Component query in the Murphy A40C Radio Chassis. - UK Vintage Radio Repair and Restoration Discussion Forum

mister valve · 12th Jul 2021, 10:16 pm

Can someone help me please....
I am restoring a Murphy A40C Consul Radio, and I am currently replacing all the paper type Capacitors on each panel (everyone of them is electrically leaky). The problem I have is 'C58' (on panel 5), when I removed it was a '0.05MFD', but the manufacturers service sheet, parts list states that 'C58' is a '0.5MFD'. My question is - should 'C58' be a '0.05MFD' or a '0.5MFD' ?
Obviously they would have to be replaced with either '0.047MFD' or '0.47MFD' preferred values.
It is in the A.T.C. circuit, so I can't even guess what the correct value should be, as I really don't know how the A.T.C. circuit works.

ms660 · 13th Jul 2021, 10:28 am

If it's any help, the Murphy A28c which uses a similar ATC circuit to the A40c has a 0.05uF in that circuit position in the schematic.

Lawrence.

mister valve · 13th Jul 2021, 11:38 am

Many thanks for that information, it is much appreciated.

It was probably a typing error in the parts list !!

I'll replace C58 with a 0.05 MFD (0.047 MFD) as in the A28C.

Regards, Kevin.

turretslug · 13th Jul 2021, 12:14 pm

Perhaps a value that was subject to change during production to optimise performance? No doubt the designer of pricey and prestigious sets like these would have done their best to analyse the circuit effects and necessary time constants but I can well see that complete prediction would be difficult. Slight off-tuning through drift and subsequent ATC correction would likely also result in a change of AVC voltage and consequent change in current drawn by the controlled valves. HT voltage changes slightly, the oscillator shifts slightly (in a "good" or "bad" way?), the ATC responds.... Quite a few time-constants possibly in play, plus valve characteristic and ageing spread, the behaviour of the corrected oscillator over different bands and ends of bands. Yes, plenty of scope for optimum tweakage to minimise hunting/flutter etc, possibly after early-adopter experience in use.

12th Jul 2021, 10:16 pm	#1
mister valve Hexode Join Date: Aug 2009 Location: Blackpool, Lancashire, UK. Posts: 311	Component query in the Murphy A40C Radio Chassis. Can someone help me please.... I am restoring a Murphy A40C Consul Radio, and I am currently replacing all the paper type Capacitors on each panel (everyone of them is electrically leaky). The problem I have is 'C58' (on panel 5), when I removed it was a '0.05MFD', but the manufacturers service sheet, parts list states that 'C58' is a '0.5MFD'. My question is - should 'C58' be a '0.05MFD' or a '0.5MFD' ? Obviously they would have to be replaced with either '0.047MFD' or '0.47MFD' preferred values. It is in the A.T.C. circuit, so I can't even guess what the correct value should be, as I really don't know how the A.T.C. circuit works. __________________ If you live, the chassis was dead; if you're dead, the chassis was live !!!!

13th Jul 2021, 10:28 am	#2
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Component query in the Murphy A40C Radio Chassis. If it's any help, the Murphy A28c which uses a similar ATC circuit to the A40c has a 0.05uF in that circuit position in the schematic. Lawrence.

13th Jul 2021, 11:38 am	#3
mister valve Hexode Join Date: Aug 2009 Location: Blackpool, Lancashire, UK. Posts: 311	Re: Component query in the Murphy A40C Radio Chassis. Many thanks for that information, it is much appreciated. It was probably a typing error in the parts list !! I'll replace C58 with a 0.05 MFD (0.047 MFD) as in the A28C. Regards, Kevin. __________________ If you live, the chassis was dead; if you're dead, the chassis was live !!!!

13th Jul 2021, 12:14 pm	#4
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,397	Re: Component query in the Murphy A40C Radio Chassis. Perhaps a value that was subject to change during production to optimise performance? No doubt the designer of pricey and prestigious sets like these would have done their best to analyse the circuit effects and necessary time constants but I can well see that complete prediction would be difficult. Slight off-tuning through drift and subsequent ATC correction would likely also result in a change of AVC voltage and consequent change in current drawn by the controlled valves. HT voltage changes slightly, the oscillator shifts slightly (in a "good" or "bad" way?), the ATC responds.... Quite a few time-constants possibly in play, plus valve characteristic and ageing spread, the behaviour of the corrected oscillator over different bands and ends of bands. Yes, plenty of scope for optimum tweakage to minimise hunting/flutter etc, possibly after early-adopter experience in use.