BUSH AC31 Low volume/gain - UK Vintage Radio Repair and Restoration Discussion Forum

richieyork · 31st Aug 2015, 5:59 pm

Hi all
I've been asked to restore a rather sad looking but complete AC31. Initial inspection revealed lost of dust so felt hopeful it hadn't been meddled with, well if so it was a very long time ago. The radio had been stored in a damp environment at some stage - the output transformer metalwork is very rusty and the speaker cone is dragging ever so slightly but not enough to cause the low volume.

There are several 'waxies' so I set about changing them before powering it up.
I powered it up via a lamp limiter. Heaters lit but no sound whatsoever. Quick check on the output transformer showed zero volts both sides. Further check revealed R16 on reservoir cap was o/c. I found something close enough in value and tried again. This time low level crackle from speaker esp when aerial socket touched with wire. I found one or two stations on Med wave (literally one or two) but the volume is very low. I left it running for half an hour then removed the lamp limiter and gave it full mains. The volume came up quite a bit but is still very low. I'm seeing around 220V at the output transformer.

Anyway, here's a summary of the faults:
*Low volume & seems very insensitive
* Volume control has been replaced at some stage - this one has a plastic shaft but it is correct value at 500k/LOG
* Volume control correctly increases volume as you turn it clockwise but just beyond halfway round it then begins to go quieter as you turn it further and the sound gets distorted
*The gram input is also very low and not very responsive to touching the input socket - i'd expect it to hum loudly but all I get is a faint click

The trader service sheet suggests that V1 ECH42 is also a gram preamplifier. I'm suspecting maybe faults in either this valve OR the EL41 output valve

Also this set has an EBF89 IF valve but the trader sheet says EBF80. Are they compatible and has someone perhaps 'been at it' before I got to it?

Before I go off on a valve buying spree can anyone suggest what's going on and where I might want to check beforehand

Many thanks
Rich

ms660 · 31st Aug 2015, 6:15 pm

If you have a reasonable antenna then check all the valve voltages and report them in this thread.

Lawrence.

Boater Sam · 31st Aug 2015, 6:40 pm

You say the volume control has been replaced, ensure it is wired correctly. I have had them with crossed connections before.
Voltages on the output valve will be instructive I think!

richieyork · 8th Sep 2015, 11:25 am

UPDATE: Connections to volume control confirmed OK and it's the correct value/type. Checked voltages around EL41 - cathode was 10v. Anode was initially 210 volts but after a while slowly dropped to 170v. G2 dropping too. Filter resistor (1.5k) getting very hot - this was open circuit originally and so this is the replacement overheating. I disconnected the G1 feed and touched the wire - the hum was weak and distorted. Also checked resistance of output transformer - around 400ohm so that seems ok. Finally connected a test amp to the output from the radio side and signal loud and clear. Now strongly suspect the EL41 is dragging the whole show down - the valve looks 'cooked' (I know they run hot). So, the EL41 confined to the bin and another ordered. Fingers crossed.

More to follow once the new valve in place as I'm still unhappy with the sensitivity.

boxdoctor · 8th Sep 2015, 3:05 pm

Is there a positive voltage on the top end of the volume control? That will cause the volume reduction as you advance the control, as it will progressively upset the bias on th EL41. I am not familiar with this model, but that was my first thought on reading your post. Tony

richieyork · 9th Sep 2015, 8:48 pm

Final update : New EL 41 arrived today. Fitted it and immediately the sound and volume are restored to as new. I suspect the old valve had seriously gone downhill and was drawing so much current it caused the ht filter resistor to burn out. Anyway, one happy 1952 Bush and an even happier owner. Thanks to all for your help and contributions.

crackle · 10th Sep 2015, 4:04 pm

Glad its working better, have the voltages come up now, it may also be worth checking the EL41 cathode resistor.

Mike

richieyork · 11th Sep 2015, 8:30 pm

Pleased to say all voltages are now fine. Problem solved!!

AlanC · 15th Sep 2015, 9:21 pm

Great repair. Mine had leaky caps on the AGC, a faulty tuning cap in one of the IF transformers, and the oscillator anode load resistor gone high. Because these are a 'short superhet' with no audio amp stage before the output, every ounce of gain matters!

31st Aug 2015, 5:59 pm	#1
richieyork Pentode Join Date: Dec 2010 Location: York, North Yorkshire, UK. Posts: 212	BUSH AC31 Low volume/gain Hi all I've been asked to restore a rather sad looking but complete AC31. Initial inspection revealed lost of dust so felt hopeful it hadn't been meddled with, well if so it was a very long time ago. The radio had been stored in a damp environment at some stage - the output transformer metalwork is very rusty and the speaker cone is dragging ever so slightly but not enough to cause the low volume. There are several 'waxies' so I set about changing them before powering it up. I powered it up via a lamp limiter. Heaters lit but no sound whatsoever. Quick check on the output transformer showed zero volts both sides. Further check revealed R16 on reservoir cap was o/c. I found something close enough in value and tried again. This time low level crackle from speaker esp when aerial socket touched with wire. I found one or two stations on Med wave (literally one or two) but the volume is very low. I left it running for half an hour then removed the lamp limiter and gave it full mains. The volume came up quite a bit but is still very low. I'm seeing around 220V at the output transformer. Anyway, here's a summary of the faults: Low volume & seems very insensitive Volume control has been replaced at some stage - this one has a plastic shaft but it is correct value at 500k/LOG * Volume control correctly increases volume as you turn it clockwise but just beyond halfway round it then begins to go quieter as you turn it further and the sound gets distorted *The gram input is also very low and not very responsive to touching the input socket - i'd expect it to hum loudly but all I get is a faint click The trader service sheet suggests that V1 ECH42 is also a gram preamplifier. I'm suspecting maybe faults in either this valve OR the EL41 output valve Also this set has an EBF89 IF valve but the trader sheet says EBF80. Are they compatible and has someone perhaps 'been at it' before I got to it? Before I go off on a valve buying spree can anyone suggest what's going on and where I might want to check beforehand Many thanks Rich

31st Aug 2015, 6:15 pm	#2
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: BUSH AC31 Low volume/gain If you have a reasonable antenna then check all the valve voltages and report them in this thread. Lawrence.

31st Aug 2015, 6:40 pm	#3
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: BUSH AC31 Low volume/gain You say the volume control has been replaced, ensure it is wired correctly. I have had them with crossed connections before. Voltages on the output valve will be instructive I think!

8th Sep 2015, 11:25 am	#4
richieyork Pentode Join Date: Dec 2010 Location: York, North Yorkshire, UK. Posts: 212	Re: BUSH AC31 Low volume/gain UPDATE: Connections to volume control confirmed OK and it's the correct value/type. Checked voltages around EL41 - cathode was 10v. Anode was initially 210 volts but after a while slowly dropped to 170v. G2 dropping too. Filter resistor (1.5k) getting very hot - this was open circuit originally and so this is the replacement overheating. I disconnected the G1 feed and touched the wire - the hum was weak and distorted. Also checked resistance of output transformer - around 400ohm so that seems ok. Finally connected a test amp to the output from the radio side and signal loud and clear. Now strongly suspect the EL41 is dragging the whole show down - the valve looks 'cooked' (I know they run hot). So, the EL41 confined to the bin and another ordered. Fingers crossed. More to follow once the new valve in place as I'm still unhappy with the sensitivity.

8th Sep 2015, 3:05 pm	#5
boxdoctor Rest in Peace Join Date: Sep 2010 Location: Ripley, Derbyshire, UK. Posts: 785	Re: BUSH AC31 Low volume/gain Is there a positive voltage on the top end of the volume control? That will cause the volume reduction as you advance the control, as it will progressively upset the bias on th EL41. I am not familiar with this model, but that was my first thought on reading your post. Tony

9th Sep 2015, 8:48 pm	#6
richieyork Pentode Join Date: Dec 2010 Location: York, North Yorkshire, UK. Posts: 212	Re: BUSH AC31 Low volume/gain Final update : New EL 41 arrived today. Fitted it and immediately the sound and volume are restored to as new. I suspect the old valve had seriously gone downhill and was drawing so much current it caused the ht filter resistor to burn out. Anyway, one happy 1952 Bush and an even happier owner. Thanks to all for your help and contributions.

10th Sep 2015, 4:04 pm	#7
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: BUSH AC31 Low volume/gain Glad its working better, have the voltages come up now, it may also be worth checking the EL41 cathode resistor. Mike Last edited by crackle; 10th Sep 2015 at 4:15 pm.

11th Sep 2015, 8:30 pm	#8
richieyork Pentode Join Date: Dec 2010 Location: York, North Yorkshire, UK. Posts: 212	Re: BUSH AC31 Low volume/gain Pleased to say all voltages are now fine. Problem solved!!

15th Sep 2015, 9:21 pm	#9
AlanC Hexode Join Date: Jul 2006 Location: Sheffield, South Yorkshire, UK. Posts: 312	Re: BUSH AC31 Low volume/gain Great repair. Mine had leaky caps on the AGC, a faulty tuning cap in one of the IF transformers, and the oscillator anode load resistor gone high. Because these are a 'short superhet' with no audio amp stage before the output, every ounce of gain matters! __________________ Always expect the unexpected...