HMV 495 beating - UK Vintage Radio Repair and Restoration Discussion Forum

humphrey · 12th Sep 2008, 6:55 pm

I just finished recapping the set but I now have beats. About four per second and not when a strong station is tuned in. I did not change any of the smaller caps below 1000pf. Any ideas? Using Trader sheet 555. It affects all wavebands is well as the gram.

Steve_P · 12th Sep 2008, 7:09 pm

Have you put anything back in the 'wrong' place, so to speak. Even wires. Something is picking up something else.

C24 and C28 look likely to me. And try adjusting/leaving out C49. If it does it on gram as well, then to me it's past the P.U. sockets.

Cheers,

Steve P.

humphrey · 12th Sep 2008, 7:47 pm

I just replaced C24 no change.
Took out the tone control c49 and adjusting makes no difference.
C28 was replaced and looks fine.
No improvement yet.
I will try again tomorrow to see if there is anything I missed.

humphrey · 12th Sep 2008, 8:37 pm

The only "wrong" I can think of is the placement of the new electrolytics. Are there any things to look out for?

Steve_P · 12th Sep 2008, 8:43 pm

Anything touching or near to wires carrying I.F. or R.F. ! Just put them exactly where the old ones were and see if that helps.

Cheers,

Steve P.

flyingtech55 · 12th Sep 2008, 10:35 pm

Did the set work before the recapping?

TimR

Top Cap · 13th Sep 2008, 12:11 am

Check the decouplers are in their correct circuit positions, its easy to make a mistake, we have all done it at some time. The main ones appear to be C11, C12,C13,C14,C15,C16 and C21. However, before you start warming up the soldering iron, check that the motor boating is not due to poor connections to the valves outer metallised coating. Sometimes you can make the instability vanish by placing your hand on the metallising of the culprit valve.
My usual cure for this is to wrap a few evenly spaced turns of tinned copper wire around the valve starting where the valve base meets the glass envelope. If the earthing wire has broken then it is necessary to run a wire from the winding you have made to a convenient earthing point. Sometimes you can run a thin wire down the outside of the base and wrap it round an earthy valve pin. Just make sure you don't accidentally wrap it round the anode pin

Les

humphrey · 13th Sep 2008, 8:04 am

The radio was not working but it did have a hummy gram. There were no beats then. There is no metallizing as they are shrouded by cans. Unfortunately I snipped out the electrolytics as they had been done before while waiting for replacements, thinking that it was a simple job.
The other caps were replaced individually, but you know how it is when it looks easy to replace two or three at once.
When tuned into a strong signal, it sounds very impressive.

Top Cap · 13th Sep 2008, 11:55 am

The service sheet I have is for another model supposed to be equivalent to the HMV495 but I have my doubts, so the capacitor references I quoted may be incorrect. The chassis view shows vacant valve holders so I checked up on the quoted valve types, found they were metallised and made the wrong assumption, sorry about that. I have had fun and games in the past with similar faults to this, one thinks that perhaps it could be related to the agc circuit due to the fault going on strong signals. But it could of course be the very fact that agc is present and lowering the gain of the rf circuits and thus clearing the fault that way. If all the decouplers have been changed then the oscillation is certain to be the result of a feed back mechanism somehow. I have a 1930's Pye that plays for hours then suddenly goes off into a howl just for a brief second, then continues to play perfect for another couple of hours. I am still trying to find the cause of that one, I hate intermittent faults like that but all part of the fun I suppose

.

humphrey · 13th Sep 2008, 11:59 am

Heh heh, I just turned the set over after wracking what brains I have left and so I thought I would check the voltages on the valve caps. The first one (V5) I touched with the probe cured the beating.
There was a shield missing on the set so I took one off V4 which cured it.

humphrey · 13th Sep 2008, 4:49 pm

Thanks for your help chaps. I checked all the circuits and tidied up as much as I could. When I actually found the real cause I could not believe it was so simple. I will be looking around for another shield but there is no indication that it will be needed. My next problem is the Defiant M900 but that is for tomorow. Almost total silence.

12th Sep 2008, 6:55 pm	#1
humphrey Retired Dormant Member Join Date: Aug 2008 Location: Bellac, 87. France. Posts: 357	HMV 495 beating I just finished recapping the set but I now have beats. About four per second and not when a strong station is tuned in. I did not change any of the smaller caps below 1000pf. Any ideas? Using Trader sheet 555. It affects all wavebands is well as the gram.

12th Sep 2008, 7:09 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: HMV 495 beating Have you put anything back in the 'wrong' place, so to speak. Even wires. Something is picking up something else. C24 and C28 look likely to me. And try adjusting/leaving out C49. If it does it on gram as well, then to me it's past the P.U. sockets. 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...?

12th Sep 2008, 7:47 pm	#3
humphrey Retired Dormant Member Join Date: Aug 2008 Location: Bellac, 87. France. Posts: 357	Re: HMV 495 beating I just replaced C24 no change. Took out the tone control c49 and adjusting makes no difference. C28 was replaced and looks fine. No improvement yet. I will try again tomorrow to see if there is anything I missed.

12th Sep 2008, 8:37 pm	#4
humphrey Retired Dormant Member Join Date: Aug 2008 Location: Bellac, 87. France. Posts: 357	Re: HMV 495 beating The only "wrong" I can think of is the placement of the new electrolytics. Are there any things to look out for?

12th Sep 2008, 8:43 pm	#5
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: HMV 495 beating Anything touching or near to wires carrying I.F. or R.F. ! Just put them exactly where the old ones were and see if that helps. 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...?

12th Sep 2008, 10:35 pm	#6
flyingtech55 Octode Join Date: Nov 2005 Location: Ayrshire, UK. Posts: 1,096	Re: HMV 495 beating Did the set work before the recapping? TimR __________________ All those moments will be lost in time, like tears in rain.

13th Sep 2008, 12:11 am	#7
Top Cap Octode Join Date: Mar 2006 Location: Watford, Hertfordshire, UK. Posts: 1,270	Re: HMV 495 beating Check the decouplers are in their correct circuit positions, its easy to make a mistake, we have all done it at some time. The main ones appear to be C11, C12,C13,C14,C15,C16 and C21. However, before you start warming up the soldering iron, check that the motor boating is not due to poor connections to the valves outer metallised coating. Sometimes you can make the instability vanish by placing your hand on the metallising of the culprit valve. My usual cure for this is to wrap a few evenly spaced turns of tinned copper wire around the valve starting where the valve base meets the glass envelope. If the earthing wire has broken then it is necessary to run a wire from the winding you have made to a convenient earthing point. Sometimes you can run a thin wire down the outside of the base and wrap it round an earthy valve pin. Just make sure you don't accidentally wrap it round the anode pin Les __________________ Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way!

13th Sep 2008, 8:04 am	#8
humphrey Retired Dormant Member Join Date: Aug 2008 Location: Bellac, 87. France. Posts: 357	Re: HMV 495 beating The radio was not working but it did have a hummy gram. There were no beats then. There is no metallizing as they are shrouded by cans. Unfortunately I snipped out the electrolytics as they had been done before while waiting for replacements, thinking that it was a simple job. The other caps were replaced individually, but you know how it is when it looks easy to replace two or three at once. When tuned into a strong signal, it sounds very impressive.

13th Sep 2008, 11:55 am	#9
Top Cap Octode Join Date: Mar 2006 Location: Watford, Hertfordshire, UK. Posts: 1,270	Re: HMV 495 beating The service sheet I have is for another model supposed to be equivalent to the HMV495 but I have my doubts, so the capacitor references I quoted may be incorrect. The chassis view shows vacant valve holders so I checked up on the quoted valve types, found they were metallised and made the wrong assumption, sorry about that. I have had fun and games in the past with similar faults to this, one thinks that perhaps it could be related to the agc circuit due to the fault going on strong signals. But it could of course be the very fact that agc is present and lowering the gain of the rf circuits and thus clearing the fault that way. If all the decouplers have been changed then the oscillation is certain to be the result of a feed back mechanism somehow. I have a 1930's Pye that plays for hours then suddenly goes off into a howl just for a brief second, then continues to play perfect for another couple of hours. I am still trying to find the cause of that one, I hate intermittent faults like that but all part of the fun I suppose . __________________ Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way!

13th Sep 2008, 11:59 am	#10
humphrey Retired Dormant Member Join Date: Aug 2008 Location: Bellac, 87. France. Posts: 357	Re: HMV 495 beating Heh heh, I just turned the set over after wracking what brains I have left and so I thought I would check the voltages on the valve caps. The first one (V5) I touched with the probe cured the beating. There was a shield missing on the set so I took one off V4 which cured it. Last edited by humphrey; 13th Sep 2008 at 12:25 pm.

13th Sep 2008, 4:49 pm	#11
humphrey Retired Dormant Member Join Date: Aug 2008 Location: Bellac, 87. France. Posts: 357	Re: HMV 495 beating Thanks for your help chaps. I checked all the circuits and tidied up as much as I could. When I actually found the real cause I could not believe it was so simple. I will be looking around for another shield but there is no indication that it will be needed. My next problem is the Defiant M900 but that is for tomorow. Almost total silence.