HMV 1121 sound fading away. - UK Vintage Radio Repair and Restoration Discussion Forum

MurphyNut · 29th Apr 2017, 2:27 am

I restored this set last year, I replaced all the wax capacitors but not the electrolytic ones as they seem to read OK and got it working.
The problem is it works really well but after about half an hour it starts to get quieter and quieter until there is vertically no sound.
I have a similar set with the same valve set up and have swapped these valves over one at a time but still the sound fades away.
If I switch the set off and return to it the next day it works really well again and then the same problem arises.
Could it be I need to revisit the electrolytic capacitors again?

julie_m · 29th Apr 2017, 3:21 am

Could you try some DC voltage measurements with the set behaving "properly" and with the fading manifesting? Tack-solder some lengths of wire from strategic points (Try: rectifier cathode, other end of smoothing resistor; O/P valve anode, cathode, screen, grid; audio preamp anode, cathode and grid; oscillator grid) to a piece of tag strip or terminal block, so as you can move quickly and safelyfrom one test point to the next. It will be worth it for the improvement in fault-findability.

By seeing what voltages are affected when the fault manifests, you should be able to get a clue what might be going wrong.

I'd better stop here, bscause now I'm getting ideas about building a multi-channel ADC dara-logging adaptor that would plug into a laptop .....

Boater Sam · 29th Apr 2017, 8:44 am

In the past I have found this to be the local oscillator drifting off frequency or even shutting down completely.
Possibly a resistor or even a cap getting heated and drifting.

Freya · 29th Apr 2017, 8:47 am

Does it do the same on the Pick up input.

Steve_P · 29th Apr 2017, 11:37 am

Does anything get hotter than it should?

Cheers,

Steve P.

MurphyNut · 30th Apr 2017, 2:08 am

Quote:

Originally Posted by Freya

Does it do the same on the Pick up input.

Good point, I'll try this out and report back.

ColinB · 30th Apr 2017, 5:14 am

If it fades over a second or two it sounds like a heater opening up...

Boater Sam · 30th Apr 2017, 8:15 am

Or a heater to cathode hot short on a valve?

29th Apr 2017, 2:27 am	#1
MurphyNut Heptode Join Date: Apr 2016 Location: Croxley Green, Hertfordshire, UK. Posts: 979	HMV 1121 sound fading away. I restored this set last year, I replaced all the wax capacitors but not the electrolytic ones as they seem to read OK and got it working. The problem is it works really well but after about half an hour it starts to get quieter and quieter until there is vertically no sound. I have a similar set with the same valve set up and have swapped these valves over one at a time but still the sound fades away. If I switch the set off and return to it the next day it works really well again and then the same problem arises. Could it be I need to revisit the electrolytic capacitors again? __________________ Clive

29th Apr 2017, 3:21 am	#2
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: HMV 1121 sound fading away. Could you try some DC voltage measurements with the set behaving "properly" and with the fading manifesting? Tack-solder some lengths of wire from strategic points (Try: rectifier cathode, other end of smoothing resistor; O/P valve anode, cathode, screen, grid; audio preamp anode, cathode and grid; oscillator grid) to a piece of tag strip or terminal block, so as you can move quickly and safelyfrom one test point to the next. It will be worth it for the improvement in fault-findability. By seeing what voltages are affected when the fault manifests, you should be able to get a clue what might be going wrong. I'd better stop here, bscause now I'm getting ideas about building a multi-channel ADC dara-logging adaptor that would plug into a laptop ..... __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

29th Apr 2017, 8:44 am	#3
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: HMV 1121 sound fading away. In the past I have found this to be the local oscillator drifting off frequency or even shutting down completely. Possibly a resistor or even a cap getting heated and drifting.

29th Apr 2017, 8:47 am	#4
Freya Octode Join Date: Jan 2009 Location: Falmouth, Cornwall, UK. Posts: 1,989	Re: HMV 1121 sound fading away. Does it do the same on the Pick up input. __________________ Stephen _________"It`s only an old telly" ___

29th Apr 2017, 11:37 am	#5
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: HMV 1121 sound fading away. Does anything get hotter than it should? Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

30th Apr 2017, 5:14 am	#7
ColinB Heptode Join Date: Apr 2003 Location: Newton Abbot, Devon, UK. Posts: 761	Re: HMV 1121 sound fading away. If it fades over a second or two it sounds like a heater opening up...

30th Apr 2017, 8:15 am	#8
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: HMV 1121 sound fading away. Or a heater to cathode hot short on a valve?