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Old 5th Jan 2021, 8:07 pm   #26
em536716
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Join Date: Dec 2017
Location: South East London
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Default Re: My newly found Marconi 707

Quote:
Originally Posted by beery View Post
Hi Ed,
R86 is 150K, 0.15M is just another way of writing it. It is a bit like the way that a 100nF decoupling capacitor is often labled 0.1uF.
The Low EHT is probably failing EHT caps, you might want to try disconnecting them to give the poor old transformer a chance. A gassey rectifier can also be a problem, but it would glow purple though.
R59, 350K is another one to check.

Ultimatley I would reccomend that you replaced the beader chain with something more trustworthy. You will find that apart from the 1/4W and 1/2W values, EMI resistors are not very good.

For the EHT smothing caps I used a couple of RS no. 384-7795

Cheers
Andy
Thanks Andy for letting me know and your suggestions. Yes I fear you're right about the caps, but I’m suspecting a tired EHT valve first. Made an error with the earth connection for my probe, but still the highest I seem to get is 1.7KV.

Disconnecting the top cap, lots of AC is there!

I don’t know if I have a spare U17, I’ll have to check my pile. It’s a little too old for my work.

Is there any reproduction or old-looking resistors which I could purchase for the bleed chain?

I would like to put a 50ma fuse in line with the EHT, just in case something does short. Dreading this transformer going up in smoke.

I want to confirm the EHT and my CRT are good before I continue with the rest of it. If my tube is tired/bad, I might as well put it back together until I can find a suitable replacement.

I’ve disconnected the smoothing caps, not much of a difference really!

Before I declare the EHT caps bad, I’ll swap the rect valve with a known good one. I have a few replacement cans here which I can use to replace them with anyway if need be.
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