Quote:
Originally Posted by Maarten
Could it be a problem with the rated working voltage of the pot and/or the resistor?
Also, the rated dissipation of the pot might still be exceeded dependent on its setting as it actually varies with the setting.
Both fixed and variable resistors do have a SOA just as semiconductors.
|
Hi Maarten,
Whatever the failure mode is, it obviously isn't one that the designers foresaw. They would have looked at the component ratings supplied by the manufacturer and been happy that the 1 meg pot and 330K resistor were well within their ratings
when new. What they would not have known was how the parameters of each component would change once they had been in service for a while. I wonder how long after manufacture the problem took to appear?
While I was investigating the not-quite-enough-width problem, I found that the screen voltage on the PL504 line O/P valve was rather low at 160V. This is fed from the inevitable 2K2 wirewound resistor from the 225V HT rail. The HT rail was about right, so something was drawing too much current. The screen grid supply is decoupled by C115, another orange TCC 1MFD thing. I replaced this and the measured voltage actually went
down to 125V!
I dug out another PL504 and switched on again. I was greeted by much hissing from the tripler & eht cap and a superwide picture. I grabbed a screwdriver and wound the width pot down and all became calm again. With the width set correctly, the screen voltage was now a much more respectable 207V. So the poor old Mazda PL504, which may have been the original one, as all the other valves are Mazda, has come to the end of its life.
So things are getting better but the picture is still very 'twitchy'. The frame scan keeps contracting in the middle and if you give the frame O/P transformer a whack, it either corrects it or the scan collapses to a line momentarily. There are no obvious dry joints on the transformer or anywhere else, but I resoldered it anyway without much hope of any improvement. Sure enough there wasn't any!
I think I'll remove the transformer and examine its leadout wires where they solder to the pins. Also on the list is a good clean of the valve pins & bases and the system switch - all of which seem sensitive to being tapped.
Also the sound has yet to put in an appearance - all I've got so far is a hum from the speaker and an almighty crackling if I wiggle the 30PL1 sound O/P valve - I've not investigated this yet.
Still, it's all good fun!
Cheers
Nick