Pye V4 Sync Discriminator Diodes - UK Vintage Radio Repair and Restoration Discussion Forum

Ghostuser · 17th Jul 2006, 8:00 pm

The V4/V7 uses a twin diode discriminator to measure the drift between pulses from the LOPT and the line sync pulses from the sync separator. The resulting voltage is used to pull the line oscillator back into step. My set suddenly lost all line sync and I have checked the diodes which are listed as M3s. One has nearly infinite forward resistance on the Ohmsx100 range of an AVO 8. The other looks much more like a diode with forward resistance of a few K. Any ideas of what I could use as a replacement please or are M3s still available?
Nigel

Heatercathodeshort · 17th Jul 2006, 10:34 pm

Hello,
The M3 'metal pea' diodes have not been available for a long time. Any small silicon diodes such as a pair of 1N4001 series will make a good replacement. Regards John

ronbryan · 17th Jul 2006, 10:51 pm

Hi Nigel

I've got some unused M3s diodes if you want them.

Ron

Heatercathodeshort · 18th Jul 2006, 8:16 am

Hello Ron,
Its a long time since I saw a new one of those! They may be a bit variable due to age. They consisted of a couple of circular selenium rectifier units squeezed into the aluminium case. Contact was not the best when new and caused Pye horrendous problems in the V4 and V14C line sync circuit. They would keep the chassis looking original if a reasonably well matched pair could be found. Regards John.

Ghostuser · 18th Jul 2006, 9:36 am

Hi to All,
Thanks for the info, particularly that M3s were pretty unreliable when new and why. I have the Pye service book for the V4 and in case of loss of line sync, it immediately points the finger at the discriminator diodes, so that bears out John's comments. I have had to replace all Hunts caps under chassis, as I don't know how to rebuild moldseals in the way that one can with card tube waxed paper caps. Therefore, originality of appearance is not an issue with this set. Given the uncertainty of matching a pair of M3s, I will try a couple of silicon diodes. I have some 1N4007s to hand, although they are probably an overkill, but I will try those. If there is no joy, I will take up Ron's offer.
The forum never fails to please.
Regards
Nigel

Ed_Dinning · 18th Jul 2006, 12:19 pm

Hi Nigel, it may be better to use Ge diodes or lower voltage types such as 1N4001. The forward voltage of 1N4007's may be too high for good operation.

Ed

ppppenguin · 18th Jul 2006, 12:56 pm

Quote:

Originally Posted by Ed_Dinning

it may be better to use Ge diodes or lower voltage types such as 1N4001. The forward voltage of 1N4007's may be too high for good operation.

I agree that Ge or schottky diodes will have a lower voltage drop than ordinary silicon diodes. I'm not at all sure that a 1N4007 will have a greater drop than a 1N4001. I tested a few rectifier diodes at random from my drawer and they all seemed to be around 0.5V drop at the rather low current my DMM uses for diode testing. I was a bit surprised at how low this was - I was expecting 0.6V to 0.7V. Obviously the drop will be greater when they are handling "real" current.

The possible objection to 1N400x series diodes is that they are not designed for high frequency. 10kHz is high frequency in this context. The slow turn off time may give poor performance.

I don't know what sort of voltage rating is needed in this circuit but provided it's adequate you certainly won't do any harm by trying what you've got in your junk box. If it works then you've solved the problem.

Ghostuser · 18th Jul 2006, 1:37 pm

The 1N4007s have done the trick! Line hold is rock solid and there was no need to touch the preset line hold adjustment, so the line oscillator is getting the average voltage it expects. As to the question of high frequency operation, I gather that the junction capacitance is only about 15pF, for what that is worth.
Thanks to all for the very helpful advice.
Regards
Nigel

ppppenguin · 18th Jul 2006, 2:36 pm

Quote:

Originally Posted by Ghostuser

As to the question of high frequency operation, I gather that the junction capacitance is only about 15pF, for what that is worth.

It's not the capacitance that hurts at 10kHz. The problem is that it takes time for the holes and electrons to vacate the junction area and that delays the switch off. That's a really appalling simplified explanation but it's too hot to do it any better.

Anyway, congratulations on getting it working and also for proving that 1N400x are a good substitute for those discriminator diodes.

SPCh · 18th Jul 2006, 5:19 pm

Don't apologise for the simple explanation, Jeffrey: that's about the level some of us pre-transistor types can absorb. (But I sympathise with the flywheel problem - horror movies from my youth !)

17th Jul 2006, 8:00 pm	#1
Ghostuser Pentode Join Date: Oct 2003 Location: Alton, Hampshire, UK. Posts: 137	Pye V4 Sync Discriminator Diodes The V4/V7 uses a twin diode discriminator to measure the drift between pulses from the LOPT and the line sync pulses from the sync separator. The resulting voltage is used to pull the line oscillator back into step. My set suddenly lost all line sync and I have checked the diodes which are listed as M3s. One has nearly infinite forward resistance on the Ohmsx100 range of an AVO 8. The other looks much more like a diode with forward resistance of a few K. Any ideas of what I could use as a replacement please or are M3s still available? Nigel

17th Jul 2006, 10:34 pm	#2
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Pye V4 Sync Discriminator Diodes Hello, The M3 'metal pea' diodes have not been available for a long time. Any small silicon diodes such as a pair of 1N4001 series will make a good replacement. Regards John

17th Jul 2006, 10:51 pm	#3
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Pye V4 Sync Discriminator Diodes Hi Nigel I've got some unused M3s diodes if you want them. Ron

18th Jul 2006, 8:16 am	#4
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Pye V4 Sync Discriminator Diodes Hello Ron, Its a long time since I saw a new one of those! They may be a bit variable due to age. They consisted of a couple of circular selenium rectifier units squeezed into the aluminium case. Contact was not the best when new and caused Pye horrendous problems in the V4 and V14C line sync circuit. They would keep the chassis looking original if a reasonably well matched pair could be found. Regards John. Last edited by Heatercathodeshort; 18th Jul 2006 at 8:30 am.

18th Jul 2006, 9:36 am	#5
Ghostuser Pentode Join Date: Oct 2003 Location: Alton, Hampshire, UK. Posts: 137	Re: Pye V4 Sync Discriminator Diodes Hi to All, Thanks for the info, particularly that M3s were pretty unreliable when new and why. I have the Pye service book for the V4 and in case of loss of line sync, it immediately points the finger at the discriminator diodes, so that bears out John's comments. I have had to replace all Hunts caps under chassis, as I don't know how to rebuild moldseals in the way that one can with card tube waxed paper caps. Therefore, originality of appearance is not an issue with this set. Given the uncertainty of matching a pair of M3s, I will try a couple of silicon diodes. I have some 1N4007s to hand, although they are probably an overkill, but I will try those. If there is no joy, I will take up Ron's offer. The forum never fails to please. Regards Nigel

18th Jul 2006, 12:19 pm	#6
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Pye V4 Sync Discriminator Diodes Hi Nigel, it may be better to use Ge diodes or lower voltage types such as 1N4001. The forward voltage of 1N4007's may be too high for good operation. Ed

18th Jul 2006, 1:37 pm	#8
Ghostuser Pentode Join Date: Oct 2003 Location: Alton, Hampshire, UK. Posts: 137	Re: Pye V4 Sync Discriminator Diodes The 1N4007s have done the trick! Line hold is rock solid and there was no need to touch the preset line hold adjustment, so the line oscillator is getting the average voltage it expects. As to the question of high frequency operation, I gather that the junction capacitance is only about 15pF, for what that is worth. Thanks to all for the very helpful advice. Regards Nigel

18th Jul 2006, 5:19 pm	#10
SPCh Rest in Peace Join Date: Nov 2004 Location: Antwerp, Belgium Posts: 345	Re: Pye V4 Sync Discriminator Diodes Don't apologise for the simple explanation, Jeffrey: that's about the level some of us pre-transistor types can absorb. (But I sympathise with the flywheel problem - horror movies from my youth !)