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Old 6th Nov 2019, 6:43 pm   #1
Cathode Ray
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Default Murphy A372 VHF restoration

Hello follow members, it's been a while.

I have come across a fairly tidy 1957 Murphy A372 set with MW, LW and VHF.

https://www.radiomuseum.org/r/murphy_a372_a_372.html

It's in good shape, but the mains rectifier is missing along with it's output smoothing cap.
Someone removed them but didn't go any further, there is an ample layer of dust inside the chassis to indicate it's not been removed (thankfully).

The smoothing cap must have leaked and shorted, the remaining wires are burnt and there is blackened scorch marks on the chassis.
Hopefully that's all the matter.

'That cap' Af coupling cap (C73) is first item to be replaced, after replacing the rectifier, which a fellow member has to hand!!! cheers John Joe.

I have the schematics from radiomuseum... But no component locator or alignment guide.

If anyone can supply these it will help a lot.


Thanks
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Old 6th Nov 2019, 9:15 pm   #2
AC/HL
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Default Re: Murphy A372 VHF restoration

The full manufacturer's manual is here: https://www.service-data.com/product...14/5139/m13914
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Old 6th Nov 2019, 11:12 pm   #3
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Default Re: Murphy A372 VHF restoration

Cheers Bill, got it, great manual.
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Old 7th Nov 2019, 6:26 pm   #4
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Default Re: Murphy A372 VHF restoration

These are great radios Ray, my A362 is in daily use in the kitchen. Be prepared to replace lots of Hunt's capacitors as they will be leaky and crumbling.

If you need any help I've got all my notes.

If you run it with the chassis out of the cabinet don't let the speaker short against it.
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Old 8th Nov 2019, 10:59 pm   #5
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Default Re: Murphy A372 VHF restoration

Hi Ken, your dead right.... hunts galore!

AM and LW are working fine now.

But VFH is just noise, no stations, any tips would be appreciated ;-)
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Old 8th Nov 2019, 11:35 pm   #6
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Default Re: Murphy A372 VHF restoration

I should add that I have another one of these sets (my wife thinks they are just beautiful).
I'm restoring this one as a 50th birthday gift.

So I swapped the UCC85 but no difference.
When tuning on VHF there are some low carriers detectable, along with sharp increases in hiss in certain spot... this may indicate the oscillator is working and the issue may be in the Rf amp?

thoughts anyone?
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Old 9th Nov 2019, 6:30 am   #7
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Default Re: Murphy A372 VHF restoration

Could it be a problem in the discriminator (FM detector) circuit?
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Old 9th Nov 2019, 9:12 am   #8
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Default Re: Murphy A372 VHF restoration

Possibly a faulty discriminator bypass capacitor. These are usually between 2 & 5uf, connected with the [positive terminal to chassis.
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Old 9th Nov 2019, 1:31 pm   #9
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Default Re: Murphy A372 VHF restoration

Ok, thanks for the advice chaps.
I had already changed the discriminator bypass in my earlier test/replacement process.

I decided to focus on the FM Rf section and found C13 was open.
C13 (500pf) is described as a very critical value which " too high a value will result in loss of gain, too low will result in self oscillation"
Its function is to introduce positive feedback to the first IF trafo.
It was only reading 80 pf.
I didn't have a high frequency 500pf cap rated at 200v, so I made one with two 1nf in series, green dip type. Probably not the best fix, but it has worked.
VHF is now working.

Two issues now ramain:
1. A fairly loud mains hum on VFH, not so noticeable on AM bands, but that may be because VFH needs more volume.

2. VHF reception dies when I place my hand to one side of the radio but improves when I place my hand to the other side... unusual eh!!
The internal aerial is damaged (tin foil), torn away from the sides, leaving only a small section in the middle... so that's my next repair.

Any ideas on the mains hum? The rectifier valve was a free donation from a 'used' box. Can a weak/bad U404 cause mains hum?

cheers
ray
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Old 9th Nov 2019, 2:23 pm   #10
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Default Re: Murphy A372 VHF restoration

Somewhere in the back of my mind l remember someone had a similar situation which was cured by taking the aluminium cover showing on the left hand side of the first picture.
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Old 9th Nov 2019, 2:25 pm   #11
HamishBoxer
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Default Re: Murphy A372 VHF restoration

I have never had a rectifier valve cause hum,though I guess always a first time.

Heater cathode leak maybe?
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Old 9th Nov 2019, 3:13 pm   #12
Leon Crampin
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Default Re: Murphy A372 VHF restoration

A misaligned discriminator will cause tuneable hum due to its poor AM rejection properties. Before realigning, check carefully the balance of the two high perveance diodes in the EABC80. I remember having to realign the final IF transformer on my A272, with good results. These sets work well when properly set up. Note that the AM aerial coils have no screening can, and the first FM IF transformer does - they often get transposed, which screws up AM and FM alignment.

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Old 9th Nov 2019, 8:06 pm   #13
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Default Re: Murphy A372 VHF restoration

Quote:
Originally Posted by Leon Crampin View Post
Note that the AM aerial coils have no screening can, and the first FM IF transformer does - they often get transposed, which screws up AM and FM alignment.

Leon.
Thanks Leon, that's what I was trying to say.
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Old 9th Nov 2019, 11:30 pm   #14
Cathode Ray
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Default Re: Murphy A372 VHF restoration

Cheers guys.
The IF cans are in correct places, checked.

The discriminator may well be off, but the core is frozen solid and a dare not force it.
Same with most of the other cores, it seems they have locked up really well over time.

I may just gave to live with it...

Case is looking good now, and the antenna is repaired with self adhesive copped tape.
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Old 13th Nov 2019, 8:49 pm   #15
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Default Re: Murphy A372 VHF restoration

Just a final note on this radio.

A small re-rune of the FM IF trafo resolved the FM sensitivity issue improved the reception.
Mains hum is less noticeable as a result, practically gone on strong stations.

The antenna had a bad connection which resulted in the attenuation effect when placing hands near the set, the two pin brass antenna connector needed re-soldering and a good clean along with the socket which was not making good contact.

Radio is now working well enough to present as a gift, thanks to all for their help.

Cheers

Ray
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