HMV 1107 with hum pickup - UK Vintage Radio Repair and Restoration Discussion Forum

davidgem1406 · 2nd Oct 2017, 8:49 pm

I have an HMV 1107 that I have, after a very long time, in years, now completed its refurbishment.
I have included 2 pictures of the chassis, that was totally stripped out, cleaned, painted and rebuilt.
The chassis I did a long time ago but only recently completed the cabinet.
I bought it originally as being a Marconi 888 but the wiring did not agree with that.

It is all now fully working but has a hum problem that is quite distracting at low volume. The hum I believe is being picked up by the audio amplifier section of V3. This is what I have tried in an effort to cure/locate it.

Disconnecting the feed to V3 grid stops it immediately and I can produce the same hum by slowly moving my hand towards V3. At about 50mm from the valve it picks up the same hum from me, V3 seems to have a very high gain.

The AC On/Off switch is on the back of the V.C. so I removed the AC wiring from the switch. The hum was reduced slightly but still apparent.

Next, I added a fully screened top cap to V3 and wrapped the valve in metal foil and grounded it. Result zero change.

I then decided to move the AC wiring as far away from the V.C. as I could sensibly do. I changed the tone control, this is mounted on the side of the chassis not the front, for one that had a DP switch that I connected the AC wiring to. Still no change.

Lastly, I changed the screened wire from the V.C. to V3 grid, ever hopeful, grounded it at various earth points to no avail.

Now I have only one other idea in mind but it entails some awkward rewiring.
I noticed that the heater wiring is using only one wire to connect to V1 to V4 heaters, whilst the other heater pin for each valve is connected to chassis.

The heater current total is 1.8 Amp, V1 to V3 at 0.3 and V4 at 0.9.
My thought on this is that the hum is being developed within the chassis itself with an equivalent current path that totals 1.8 Amp.
If this is the case then a rewire of the heater chain using the twisted pair should cure the problem as hum reduction is the main reason for using twisted pairs.

My question is how likely is that to be the cause of the problem? or does anybody have a fresh idea that may be the cause?

I have also added a pdf document that contains the drawings I made during the chassis refurbishment. It clearly shows the heater wiring.

Best regards to all
Dave

Boater Sam · 2nd Oct 2017, 9:01 pm

I would suggest that it is the single heater wire straggling around the chassis that is the problem.
A twisted pair laid close to the chassis running in the corners wherever possible is the normal method with neither side grounded to the chassis ( provided that all valves are indirectly heated )
A possible centre tap on the mains transformer heater winding could be grounded.

'LIVEWIRE?' · 3rd Oct 2017, 9:17 am

Dave-As you may know some designs feature a 'hum-bucking circuit, consisting of a pot. wired across the heater winding, with it's wiper connected to chassis. The pot. is then adjusted for minimum hum. I don't recall what value of pot. is normally used for this purpose, but other members no doubt will know.

ms660 · 3rd Oct 2017, 12:05 pm

Typical value for a humdinger pot would be 100 ohms or even lower, the one in my old HRO receiver was 64 ohms.

All suitably rated of course.

Lawrence.

Herald1360 · 3rd Oct 2017, 12:08 pm

The pot across the LT with slider grounded is usually called a "humdinger". It can be very effective but it balances the heater volts to ground to reduce heater cathode pickup rather than bucking HT ripple current in the output valve anode circuit which is what a "humbucker" winding in the output TX does.

Fitting a humdinger in my youth to a PA amp I had concocted was like magic- a lot of nasty snarly hum just disappeared with a twiddle!

'LIVEWIRE?' · 3rd Oct 2017, 12:16 pm

Apologies for the incorrect trerminology, folks. I know that 'Humbucker' is the name for a winding or tapping on an o/p transformer, but had forgotten what the pot. across a heater winding was called.

davidgem1406 · 3rd Oct 2017, 9:47 pm

Hello all,

I don't think a humdinger pot could be used without a rewire of the heater as the pot would normally be across the 6.3V supply to the heaters with the wiper taken to ground.
It's looking more like a rewire to even find out if that is the problem.
Incidentally, the heater wiring is actually original as is and not part of my refit.

Dave.

Herald1360 · 3rd Oct 2017, 10:12 pm

Quote:

Originally Posted by davidgem1406

I don't think a humdinger pot could be used without a rewire of the heater as the pot would normally be across the 6.3V supply to the heaters with the wiper taken to ground.

You're right, it can't. You'll need a nicely dressed twisted pair rewire to the heaters before you can ding any remaining hum.

davidgem1406 · 7th Oct 2017, 3:59 pm

Thanks, OK, that's the way it will go, heater rewire coming up.

Dave

crackle · 7th Oct 2017, 4:02 pm

You should not need to do a heater rewire, something else is wrong.

Mike

crackle · 7th Oct 2017, 6:16 pm

These are not a hummy radio, here is a short demonstration of my 1107 https://youtu.be/s0YwYggxrOE

Mike

crackle · 7th Oct 2017, 8:16 pm

Just had a thought, have you fitted a 3 core mains cable and earthed the chassis. My radio still has a 2 core mains cable.

Mike

yestertech · 8th Oct 2017, 9:05 am

Don't know if it's relevant , but your wiring diagram shows the inner of the coax to V3 grid as connected to the 'oute'r of the pot, rather than the slider. This could mean that the grid is fully 'open' at all volume settings, rather than nearer to ground at minimum volume settings.
It would be a quick job to swap the two coax inners and see if there's any difference ?

Andy

davidgem1406 · 9th Oct 2017, 10:20 pm

Hello Andy,

You are quite right with what you say, I had not spotted that one.
I drew those drawings from the original chassis and I have just checked the original pictures I took before it was stripped down.
Unfortunately, I cant see where the 2 cables go once they leave the pot as they are hidden under other components and wires. Looks like I could have marked them up incorrectly as to their destinations. Then, of course, I rebuilt the chassis using those drawings.

I just checked the circuit diagram and yes the TC of V3 should be on the wiper.
Having now corrected that all is now silent as it should be, but not before I had completed the heater rewire.

Thank you very much Andy for spotting that, I can now wrap it up as completed.

My thanks to everyone else for your input it is all much appreciated.

I will call it another one of "Lifes Experiences"

Best regards to all
Dave.

Boater Sam · 10th Oct 2017, 8:08 am

Good, job done.
Just to be clear, did re-routing the heater wiring in twisted pair prove to make a significant difference to the hum?
Sam.

crackle · 10th Oct 2017, 1:37 pm

Well spotted Andy.

mike

davidgem1406 · 12th Oct 2017, 7:59 pm

Hello Sam,

It made no difference at all as far as I know.
I did the rewire before I knew of the error in the diagram and consequently the volume control.

Dave

Boater Sam · 12th Oct 2017, 8:05 pm

Interesting. With audio amps it has long been considered to be essential to wire in twisted pair. Seems that it is not so vital.

davidgem1406 · 16th Oct 2017, 11:15 pm

I think it is equipment dependant as to twisted pair or not.
When I worked at Solatron, a long time ago now, every instrument that used valves, most of them back at that time, always had twisted pairs for the heater wiring and all other AC wiring.

With commercial valve radios it seems some did and some didn't use twisted pair.

Dave.

2nd Oct 2017, 9:01 pm	#2
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: HMV 1107 with hum pickup I would suggest that it is the single heater wire straggling around the chassis that is the problem. A twisted pair laid close to the chassis running in the corners wherever possible is the normal method with neither side grounded to the chassis ( provided that all valves are indirectly heated ) A possible centre tap on the mains transformer heater winding could be grounded.

3rd Oct 2017, 9:17 am	#3
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: HMV 1107 with hum pickup Dave-As you may know some designs feature a 'hum-bucking circuit, consisting of a pot. wired across the heater winding, with it's wiper connected to chassis. The pot. is then adjusted for minimum hum. I don't recall what value of pot. is normally used for this purpose, but other members no doubt will know.

3rd Oct 2017, 12:05 pm	#4
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: HMV 1107 with hum pickup Typical value for a humdinger pot would be 100 ohms or even lower, the one in my old HRO receiver was 64 ohms. All suitably rated of course. Lawrence.

3rd Oct 2017, 12:08 pm	#5
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: HMV 1107 with hum pickup The pot across the LT with slider grounded is usually called a "humdinger". It can be very effective but it balances the heater volts to ground to reduce heater cathode pickup rather than bucking HT ripple current in the output valve anode circuit which is what a "humbucker" winding in the output TX does. Fitting a humdinger in my youth to a PA amp I had concocted was like magic- a lot of nasty snarly hum just disappeared with a twiddle! __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

3rd Oct 2017, 12:16 pm	#6
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: HMV 1107 with hum pickup Apologies for the incorrect trerminology, folks. I know that 'Humbucker' is the name for a winding or tapping on an o/p transformer, but had forgotten what the pot. across a heater winding was called.

3rd Oct 2017, 9:47 pm	#7
davidgem1406 Heptode Join Date: Feb 2011 Location: Sheerness, Kent, UK. Posts: 936	Re: HMV 1107 with hum pickup Hello all, I don't think a humdinger pot could be used without a rewire of the heater as the pot would normally be across the 6.3V supply to the heaters with the wiper taken to ground. It's looking more like a rewire to even find out if that is the problem. Incidentally, the heater wiring is actually original as is and not part of my refit. Dave.

7th Oct 2017, 3:59 pm	#9
davidgem1406 Heptode Join Date: Feb 2011 Location: Sheerness, Kent, UK. Posts: 936	Re: HMV 1107 with hum pickup Thanks, OK, that's the way it will go, heater rewire coming up. Dave

7th Oct 2017, 4:02 pm	#10
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: HMV 1107 with hum pickup You should not need to do a heater rewire, something else is wrong. Mike

7th Oct 2017, 6:16 pm	#11
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: HMV 1107 with hum pickup These are not a hummy radio, here is a short demonstration of my 1107 https://youtu.be/s0YwYggxrOE Mike

7th Oct 2017, 8:16 pm	#12
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: HMV 1107 with hum pickup Just had a thought, have you fitted a 3 core mains cable and earthed the chassis. My radio still has a 2 core mains cable. Mike

8th Oct 2017, 9:05 am	#13
yestertech Nonode Join Date: Dec 2002 Location: Coulsdon, Surrey, UK. Posts: 2,130	Re: HMV 1107 with hum pickup Don't know if it's relevant , but your wiring diagram shows the inner of the coax to V3 grid as connected to the 'oute'r of the pot, rather than the slider. This could mean that the grid is fully 'open' at all volume settings, rather than nearer to ground at minimum volume settings. It would be a quick job to swap the two coax inners and see if there's any difference ? Andy

9th Oct 2017, 10:20 pm	#14
davidgem1406 Heptode Join Date: Feb 2011 Location: Sheerness, Kent, UK. Posts: 936	Re: HMV 1107 with hum pickup Hello Andy, You are quite right with what you say, I had not spotted that one. I drew those drawings from the original chassis and I have just checked the original pictures I took before it was stripped down. Unfortunately, I cant see where the 2 cables go once they leave the pot as they are hidden under other components and wires. Looks like I could have marked them up incorrectly as to their destinations. Then, of course, I rebuilt the chassis using those drawings. I just checked the circuit diagram and yes the TC of V3 should be on the wiper. Having now corrected that all is now silent as it should be, but not before I had completed the heater rewire. Thank you very much Andy for spotting that, I can now wrap it up as completed. My thanks to everyone else for your input it is all much appreciated. I will call it another one of "Lifes Experiences" Best regards to all Dave. Last edited by davidgem1406; 9th Oct 2017 at 10:21 pm. Reason: Minor correction

10th Oct 2017, 8:08 am	#15
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: HMV 1107 with hum pickup Good, job done. Just to be clear, did re-routing the heater wiring in twisted pair prove to make a significant difference to the hum? Sam.

10th Oct 2017, 1:37 pm	#16
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: HMV 1107 with hum pickup Well spotted Andy. mike

12th Oct 2017, 7:59 pm	#17
davidgem1406 Heptode Join Date: Feb 2011 Location: Sheerness, Kent, UK. Posts: 936	Re: HMV 1107 with hum pickup Hello Sam, It made no difference at all as far as I know. I did the rewire before I knew of the error in the diagram and consequently the volume control. Dave

12th Oct 2017, 8:05 pm	#18
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: HMV 1107 with hum pickup Interesting. With audio amps it has long been considered to be essential to wire in twisted pair. Seems that it is not so vital.

16th Oct 2017, 11:15 pm	#19
davidgem1406 Heptode Join Date: Feb 2011 Location: Sheerness, Kent, UK. Posts: 936	Re: HMV 1107 with hum pickup I think it is equipment dependant as to twisted pair or not. When I worked at Solatron, a long time ago now, every instrument that used valves, most of them back at that time, always had twisted pairs for the heater wiring and all other AC wiring. With commercial valve radios it seems some did and some didn't use twisted pair. Dave.