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Old 19th Jan 2022, 8:08 pm   #1
6SN7WGTB
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Default Bush DAC90A - volume pot way out of spec

I see my volume pot is more like 200k than 500k.

Given the volume appears to be very acceptable even at low settings, and that the pot (including a DPDT 250V switch) is hard to find, I was minded to simply put a resistor in series with the top leg of the track.

Probably 150k or so.

Anyone see a problem with that?
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Old 19th Jan 2022, 8:17 pm   #2
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Default Re: Bush DAC90A - volume pot way out of spec

It will reduce the gain of the AF stages, but that may not matter.

If it sounds OK as it is, I would leave it alone.
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Old 19th Jan 2022, 8:19 pm   #3
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Default Re: Bush DAC90A - volume pot way out of spec

It will reduce your available volume quite a bit.
If you leave ir as it is it will do no harm.

It might be worth measuring again with it out of circuit in case there are any parallel paths

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Old 19th Jan 2022, 8:29 pm   #4
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Default Re: Bush DAC90A - volume pot way out of spec

Thanks chaps.

The 'available volume' is probably excessive, hence my temptation to put something in series.

As you might well expect, down the lower end of the control I bump into the switch mechanism a bit too quickly also.
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Old 19th Jan 2022, 8:54 pm   #5
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Default Re: Bush DAC90A - volume pot way out of spec

Quote:
It will reduce your available volume quite a bit.
By about 7dB, not much and most radios had too much gain for comfortable listening at full. All specmanship. I find most of my radios have enough volume at 20% or so wasting 80% of the pot. My Hacker has a resistor added by me in series with the pot to make 50% about right.
 
Old 20th Jan 2022, 1:46 pm   #6
Ian - G4JQT
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Default Re: Bush DAC90A - volume pot way out of spec

I can't see how the volume pot can reduce in value. Something weird is going on unless the 500k pot has been changed, or the wrong track put in during manufacture - something I've never heard of.

I'd disconnect two of the connections and check again resistance if you haven't done this already. Maybe the cap across it is very leaky, although at 100pF that too seems unlikely.

Or maybe it's got covered in conductive dirt, but if that were the case I'd expect other problems elsewhere.

Last edited by Ian - G4JQT; 20th Jan 2022 at 1:59 pm.
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Old 20th Jan 2022, 2:21 pm   #7
6SN7WGTB
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Default Re: Bush DAC90A - volume pot way out of spec

It does say 500k LOG on it and appears original.

It's been cleaned.

I will double-check in case of unexpected leakage, but I assumed same re the bypass.

My suggestion as to why is: these radios appeared to be quite loud and so I imagine the pot has spent 70 years scrubbing around the low end, which will have thinned the carbon track and therefore 'lost' resistance where it had most.
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Old 20th Jan 2022, 2:52 pm   #8
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Default Re: Bush DAC90A - volume pot way out of spec

I have found very large variations in the measured resistance of some NOS vintage potentiometers when compared to the markings stamped onto the housings.

I have read that distortion is increased if the load on the standard diode demodulator circuit is reduced.

I would prefer to add a resistor to the potentiometer if there is plenty of volume available.

If a potentiometer is worn and conductive material has been removed from the track, the end to end resistance of the potentiometer should increase.
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Old 20th Jan 2022, 6:23 pm   #9
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Default Re: Bush DAC90A - volume pot way out of spec

It is something that is often overlooked as a wide variation in resistance appears to have little effect and does no harm. I have had 500K pots read over 2M mostly in TV receivers. If it works ok best leave it alone. Think of Pandora's Box. J.
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Old 20th Jan 2022, 6:37 pm   #10
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Default Re: Bush DAC90A - volume pot way out of spec

From Radiomuseum: A common Problem is: Volume doesn't go to zero on a strong signal if Potentiometer is at the CCW stop. Remedy: a 330 picofarad capacitor between Grid and Kathode of the UBC41. Short leads are important, and perhaps replacement of the big 100k resistor to the grid by a smaller sized one of the same value.
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Old 20th Jan 2022, 8:49 pm   #11
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Default Re: Bush DAC90A - volume pot way out of spec

Quote:
Originally Posted by 6SN7WGTB View Post
I imagine the pot has spent 70 years scrubbing around the low end, which will have thinned the carbon track and therefore 'lost' resistance where it had most.
Possibly... And in that case, there'll be even more of the control range crammed into the first quarter-turn!
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Old 22nd Jan 2022, 11:49 pm   #12
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Default Re: Bush DAC90A - volume pot way out of spec

It seems unusual for the track to lose resistance, it usually goes the other way.
A relatively cheap way to temporarily fix things is with a HB pencil on the track, a minute of scribbling can bring things back to where it should be but this only applies to a pot that's got crackly low end.
I usually test things with the valves removed, never had major issues doing meter checks this way.
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Old 23rd Jan 2022, 9:57 am   #13
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Default Re: Bush DAC90A - volume pot way out of spec

Yes, was incorrect in my thinking re track changing resistance.

I have measured it fully out of circuit and sure enough 200K. 150K now in top leg and seems to work fine.
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