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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 28th May 2018, 1:36 pm   #1
ukcol
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Default Murphy A372. Success Story? You decide.

Back in September 2017 I took a trip to Ilkeston to collect a Marconi 219 that I had bought from Steve (Colourstar). While I was there I noticed a Murphy A372 and mentioned that I've always liked the look of that particular model. Steve said that he had no intention of doing anything with it and very generously said I could have it FOC.

The A372 is a quite small table radio from 1957 and is almost identical to the A362 which came out in 1955. About the only significant difference in the models is that the A372 uses a 10F18 valve in the IF amplifier stage, the A362 having used a 10F9.

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

The set uses a mains auto-transformer which makes it live chassis but only suitable for AC mains. It covers the VHF/FM band 88 – 100 MHz (only) as well as MW & LW.

When I eventually got around to looking at it an initial survey suggested that apart from the usual suspects (paper dielectric capacitors) there was not a lot wrong with it. One knob bright was missing and the 10C1 frequency changer valve had a control grid leak in the mixer section. I had a Murphy A192T on my “to do” pile that also used a 10C1 and so I “borrowed” it and tested it in my Mullard E7600-4 tester. This 10C1 also showed exactly the same problem and since I had thoroughly overhauled the tester and had never had cause to doubt it I ordered three 10C1 valves, one for each Murphy and one for stock. The new valves all tested good. The only cosmetic problem was that missing knob bright. Mike Barker was able to supply me with a replacement knob complete with bright.

I replaced all the wax/paper capacitors and not unexpectedly they all tested leaky. I also set about changing the Hunts paper capacitors although they all tested OK. The cases of these 60 year old Hunts capacitors commonly split and let moisture in but the cases of those in this set were still intact which is probably why they were still OK. I also replaced a couple of resistors that had changed value beyond their specification.

When I came to give the set its first power up it worked very well on VHF but on strong AM stations, particularly from my SStran AMT3000 transmitter*, it was near impossible to turn the volume down to a low level. The track of the volume control was badly worn near the bottom end and did not respond to repair attempts. I ordered a replacement on/off volume control from Bowood Electronics and in the meantime lashed in a non-switched pot while I continued with the work. The volume symptom was still present which probably explains how the volume control track got so badly worn.

This set uses a very simple AGC (AVC) system. There is no delay voltage employed and the AGC uses the voltage from the AM detector diode. On the strongest signal I can receive there is about minus 10 volts at the detector. With a 0.1uF capacitor connected across the RF section of the tuning capacitor the AGC voltage under no signal conditions was about 0.25 volts POSITIVE! Examination of the circuit did not reveal any likely sources of this positive voltage with the exception of the mixer and IF valves that had already been tested. To eliminate the valves completely from suspicion I removed them all from the set and applied a 200 volt HT supply to the set from an external power supply. The AGC line went up to about 0.4 volts positive! The AGC line was slightly more positive at the point where it feeds off to the control grid of the IF amplifier valve (see the first circuit below), so I concentrated my search in that area. That particular point on the AGC line is connected to a tag at the end of the main tag strip that runs down the middle of the chassis. Next to the tag is an unused one and next to that a tag with HT on it. I put my DMM on the empty tag and measured 5 volts positive - THE TAG STRIP WAS LEAKY. Confidently I removed the AGC connection from the tag strip and measured the AGC line again, IT WAS STILL POSITIVE and had only dropped by a small amount.
To cut an already long story down a bit the culprit turned out to be the first AM IF transformer, it was leaking between primary and secondary. Removed from the set it read a leak of about 300meg ohms between primary and secondary using an insulation tester on the 250 volt DC range.
I then built a modge** (see second circuit below). This change removed the no signal positive voltage from the AGC line BUT IT MADE NO PERCEIVABLE difference in the symptom. The volume level being higher on AM then on FM in sets from this period is very common of course but this difference was of another order. Maybe the AGC performance of this model is quite poor. I continued to investigate the AGC circuit after the new volume pot was fitted including temporarily substituting a germanium diode for the detector but I never managed to improve it.
I have now to admit to another modge; I have added a 1 meg ohm resistor in the feed to the volume control on AM only and that gives a reasonable result. I left the first modge in place as, although it didn’t cure the problem, I felt it was a good idea to keep the leakage fault away from the AGC line.
I don’t know what is causing the leakage, my first thought was that moisture penetration was the cause; however I have applied a lot of heat to the tag strip with a heat gun (leaving it to dry out for days in between) but it still leaks. The IF transformer is a sealed unit with an outside construction similar to that of a “can type” electrolytic capacitor and I didn’t want to break into it and risk making matters worse. (See picture below).

A success story?
I’ll leave you to judge.

*This transmitter is located in my house and my workshop is out in the garden. I have listened to it on many different sets, some with quite sophisticated AGC systems and some with simple AGC and none have had the symptoms this Murphy suffers.

** modge = modification/bodge. i.e. a modification to get around a fault.
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Last edited by ukcol; 28th May 2018 at 1:41 pm.
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Old 28th May 2018, 1:53 pm   #2
Boater Sam
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Default Re: Success Story? You decide.

Off the top of my head, could there be an instability which is causing the IF to greatly increase in sensitivity?
I have had some near similar odd results and have done as you have, installed grid stoppers, with great success. No, its not really repairing but if the results justify the Modge ( like that ) so what?
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Old 28th May 2018, 3:47 pm   #3
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Default Re: Success Story? You decide.

Quote:
Originally Posted by Boater Sam View Post
Off the top of my head, could there be an instability which is causing the IF to greatly increase in sensitivity?
I hadn't considered that possibility Sam, I will re-visit the set and double check the decoupling etc.
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Old 28th May 2018, 5:03 pm   #4
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Default Re: Murphy A372. Success Story? You decide.

Out of interest with 10 volts -ve out of the detector (your strongest signal) what's the control grid voltages?

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Old 28th May 2018, 7:03 pm   #5
ukcol
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Default Re: Murphy A372. Success Story? You decide.

I can't make any meaningful measurements of the control grid voltages directly Lawrence because measurement would damp the signal and change the AGC voltage. There is however -10volts at either end of R37 less the small drop caused by the loading effect of the DMM in this high impedance circuit. It is not possible to accurately predict this voltage drop using ohms law however because AGC action conspires to defeat us again.

Sam - I have checked for evidence of instability and all my observations suggest there is none. I also double checked that I hadn't made any mistakes fitting the capacitors.

It certainly was worth mentioning though as I hadn't considered the possibility of instability at all.
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Old 28th May 2018, 7:35 pm   #6
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Default Re: Murphy A372. Success Story? You decide.

Quote:
Originally Posted by ukcol View Post
I can't make any meaningful measurements of the control grid voltages directly Lawrence because measurement would damp the signal and change the AGC voltage. There is however -10volts at either end of R37
Yes, sorry, I was a bit slack Should have meant the cold end of the coils, having said that I see one of the mods would prevent that on one of them.

Often the symptom is not enough volume, not too much

Lawrence.
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Old 28th May 2018, 9:11 pm   #7
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Default Re: Murphy A372. Success Story? You decide.

Is it worth putting a capacitor from L17/C47 to chassis instead of a direct connection? If the leak gets worse it could burn out the IF transformer.
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Old 28th May 2018, 10:34 pm   #8
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Default Re: Murphy A372. Success Story? You decide.

LOL-I like Modge. I'll start using that one.
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Old 29th May 2018, 3:48 pm   #9
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Default Re: Murphy A372. Success Story? You decide.

I wonder if the A362 suffers the same, very similar circuit but 10F9 instead of 10F18.

Lawrence.
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Old 29th May 2018, 7:21 pm   #10
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Default Re: Murphy A372. Success Story? You decide.

Quote:
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LOL-I like Modge. I'll start using that one.
Yes a nice concise expression; I make no claim to originality, its been around a long time.
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