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Sobell T192 again.
 
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Sobell T192 again.

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Doz
 Doz
(@doz)
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"Your telly's gone pop and there's no sound" shouts SWMBO from the dining room.

Damn....

It's my favourite set. The Sobell T192 fitted with wonderfully bright and sharp AW43-88 tube. Circuit can be found here http://www.technicalibrary.radios-tv.co ... =23&sort=0

I cart it into the workshop and remove the back. I extensively re-capped the set some time ago, and it's been well behaved until today.

A quick wiggle around valve holders and pop, up comes some sound, accompanied with some vision on sound to. It's V9 a PCL84. Swap a new one in, a quick tweak of the sound IF transformer, and all's well... for a minute or so. Crackle, pop and no sound....

Out with the scope and trace the audio from MR3, an OA81 Audio detector. Yep, fine here, moving along to the grid of V7b , yes good here too. Anode of V7b is good too, HT and audio in quantity. Over the other side of the chassis to The grid of the pentode bit of V9 (PCL82) and audio is fine, but sitting on a nasty 43V.. where's that coming from? C71. That's were. Replaced. Plenty of audio. But my video buzz is back.

Just about to set about aligning the IF , when the picture balloons and fades away! OK. It's going to be like this is it?!?! Turn the brightness down and the pictures back but ballooning all over the place.

Open up the line output screen and replace the DY86. Switch back on. EHT now steady, but not enough width. Pull PL81, and it's looking very brown and unpleasant. New one in and width better but still not quite enough, but back to the IF for now ....

A complete re-alignment as described in the manual didn't provide any improvement. Coils tweaked, capacitors checked, none improving the rejection of vision from audio. Removal of the OA81 , and inserting audio here proved it wasn't pickup from the frame stage or elsewhere. Re-inserting the OA81 and no sound! :bbd

I replaced the OA81 with a new one, and sound returned! A few tweaks again of the sound IF and performance was restored!

Low width failed to improve with a new PY81, but thankfully responded to the "set width" tags getting set up on the lopt.

.... now where's that VT4 .....

 
Posted : 05/04/2014 10:06 pm
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