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Old 16th Aug 2019, 11:16 am   #41
David G4EBT
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Default Re: DAC90A with a faulty volume control – no surprise!

Quote:
Originally Posted by John G8MWF View Post
I am pretty sure this is the correct value - see enclosed chart.
Which confirms that C16 is 470pF, which is 0.00047 uF - not '0.047uF' as stated in posts 21 & 29.

Quote:

"C16 is most definitely a 0.047uF - 473pF."

Unquote.

0.047 uF would be 47,000pF.

Not arguing - just saying.
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Last edited by David G4EBT; 16th Aug 2019 at 11:29 am.
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Old 16th Aug 2019, 11:43 am   #42
ajgriff
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Default Re: DAC90A with a faulty volume control – no surprise!

As David says but C16 (Trader Sheet) is given as 0.05uF so code 473 is 0.047uF (47,000pF) which is fine.

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Old 16th Aug 2019, 11:58 am   #43
John G8MWF
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Default Re: DAC90A with a faulty volume control – no surprise!

I have just sent pictures of the caps in use along with a measurement.

Hopefully this one will get through.
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Old 18th Aug 2019, 11:24 am   #44
John G8MWF
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Default Re: DAC90A with a faulty volume control – no surprise!

I am still not 'happy' with my results and found this paragraph on one of the related WEB pages -

http://vintage.timspencerweb.co.uk/bush-dac90a

Modifications
After my experience with the AC11, I was expecting UL41 problems. Luckily it is OK - at the moment! I have however carried out a modification to reduce (but not eliminate) the problems due to internal shorts in these valves. There is a spare pin on the UL41 holder which in this set was used to terminate the grid stopper resistor, the grid leak resistor and a connection from the grid coupling cap. These parts no longer terminate here and instead go to the tag strip where the grid coupling cap is. This is quite easy as the leads on these parts will reach as long as you cut them as near to the spare UL41 terminal as possible.

My intention is to try this modification out if it is applicable to the version of my sets and see what, if any, difference there is.
It certainly looks like this is a well known problem - just not to me!
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Old 18th Aug 2019, 11:42 am   #45
Nuvistor
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Default Re: DAC90A with a faulty volume control – no surprise!

This thread has some mods you can think about trying. The pdf is at the bottom of the first post.
https://www.vintage-radio.net/forum/...ad.php?t=35784
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Old 18th Aug 2019, 12:03 pm   #46
John G8MWF
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Default Re: DAC90A with a faulty volume control – no surprise!

Thanks frank.

I am pretty sure that I tried those mods some years ago and didn't find that they worked too well for me.

Have you tried them and what did you think of the difference if any?
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Old 18th Aug 2019, 3:27 pm   #47
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Default Re: DAC90A with a faulty volume control – no surprise!

I have not tried them, found the reference and thought it may be useful.
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