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Originally Posted by SunSPARC
C52 12uF (4uF in early sets) which decouples HT4 often gets overlooked. It lurks down near the 30PL1 in the sound section. It will probably be a Callins horror.
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Ah ha! SunSparc, you are spot on - well done!
I looked more closely at the circuit last night and discovered they had put some effort into keeping 50Hz out of the line oscillator & sync DC amp valve. It's got its own HT supply, HT4 which is decoupled by C52. It was a 4mfd in my set and a rather odd orange TCC made thing with a green plastic base. I've not seen one of those before. Replacing that with a 4.7mfd got rid of almost all of the ripple on the scan. A 22mfd made things even better!
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On 1400's and early 1500's there was a must do modification on the width chain to prevent a fire.
The width control changed to 2.2Meg with the feed resistor increased to 680K.
many years ago I had a really good 1400 go up in flames because of this.
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That's interesting, Simon. I've often wondered why the original 1 meg with 330K in series would cause any problems. After all, even though it's fed from the boost HT rail of 700V give or take, the current through the resistor & preset would only be 0.526ma. That equates to a power of 0.23W in the preset & 0.091W in the 330K resistor, assuming I've done that right!
The only thing I can think of is that presets & pots in general almost always seem to go
low in value, whereas fixed resistors always go high. I wonder if the 1 meg preset gradually went lower in value causing the current through it to increase?
Anyway, my set had a 330K fitted, so I assume originally it would have had a 1 meg pot. Someone in the past had removed the 2.2 meg 'local distant' preset and fitted this in place of the width control - quite inventive I thought! That 2.2 meg has gone down to 1.5 meg!
Hopefully I can find a 2.2 meg that actually measures 2.2 meg ONO!
Thanks for the replies,
Cheers
Nick