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Old 13th Aug 2017, 9:19 pm   #1
Mazda Man
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Default Baird M676 console set

Evening All.

Just started on this Baird 24 inch console. It's the first pre Thorn Baird I have worked on. It's a good looking set and with the large screen should make a good set to watch. Mazda CRT tests good so that's encouraging. Initial power up gives HT but no heaters. 195R dropper section O?C so got to find a section to bridge it with unless anybody has one here. Will update as progress is made.

Dave
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Old 13th Aug 2017, 9:34 pm   #2
Focus Diode
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Default Re: Baird M676 console set

Wow, that's a nice looking set.
I'd imagine the CRT will be a 23" type or perhaps even 25". Not sure if RR/Baird made any 25" B&W models however.

Look forward to hearing more.

Cheers
Brian
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Old 13th Aug 2017, 9:59 pm   #3
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Default Re: Baird M676 console set

That's the only problem with old stuff you can't get the parts. I've just started on my Bush TV109 and 3 sections of dropper are gone, but your set looks a better prospect than mine so shouldn't be much of a problem to get it working well .

Cheers
Neil.
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Old 13th Aug 2017, 10:42 pm   #4
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Default Re: Baird M676 console set

You can mod the circuit for a Thorn 1400 LOPT if you get stuck.
My set is the same model no as yours, but has 2-part folding doors.
Television mag Dec 1974 & Jan 1975 covers it.

It has Mullard A59-11W 23" tube.
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Old 13th Aug 2017, 11:17 pm   #5
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Default Re: Baird M676 console set

Your photo shows the transistorised IF chassis. The dropper for this is 100+276+11+22.
A meaty 270 ohm should get you going.
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Old 14th Aug 2017, 8:47 am   #6
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Default Re: Baird M676 console set

Hi all,
Restoration73 thanks for the details on the loptx mod. I hope not to need to try it. were these sets prone to loptx failure? my set did have bi fold doors on the front but they were damaged so i removed them. By chance i found a copy of the dec 73 Tv mag and saw the article on the Baird which is what prompted me to start the renovation of this set. I just need to locate the Jan 74 one now for the rest of the write up. I should have it here somewhere.
AC/HL
Thanks for the value on the dropper. the schematic for the all valve version shows 100+195+11+22. is the one with the solid state IF panel different with a higher value section?
Dave
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Old 14th Aug 2017, 9:26 am   #7
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Default Re: Baird M676 console set

Quote:
Originally Posted by Mazda Man View Post
Hi all,
Were these sets prone to loptx failure?
Thanks for the value on the dropper. The schematic for the all valve version shows 100+195+11+22. Is the one with the solid state IF panel different with a higher value section?
Dave
The mains dropper in the two chassis versions are:

660 series d/std, valve, RR stores part number RE4033, 100+195+11+22

680 series d/std, hybrid, RR stores part number RE4036, 100+276+11+22.


The LOPTs in this series were prone to failure and three different types were eventually produced:

660/670/680 original black wax EHT o/wind type, TR4133

680 series (later type, with clear silicone potted EHT overwind), TR4140

660 series (final version, Plessey made), PLY 62/001/H3, TR4150.

There was also a mod for early versions of the Set Boost circuit. It's likely that you hybrid chassis will already have this mod, as it was introduced on sets prior to the 660 and 680 series, with almost identical line output circuitry.
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Old 14th Aug 2017, 9:50 am   #8
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Default Re: Baird M676 console set

Thanks dazzlevision that's some detailed info. I guess it's obvious on reflection that the solid state signals version would have a higher resistance section as less valve heaters to feed. Will update once I get the section bridged and the set fired up again. Mine has the pitch covered loptx so maybe not a good sign. Fingers crossed !
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Old 14th Aug 2017, 10:29 am   #9
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Default Re: Baird M676 console set

I've always liked this Baird hybrid chassis, which displays a very good picture, especially on 625. Interesting that they retained a valve VHF tuner with a Silicon transistor IF strip. I'm sure suitable transistor VHF turret tuners were available by then, as the dual standard Philips T-Vette used one (made by Philips/Mullard, of course!). Baird did use Philips valved UHF tuners in their early dual standard sets.
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Old 14th Aug 2017, 9:47 pm   #10
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Default Re: Baird M676 console set

Agreed. A lovely looking set. Very similar in style to the 700 dual standards. Even that IF strip looks very familiar!
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Old 14th Aug 2017, 10:16 pm   #11
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Default Re: Baird M676 console set

They are indeed lovely sets, I have the earlier all valve M664 which I restored several years ago. It is a very watchable set being a 23 inch CRT.

Cheers
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Old 15th Aug 2017, 3:41 pm   #12
Edward Huggins
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Default Re: Baird M676 console set

A nice Console cabinet model. But, my goodness, what a tidgy table model type speaker!
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Old 16th Aug 2017, 3:28 am   #13
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Default Re: Baird M676 console set

A missed opportunity, perhaps. With the separate lower compartment, it would have been possible to fit a bigger speaker, with a bigger magnet and with some semblance of proper acoustic loading. A further “plus” would have been a small speaker as a tweeter placed at the top of the control panel. Nothing too elaborate, but perhaps at the level represented by the better mass-produced radiograms. But of course, cost control was king – perhaps the appearance of having better sound was seen to be enough, with the promise not needing to be backed up by actuality.

Quote:
Originally Posted by dazzlevision View Post
Interesting that they retained a valve VHF tuner with a Silicon transistor IF strip. I'm sure suitable transistor VHF turret tuners were available by then, as the dual standard Philips T-Vette used one (made by Philips/Mullard, of course!). Baird did use Philips valved UHF tuners in their early dual standard sets.
I’d suspect supply-side reasons here, such as cost and inventory, and perhaps even inertia, but one cannot completely outrule performance reasons. Quite a few American TV makers retained valved VHF tuners into the very early 1970s, at least for their better models, because bipolar solid-state tuners had significantly inferior cross-modulation performance. It wasn’t until dual-gate mosfets came into use that solid-state VHF tuners were able to match or better their valved predecessors.

Poor spurious response may have been seen as more of an American problem given that in most reception areas, there were multiple VHF TV transmitters. (When I lived in the DFW area, there were 5, later 6 VHF TV stations.) But the empirical evidence says otherwise. Twice here in NZ, once in Auckland in the 1970s and again in Wellington in the 1980s, I saw serious cross-modulation with Philips TV receivers that had bipolar VHF tuners. The first was a K9, the second a later model whose designation I do not recall. In both cases there was but one Band I channel and one Band III channel, so the conditions were hardly severe. In the Auckland case, something like 18 dB of aerial signal attenuation was required to clear the problem; an older HMV monochrome valved receiver had handled the whole signal with ease. So, I’d say that whether deliberately or accidentally, Baird was wise not to have used a Philips bipolar VHF tuner, something I’d say that one should run, not walk away from. (Philips NZ were remiss not to have fitted a better tuner for NZ use, which in those days was VHF only. Curiously, Pye NZ fitted a mosfet VHF tuner to some of its models from the mid-1970s.)


Cheers,
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Old 16th Aug 2017, 12:00 pm   #14
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Default Re: Baird M676 console set

Synchrodyne wrote: "A missed opportunity, perhaps. With the separate lower compartment, it would have been possible to fit a bigger speaker, with a bigger magnet and with some semblance of proper acoustic loading. A further “plus” would have been a small speaker as a tweeter placed at the top of the control panel. Nothing too elaborate, but perhaps at the level represented by the better mass-produced radiograms. But of course, cost control was king – perhaps the appearance of having better sound was seen to be enough, with the promise not needing to be backed up by actuality".

The Baird 638 schools set employed an add-on push-pull audio amplifier unit which simply plugged into the PCL82 socket. A mains isolation transformer was a safety requirement demanded by certain education authorities so this was fitted in the special receiver. Note the BY100 silicon full wave rectifiers which are used to supply the unmodified 640 chassis.
It's possible the loudspeaker was remote from the schools receiver and in fact Baird Television did offer an extension loudspeaker isolation transformer for all 640 series models.

DFWB.
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Old 24th Aug 2017, 5:58 am   #15
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Default Re: Baird M676 console set

Perhaps it’s a pity that those options, the mains isolating transformer and the push-pull audio amplifier, were not available on the domestic version. I guess that for the “schools” version, a bigger amplifier was required given that it would often be used in rooms that were fairly large as compared with domestic living rooms.

Back in the 1950s, the Dynatron TV32 domestic receiver (405-line only) featured a mains isolating transformer and a push-pull audio output, using an ECL82 pair.

And in 1967, the Decca “Professional 23” domestic dual-standard receiver had both an isolated power supply and a superior audio section, with a push-pull transistor audio output, albeit of a mere 3 watts. The Decca also had an audio “line” output suitable for feeding into a hi-fi system, and optionally, video baseband inputs and outputs, making it a receiver/monitor. That would have made it an early example of a domestic, as distinct from professional or industrial, receiver /monitor. The earliest example of that species that I can trace is the Sony CVM-2300U (US version) of c.1965.

Cheers,
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Old 24th Aug 2017, 9:16 pm   #16
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Default Re: Baird M676 console set

Well a little progres has been made in re awaking the Baird. A 270 ohm dropper section has been wired in and a soft DY802 replaced. Switch on and we have a cramped raster and loud motor boating sound from the speaker. So at least the lopt and crt look ok. Time to check some caps in the frame stage and locate why the sound is motor boating.
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Old 24th Aug 2017, 10:28 pm   #17
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Default Re: Baird M676 console set

This Baird (MRG?) design was the set I cut my teeth on. We had dozens of them on rental. The school model had a very sustainable hardwood frame and giant castors. I also remember more frame faults than anything else. I think they were a bit fussy about the PCL85. The system switches were fine unless someone flooded them with elecrolube which they didn't seem to like. My first job was cutting the cardboard backs and fitting coin meters to them.
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Old 25th Nov 2017, 11:40 am   #18
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Default Re: Baird M676 console set

Well after a long battle the Baird is now working very well on both line standards with a lovely sharp picture. It's shows an unusually good 625 line picture for a set of this age.
The frame stage had many issues even after the thermis tor on the scan coils was linked out. For example every time the frame scan was increased to the correct amplitude it would Loose frame sync. Many resistors were found to be out of tolerance. The interlace diode was replaced etc etc. Strange audio fault due to screened cables to volume control not correctly earthed. UHF tuner problems. System switch cam problem.
It's still on its original LOPTX which is date stamped august 1968. I found a Radio Rentals mod to reduce LOPT failures by adding an extra 30pF tuning cap on the LOPT. This reduces the EHT by 2kV but has not reduced picture quality. So fingers crossed the old pitch covered LOPT will last another 50 years of occasional use. Its been well worth all the effort. Thanks to everybody that gave servicing advice.
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Old 25th Nov 2017, 1:13 pm   #19
Edward Huggins
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Default Re: Baird M676 console set

That looks vey nice. BTW, and noting earlier Postings, how does it sound now?
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Old 25th Nov 2017, 2:54 pm   #20
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Default Re: Baird M676 console set

Hi.

Do you still have the frame sync/height control fault? If you do the PCL85 cathode has two resistors connected in series. The top one has a small cermica cap tubular type across it. Check, this for a very high restance across it.

Regards, Derrick.
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