Pye v4 - Page 15 - UK Vintage Radio Repair and Restoration Discussion Forum

Boom · 5th May 2010, 5:20 pm

That 84.5 Ohm if correct indicates that probably another resistor will have overheated severely as the feed resistors are higher than that. It might be worth simply looking for burnt ones.

Boom · 5th May 2010, 5:25 pm

That's not a feed through capacitor is it Chris?

Dave

Lloyd 1985 · 5th May 2010, 5:28 pm

I think I've found C75 on your picture:

what does anyone else think?

Boom · 5th May 2010, 5:30 pm

I make it under the diode here:-

Lloyd 1985 · 5th May 2010, 5:34 pm

ah, yes it could be, the one I've circled is looking more like C76 8.2pf.

PJL · 5th May 2010, 5:36 pm

I would agree with Boom.

Measure resistance from the tag with the 6K8 to chassis to test C75.

Boom · 5th May 2010, 5:39 pm

That's interesting though Lloyd. I make your one C73 a .003 decoupler as well so Chris is looking for an identical one under the diode.

Sorry, totally messed that up. The round disc one connected to pin 8 is C73 so one of those is what Chris is looking for (under the diode).

oldticktock · 5th May 2010, 5:48 pm

Tag with 6.8K resistance reads oddly, 0.386R

oldticktock · 5th May 2010, 5:49 pm

Quote:

Originally Posted by Boom

I make it under the diode here:-

That's a 10pF

PJL · 5th May 2010, 5:50 pm

Well, we have found the problem then. The solution is going to be another story! You will have to try to snip the capacitor off the tag.

It's under the C76 8.2pF and has a wire running to the tag with the 6K8.

oldticktock · 5th May 2010, 5:52 pm

C75 is supposed to be a 0.003 everywhere else they've been hunts. Maybe due to the RF side they used something else. I will be interested to see what Trevor has in his.

oldticktock · 5th May 2010, 5:53 pm

I can see it and it was hiding under as you say and it's the round flat disc jobby.

Excuse while I do a Homer Simpson run over the Durham Dales screaming.

PJL · 5th May 2010, 6:00 pm

It is difficult to tell from the picture - there is something under there?

I use a very fine pair of medical tweezers for this sort of work. You may need to remove the other connections from the tag to get at it but be careful as the coil has a wire to it.

On the other hand you could just follow the homer approach and reach for the hammer.

oldticktock · 5th May 2010, 6:09 pm

Phew I'm back!

Ok I got it out, now I'm going to try and get a new one in.

Working on those clocks over the years has finally served it purpose, lots of little

bits to fiddle with

I suppose on a positive note at least I know that the caps I cannot find are these types. Trevors is March 1953 this set is July 1953 we shall see if it's the same.

PYE really could not have made this more difficult if they had tried

Boom · 5th May 2010, 6:12 pm

Does it measure s/c Chris? I thought these were pretty reliable.

Dave

oldticktock · 5th May 2010, 6:18 pm

0.39R I was under the same impression and was always intending to leave them well alone and with their difficult access.

I think I need a glass of your amber stuff Boom, for the next bit coming up.

Hope you don't all think me stupid but can someone explain what was happening, why this caused the massive over heat on R131, I have to understand otherwise it's just a swap, move on and I learn nothing for next time.

Chris

Boom · 5th May 2010, 6:27 pm

You can have a drop of my amber stuff when we see a picture on it.

Dave

oldticktock · 5th May 2010, 6:52 pm

Oh and this was the latest on screen before entering into the R131 battle, which I forgot to post

PJL · 5th May 2010, 6:54 pm

It would be a good idea to check the resistance at C18 again.

C75 was a short circuit to earth. There is HT voltage of 200V at C102. There is a current path from C102 -> R131 -> R94 -> C75 -> chassis.

R131=131 ohms, R94=150 ohms, C74=0 ohms (a short circuit). The total resistance for the current path is 131+150+0=281 ohms.

Ohms law says V=I*R. I=V/R, I=200/281=0.71A.
Power is given by W=V*I. But V=I*R. So power dissipated in a resistor is W=I*R*I
Power dissipated in the 311 ohm resistor=0.71*311*0.71=157W
So the resistor gets VERY hot VERY quickly and pulls down the HT rail for a bit before it melts...

oldticktock · 5th May 2010, 7:17 pm

Thanks PJL thats what I was looking for.

Wiring in this replacement cap is difficult but not impossible. I can get the connection to the top of the 6.8K fine the original earth point is a no go. I will just end up melting everything or having to disturb too much and the coil worries me. I think I shall just pick the nearest earth point which will be easier and hopefully will not put the circuit out as I know these circuits are designed around optimum placement. However the guy who designed this was very creative in his component placement and i'm uncertain he had optimum in mind, I'm sure me moving one earth point should be ok.

5th May 2010, 5:20 pm	#281
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: Pye v4 That 84.5 Ohm if correct indicates that probably another resistor will have overheated severely as the feed resistors are higher than that. It might be worth simply looking for burnt ones.

5th May 2010, 5:25 pm	#282
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: Pye v4 That's not a feed through capacitor is it Chris? Dave

5th May 2010, 5:28 pm	#283
Lloyd 1985 Nonode Join Date: Jun 2008 Location: Coningsby, Lincolnshire, UK. Posts: 2,820	Re: Pye v4 I think I've found C75 on your picture: what does anyone else think? Attached Thumbnails

5th May 2010, 5:30 pm	#284
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: Pye v4 I make it under the diode here:- Attached Thumbnails

5th May 2010, 5:34 pm	#285
Lloyd 1985 Nonode Join Date: Jun 2008 Location: Coningsby, Lincolnshire, UK. Posts: 2,820	Re: Pye v4 ah, yes it could be, the one I've circled is looking more like C76 8.2pf.

5th May 2010, 5:36 pm	#286
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Pye v4 I would agree with Boom. Measure resistance from the tag with the 6K8 to chassis to test C75.

5th May 2010, 5:39 pm	#287
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: Pye v4 That's interesting though Lloyd. I make your one C73 a .003 decoupler as well so Chris is looking for an identical one under the diode. Sorry, totally messed that up. The round disc one connected to pin 8 is C73 so one of those is what Chris is looking for (under the diode). Last edited by Boom; 5th May 2010 at 5:44 pm. Reason: Put right

5th May 2010, 5:48 pm	#288
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Pye v4 Tag with 6.8K resistance reads oddly, 0.386R

5th May 2010, 5:50 pm	#290
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Pye v4 Well, we have found the problem then. The solution is going to be another story! You will have to try to snip the capacitor off the tag. It's under the C76 8.2pF and has a wire running to the tag with the 6K8.

5th May 2010, 5:52 pm	#291
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Pye v4 C75 is supposed to be a 0.003 everywhere else they've been hunts. Maybe due to the RF side they used something else. I will be interested to see what Trevor has in his.

5th May 2010, 5:53 pm	#292
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Pye v4 I can see it and it was hiding under as you say and it's the round flat disc jobby. Excuse while I do a Homer Simpson run over the Durham Dales screaming.

5th May 2010, 6:00 pm	#293
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Pye v4 It is difficult to tell from the picture - there is something under there? I use a very fine pair of medical tweezers for this sort of work. You may need to remove the other connections from the tag to get at it but be careful as the coil has a wire to it. On the other hand you could just follow the homer approach and reach for the hammer.

5th May 2010, 6:09 pm	#294
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Pye v4 Phew I'm back! Ok I got it out, now I'm going to try and get a new one in. Working on those clocks over the years has finally served it purpose, lots of little bits to fiddle with I suppose on a positive note at least I know that the caps I cannot find are these types. Trevors is March 1953 this set is July 1953 we shall see if it's the same. PYE really could not have made this more difficult if they had tried Attached Thumbnails Last edited by oldticktock; 5th May 2010 at 6:16 pm.

5th May 2010, 6:12 pm	#295
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: Pye v4 Does it measure s/c Chris? I thought these were pretty reliable. Dave

5th May 2010, 6:18 pm	#296
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Pye v4 0.39R I was under the same impression and was always intending to leave them well alone and with their difficult access. I think I need a glass of your amber stuff Boom, for the next bit coming up. Hope you don't all think me stupid but can someone explain what was happening, why this caused the massive over heat on R131, I have to understand otherwise it's just a swap, move on and I learn nothing for next time. Chris Last edited by oldticktock; 5th May 2010 at 6:27 pm.

5th May 2010, 6:27 pm	#297
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: Pye v4 You can have a drop of my amber stuff when we see a picture on it. Dave

5th May 2010, 6:52 pm	#298
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Pye v4 Oh and this was the latest on screen before entering into the R131 battle, which I forgot to post Attached Thumbnails

5th May 2010, 6:54 pm	#299
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Pye v4 It would be a good idea to check the resistance at C18 again. C75 was a short circuit to earth. There is HT voltage of 200V at C102. There is a current path from C102 -> R131 -> R94 -> C75 -> chassis. R131=131 ohms, R94=150 ohms, C74=0 ohms (a short circuit). The total resistance for the current path is 131+150+0=281 ohms. Ohms law says V=IR. I=V/R, I=200/281=0.71A. Power is given by W=VI. But V=IR. So power dissipated in a resistor is W=IRI Power dissipated in the 311 ohm resistor=0.71311*0.71=157W So the resistor gets VERY hot VERY quickly and pulls down the HT rail for a bit before it melts...

5th May 2010, 7:17 pm	#300
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Pye v4 Thanks PJL thats what I was looking for. Wiring in this replacement cap is difficult but not impossible. I can get the connection to the top of the 6.8K fine the original earth point is a no go. I will just end up melting everything or having to disturb too much and the coil worries me. I think I shall just pick the nearest earth point which will be easier and hopefully will not put the circuit out as I know these circuits are designed around optimum placement. However the guy who designed this was very creative in his component placement and i'm uncertain he had optimum in mind, I'm sure me moving one earth point should be ok.