Murphy A146CM

[Michael Maurice]

I see there is a thread about the C version, this is different, its the CM version with a full mains transformer.

I have been restoring it, this involved replacing the wiring between the power chassis and the main chassis, replacing all the capacitors and resistors and testing.

The main electrolytic is fine, the set was sort of in use when I got it.

I have replaced all the waxies and out of tolerance resistors.

The problem I now have in that it is nowhere near sensitive enough, you need a very powerful aerial to get anything, there is loads of instability, you cant pick up anything on LW and what you get on MW is rather distorted. Certainly music is.

Some voltages which seem odd to me.

The AGC line sits at approx -16V even with no signal/aerial removed, goes to -18V with a strong signal. The three resistors in the circuit, R3, R7 and R8 although theyve gone a bit high are not that bad and haven't been changed. C22 which decouples this line has.

What does concern me is that the voltage on the G2/G4 of the hexode V1a and G2 of the pentode V2 is at 198V instead of 92V. R1 15K has been replaced. What would cause this? Does that mean the V1 and V2 are low emission? Cathode volts on V1 is 1.8V, the manual says it should be 3.2V

Where do I go next?

[ronbryan]

With such a large (18V) negative bias on the AGC line, the AGC controlled valves will be virtually cut off, causing the screen grids to be at 198V as you report.

It seems that the set is oscillating furiously. Have a look with a scope on the final IF valve anode to see what is going on.

You could re-check R7 (Murphy manual) the 680k AGC diode load to see if it is open circuit. That would cause the AGC line voltage to build up negative, charging the AGC decoupler, with nowhere to discharge.

Ron

[cathoderay57]

Ron's diagnosis sounds very plausible. You could maybe confirm it by temporarily disconnecting the AGC feed at R8, earthing the AGC line, and see what happens. The root cause of the oscillation could be that C37 (16uF) has lost capacity (even though it might appear to be "OK"). It sounds like you've already replaced the screen grid decoupler C10 (0.1uF). Also worth checking C24 (56pF) is not leaking, albeit unlikely. Cheers, Jerry

[Michael Maurice]

Jerry's prognosis was right on the button. Replaced C37 and all came good.

[Radio Wrangler]

An illustration of why a scope is sometimes a better thing to poke around with than a voltmeter. You don't need voltmeter precision in checking an AGC line or most things in a receiver, but being able to see oscillation rather than having to deduce it can be a good time-saver.

Of course, you need a probe good to the voltages in valve sets.

David

[sparkymike]

I am also restoring a Murphy "Batwing" but mine is the earlier version.
On mine the tuning indicator/pointer seems to be catching on something and at the moment I can't figure it out . Does the holder part of the pointer clip on the rail that is inside the set or does it just skate on the top of the rail , and is the holder for the pointer at right angles to the part that connects to the drive cord ? A photo would help of rail and pointer if possible.
On mine the reception on medium wave was quite bad but since I cleaned the pins on the first tuner valve (oscillator) I am getting more stations.
Mike.

[sparkymike]

I am now replacing the interconnect wires between the tuner and amplifier.
How on earth do you get to the wires marked in the photo below.
They vanish beneath the alloy cased cap on the left with a white wire which cannot be seen in photo.
The tuner is definitely crammed full to capacity..
I have also enclosed photo of the indicator carrier. Has the end where arrowed been bent ? It is not at right angles which I thought it should be ?
The 8 pin socket which fits into the receptacle on the amplifier has a cap on it which is a pig to remove. Not meant to come undone, but pity it was not a screw cap cover.