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Old 12th Aug 2019, 8:59 pm   #21
John G8MWF
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Default Re: DAC90A with a faulty volume control – no surprise!

I also have R6 as 330 ohm - it measures 398 ohms which is about 20% high and for this type of resistor doesn't seem abnormal to me.

C16 is 473pF (0.047) and I checked continuity between it and R6 and that both legs are earthed.

I have also placed a 331pF across C13 for increased IF bypass.
R9 measures 111K Ohm, R12 507K Ohm, R14 165 Ohm - all round 20% high but typical values.

Hmm!
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Old 14th Aug 2019, 10:01 pm   #22
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Default Re: DAC90A with a faulty volume control – no surprise!

I am still scratching my head on this one and the only thing that I have come up with is that there is a high resistance socket on the valve base?

Not my most favorite task and if there is an easier fix that I haven't used yet then I would much rather try that first and I be grateful for any other options.

Cheers

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Old 15th Aug 2019, 7:46 am   #23
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Default Re: DAC90A with a faulty volume control – no surprise!

How loud is it on low volume? I have seen many sets where there is still a clear whisper of sound with the volume turned right down.
Is it worth messing with the valve socket?

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Old 15th Aug 2019, 6:32 pm   #24
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Default Re: DAC90A with a faulty volume control – no surprise!

Loud enough to be annoying if you are on the phone.
And also a bit of ego damage on my part for not being able to find the cause!
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Old 15th Aug 2019, 6:48 pm   #25
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Default Re: DAC90A with a faulty volume control – no surprise!

What happens if you short the slider/wiper/centre contact to the 'earthy' end of the pot (the tag connected to the 330 ohm)? That should simulate turning the volume control to minimum. If the volume drops to almost nothing, it would seem that the replacement pot is not reaching a low enough resistance at the earthy end. Not sure what you can do if that is the case other than try another pot.....

If I can, I'll nip up into the workshop tonight and see how low the volume goes on mine. I can't say I've ever noticed a problem.
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Old 15th Aug 2019, 7:19 pm   #26
John G8MWF
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Default Re: DAC90A with a faulty volume control – no surprise!

Enclosed is a picture of my 'helper' who looks rather like the one in your picture!

With the POT turned to minimum I have just shorted the slider to the earth side of the pot without any change in volume.
With volume turned up the volume decreases to the 'normal' minimum when shorted.
When the POT is at minimum there is 0 ohms between slider and earth.

Just a thought - obviously the strongest stations are more noticeable and I wonder if this is an AGC related fault? i.e. the AGC is at full gain?
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Old 15th Aug 2019, 8:25 pm   #27
John G8MWF
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Default Re: DAC90A with a faulty volume control – no surprise!

FYI - just measured the DC voltage on the AGC line R4/L6 and this goes up and down nicely which changes to about -2.5V on a really strong signal - note that I am running the radio via a safety lamp so I am not getting the full power.

I have run it on full power and I still get the same problem and this is just a safety measure that I like to use when the set is out of the case.

The audio level does drop when I place a meter on the junction of L6/V2 so the AGC appears to be working.
I put this down to the meter resistance and pick up on the leads.
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Old 15th Aug 2019, 10:05 pm   #28
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Default Re: DAC90A with a faulty volume control – no surprise!

Post 21,. 'c16 473pf (0.047)' ??
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Old 15th Aug 2019, 10:25 pm   #29
John G8MWF
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Default Re: DAC90A with a faulty volume control – no surprise!

C16 is most definitely a 0.047uF - 473pF.

Not sure what you mean by Post 21?

I have included a couple of pictures of the radio which is a fairly standard cap replacement fix.
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Old 15th Aug 2019, 10:29 pm   #30
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Default Re: DAC90A with a faulty volume control – no surprise!

Well I checked mine earlier and at minimum volume there is a VERY faint sound still audible. It is hardly intelligible at more than a foot or so from the speaker. I suppose in a very quiet room it might sound like faint whispering but it's hardly disturbing and the slightest noise would mask it.
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Old 16th Aug 2019, 12:32 am   #31
John G8MWF
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Default Re: DAC90A with a faulty volume control – no surprise!

Thanks for the test.

I have another DAC90A that doesn't do the same as the one that is giving problems.

The problem appears to be with RF break through seeing as the audio input path is shorted to earth.
Maybe it is just a case of tolerances?
Or the type of replacement capacitors are somehow allowing break through?

Or perhaps as I am sitting only 2 miles away from 1413, Premier Christian Radio, Dartford Marshes, 0.5 kW and it is blasting through? But Gold Radio also breaks through but not as loud!

You have to admit it is a little bit strange and it would be nice just to identify the real cause especially as it is a very basic radio.
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Old 16th Aug 2019, 1:36 am   #32
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Default Re: DAC90A with a faulty volume control – no surprise!

My DAC90A hasn't got a viable output bottle so I can't try that, but I've just tried my DAC90 which has a similar volume control circuit, and I can't really hear any sound at zero volume, but if I tune through a strong station I can briefly hear the station as if the varying AGC DC voltage is letting the audio through.
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Old 16th Aug 2019, 8:46 am   #33
David G4EBT
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Default Re: DAC90A with a faulty volume control – no surprise!

Quote:
Originally Posted by John G8MWF View Post
C16 is most definitely a 0.047uF - 473pF.
Not sure about that - a picofarad is a millionth of a microfarad, so six decimal places different.

Hence, 0.047uF is surely 47,000 pF.

473pF would be 0.000473uF wouldn't it?
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Old 16th Aug 2019, 9:09 am   #34
John G8MWF
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Default Re: DAC90A with a faulty volume control – no surprise!

I am pretty sure this is the correct value - see enclosed chart.
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Old 16th Aug 2019, 9:11 am   #35
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Default Re: DAC90A with a faulty volume control – no surprise!

Worth verifying the values of all the replacement capacitors. If some of them are out be a factor of 100 that could induce all manner of odd behaviour.

Alan

PS The chart correctly says that 470pF is .00047uF.
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Old 16th Aug 2019, 9:45 am   #36
John G8MWF
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Default Re: DAC90A with a faulty volume control – no surprise!

Just fitted a third UBC41 and this has sorted out the problem!

I put the 'problematic' UBC41' back in another DAC 90A and that performs correctly and the minimum volume problem is NOT present on this set!!

I would really like to thank everyone for their help on this very strange 'fault' and if I will try and return the favour if I can.

Cheers

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Old 16th Aug 2019, 9:47 am   #37
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Default Re: DAC90A with a faulty volume control – no surprise!

Hi chart says 0.047 = code 473 this is not 473pf but 47 and 3 zeros pf ie 47000pf
the last number is the number of zeros, hope this helps Mick
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Old 16th Aug 2019, 10:14 am   #38
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Default Re: DAC90A with a faulty volume control – no surprise!

Ahh I was going to suggest another UBC41 but since you'd already tried another you might have thought I was going mad! I had a similar problem a few years ago with an EABC80 in an FM radio. I had to fit a brand new one to clear the problem. It must be something to do with either slight leakage in the valve or a change in characteristics affecting the grid base (cut-off) of the valve.
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Old 16th Aug 2019, 11:03 am   #39
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Default Re: DAC90A with a faulty volume control – no surprise!

Quote:
Originally Posted by mickm3for View Post
Hi chart says 0.047 = code 473 this is not 473pf but 47 and 3 zeros pf ie 47000pf
the last number is the number of zeros, hope this helps Mick
Thank you for highlighting that. At least I can now see how the confusion has arisen. Glad I wasn't the only one who was befuddled.

Alan
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Old 16th Aug 2019, 11:08 am   #40
John G8MWF
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Default Re: DAC90A with a faulty volume control – no surprise!

Even stranger that the other valves work fine in different DAC 90A radios!

However, I think that this radio had 2 problems causing this. The original POT will only go down to a minimum of around 8K ohms AND the tolerance of V3 was also a factor.
So I had changed V3 and got the same problem ergo the POT must be faulty - keep original V3 - change POT - same problem arghh!

I have also noticed that the replacement V3 has a metal skirt but it still doesn't explain why the original valve works nicely in an identical radio.
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