Page 1 of 1

RBM A640 Transformer tussles

PostPosted: Sat May 21, 2016 12:01 pm
by Colourstar
This Murphy V2015D has been mentioned before. To recap, it was extricated from a friend's garage as he wanted rid of it. The case is tatty but the crt is a corker. Unfortunately it showed the classic problem of a raster that after a few minutes use grew dimmer and 'blew out'.

A prime candidate then for the now tried-and-tested method of overwind revival. This was duly done with the inner core being wound with as many layers of Kapton tape as possible. On reassembly I was greeted with a nice bright picture of ample width, which looked set to hold steady. Alas, it was not to be. After a few minutes there was a nasty internal arcing noise from the LOPT and the raster disappeared. No physical signs of distress could be seen around the LOPT and the overwind was cool to the touch.

Once the set has cooled, the whole performance is repeated. Nice bright raster, then after a short time we're in trouble again. A close monitoring of events reveals sounds of faint fizzing within the LOPT after a few minutes. This grows a little more intense as series of white dashes dance in a vertical line on screen about a quarter of the way in on the left of the picture. The comes the sudden breakdown with the raster vanishing as the LOPT squeals and crackles in protest. There is a distinct smell of burning, so something really isn't happy.

There's nothing for it but to whip the LOPT out again and pull it apart to see what may be happening. Meanwhile any thoughts would be welcome!

Steve

Re: RBM A640 Transformer tussles

PostPosted: Sat May 21, 2016 12:52 pm
by Focus Diode
May be worth removing the overwind then re-fitting the Lopt. If the arcing persists we'll know the primary will be at fault. Out of interest does the sound remain when the raster collapses?

Cheers
Brian

Re: RBM A640 Transformer tussles

PostPosted: Sat May 21, 2016 2:16 pm
by Colourstar
Hi Brian

Yes the sound goes when the raster disappears. I know the sound and vision stages are dependent upon the LOPT as they derive their supplies from it.

I left the set for ten minutes then tried it with the top cap off the DY802. As soon as the line whistle started up, the LOPT began squealing badly.

Steve

Re: RBM A640 Transformer tussles

PostPosted: Sat May 21, 2016 2:53 pm
by Focus Diode
Hi Steve, The overwind will have to be removed from the core as it will damp down the rest of the Lopt even when disconnected. This was the the case with Till's Ekco CT104 of course.

It sounds to me that the insulation is breaking down on the primary unfortunately. It would do no harm to check the condition of the line drive coupling capacitor and the negative volts at the control grid of the PL504.

Cheers
Brian

Re: RBM A640 Transformer tussles

PostPosted: Sun May 22, 2016 8:54 pm
by davidh1041
Hi Steve, i was in Norwich this weekend and searched for a LOPT for you, the only one i could find was for the early version of the 141 series that has a lower EHT. I will have another look for you next week. If it runs ok with the overwind removed you could use a tripler.
Cheers David

Re: RBM A640 Transformer tussles

PostPosted: Tue May 24, 2016 10:55 pm
by Colourstar
Hi David, that's very kind of you. Thanks for looking.

Hopefully I may be able to get back to the set again this coming weekend when I have some spare time.

Steve