Sobel T143 evaluation

[Murphyv310]

Hi.
ECL80's have a common cathode between both triode and pentode, there is no direct equivalent, that said the PCL83 and 30PL1 can work if you strap pins 3 & 7 together, they often are and you may find that either will just be a drop in replacement, I'd not use them in the frequency changer though, OK for audio and frame. The cogging will be C22 having a very small leak. The DY87 is a B9A based valve so could be an awkward replacement, I'd use an EHT stick or diode which will be an easier part to fit.

[djrm]

Thanks for the tip regarding the capacitor in the sync circuit, sadly changing it hasn't helped or not much anyway. I'm going to check the video input as I'm not sure the amplifier is working at the corect input level as it is being fed with the video directly. Things are looking reasonable now anyway apart from the poor EHT regulation. It looks as if I'm going to dive into the line output compartment to check the valves if nothing else.

Isn't Frank's test card great! I'm going to see where the highest frequency grating is getting lost.

The line linearity and width controls are sitting comfortably in the mid area of their range, I have found the picture centering adjusters on the tube yolk.
There is a preset of some kind above the scan coils, I'm not sure what it is and I didnt notice any change in the picture when I turned it.

   

[Refugee]

Does that cogging run thru or is it fixed?
It could be from your pattern generator if it is not running thru.

[djrm]

Hi refugee,
I don't know what you man by 'runing through' but the problem isn't evident on my other TV when fed by the same generator into it by similar means.
Actually, I removed the diode in the other TV when I fed the signal in directley - I'll try that now.
David.

[ppppenguin]

"Running through" means moving with respect to the picture. Like a hum bar. I strongly suspect it's fixed wrt picture so it's a sync separation problem.

[pwdrive]

The cogging might be due to the line sync getting upset by the castellations, video in sync, sometimes the video lead from the amp to CRT can radiate and be picked up by the line osc stage kicking that out of kilter.

Lawrence.

[Terry]

From your picture, it is obviously related to the castellations on the left hand side or, in your case, the right hand side because you've got the scan coils upside down!



The cogging points to a sync separator clipping problem, most likely caused by C22 being leaky and overcoming the negative grid bias, although Lawrence might have a point here.

Incidentally, unless I'm  getting senile, there are two errors that I've spotted. The first is that the field sync pulses are connect directly to earth(!) and the line hold control is connected to the field output valve cathode so that, when P2 slider is at the earthy end of its travel, it shorts out the cathode bias! (It wouldn't do the pot much good, either!)

Anybody got any ideas about what is going on here?

[Murphyv310]

I'd say its still sync separator troubles. Its not the coupling cap but it could be out of tolerance resistors upsetting the DC conditions. Don't forget that feeding the generator to the video amp that the level is likely to be wrong and of course there could well be high frequency transients way above what both video amp and sync separator would normally handle.

C22 has been replaced Terry

[ppppenguin]

The whole point of the castellations is to put maximum stress on the sync separator. Several possible problems, mostly as Lawrence says, video getting through the sync separator. Old time TV service guys usually had a pretty good idea of what was causing this sort of fault. I tend to reach for the 'scope and look at the waveforms around the sync sep.

I assume this set is "live chassis" with one side of the mains conencted to the chassis. If this is the case, the correct and safe way to conenct a 'scope is to use an isolating transformer to supply mains to the set. There are other methods that are less safe (old hands may well have used them) but I won't describe them here.

[Terry]

From the 1953/54 R&T servicing book two not very good scans of the circuit diagram and component lists of the Sobell T143.

Considering the difficulty of scanning a bound book on a flat bed scanner (I wish they'd make them with the scanning area reaching right to the side so that you could hold the book flat with the other half hanging over the side!) I think you did a damned good job!

However, as I hate it when a circuit diagram has been cut in half like this one, I thought I would try to 'glue' the two halves back together again.

While I was at it, I decided to use the techniques I developed for cleaning up some scans for this thread, and tidy up the 'curvy' bits!

It's come out quite well but only because of the high quality of your original scan!

I haven't posted it here because when it's full size there doesn't seem to be any way of moving sideways but you can download it from here.