Invicta 33...volume won't reduce to zero - UK Vintage Radio Repair and Restoration Discussion Forum

Tim · 4th Dec 2007, 8:57 pm

Hi all.
Have an example of above set on the bench, but the volume won't reduce to zero. In fact it remains quite audible even at minimum setting of the volume control. Shorting the wiper of the volume control to chassis makes no difference, all waxies and the EL41's cathode resistor and bypass capacitor have been replaced, as have all the valves(with N.O.S). Shunting the EBC41's cathode resistor (which is within value) with a 22uF cap, cures the problem, but the tone control is then rendered inoperative. I haven't yet tried a replacement volume control, but I should be able to connect one in parallel for testing?
I have the trader sheet.
Any pointers please?

Thanks
Tim

Steve_P · 4th Dec 2007, 11:01 pm

Try a new EBC41. This is quite common! If it is this, keep the original valve for testing or maybe in another set it'll be OK.

Cheers,

Steve P.

paulsherwin · 4th Dec 2007, 11:09 pm

I suspected the EBC41 too, but you say you've changed it. I suppose you might have been unlucky and bought a duff one. If you're confident the EBC41 is OK try shorting the volume wiper to chassis. If this mutes the output you need to investigate the volume control.

Paul

Tim · 5th Dec 2007, 12:14 am

Hi Paul and Steve
Thanks for the advice. The set was supplied with brand new(old stock) valves, in addition to the originals. Symptoms are the same with both EBC41's.
Shorting the wiper of the volume control to chassis makes no difference.

Hence my visit here!

I haven't had time to test the valves yet, but will do ASAP.

PJL · 5th Dec 2007, 12:30 am

The circuit is a bit unusual in not having any cathode bypass. The detector current will change the cathode voltage - but it's too late in the day for me to work the maths out to see if you are getting what's expected...Peter

Steve_P · 5th Dec 2007, 1:20 am

Lift R9 - See if it goes away.

C19. C20. C17. C21.

It is likely to be the valve. The same cathode is used for both the Diode and the Triode. You can get leakage inside the valve itself, and some valves are better than others and some sets are better than others here. Try one from a working set to be sure about the valve.

Cheers,

Steve P.

Tim · 5th Dec 2007, 1:28 am

Thanks Steve. I'll try that and report back.
All waxies have been replaced with RS "yellow" caps.

PJL · 5th Dec 2007, 10:40 am

Hi Tim,

The cathode bias stopped the sound therefore it's not an EBC41 diode/grid leak.

The design will result in some sound when the grid is shorted to earth as AF current is flowing in the detector diode which is changing the cathode volts. The ratio of grid/cathode resistance is 700:1 which sounds like it would be audible.

The effect would be made worse if the AGC wasn't working properly. I haven't worked out if the signal is in/out of phase with the grid.

Tim · 5th Dec 2007, 2:33 pm

The sound is quite loud, in fact probably normal listening level.
The 1 meg resistors on the AGC check out OK(1.1 meg each approx) but I'll see if there is a voltage there.

Steve P

Lifting R9(680k) made no difference.

(C17 not tested-inside IF can& in-sufficient time )
C19,20,21 check out OK.

3 EBC41's now tried (two of which are new).
I might try connecting another volume control anyway, just for elimination purposes.

I don't suppose there could be anything amiss in the tone controls?

Thanks for your help so far. The mystery deepens!!

PJL · 5th Dec 2007, 3:29 pm

R9 is the diode load. If diodes and caps were 'perfect' you should have no audio output as the time constant would be too long...probably just works on leakage but it might be worth measuring resistance from diode to chassis without R9 in case it is severe.

Your bypass cap test implies it's too much AF on the EBC41 cathode.

Wiring error? Cathode/Anode resistors for EBC41? No AGC due to leaky grid on V1/V2? Perhaps poor regulation due to HT electrolytic gone?

You could try disconnecting the feedback path but I can't see how unwanted AF could end up there...Peter

Steve_P · 5th Dec 2007, 3:58 pm

C23 if not already changed. If R9 out of circuit that would eliminate the tone controls.

Maybe tracking somehow across the wavechange switch?

C8 possibly.

Earthing to chassis - check!

This has got to be something really stupid!

Cheers,

Steve P.

Tim · 5th Dec 2007, 5:33 pm

Quote:

This has got to be something really stupid!

Something so stupid I can't see it!

Quote:

Maybe tracking somehow across the wavechange switch?

I am starting to suspect something like that. This set uses those horrid paxolin valve holders, which are not immune to tracking. It wouldn't have to be much of a leak to affect this part of the circuit. It all looks fairly clean though.

About -ve 5 volts (DVM)on the AGC when tuned into a station. Volume control checks out with a meter.

Interestingly shorting the "top" of the volume control to chassis has no effect either.I'll see what happens on the Gram position.

Thanks Steve and PJL

ALL valves are new, so I'll pull 'em all exept the rectifier and check for leakage.
Wiring error is possible, but it did this befor I changed the waxies so hopefully the error isn't mine!!

I'll also try subbing out the HT caps, or stick a 0.1uF across the HT.

PJL · 6th Dec 2007, 11:39 am

Did you check the EBC41 cathode resistance R13 (1K)? also check R9 (680K) and the correct vol. control (1M).

Temporarily disconnect the AGC feed capacitor C20 to confirm it isn't coming from the AGC diode.

Are the voltages OK on the EBC41?

Tim · 6th Dec 2007, 12:28 pm

Quote:

Did you check the EBC41 cathode resistance R13 (1K)? also check R9 (680K) and the correct vol. control (1M).

Yes(replaced),yes(within tolerance), and yes(1.1 Meg).

EBC 41 Voltage not as per trader, but reasonable.I think it's down to the meter I am using 'cos cathode volts about right.
Anode resistor slightly high(~270k)

Next time I get to the set I'll try the C20 trick.
Thanks

PJL · 6th Dec 2007, 9:38 pm

Might be easier to short the diode to chassis...very odd design.

Steve_P · 6th Dec 2007, 10:22 pm

Change the valve base.

Cheers,

Steve P.

Tim · 10th Dec 2007, 8:24 pm

Quote:

Change the valve base.

I might just do that Steve. It's looking more and more likely. It'll be fun though, as the original is riveted in. Not sure how many of these bases I have. I might try an EABC80/EBC81, etc. perhaps on flying leads to start with.

What I found today:-

An original wiring error meant that R9(680K) was taken direct to chassis and not to EBC41 cathode. Now corrected, and slightly better. Disconnecting c/t of volume control-signal still there at audible level.

Another control tried-no change.

Even more spooky, disconnecting the other end of this co-ax(replaced by modern plastic) from the EBC41 grid---signal STILL there!!!!
With NOTHING connected to the triode grid-program clearly audible.

Changed detector diode for 1n4148, no change-signal still audible.

The problem does not show on Gram,(volume works normally) so as some previous posters have suggested I think it likely a problem with the detector/diodes.. With no diodes connected, no signal, but amp OK.

0.1 uF connected across HT-no change.

I hope I'm getting to the bottom of this, as I don't have much hair to start with!!

Thanks to everybody with your help so far.

PJL · 10th Dec 2007, 8:55 pm

The circuit I have shows R9 680K from junction of C18,C19 and R8 and it goes to chassis. R9 to the cathode would seem to make more sense but C19 will still result in AC current that will pass through the detector on to the cathode and induce a signal regardless of the grid.

Did you try disconnecting the AGC feed, C20?

Tim · 10th Dec 2007, 9:00 pm

Quote:

The circuit I have shows R9 680K from junction of C18,C19 and R8 and it goes to chassis.

Different to my Trader sheet.

Quote:

Did you try disconnecting the AGC feed, C20?

No not yet, but will do tomorrow. Shorting out either diode makes no difference

PJL · 10th Dec 2007, 9:13 pm

Shorting out the diodes has no effect!!! So something else is doing the detecting

4th Dec 2007, 8:57 pm	#1
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Invicta 33...volume won't reduce to zero Hi all. Have an example of above set on the bench, but the volume won't reduce to zero. In fact it remains quite audible even at minimum setting of the volume control. Shorting the wiper of the volume control to chassis makes no difference, all waxies and the EL41's cathode resistor and bypass capacitor have been replaced, as have all the valves(with N.O.S). Shunting the EBC41's cathode resistor (which is within value) with a 22uF cap, cures the problem, but the tone control is then rendered inoperative. I haven't yet tried a replacement volume control, but I should be able to connect one in parallel for testing? I have the trader sheet. Any pointers please? Thanks Tim __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."

4th Dec 2007, 11:01 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Invicta 33...volume won't reduce to zero Try a new EBC41. This is quite common! If it is this, keep the original valve for testing or maybe in another set it'll be OK. 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...?

4th Dec 2007, 11:09 pm	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,944	Re: Invicta 33...volume won't reduce to zero I suspected the EBC41 too, but you say you've changed it. I suppose you might have been unlucky and bought a duff one. If you're confident the EBC41 is OK try shorting the volume wiper to chassis. If this mutes the output you need to investigate the volume control. Paul

5th Dec 2007, 12:14 am	#4
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: Invicta 33...volume won't reduce to zero Hi Paul and Steve Thanks for the advice. The set was supplied with brand new(old stock) valves, in addition to the originals. Symptoms are the same with both EBC41's. Shorting the wiper of the volume control to chassis makes no difference. Hence my visit here! I haven't had time to test the valves yet, but will do ASAP. __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."

5th Dec 2007, 12:30 am	#5
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Invicta 33...volume won't reduce to zero The circuit is a bit unusual in not having any cathode bypass. The detector current will change the cathode voltage - but it's too late in the day for me to work the maths out to see if you are getting what's expected...Peter

5th Dec 2007, 1:20 am	#6
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Invicta 33...volume won't reduce to zero Lift R9 - See if it goes away. C19. C20. C17. C21. It is likely to be the valve. The same cathode is used for both the Diode and the Triode. You can get leakage inside the valve itself, and some valves are better than others and some sets are better than others here. Try one from a working set to be sure about the valve. 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...?

5th Dec 2007, 1:28 am	#7
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: Invicta 33...volume won't reduce to zero Thanks Steve. I'll try that and report back. All waxies have been replaced with RS "yellow" caps. __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."

5th Dec 2007, 10:40 am	#8
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Invicta 33...volume won't reduce to zero Hi Tim, The cathode bias stopped the sound therefore it's not an EBC41 diode/grid leak. The design will result in some sound when the grid is shorted to earth as AF current is flowing in the detector diode which is changing the cathode volts. The ratio of grid/cathode resistance is 700:1 which sounds like it would be audible. The effect would be made worse if the AGC wasn't working properly. I haven't worked out if the signal is in/out of phase with the grid.

5th Dec 2007, 2:33 pm	#9
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: Invicta 33...volume won't reduce to zero The sound is quite loud, in fact probably normal listening level. The 1 meg resistors on the AGC check out OK(1.1 meg each approx) but I'll see if there is a voltage there. Steve P Lifting R9(680k) made no difference. (C17 not tested-inside IF can& in-sufficient time ) C19,20,21 check out OK. 3 EBC41's now tried (two of which are new). I might try connecting another volume control anyway, just for elimination purposes. I don't suppose there could be anything amiss in the tone controls? Thanks for your help so far. The mystery deepens!! __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."

5th Dec 2007, 3:29 pm	#10
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Invicta 33...volume won't reduce to zero R9 is the diode load. If diodes and caps were 'perfect' you should have no audio output as the time constant would be too long...probably just works on leakage but it might be worth measuring resistance from diode to chassis without R9 in case it is severe. Your bypass cap test implies it's too much AF on the EBC41 cathode. Wiring error? Cathode/Anode resistors for EBC41? No AGC due to leaky grid on V1/V2? Perhaps poor regulation due to HT electrolytic gone? You could try disconnecting the feedback path but I can't see how unwanted AF could end up there...Peter Last edited by PJL; 5th Dec 2007 at 3:42 pm.

5th Dec 2007, 3:58 pm	#11
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Invicta 33...volume won't reduce to zero C23 if not already changed. If R9 out of circuit that would eliminate the tone controls. Maybe tracking somehow across the wavechange switch? C8 possibly. Earthing to chassis - check! This has got to be something really stupid! 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...?

6th Dec 2007, 11:39 am	#13
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Invicta 33...volume won't reduce to zero Did you check the EBC41 cathode resistance R13 (1K)? also check R9 (680K) and the correct vol. control (1M). Temporarily disconnect the AGC feed capacitor C20 to confirm it isn't coming from the AGC diode. Are the voltages OK on the EBC41?

6th Dec 2007, 9:38 pm	#15
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Invicta 33...volume won't reduce to zero Might be easier to short the diode to chassis...very odd design.

6th Dec 2007, 10:22 pm	#16
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Invicta 33...volume won't reduce to zero Change the valve base. 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...?

10th Dec 2007, 8:55 pm	#18
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Invicta 33...volume won't reduce to zero The circuit I have shows R9 680K from junction of C18,C19 and R8 and it goes to chassis. R9 to the cathode would seem to make more sense but C19 will still result in AC current that will pass through the detector on to the cathode and induce a signal regardless of the grid. Did you try disconnecting the AGC feed, C20? Attached Thumbnails Last edited by PJL; 10th Dec 2007 at 9:12 pm.

10th Dec 2007, 9:13 pm	#20
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Invicta 33...volume won't reduce to zero Shorting out the diodes has no effect!!! So something else is doing the detecting