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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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3rd Oct 2017, 6:00 pm | #21 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Frame cramp Baird M676 help
That VA1099 is a bit thin on the ground doing an internet search...?
Lawrence. |
3rd Oct 2017, 7:17 pm | #22 |
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
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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. |
3rd Oct 2017, 7:43 pm | #23 |
Octode
Join Date: Jan 2006
Location: Borough of Gateshead, UK.
Posts: 1,420
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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 Brian |
3rd Oct 2017, 7:50 pm | #24 |
Triode
Join Date: Feb 2017
Location: Poole, Dorset, UK.
Posts: 44
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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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3rd Oct 2017, 8:31 pm | #25 |
Octode
Join Date: Nov 2005
Location: Ayrshire, UK.
Posts: 1,096
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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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3rd Oct 2017, 9:57 pm | #26 |
Dekatron
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
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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. |
4th Oct 2017, 9:08 am | #27 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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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. |
4th Oct 2017, 9:32 pm | #28 |
Triode
Join Date: Feb 2017
Location: Poole, Dorset, UK.
Posts: 44
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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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4th Oct 2017, 10:05 pm | #29 |
Octode
Join Date: Jan 2006
Location: Borough of Gateshead, UK.
Posts: 1,420
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Re: Frame cramp Baird M676 help
Fantastic, well done!
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4th Oct 2017, 11:13 pm | #30 |
Rest in Peace
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Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
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Re: Frame cramp Baird M676 help
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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5th Oct 2017, 8:12 am | #31 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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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. |
7th Oct 2017, 2:32 pm | #32 |
Nonode
Join Date: Jan 2004
Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area.
Posts: 2,965
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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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5th Nov 2017, 6:39 pm | #33 | |
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Re: Frame cramp Baird M676 help
Quote:
Baird 700 |
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