Baird M708 Dual Standard Colour Television

Looks new. ? 

Posted by: PYE625

I've finished effecting a temporary repair to the mains transformer [..]

Very nice - but should the worst happen, I'm sure Mr. Barker will lend a hand...

Posted by: Nuvistor

Looks new. ? 

Looks can be deceiving Frank.... I wish it was as new inside, but a less grand sight I dare not photograph lurks beneath !

Thanks Mike, at least it should be ok for first light. That is a little way off yet though.

So the clean-up begins, with a whiff of white spirit in the air.... This nice lump of wax will be useful for re-stuffing wax capacitors in the future.

A view of the dropper and associated components, it looks to be in better condition than some I have seen !

Oh, and the up and down thingy....aka the system switch solenoid....

The screen mask mystery continues with the discovery of more likely material from the possibly original type of surround fitted. This was found in the cabinet base and I can't think of anything else it could have come from.

Both big cans have now reformed on all 400uf sections (took pic below before I disconnected the sections) and a successful 6.3v test on the CRT heater's proves there is at least a vacuum. I'm not sure if the CRT is original as I can see no label.

Of course, those bits of mask might not even have anything to do with this set - they could be refugees from something else that was in the shop while this set awaited its new tube? ❓

No mystery and its a 100% certainty those fragments are from the sets original mask.

If you check my 718 thread or blog report, you would have seen the original bronze mask and the repair I had to make to it. I have the exact same mask on a my 25" Baird 8724, it too has  suffered the same breaks. Also Mikey405 again had the same problem with his set, see here. That set went to Tas then onto Graham (parabola/volte-face) where it resides now.

These masks used by Radio Rentals, were very brittle and were easily damaged. But the main problem lay in tube replacement. The original tubes were not push through and had a metal rounded shroud so the mask was a snug fit with little pressure placed upon it. When the tube was replaced with the newer push through type, the mask couldn't take the strain and eventually cracked.

There is also the grey (I have one fitted on my 25" 702) type which was a much more flexible and is what is fitted to Andrews after his was obviously broken either by accident or due to a tube replacement.

There is a good circuit description of the degauss circuit in the manual, worth a read when you replace the N600 thermistor.

Posted by: Nuvistor

There is a good circuit description of the degauss circuit in the manual, worth a read when you replace the N600 thermistor.

 

Hi, some of my post's were lost so just to update, thermistor N600 fell away from it's fragile solder connections and will have to be replaced. Chris will kindly be sending me an E299DH to modify the degaussing circuit with.

The paxolin panel above the LOPT has degraded in the same place as the one in Chris's set due to heat and high voltage so will have to be replaced. I ordered some glass fibre epoxy resin board for this as the insulation property is very good.

I also have refitted the transformer and PSU components. Today, I hope to progress by exploring more of the chassis, LOPT and tripler.

Apologies for the loss of posts Andrew I feel bad but it was unavoidable.

Just to recap on what I said yesterday to Andrews query about the degauss thermistor used in the 708/718. He made a query about the thermistor specs.

I had trod this same path as N600 had failed o/c on my 718 too, well only one connection had popped off unlike Andrews two. N600 is a Mullard Va1103 ( specs below) these are difficult to come by these days but there is a mod. That mod is to do away with the Va1103 and fit an E299DH (as used on the BRC 3000 and other sets) up on the tag strip on the CRT. You also have to reposition R152.

The likely cause of the N600 connections breaking away from its body are the thermals generated due to close proximity to the larger dropper. Therefore the mod makes perfect sense as it mounts the new degauss thermistor way up out the way of all that heat.

Andrew also asked if a Va1104 could be used, I too wondered this at the time on mine as I had a few in stock. Unfortunately the specs are wildly different and as the degauss has to fade at a correct rate, I didn't think it was wise to try it.

Posted by: Chris

Apologies for the loss of posts Andrew I feel bad but it was unavoidable.

Relax.... no problem ! 🙂

Here is the broken N600 and the damaged paxolin board (top pic) above the LOPT. I think that 2k7 resistor could do with being replaced too.

Looking more carefully at the chassis, it is apparent that quite a bit of work has been done with the decoder panel. It is meant to be a light tan colour, but localised heat around the valves has darkened it considerably. A track to the PFL200 base has been removed and bridged by a wire. (Timebase panel will follow shortly).

The IF panel looks relatively untouched but not sure if this is original....

Some pictures of the decoder panel. Earth straps from the chassis have been removed (and left off) and glue is helping to hold the RGB terminals!

Posted By: PYE625

The IF panel looks relatively untouched but not sure if this is original..

Yes its original.

As I'm sure you're aware its the signal input to the I.F.  The plug is what makes the 700/710 series chassis so easy to fully remove, all connections are plug N socket. 

p.s That's also the best point to feed in a signal for I.F. alignment should you find you need to.

Thanks Chris 🙂

The timebase board looks relatively ok with just a little resistor bodgery .....

If in doubt, snap it in half and just slap another across it !!   ? 

Resistors in connection with the EHT VDR have been hastily replaced too and the little strip of protective card torn away. Thank goodness for Chris's photo's of his set....they will certainly help me in correcting some issues here.

Today, the new epoxy glass fibre board arrived so this afternoon, I have been busy replacing the old paxolin LOPT panel. I used the old panel as a template and cut the new one around it. I then drilled out the holes and fitted eyelets from a tag-strip. I also replaced the boost capacitor.

The old boost cap and below you can see within the old panel where it had been arcing and burning across. A nasty carbonised resistive path has formed between the old eyelet terminals.

Andrew seeing as you're doing loads of precautionary work, you may also want to check the three high value boost line resistors, R486A/B and R480.  If these go high you will be left with an ever reducing width raster, giving similar symptoms of saturated LOPT,  when it is in fact these three gone high. Also if they have drifted high you may find L405 has been wound out to compensate leaving very little room for adjustment.  They are 2 x 2.7M an 1 x 1.8M.

Cheers Chris, a valuable tip.

I've more or less finished the pre-first light work, enough to be reasonably satisfied that nothing too nasty will happen and cause more damage. I just want to look inside the tripler, but I'm sure it is likely to be ok. Oh, and I mustn't forget the mains RF bypass cap in the control panel.