Frame collapse, BRC950 chassis - UK Vintage Radio Repair and Restoration Discussion Forum

stuie319 · 28th Jan 2007, 1:57 am

Hello folks,

I'm seeking some help with a field problem on this chassis. I've got roughly an inch of scan, variable, to a modest degree by the height pot.
All the 'usual suspects' appear to have been attended to.
HT is correct, in fact, all the DC conditions around the PCL85 (known good) are spot on. Field sawtooth on pin 1 looks perfect, and I've got high amplitude waveform on pin 6 feeding the field output tx.

I'm a bit stumped here

, and casting accusing glances at the field tranny and scannies, although cold resistance checks suggest all is well. I've gone over the work done prior to my acquisition of the set, even tested replacement caps out of circuit, and it's all fine. Cosmetically this 25" model is astonishingly good; it could be brand new, with its dual rotary tuners all chromy and shiny - it needs to live!

My borrowed modulator and 405 recordings (thanks to Johnmarshman) wait like coiled springs......

Does anyone have any ideas? any suggestions will be very gratefully received,

Best regards, Stu

Heatercathodeshort · 28th Jan 2007, 10:11 am

Hello Stu,
The Thorn 950 chassis so favoured by GRANADA should be an easy one to solve. First items for lack of height are:

R102 330k feeding height control from boost H.T.
R101 The height control itself.
R127 220k boost feed to height.
C100 1uf electrolitic decoupling boost rail volts to height control.
Z1 VDR boost rail stabilizer. [Disconnect and height will fly out if faulty]
Z2 The VDR mounted inside the scan coils.[Short out to prove if faulty]. The connections for this are terminated on the scan coil tag board.

The following capacitors are generally reliable but I have had a few
go wobbly when I used to service literally hundreds of these.

C90 .03uf
C88 .01uf
C81 .02uf
C80 .01uf.

Is the scan, or rather what there is of it, linear?

The scan coils and frame output transformer are very reliable and I never replaced one. Other than checking the resistor values and the presets that were horrible, that should be a good start.

Regards John.

stuie319 · 28th Jan 2007, 2:02 pm

Hi John,
Thanks for the prompt reply!

The resistors and capacitors you mention all declare themselves innocent
when tested.
I ran the set this morning with Z1 out of circuit and it, too, is blameless.
Z2 across the primary of the field tranny looks ok as well.

Now this X2, the thermistor in the yoke assembly, I presume its there to counter the effect of the scan coils warming? I'm not altogether sure which tags it's on.
Looking at the diagram if it was to go o/c it would render the frame coils o/c as well so Ii'm wondering if it's our culprit. I get 9 ohms across the winding, but this was measured with the output tranny still in circuit. The parallel cap has been replaced by the looks of it.

Do you know where on the tagstrip this thermistor is terminated? I've attached the best pic I could get of the yoke. As for linearity I was over generous with 'an inch' of scan, it's about half that, and thus very difficult to gauge the lin.

Thanks once again,
Regards Stu

MALC SCOTT · 28th Jan 2007, 2:08 pm

The thermistor can be shorted out and the set will run o.k. I have done this many times without any problems.

Steve_P · 28th Jan 2007, 2:13 pm

Hi Stu.

Well last night when I couldn't sleep, I got the Newnes Book out and had a look.

The capacitors may look okay, but it's best to swap them out for a real test. But everything that HKS said is true of course.

Where exactly are you getting 9 ohms? The transformer secondary should be 0.1 ohms according to my circuit. (Primary is 220 ohms by the way.) The transformer itself is located on the left hand panel - the one with the controls on it. It's the middle one.

Cheers,

Steve P

stuie319 · 28th Jan 2007, 2:48 pm

Thanks for the replies chaps,

I'm measuring 9 ohms across the secondary of the field output tranny, with the deflection coil in circuit. My manual show the resistance as 260 on the primary and 8.2 ohms across the secondary.

This thermistor in the yoke is looking more and more likely, i just need to identify which pin on the tags strip it lives on to short it out and prove it one way or t'other.

As regarding caps, the electrolytic ones have been replaced, the low value ones i've measured out of circuit for DC leakage and for capacitance value, ideally yes they should be swapped out, but i think my problem lies elsewhere since all the DC conditions around v8 are as should be.......

Regards Stu

Steve_P · 28th Jan 2007, 3:41 pm

It's a 950 Mk1 not a Mk2 isn't it. Pauses to bang head on desk....

I think you've got to take the base off to get to this part. Before you do this, can anyone else verify this, as I can see no other way.

Steve P

stuie319 · 28th Jan 2007, 3:55 pm

Hi Steve,

hmmmmmm, the manual I have is a generic Thorn one for the MKII, it's not model specific. If it's any help the set has the pentode flywheel sync board fitted, and the frame tranny is on the chassis left hand panel, not on the customer control assembly

Regards Stu

stuie319 · 28th Jan 2007, 4:05 pm

Just checked against the model number its def the MK2 version

Stu

Steve_P · 28th Jan 2007, 4:19 pm

What is the make and model no. of the set?
I'll check it.

Cheers,

Steve P

stuie319 · 28th Jan 2007, 4:27 pm

Hi Steve,

Its an Ultra 6638 model

Regards Stu

Heatercathodeshort · 28th Jan 2007, 6:32 pm

Hi Stu again,
Oh boy; it's been a few years since I did one of these.

I think you will find that if you have a careful look at the frame connections on the coils, you will see one tag that is not connected with the windings, and is connected to the FOT. This will be one end of the thermistor.
It should be easy to trace where the other end comes out on the scan coil board.

The 950 Mk1 had a half wave eht 'tray' [Red plastic jellypot]

The Mk2 had a tripler.[Grey or white jellypot]

They are virtually the same chassis other than this.

Regards John.

stuie319 · 28th Jan 2007, 11:28 pm

Gents,

Its huge thank you time

I had a closer look at the yoke as John suggested, and there it was,
the thermistor wasnt buried in the coil assembly but clipped next to it.

There wasnt any need to measure it, it had fallen apart! We have frame scan

.
I wasnt too far away in thinking scan coils....but the answer ultimately was found on here. Once again thanks ever so much for all the responses and sound (and correct) advice!

Regards, Stu

Steve_P · 28th Jan 2007, 11:51 pm

I'm glad it's sorted, saves me taking mine apart for a look. By the way, it's a Mk1 chassis version.

Never had that one myself!

Cheers,

Steve P

28th Jan 2007, 1:57 am	#1
stuie319 Pentode Join Date: Jan 2007 Location: Stockport, Greater Manchester, UK. Posts: 229	Frame collapse, BRC950 chassis Hello folks, I'm seeking some help with a field problem on this chassis. I've got roughly an inch of scan, variable, to a modest degree by the height pot. All the 'usual suspects' appear to have been attended to. HT is correct, in fact, all the DC conditions around the PCL85 (known good) are spot on. Field sawtooth on pin 1 looks perfect, and I've got high amplitude waveform on pin 6 feeding the field output tx. I'm a bit stumped here , and casting accusing glances at the field tranny and scannies, although cold resistance checks suggest all is well. I've gone over the work done prior to my acquisition of the set, even tested replacement caps out of circuit, and it's all fine. Cosmetically this 25" model is astonishingly good; it could be brand new, with its dual rotary tuners all chromy and shiny - it needs to live! My borrowed modulator and 405 recordings (thanks to Johnmarshman) wait like coiled springs...... Does anyone have any ideas? any suggestions will be very gratefully received, Best regards, Stu Last edited by Mike Phelan; 25th Oct 2007 at 4:46 pm. Reason: Slight editing for lack of white space

28th Jan 2007, 10:11 am	#2
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Bid for help, BRC950 chassis Hello Stu, The Thorn 950 chassis so favoured by GRANADA should be an easy one to solve. First items for lack of height are: R102 330k feeding height control from boost H.T. R101 The height control itself. R127 220k boost feed to height. C100 1uf electrolitic decoupling boost rail volts to height control. Z1 VDR boost rail stabilizer. [Disconnect and height will fly out if faulty] Z2 The VDR mounted inside the scan coils.[Short out to prove if faulty]. The connections for this are terminated on the scan coil tag board. The following capacitors are generally reliable but I have had a few go wobbly when I used to service literally hundreds of these. C90 .03uf C88 .01uf C81 .02uf C80 .01uf. Is the scan, or rather what there is of it, linear? The scan coils and frame output transformer are very reliable and I never replaced one. Other than checking the resistor values and the presets that were horrible, that should be a good start. Regards John.

28th Jan 2007, 2:02 pm	#3
stuie319 Pentode Join Date: Jan 2007 Location: Stockport, Greater Manchester, UK. Posts: 229	Re: Bid for help, BRC950 chassis Hi John, Thanks for the prompt reply! The resistors and capacitors you mention all declare themselves innocent when tested. I ran the set this morning with Z1 out of circuit and it, too, is blameless. Z2 across the primary of the field tranny looks ok as well. Now this X2, the thermistor in the yoke assembly, I presume its there to counter the effect of the scan coils warming? I'm not altogether sure which tags it's on. Looking at the diagram if it was to go o/c it would render the frame coils o/c as well so Ii'm wondering if it's our culprit. I get 9 ohms across the winding, but this was measured with the output tranny still in circuit. The parallel cap has been replaced by the looks of it. Do you know where on the tagstrip this thermistor is terminated? I've attached the best pic I could get of the yoke. As for linearity I was over generous with 'an inch' of scan, it's about half that, and thus very difficult to gauge the lin. Thanks once again, Regards Stu Attached Thumbnails

28th Jan 2007, 2:08 pm	#4
MALC SCOTT Octode Join Date: Aug 2006 Location: Willington, County Durham, UK. Posts: 1,499	Re: Bid for help, BRC950 chassis The thermistor can be shorted out and the set will run o.k. I have done this many times without any problems. __________________ Malc Scott

28th Jan 2007, 2:13 pm	#5
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Bid for help, BRC950 chassis Hi Stu. Well last night when I couldn't sleep, I got the Newnes Book out and had a look. The capacitors may look okay, but it's best to swap them out for a real test. But everything that HKS said is true of course. Where exactly are you getting 9 ohms? The transformer secondary should be 0.1 ohms according to my circuit. (Primary is 220 ohms by the way.) The transformer itself is located on the left hand panel - the one with the controls on it. It's the middle one. 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...?

28th Jan 2007, 2:48 pm	#6
stuie319 Pentode Join Date: Jan 2007 Location: Stockport, Greater Manchester, UK. Posts: 229	Re: Bid for help, BRC950 chassis Thanks for the replies chaps, I'm measuring 9 ohms across the secondary of the field output tranny, with the deflection coil in circuit. My manual show the resistance as 260 on the primary and 8.2 ohms across the secondary. This thermistor in the yoke is looking more and more likely, i just need to identify which pin on the tags strip it lives on to short it out and prove it one way or t'other. As regarding caps, the electrolytic ones have been replaced, the low value ones i've measured out of circuit for DC leakage and for capacitance value, ideally yes they should be swapped out, but i think my problem lies elsewhere since all the DC conditions around v8 are as should be....... Regards Stu

28th Jan 2007, 3:41 pm	#7
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Bid for help, BRC950 chassis It's a 950 Mk1 not a Mk2 isn't it. Pauses to bang head on desk.... I think you've got to take the base off to get to this part. Before you do this, can anyone else verify this, as I can see no other way. Steve P __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

28th Jan 2007, 3:55 pm	#8
stuie319 Pentode Join Date: Jan 2007 Location: Stockport, Greater Manchester, UK. Posts: 229	Re: Bid for help, BRC950 chassis Hi Steve, hmmmmmm, the manual I have is a generic Thorn one for the MKII, it's not model specific. If it's any help the set has the pentode flywheel sync board fitted, and the frame tranny is on the chassis left hand panel, not on the customer control assembly Regards Stu

28th Jan 2007, 4:05 pm	#9
stuie319 Pentode Join Date: Jan 2007 Location: Stockport, Greater Manchester, UK. Posts: 229	Re: Bid for help, BRC950 chassis Just checked against the model number its def the MK2 version Stu

28th Jan 2007, 4:19 pm	#10
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Bid for help, BRC950 chassis What is the make and model no. of the set? I'll check it. 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...?

28th Jan 2007, 4:27 pm	#11
stuie319 Pentode Join Date: Jan 2007 Location: Stockport, Greater Manchester, UK. Posts: 229	Re: Bid for help, BRC950 chassis Hi Steve, Its an Ultra 6638 model Regards Stu

28th Jan 2007, 6:32 pm	#12
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Bid for help, BRC950 chassis Hi Stu again, Oh boy; it's been a few years since I did one of these. I think you will find that if you have a careful look at the frame connections on the coils, you will see one tag that is not connected with the windings, and is connected to the FOT. This will be one end of the thermistor. It should be easy to trace where the other end comes out on the scan coil board. The 950 Mk1 had a half wave eht 'tray' [Red plastic jellypot] The Mk2 had a tripler.[Grey or white jellypot] They are virtually the same chassis other than this. Regards John.

28th Jan 2007, 11:28 pm	#13
stuie319 Pentode Join Date: Jan 2007 Location: Stockport, Greater Manchester, UK. Posts: 229	Re: Bid for help, BRC950 chassis Gents, Its huge thank you time I had a closer look at the yoke as John suggested, and there it was, the thermistor wasnt buried in the coil assembly but clipped next to it. There wasnt any need to measure it, it had fallen apart! We have frame scan . I wasnt too far away in thinking scan coils....but the answer ultimately was found on here. Once again thanks ever so much for all the responses and sound (and correct) advice! Regards, Stu Attached Thumbnails

28th Jan 2007, 11:51 pm	#14
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Bid for help, BRC950 chassis I'm glad it's sorted, saves me taking mine apart for a look. By the way, it's a Mk1 chassis version. Never had that one myself! 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...?