to the signal path, such as the connection between V1 plate and IF1.

I'd like to confirm a couple of things about the power supplies.

There are two 6.3v taps on the transformer - one is rated .6amps, the other 1.8amps. One feeds the dial lamps and all heaters except V5 (rectifier). The other, L14 feeds only the heater on V5 and is not grounded to chassis.

1. I think the V5 is currently on the .6amp tap, the other heaters and lamps on the 1.8amp tap. Is this correct? The schematic information does not make clear which is which.

2. The rectifier heater tap, L14, is also tied to the B+ output from the rectifier. That seems odd to me that 6vac is connected to 250vdc, but it seems to match the schematic. Is that correct?

There is no drop in the heater voltages from the transformer - I doubt there is a leak. The measured voltage of 5.6vac may be measurement error on my DMM? Or could it indicate the wrong tap is being used?

B+ to chassis reads 250vdc from the transformer, about 232vdc after the filter choke L9.

There are no old resistors or caps left in the RF/IF stages EXCEPT those inside IF2.

Something is reacting to surges in some conditions that might include temperature. The only remaining target seems to be the old resistors in the volume control circuitry between IF2 and V3.

Maven