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Old 28th May 2007, 4:30 pm   #1
Focus Diode
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Default Ferguson 306T- EHT fade out

I have one of these 1957 Ferguson 17" beauties. It works well after replacing the usual capacitors and the likes but there's one problem.

All is OK on warm up. After a few minutes however the EHT starts to fade with the usual symptoms, the height increases as the width narrows, becomes defocused etc. This gets progressively worse until hardly anything can be seen on the screen.

The usual troublesome decoupling capacitor known to cause this fault has been replaced, as have the valves including the PY32 HT Rectifier. I suspect the EHT overwind on the LOPT but is there anything else I may have missed?

Cheers,
Brian
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Old 28th May 2007, 4:42 pm   #2
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Default Re: Ferguson 306T- EHT fade out

In a case like this, I check everything else first. Usually, when it's just the EHT, the picture size - height and width, both increase as the EHT fades away.

Does the screen grid of the PL81 vary as the fault occurs? If yes, try replacing V5 and C123. Then, go for C125. It might also be worth changing/checking the EY86, heater winding and R157. (This resistor is usually in the base socket.)

Also, R136, C122, C124, C126.

Cheers,

Steve P
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Old 28th May 2007, 5:40 pm   #3
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Default Re: Ferguson 306T- EHT fade out

This was a very common fault with the Ferguson 306T. The original models used an unreliable coupling capacitor to the PL81 control grid. This was .001uf but was later modified and all models were fitted with a .01uf component. The anode usually glows a dull red when the fault slowly appears.

STEVE P has probably mentioned this, but I cannot recall the actual circuit numbers.

EY86 failure, after only a short period of use, was very common with early models. The transformer supplied around 7v instead of 6.3v and Mullard produced a special valve for this set known as the TY86F [The 'F' standing for Ferguson] This had a 7.3 volt heater, if I remember correctly. My bet's on the coupling cap.
Regards John.

Last edited by Mike Phelan; 4th Mar 2008 at 8:51 am. Reason: S&P
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Old 28th May 2007, 8:59 pm   #4
Focus Diode
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Default Re: Ferguson 306T- EHT fade out

Cheers to you both! I'm sure I've changed the said coupling capacitor, but used a .001uF component. I'll have a look at it next week when I'm off work.

Thanks for the tip re the .01uF mod John, didn't know about that! As for TY86 valves I've never seen one!

Cheers,
Brian
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