TMB272 preservation

[djrm]

Thanks for the assistance Trevor.
The EHT diode is dimly lit, I've never noticed it before.
I can draw about 4 to 5 mm from the diode anode when I bring an insulated screwdriver near to it.

The line output drive looks ok, I think. 'scope picture attached.
0 volts is the line just below the horizontal part of the trace. I've already changed the drive coupling capacator.
I'll swap the boost capacitor in the morning. I tried one in parallel previousley but havn't removed it yet.
   

[Murphyv310]

Hi David.
4-5mm and a dull heater on the EY51 sounds like the output stage is struggling, although the drive waveform looks OK I think its a little low, I'd need to look at the diagram or check mine to be doubly sure. Not got time today but will endeavour to look tomorrow, and I've a parcel to post!!!

[djrm]

Hi Trevor, A new boost capacator has helped, I have about 3800 V now and the screen is now full height.
The cap Ive used is an old liqurice allsorts ive had for years. I'll get some new ones for it.
I see the voltage fades a bit after a while, I'll check out more parts for changed values.
David.

[Murphyv310]

Hi David.
I use the Vishay (or similar) yellow axials rated at 630 volts. They can easily be hidden in the old wax cap tubes if so desired.
The snag can be that even caps of 30 years will degrade, some are better than others to be fair.
Resistors too can change value and in the main they increase with age. Some values seem worse than others oddly, EG 27k 47k 100k 220k 470k and above. In the main valve circuitry is very forgiving and even wildly out of spec values wont make a noticeable difference. Frame stages and the grid leak resistor on the line output valve are ones that need to be within tolerance.

[djrm]

Hi, I've got some nice yellow 630v axial caps on order, but in the meantime I have found some suitable replacement parts in the spares box. The frame timebase is almost linear now. The EHT is still low and the line linearity is a bit off and the width is too much. but we have a picture just about in focus - it drifts off as it warms up, mainly I think because the EHT fades. Is there any way to center the image on these sets?  Thanks for thre loan of the video test pattern generator  Trevor, it has been very useful already. Next thing is to check more resistors in the timbase circuits then I'll see if I can get a modulator working and check the RF stages again.
   

[Terry]

The width should reduce to normal when you get the EHT voltage back up to its design level.

[Murphyv310]

Hi, I've got some nice yellow 630v axial caps on order, but in the meantime I have found some suitable replacement parts in the spares box. The frame timebase is almost linear now. The EHT is still low and the line linearity is a bit off and the width is too much. but we have a picture just about in focus - it drifts off as it warms up, mainly I think because the EHT fades. Is there any way to center the image on these sets?  Thanks for thre loan of the video test pattern generator  Trevor, it has been very useful already. Next thing is to check more resistors in the timbase circuits then I'll see if I can get a modulator working and check the RF stages again.

Hi David.
Getting there, as Terry says once the EHT is fixed the picture size will be normal.
Keep the generator David I've plenty of sources of patterns and I'm sure Frank will be pleased its of use to you.
Good luck.

[`FRANK.C]

Great progress David, apart from being oversized it looks good.

I'm sure Frank will be pleased its of use to you.

absolutely  

Frank

[djrm]

The EHT has gradually increased as I've been making improvements here and there, its now got to a point where it is high enough for the Visconel capacitor to break down. removing it from the circuit now completeley corrects the size and line linearity problems. And the brightness and focus are wonderful in comparison to how it was before.

There is still a small frame linearity problem which I think I'll try and track down next. I have an old EHT capacitor in my parts box - I wonder if it is man enough to replace the Visconel, it measures around 1nF but I cant see any voltage rating on it. I think it may be from a colour TV focus circuit. Any reccomendations for a suitable part to use for EHT smoothing?

First pictuer shows latest screenshot, secon shows capacitor I'm thinking of trying. David.

   
   

[peter scott]

Looking good!

Peter