BRC 950/960 frame hold problems - UK Vintage Radio Repair and Restoration Discussion Forum

Focus Diode · 17th Mar 2014, 3:49 pm

Well, since the "Success Stories" the set is proving to be unreliable in use. No doubt the high concentration of heat within the small cabinet won't help. The latest problem is tearing my hair out, any suggestions would be most welcome.

Basically, the frame hold is one ended. 405-lines won't lock at all unless the height is reduced. A new PCL805 only gave marginal improvement. I've so far replaced R106 (2M2) and R108 (3M9), both had risen in value by some 2M. C83 (500pF 5KV) broke away on contact. All I had was a 600pF 5KV type so it went in. Result? Absolutely no difference.....

The height control finally succumbed, as I'd intended to replace it anyway a new one was adapted into the original rectangular frame. There was a bang with much smoke emerging from the set. It turned out, confession time I'd accidentally shorted the wiper of the new height control to chassis

. So far I've been unable to find the capacitor in question. On removing the short I switched on. A new fault had developed- flyback lines more evident on high brightness and low contrast. No doubt this occured due to the s/c height control.

Any ideas?
Cheers,
Brian

Heatercathodeshort · 17th Mar 2014, 7:36 pm

Hello Brian,
Probably all down to a common cause. I think the problem is around the PFL200, video output valve and sync separator. Try a known working one. It will be R98 390K going high that will cause one ended frame hold but I don't think this is the case with your pictures.
Check C41 screen grid decoupler [Thorn circuit] 4uf, and C35 300uf cathode bypass electrolytic. Bridge it with anything near as a test. The frame flyback suppression components could be faulty but not likely. Just first moves but it's going to be a simple one. John.

ParcGwyn · 17th Mar 2014, 8:33 pm

Hi Brian,
Has your set got the diode dropper in the heater chain? If it has this may well have gone short. I had a set in the '70s and as 14 year old it took me ages to find it.

Dave
GW7ONS

Focus Diode · 17th Mar 2014, 9:20 pm

Many thanks for your advice John and Dave. I got back to the set earlier today before logging on. Thankfully I've sorted out both problems.

Two 330K resistors R102 and R107 under the PCL85 were replaced. One was just out of tolerance, the other read 1M5. This didn't cure the rolling frame though this did result in a nice flyback line free raster/picture.

R98, which John mentioned, was then replaced which thankfully sorted out the rolling frame. Now rock steady on both systems.

As for the bang and smoke this morning, this remains a mystery. All seems to be working well now.

Cheers and many thanks again,
Brian

Focus Diode · 19th Mar 2014, 4:26 pm

Two good runs shows the frame problems are thankfully at an end. Would appear these dark brown rough textured 10% and 20% tolerance resistors are prone to changing value.

The intermittent line shimmer is getting worse. I must get round to fitting those H/V diodes in the EHT tray, not applicable to this thread of course.

Cheers,
Brian

Focus Diode · 9th Dec 2014, 4:08 pm

Just when you're not in the mood for a service session more frame timebase problems emerged this afternoon. The height was varying badly and was very tap happy. It then collapsed completely, moving the PCL805 in its holder restored scan.

Naturally a new PCL805 made no difference to the conditions. A poor connection was suspected, nice and easy one would think.....Only that three wires and the height stabilising VDR were found to have broken from their tags when access was gained underneath!

Despite being wired the circuit is identical to the Thorn 950 chassis. It was established the red lead came from R113 120K, so goes to one end of the Frame Lin main control. The black and white lead is the boost feed to the height control and the orange lead from the opposite end to the height control to R102 330K. The VDR was connected across boost feed and chassis.

Thankfuly this put things right with the intermittent problems also cured. I can only think some of the wiring became loose, breaking free when pressure was applied. The fault was probably my own making after replacing some components and the height control earlier in the year.

A nice BBC2 PM5544 appeared on 625-lines UHF. Lovely display with the coloured information glowing!

Cheers,
Brian

FERNSEH · 9th Dec 2014, 4:29 pm

Hi Brian,
these 950 series do display excellent interlace and somehow they managed to do this without any complex frame sync pulse shaping circuits.
The frame sync pulse is fed to the oscillator though a simple differentiator network rather than the usual R C integrator.

DFWB.

Heatercathodeshort · 9th Dec 2014, 5:33 pm

I have never seen poor interlace on any Thorn model. They are very hard to beat at any price. John.

17th Mar 2014, 3:49 pm	#1
Focus Diode Octode Join Date: Jan 2006 Location: Borough of Gateshead, UK. Posts: 1,420	BRC 950/960 frame hold problems Well, since the "Success Stories" the set is proving to be unreliable in use. No doubt the high concentration of heat within the small cabinet won't help. The latest problem is tearing my hair out, any suggestions would be most welcome. Basically, the frame hold is one ended. 405-lines won't lock at all unless the height is reduced. A new PCL805 only gave marginal improvement. I've so far replaced R106 (2M2) and R108 (3M9), both had risen in value by some 2M. C83 (500pF 5KV) broke away on contact. All I had was a 600pF 5KV type so it went in. Result? Absolutely no difference..... The height control finally succumbed, as I'd intended to replace it anyway a new one was adapted into the original rectangular frame. There was a bang with much smoke emerging from the set. It turned out, confession time I'd accidentally shorted the wiper of the new height control to chassis . So far I've been unable to find the capacitor in question. On removing the short I switched on. A new fault had developed- flyback lines more evident on high brightness and low contrast. No doubt this occured due to the s/c height control. Any ideas? Cheers, Brian Attached Thumbnails

17th Mar 2014, 7:36 pm	#2
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: BRC 950/960 frame hold problems Hello Brian, Probably all down to a common cause. I think the problem is around the PFL200, video output valve and sync separator. Try a known working one. It will be R98 390K going high that will cause one ended frame hold but I don't think this is the case with your pictures. Check C41 screen grid decoupler [Thorn circuit] 4uf, and C35 300uf cathode bypass electrolytic. Bridge it with anything near as a test. The frame flyback suppression components could be faulty but not likely. Just first moves but it's going to be a simple one. John. Last edited by Heatercathodeshort; 17th Mar 2014 at 7:44 pm.

17th Mar 2014, 8:33 pm	#3
ParcGwyn Hexode Join Date: Jun 2005 Location: Aberystwyth, Wales, UK. Posts: 358	Re: BRC 950/960 frame hold problems Hi Brian, Has your set got the diode dropper in the heater chain? If it has this may well have gone short. I had a set in the '70s and as 14 year old it took me ages to find it. Dave GW7ONS

17th Mar 2014, 9:20 pm	#4
Focus Diode Octode Join Date: Jan 2006 Location: Borough of Gateshead, UK. Posts: 1,420	Re: BRC 950/960 frame hold problems Many thanks for your advice John and Dave. I got back to the set earlier today before logging on. Thankfully I've sorted out both problems. Two 330K resistors R102 and R107 under the PCL85 were replaced. One was just out of tolerance, the other read 1M5. This didn't cure the rolling frame though this did result in a nice flyback line free raster/picture. R98, which John mentioned, was then replaced which thankfully sorted out the rolling frame. Now rock steady on both systems. As for the bang and smoke this morning, this remains a mystery. All seems to be working well now. Cheers and many thanks again, Brian Last edited by Focus Diode; 17th Mar 2014 at 9:26 pm.

19th Mar 2014, 4:26 pm	#5
Focus Diode Octode Join Date: Jan 2006 Location: Borough of Gateshead, UK. Posts: 1,420	Re: BRC 950/960 frame hold problems Two good runs shows the frame problems are thankfully at an end. Would appear these dark brown rough textured 10% and 20% tolerance resistors are prone to changing value. The intermittent line shimmer is getting worse. I must get round to fitting those H/V diodes in the EHT tray, not applicable to this thread of course. Cheers, Brian Attached Thumbnails

9th Dec 2014, 4:08 pm	#6
Focus Diode Octode Join Date: Jan 2006 Location: Borough of Gateshead, UK. Posts: 1,420	Re: BRC 950/960 frame hold problems Just when you're not in the mood for a service session more frame timebase problems emerged this afternoon. The height was varying badly and was very tap happy. It then collapsed completely, moving the PCL805 in its holder restored scan. Naturally a new PCL805 made no difference to the conditions. A poor connection was suspected, nice and easy one would think.....Only that three wires and the height stabilising VDR were found to have broken from their tags when access was gained underneath! Despite being wired the circuit is identical to the Thorn 950 chassis. It was established the red lead came from R113 120K, so goes to one end of the Frame Lin main control. The black and white lead is the boost feed to the height control and the orange lead from the opposite end to the height control to R102 330K. The VDR was connected across boost feed and chassis. Thankfuly this put things right with the intermittent problems also cured. I can only think some of the wiring became loose, breaking free when pressure was applied. The fault was probably my own making after replacing some components and the height control earlier in the year. A nice BBC2 PM5544 appeared on 625-lines UHF. Lovely display with the coloured information glowing! Cheers, Brian Attached Thumbnails Last edited by Focus Diode; 9th Dec 2014 at 4:18 pm.

9th Dec 2014, 4:29 pm	#7
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: BRC 950/960 frame hold problems Hi Brian, these 950 series do display excellent interlace and somehow they managed to do this without any complex frame sync pulse shaping circuits. The frame sync pulse is fed to the oscillator though a simple differentiator network rather than the usual R C integrator. DFWB. Last edited by FERNSEH; 9th Dec 2014 at 4:50 pm.

9th Dec 2014, 5:33 pm	#8
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: BRC 950/960 frame hold problems I have never seen poor interlace on any Thorn model. They are very hard to beat at any price. John.