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Old 3rd Oct 2017, 5:00 pm   #21
ms660
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Default Re: Frame cramp Baird M676 help

That VA1099 is a bit thin on the ground doing an internet search...?

Lawrence.
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Old 3rd Oct 2017, 6:17 pm   #22
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Default Re: Frame cramp Baird M676 help

One bit of information that might help......is the height control roughly in the middle of its range or is it to one side, if so which side ? If the fault is in the plate load, such as the impedance going higher there (due to a faulty VDR in the yoke assy as suggested) the control would probably need to have been turned for a higher level grid drive to the pentode, which could result in the scan distortion.

Despite the fact that you have replaced the valve with a new one, this would be the type of fault seen with a low emission output valve (pentode section) too, so if you have another one, do try it too.
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Old 3rd Oct 2017, 6:43 pm   #23
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Default Re: Frame cramp Baird M676 help

I note you mention some components are new old stock. In the case of electrolytic capacitors this isn't a good idea as they age despite not being used.
I presume new components were fitted here but thought I'd mention it.

I don't know if its possible to temporarily short out the thermistor in the scan coils to rule it out?

As Argus25 says a low emission PCL85 /PCL805 can cause this problem. Have you tried them in another set if you have one?

Lovely looking set, not surprised you're keen to get it running properly.

As well as possible shorted turns in the transformer the frame side of the scan coils could be faulty but unlikely.

Cheers
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Old 3rd Oct 2017, 6:50 pm   #24
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Default Re: Frame cramp Baird M676 help

Thanks for all the suggestions everyone. I will go through it all and report back what I find. May not have time to check tonight but hoping to get back to it in the next day or so. Dave
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Old 3rd Oct 2017, 7:31 pm   #25
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Default Re: Frame cramp Baird M676 help

Here's my 2p worth. Put the height and linearity controls in the (physical) centre of their travel. See how much height you have and what the linearity is like. This may give a clue. Go through all the components in the chain R144, R145, R146, R147, R148, R150 and R151 and check that they all intact and that the values are accurate. Sometimes the carbon controls can drift dramatically. If you short R170 it will clear the thermistor.

Have you tried 'scoping the waveforms?

Good luck

TimR
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Old 3rd Oct 2017, 8:57 pm   #26
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Default Re: Frame cramp Baird M676 help

Reference to the Ultra V1781 I serviced last March, post No. 11. Bad frame linearity was traced to the series thermistor in the scanning coils.

http://www.vintage-radio.net/forum/s...ht=Ultra+V1771

Frame TB circuit is similar in some respects to the Baird 640 series.

DFWB.
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Old 4th Oct 2017, 8:08 am   #27
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Default Re: Frame cramp Baird M676 help

I was reading this with interest and was about to post when I see Colin [18] has probably posted the answer. You should be able to see the leads from the thermistor emerging from the scan coils. If you short it out this should clear your problem and give good height and linearity.

Without the thermistor the height may shrink about a half inch when the scan coils are thoroughly warmed up. [higher resistance]. Hopefully this should solve your problem. John.
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Old 4th Oct 2017, 8:32 pm   #28
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Default Re: Frame cramp Baird M676 help

Well I can't thank everyone enough for the input you have all put in. Tonight I shorted the thermister on the scanning yolk and hey presto a liner scan. I must admit I don't think I would have found that very quickly so well done to Ukol and Heater cathode short for pointing me in the right direction. Was this a common problem back in the day as I've never come across it before. There is Still some work to do as no signals on UHF and no sound. Hopefully get these cleared up overy the next week or so. I must say the picture quality is outstanding on this set and I'm looking forward to watching some vintage programmes on it. Will report with progress soon on my original thread for this set😊
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Old 4th Oct 2017, 9:05 pm   #29
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Default Re: Frame cramp Baird M676 help

Fantastic, well done!
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Old 4th Oct 2017, 10:13 pm   #30
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Default Re: Frame cramp Baird M676 help

Quote:
Originally Posted by Mazda Man View Post
..........Was this a common problem back in the day as I've never come across it before.
I don't think it was common, I never saw one go complete O/C in my working life as a TV engineer but I do remember one set with very severe frame jitter that was caused by a cracked thermistor.
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Old 5th Oct 2017, 7:12 am   #31
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Default Re: Frame cramp Baird M676 help

It wasn't exactly common but I did experience a few. The actual breakdown was caused by the connecting leads becoming detached or going high resistance from the 'element' itself. The Thorn 900/950 comes to mind.

With a complete O/C the frame collapse tended to have a kink in the straight line due to line waveform being allowed to enter the low resistance frame coils due to the O/C connection. You will only see this effect when the O/C occurs on the secondary of the output transformer, the scan coil connections or the thermistor being the usual cause. J.
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Old 7th Oct 2017, 1:32 pm   #32
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Default Re: Frame cramp Baird M676 help

Excellent news and a very interesting fault. I think I have the same component failure on my ITT/KB VC 205 chassis.
Just a further point of interest. Although I did not do much work on Baird dual standard black and white tv's the circuit of the PCL85/805 frame stage was familiar to me. I have just done some research by looking through my service manuals and found that the single standard 625 Decca black and white sets, which I did a lot of work on, uses pretty much the same circuit minus the vehicle hold control. Looks like the Baird engineers that moved to Decca after Thorn took over the Baird/Radio Rentals factory were given a free hand at Decca in the design department at Bridgnorth and not just on their famous Decca Bradford colour chassis.
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Old 5th Nov 2017, 5:39 pm   #33
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Smile Re: Frame cramp Baird M676 help

Quote:
Originally Posted by ukcol View Post
Quote:
Originally Posted by Mazda Man View Post
..........Was this a common problem back in the day as I've never come across it before.
I don't think it was common, I never saw one go complete O/C in my working life as a TV engineer but I do remember one set with very severe frame jitter that was caused by a cracked thermistor.
I remember many decades ago the Philips 1768 - the one which needed about 8 thumb screws undone to get the line trany out - had the exact same effect, frame collapse with a kink in the collapsed line!

Baird 700
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