HMV 469 Crackling - UK Vintage Radio Repair and Restoration Discussion Forum

JoshWard · 4th Nov 2019, 9:21 pm

Hi all,

I'm having a bit of trouble with my HMV 469. I suppose I can't complain as this is only the third repair the set has required since it was made!

I have dealt with persistent crackles like this before and have checked the usual. This includes bypassing the on/off switch, the internal/external speaker selector, I have checked in a dark room and can't see any arcing on valve pins, wavechange switches or indeed anywhere else! Aside from a very short burst of blue from the U50 rectifier on switch on there is nothing funny that I can see going on inside the valves.

It is not affected by the volume control and it does it when on gram so I am guessing from that the problem is not in the tuning stages. However it is muffled by the 'brilliance' control so I am guessing from this the problem lies somewhere between the two.

However I'm not sure which component between the two is the source of the problem. I am guessing it could be caused by a carbon resistor? There is also an intervalve transformer in the circuit. Not sure how I can test it though if that's arcing inside? I don't particularly want to just replace all the output stage resistors without proving they are faulty but I'm not really sure how to progress it now...!

Any help is greatly appreciated- this set is very special to me!

Josh

PJL · 4th Nov 2019, 9:44 pm

V6 anode load, R30 on Trader sheet (50K).

John10b · 5th Nov 2019, 1:32 pm

It could be your output transformer breaking down, if you have a spare one you could try hooking it up.
Cheers
John

PaulR · 5th Nov 2019, 4:31 pm

Quote:

Originally Posted by John10b

It could be your output transformer breaking down, if you have a spare one you could try hooking it up.
Cheers
John

Surely not if the brilliance control affects it?

John10b · 5th Nov 2019, 5:05 pm

It was only a thought, but you are probably correct in view of the symptoms described.
Cheers
John

Nuvistor · 5th Nov 2019, 5:13 pm

Do you have a oscilloscope? If so trace it with that. The other suggestion is a 0.1uf cap temp from grid of the audio valves to chassis, may narrow it down to V6 or V7 stages.

JoshWard · 6th Nov 2019, 10:07 pm

Quote:

Originally Posted by PJL

V6 anode load, R30 on Trader sheet (50K).

A gold star to that man! Replaced it this evening and it's now singing away beautifully once again. I can't tell you how pleased I am to have this one back on top form!

Thanks all for their words of wisdom.

John10b · 6th Nov 2019, 11:20 pm

Well done glad you found the fault following the advice of PJL.
Cheers
John

PJL · 7th Nov 2019, 9:40 am

Josh, glad its sorted. Resistors with high voltage across them tend to do this and you had done all the right ground work in your testing.

Nuvistor · 7th Nov 2019, 9:55 am

Excellent, feet up and listen to the music.

4th Nov 2019, 9:21 pm	#1
JoshWard Octode Join Date: Dec 2005 Location: Near Stowmarket, Suffolk, UK. Posts: 1,962	HMV 469 Crackling Hi all, I'm having a bit of trouble with my HMV 469. I suppose I can't complain as this is only the third repair the set has required since it was made! I have dealt with persistent crackles like this before and have checked the usual. This includes bypassing the on/off switch, the internal/external speaker selector, I have checked in a dark room and can't see any arcing on valve pins, wavechange switches or indeed anywhere else! Aside from a very short burst of blue from the U50 rectifier on switch on there is nothing funny that I can see going on inside the valves. It is not affected by the volume control and it does it when on gram so I am guessing from that the problem is not in the tuning stages. However it is muffled by the 'brilliance' control so I am guessing from this the problem lies somewhere between the two. However I'm not sure which component between the two is the source of the problem. I am guessing it could be caused by a carbon resistor? There is also an intervalve transformer in the circuit. Not sure how I can test it though if that's arcing inside? I don't particularly want to just replace all the output stage resistors without proving they are faulty but I'm not really sure how to progress it now...! Any help is greatly appreciated- this set is very special to me! Josh

4th Nov 2019, 9:44 pm	#2
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: HMV 469 Crackling V6 anode load, R30 on Trader sheet (50K).

5th Nov 2019, 1:32 pm	#3
John10b Nonode Join Date: Nov 2016 Location: Aberaeron, Ceredigion, Wales, UK. Posts: 2,887	Re: HMV 469 Crackling It could be your output transformer breaking down, if you have a spare one you could try hooking it up. Cheers John

5th Nov 2019, 5:05 pm	#5
John10b Nonode Join Date: Nov 2016 Location: Aberaeron, Ceredigion, Wales, UK. Posts: 2,887	Re: HMV 469 Crackling It was only a thought, but you are probably correct in view of the symptoms described. Cheers John

5th Nov 2019, 5:13 pm	#6
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: HMV 469 Crackling Do you have a oscilloscope? If so trace it with that. The other suggestion is a 0.1uf cap temp from grid of the audio valves to chassis, may narrow it down to V6 or V7 stages. __________________ Frank

6th Nov 2019, 11:20 pm	#8
John10b Nonode Join Date: Nov 2016 Location: Aberaeron, Ceredigion, Wales, UK. Posts: 2,887	Re: HMV 469 Crackling Well done glad you found the fault following the advice of PJL. Cheers John

7th Nov 2019, 9:40 am	#9
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: HMV 469 Crackling Josh, glad its sorted. Resistors with high voltage across them tend to do this and you had done all the right ground work in your testing.

7th Nov 2019, 9:55 am	#10
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: HMV 469 Crackling Excellent, feet up and listen to the music. __________________ Frank