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Old 12th Sep 2013, 9:08 pm   #1
Leon Crampin
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Default Murphy A372

Michael, I've restored a couple of these and know some of the wrinkles. Change all the usual capacitor suspects, but especially, the RF bypass connected to the rectifier anode, 50nF I think. There may well be a blown 250mA fuse and a knackered U404 to greet you.

The U404 can be replaced by a UY41 without any wiring changes, but it would be polite to add 90 Ohms of heater ballast to maintain the correct current. A convenient place to add this is at the heater connection to the FM front end box.

The audio is not brilliant on these sets due to the tiny 'speaker and there is some hum caused by magnetic coupling between the heater and output transformers. As usual from Murphy, the RF performance is good - on AM it comfortably out-performs the offerings from Bush.

Leon.
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Old 13th Sep 2013, 6:59 am   #2
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Default Re: Back for Murphy A372

Leon,

The RF bypass capacitor (C78) you mention (50 nF, 1 kV) was completely short circuit! This had caused the resistor R32 in series with the rectifier anode to burn out. Surprisingly the fuse F1 (250 mA) was intact. The rectifier valve also survived.

I am planning to change all the suspect capacitors.

Michael
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Old 14th Sep 2013, 4:38 pm   #3
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Default Re: Back for Murphy A372

Hello,

Capacitor C13 is a Hunts brown tubular one and is supposed to be 'critical'. It is 500 pF, 600 V. I can only find some silver mica ones here (NOS) which are 500 pF but have no voltage markings. Will one of these be OK?

Michael
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Old 14th Sep 2013, 5:08 pm   #4
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Default Re: Murphy A372

Moved from here
https://www.vintage-radio.net/forum/...ad.php?t=99410
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Old 14th Sep 2013, 6:29 pm   #5
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Default Re: Murphy A372

I wouldn't trust a silver mica at that voltage unless it said so on the tin!

Try this:
http://cpc.farnell.com/lcr-component...20This%20Range

I've used these "LCR" products before, and I reckon they work perfectly in radio restoration. This one is polystyrene, larger values tend to be polypropelyne, but the fact remains that I've used quite a few of "LCR"'s capacitors to replace old waxies and have always been happy with them.
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Old 19th Sep 2013, 4:11 pm   #6
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Default Re: Murphy A372

Hello,

All Hunts and other paper capacitors now replaced.

The set works fine on FM (with an external aerial, the internal one is pretty useless!). The volume is a bit weak though compared with AM (UCC85 trouble?).

But on AM there is a sort of strange motorboating noise. This goes away (almost) when a station is tuned in. The frequency of the noise is different between LW and MW.

Are there any clues I should look for? Thanks.

Michael

Last edited by yesnaby; 19th Sep 2013 at 4:29 pm.
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Old 19th Sep 2013, 8:21 pm   #7
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Default Re: Murphy A372

Hello,

Could this be an alignment problem? However, the AM IFTs are sealed units and do not look like they have been disturbed.

I have also just noticed that the metal cover of the FM tuner is missing, but this should not affect AM?

As the FM is OK with hardly any buzz, can the main smoothing electrolytics be ruled out?

Bit of a puzzle here.

Michael

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Old 19th Sep 2013, 11:54 pm   #8
Leon Crampin
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Default Re: Murphy A372

Michael, if the bungs are in the AM IF transformers and they look undisturbed, the alignment will usually be OK. The metal cover to the FM front end was deleted on later models and the AM aerial coil assembly should not have a can. All of the other IF coils do.

If you have AM instability, check the decoupling of the AGC line and the possibility of any mis-wired replacement capacitors.

The smoothing capacitor is usually OK unless it looks obviously distressed.

There was a modification to the tone switch, but it's unlikely that either version could cause instability - especially only on AM. If you have a spare 10C1, it might be worth a try.

Leon.
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Old 20th Sep 2013, 2:50 pm   #9
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Default Re: Murphy A372

Leon,

I am going to check for new wiring faults (though I think the set had this problem before I started on the capacitor replacement).

No spare 10C1 valve, and they seem pretty rare. I tested it on a Taylor 45C valve tester and the results for mu are -

Triode section - mu = 1.1 (should be 5.3)

Heptode section - mu = 2.1 (should be 2.5)

So maybe the triode part is faulty.

In the meantime I am going to check the AGC line. Thanks.

Michael
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Old 21st Sep 2013, 7:37 am   #10
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Default Re: Murphy A372

Hello,

Leon has indicated that the 10C1 valve may in fact be OK (see other thread).

So I am not sure where to go next. The AGC decoupling capacitors have been replaced and the resistors measure OK.

The background 'noise' on FM is nearly non-existent, but on switching to AM it is obvious that something is not right. With a strong aerial signal and by tuning exactly to a station, the noise becomes (almost) unnoticeable, but it is very evident between stations (especially on LW).

Michael
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Old 22nd Sep 2013, 6:08 pm   #11
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Default Re: Murphy A372

Hello,

I've done some checks around the 10C1 valve (local am oscillator/frequency changer). All voltages seem to be OK except that the triode section has about 30 V on the anode (should be 19.8 V) and 1.1 V against 1.5 V on the cathode. All resistors measure OK. I haven't changed the screen grid decoupling capacitor C33 (it is a Hunts) on the heptode section as it is so difficult to get at. As the screen grid voltage is OK, can this capacitor be left? The HT voltage is also correct.

Michael
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Old 22nd Sep 2013, 7:01 pm   #12
Leon Crampin
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Default Re: Murphy A372

Michael, the voltage discrepancy on the oscillator anode may be due to meter resistance if you are using a DVM. I think the A372 has a very high value feed resistor.

I suppose one can only suggest: Oscillating set -> Hunts -> decoupler = change it.

I know they're buried on this set and I didn't change them all either, but for your particular problem, I'd look around here first.

Leon.
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Old 22nd Sep 2013, 8:58 pm   #13
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Default Re: Murphy A372

Hello,

I have replaced the Hunts C33 but if anything the situation is worse!

On MW there is slightly less buzz, but LW is pretty much unusable owing to loud buzz. This disappears if a strong station is tuned in.

Is the lead length on C33 critical? I had to use long leads to connect it. If short leads are necessary this could be a problem owing to the access issue.

Michael
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Old 23rd Sep 2013, 8:50 am   #14
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Default Re: Murphy A372

Quote:
On MW there is slightly less buzz, but LW is pretty much unusable owing to loud buzz.
You're not by any chance using the radio in a room with fluorescent lighting are you? These can play havoc with AM reception.
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Old 23rd Sep 2013, 9:19 am   #15
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Default Re: Murphy A372

I was thinking exactly the same, but Michael's an experienced chap and knows all about that kind of thing.

On the other hand, we've all been caught out like that (fluorescent bench lamps, switch-mode power supplies, even thermostatic soldering irons can be culprits)!

N.
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Old 23rd Sep 2013, 9:57 am   #16
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Default Re: Murphy A372

Hello,

Thanks for the compliment! No fluorescent lamps nearby.

I have shortened the leads on C33 and the buzz is less. I might try shortening a few other similar leads on capacitors that I fitted, but they are not near the 10C1 valve so perhaps irrelevant?

There are a few trimmer capacitors in the MW/LW circuits, one at least has been fiddled with (by me! - it was very loose and I think I disturbed it accidentally). Could setting these up with a signal generator help? (I am going to steer well clear of the IFT cores.)

Michael
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Old 23rd Sep 2013, 7:46 pm   #17
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Default Re: Murphy A372

Hello,

No further progress on this. I am not sure how to proceed except to try another 10C1 valve. But no-one on the forum has one and neither does eBay. Some recently sold on eBay though for crazy prices! And what if it is not the valve after all?

Michael

PS I think the UCH42 is nearly equivalent but has a different heater voltage. I have an ECH42 here, would it be worth trying to bodge this in for a second (with appropriate modifications to the heater chain) to see if this cures the problem? I also saw that the X118 is another equivalent, available from Langrex (thanks Station X).

Michael

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Old 23rd Sep 2013, 8:10 pm   #18
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Default Re: Murphy A372

Langrex show 3 off X118's which are equivalent in stock. I have no idea of price.

http://www.langrex.co.uk/stocklist.html
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Old 23rd Sep 2013, 8:53 pm   #19
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Default Re: Murphy A372

You changed the description from motorboating to buzz, has it changed?
What happens when you short the aerial input to earth.
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Old 23rd Sep 2013, 9:01 pm   #20
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Default Re: Murphy A372

Hello,

It is more like a rapid buzz, the nearest I can describe is like a car engine running at moderately high speed.

Shorting the aerial socket to earth causes the noise to largely disappear.

Michael
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