Murphy A492 radiogram

[Parabola]

Purely as an observation -

Before going through the pain of changing the holder, if you have, or can get hold of, a B9A plug then it would be a simple matter to do cold resistance checks between plug pins and the relevant tags of the socket. If you use clip on probes then you can give the plug a good bashing / wiggling to see if the connections are sound under stress. Just a thought, before any more major surgery occurs...

Parabola

Ooooerrr, what am I doing in the radio section....

[TuningIndicator]

I go away for a few days and miss all the fun!

There appears to be a theme, C4 and C14 appear on most early Murphy VHF front-end designs I've seen.
My assumption as to their functionality is in-line with Herald's; both are for decoupling, with C14 having the additional task of keeping RF levels the same.
I assume that, as (stated by Murphy) that as C4 is super-critical, it affects neutralisation.

I also assume that C2 and C3 are for neutralisation, therefore failure of either one could cause the (unwanted) oscillations described.

It might be worth asking in the wanted section for a can, or a scrap tuner!

[ronbryan]

Michael brought the chassis to me, to see if I could help.

I did an initial test and confirmed what Jeffrey had found - i.e.

Injecting 10.7MHz signal at the first IFT (top of L9) produced an output at the FM discriminator, but no signals could be heard when the tuner was connected instead of the generator.

The 6L12 RF amp V1a did not seem to be taking current. The oscillator section V1b was definately oscillating as there was a negative bias on the C11 / R2 network connected to the grid coil.

I re-tensioned valve base pins 1-3 but it made no difference, so I changed the 6L12 for an ECC85. This sorted out the RF amp and the anode volts fell to the correct value of 80V.

I noticed that the tuner heater decoupler C67 was one of those torpedo shaped capacitors with banded colour codes that makes them look like resistors. I changed it for a 0.01uF disc ceramic, in case it was the cause of instability - it wasn't!

When the tuner valve was removed and the set powered up, I saw that there was a large reading on the AVO I had connected to the FM discriminator stabilising capacitor when doing the IF test above. This had to be IF instability, as there was no signal input at this time. An attempt to measure the screen grid volts on the 6C12 produced loud noises from the speaker. I saw that there was another of those 'torpedo' caps decoupling the screen - C23. I changed that and immediately the discriminator output fell to zero, indicating the instability had been fixed.

I could hear weak signals on FM from that point on, so I re-aligned the IF, and tuner using the procedures in the Murphy manual (damping C-R networks etc), which made a massive improvement.

There was one further 'torpedo' cap that I found -C47, coupling the discriminator output to the volume control, so I changed that as well.

It performs very well now and sounds excellent on FM, with its push-pull output.

Ron

[ppppenguin]

Ron, well done. I made the wrong assumption that the IF was OK because I could inject 10.7MHz and hear something. Apart from the non-conducting RF amp the poor old tuner was OK all along. Shows how much experience I haven't got of working on FM radios. I had a feeling that I was having to inject rather too much signal at 10.7MHz to make it audible but I had absolutely no feel for what the sensitivity should be at the IF input.

[Patrick Dixon]

Ron is the man!

[Station X]

Yes. Well done Ron.

[Steve_P]

Thank goodness that's sorted!!!!

Cheers,

Steve P.

[Michael Maurice]

Believe it or not this is my first radio restoration.

Now I know that waxies must be generally be replaced on sight especially if they're in a position where they're blocking DC but i've never heard of those torpedo type capacitors.

Of course my mistake was to think that just because a stage works on MW/LW it should be working on VHF just because the components around it are common to both.

But I've never really worked on radio or should I say RF to that extent.

My sincere thanks to Ron who had the patience to lovingly repair and realign the set for me.

I'll take some pictures tomorrow when i reassemble the whole radiogram.

[Alistair D]

Originally Posted by Alistair D

The oscillation only started after V1, V2 and V4 were changed. Sorry I dont have the cct here. Are V2 and V4 IF stages?

Slugging the coax with a probe stopping the oscillation could just as easily mean the stage after the coax is oscillating and not the front end. Perhaps a decoupling problem which could also explain the set's deafness.

Since so many components in the front end have been changed I suspect that the original dead VHF problem was due to the ECC85 connection problem. The source of the oscillation lies elsewhere.

Al

QED

[TuningIndicator]

That's good news!

[Michael Maurice]

Some pictures,

The cabinet does need touching up but I'll leave that to the owner.

Note the elongated controls on the Collaro conquest deck.

[Michael Maurice]

And yet one more picture

[Tazman1966]

A happy conclusion. Excellent news