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Old 28th Jan 2007, 1:57 am   #1
stuie319
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Default 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
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Old 28th Jan 2007, 10:11 am   #2
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Default 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.
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Old 28th Jan 2007, 2:02 pm   #3
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Default 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
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Old 28th Jan 2007, 2:08 pm   #4
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Default 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.
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Old 28th Jan 2007, 2:13 pm   #5
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Default 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
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Old 28th Jan 2007, 2:48 pm   #6
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Default 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
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Old 28th Jan 2007, 3:41 pm   #7
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Default 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
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Old 28th Jan 2007, 3:55 pm   #8
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Default 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
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Old 28th Jan 2007, 4:05 pm   #9
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Default Re: Bid for help, BRC950 chassis

Just checked against the model number its def the MK2 version


Stu
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Old 28th Jan 2007, 4:19 pm   #10
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Default Re: Bid for help, BRC950 chassis

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

Cheers,

Steve P
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Old 28th Jan 2007, 4:27 pm   #11
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Default Re: Bid for help, BRC950 chassis

Hi Steve,

Its an Ultra 6638 model

Regards Stu
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Old 28th Jan 2007, 6:32 pm   #12
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Default 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.
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Old 28th Jan 2007, 11:28 pm   #13
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Default 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
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Old 28th Jan 2007, 11:51 pm   #14
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Default 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
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