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Old 4th May 2007, 11:00 am   #1
M0TAW Tony
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Default DAC90A; un-muting volume, faulty UBC41.

I replaced the volume control pot with a new one (470k log with dpst switch) which tests fine. But at the low setting (any lower then it switches off) the radio is not muted. Do I put an additional resistor in the slider connection or to the lower pin connection of the pot - say 50k?
Thanks
Tony

Last edited by Darren-UK; 15th Jul 2011 at 1:53 pm.
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Old 4th May 2007, 11:08 am   #2
Ian - G4JQT
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Default Re: DAC90A too loud!

Sounds obvious, but are all the joints good? With the pot turned to min, the wiper should be at earth (I think one end if the volume control goes to earth -no cct in front of me right now), so volume should be very low whatever the pot's track value.

What value should the pot be? Even if 1M, 470k shouldn't make too much difference I'd have thought.

ian
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Old 4th May 2007, 11:31 am   #3
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Default Re: DAC90A too loud!

According to the Service Sheet the volume control is 500K, so 470K is quite OK. However the "bottom" end of the control is not directly connected to the chassis. There is a 300 ohm resistor R8 in series, so I suggest checking this and its bypass capacitor.

My own DAC90A fully mutes with the volume control at minimum.
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Old 4th May 2007, 11:42 am   #4
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Default Re: DAC90A too loud!

I suppose it's possible that your new pot is a bad'un. I can see how a badly made switched pot might not reach the bottom of the track before the swith operates. Easily checked with a meter.
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Old 4th May 2007, 12:11 pm   #5
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Default Re: DAC90A too loud!

Or turn the pot to min and short the wiper to the earthy end of the track.

I'm not actually sure that pots actually reach zero though. I'll have to check.
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Old 4th May 2007, 12:56 pm   #6
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Default Re: DAC90A too loud!

Thank you all for your helpful replies
I have taken the pot out of cct and it measures 434k. This is some 66k short of the listed value and so likely to be the problem? The 330R and bypass cap measure OK.
These 470k with double pole switches seem difficult to find and this one was NOS. I think I have seen a post using a 1M pot with a parallel resistor.
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Old 4th May 2007, 2:42 pm   #7
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Default Re: DAC90A too loud!

Station X has probably got the answer. With the pot either IN or OUT of circuit, turn it fully down but not off. Then measure between the wiper and the earthy side of the pot.

Or, as Station X says, turn it down but not off and short the wiper to deck.

Change R6 and C6. Then we'll be sure they're OK. Also, the screened cable - check the screen is earthed. Also, what would happen if C15 was leaky?

The 1M pot idea - put a 1M resistor across it too.

Cheers,

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Old 4th May 2007, 3:32 pm   #8
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Default Re: DAC90A too loud!

Quote:
Originally Posted by Steve_P View Post
Change R6 and C6.
I understand R6 (2.2M) between the volume control output (via C14 (0.01µF)) and chassis, but not C6. Do you mean C14?
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Old 4th May 2007, 4:58 pm   #9
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Default Re: DAC90A too loud!

C16 is in parallel with R6 on my diagram. Thanks for that. It's the Trader Sheet on the BVWS disc that I'm going off. It's 0.05uF on my circuit. 0.047 will do if it's changed!

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Old 16th May 2007, 1:47 pm   #10
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Default Re: DAC90A too loud!

Have changed the pot although slider did go to 0 ohms. Total resistance measured 434K for a stated 470K. New one measures 530K and slides to 0R. Linking slider to earthy connection makes no difference. I had already changed the 0.05uF(0.047uF) and 330R but have changed them again to eliminate this potential source. Have replaced the screened lead although it tested OK.
But I still get appreciable audio when volume control is set at its minimum. (This control operates normally when turning it up). Audio is not distorted in any way.
Any further suggestions please
Tony
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Old 16th May 2007, 2:05 pm   #11
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Default Re: DAC90A too loud!

So if the wiper goes to earthy connection, it still plays! What about linking it straight to earth? If it still makes a noise, then that means that the audio is getting through some other way.

Have you subbed V3 ?
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Old 16th May 2007, 2:09 pm   #12
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Default Re: DAC90A too loud!

Quote:
Originally Posted by Steve_P View Post
Have you subbed V3 ?
I was just going to suggest that. You can get some very strange effects when this valve goes bad.

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Old 16th May 2007, 2:12 pm   #13
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Default Re: DAC90A too loud!

Hi Tony
Is the control wired correctly? Centre to centre of coax(yellow?) one end of pot to screen of cable, and top end of pot via (black?) wire to circuitry.
TRY shorting the co-ax incase it is mis wired at the "set" end.
This really is a mysterious one, and consequently difficult to diagnose remotely via these pages.
Have you tried shorting the control grids of the individual audio valves to chassis? Short the grid of the UL41, and see what happens( use a 1o k resistor if you are worried about shorting the wrong pin). If the sound drops appreciably or disappears then move on to the UBC41.This should help to locate the fault further.
I can't help thinking something is "floating" or not making contact with chassis properly.
Also check grid stoppers and high value resistors connected from valve grids to chassis(I think 470K in the case of the UL41) for open circuits.
Good luck!
PS. I'll have one of these on the bench at the end of the week, if you want me to take some voltage readings or try to reproduce the fault for you.
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Old 16th May 2007, 2:48 pm   #14
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Default Re: DAC90A too loud!

Thank you Steve & Paul
Update: Linked earthy contact of pot to chassis earth - no difference: audio still present at min pot setting.

I dont have another UBC41. Voltage on this valve (V3) is pin 2 (Anode) 42v (which is low against the Trader spec. Cathode (pin 7) is 0.7v (also low).

Tim, I'm the novice here but the slider on the pot isn't connected to the coax. it goes to C15 (coupler) via an unshielded wire. The two outer connections on the pot are connected to the coax and thats how I read the cct. (Its not shown on the Trader sheet but is shown on the Bush Svc Instr cct diagram. But I'm ready to wear it!

I'll see if I can get a substitute UBC41.
Tony
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Old 16th May 2007, 7:15 pm   #15
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Default Re: DAC90A too loud!

For both the cathode and the anode to be low either the HT is low or the emissions of the UF41 are down. Whats the screen grid volts on the UF41 pin 5 I think?

Sounds like leakage between the detector diode and grid of the UBC41 - try cleaning the socket.
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Old 16th May 2007, 10:12 pm   #16
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Default Re: DAC90A too loud!

I haven't had much trouble with the ceramic sockets fitted to these sets, so also check for mis wiring, shorts, solder splashes, bent/touching socket pins etc. too.There is also a chance the detector circuit (feeding the volume control) may be o/c or high resistance at the earthy end.
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Last edited by Darren-UK; 26th Nov 2007 at 1:05 am. Reason: Keeping relevant.
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Old 16th May 2007, 11:14 pm   #17
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Default Re: DAC90A too loud!

Tony. I think you should try another UBC41. I've had a similar problem before. A different radio and different valve line-up (in this case an FM set with an EABC80). It would not reduce the volume to zero but a replacement valve cured the problem. It's probably developed some kind of leak (possibly why some of your voltages around the valve are low).



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Old 17th May 2007, 11:02 am   #18
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Default Re: DAC90A too loud!

I have just bought a small Murphy U198H and noticed the similar valve line up. Then in keeping with my novice status found that the 10LD3 was a direct equivelent for the UBC41 - so ...
It worked! Volume can now be controlled all the way down to mute. Its amazing that this was the only apparent indication of a faulty UBC41.
Thanks again guys - couldn't have progressed without you!
Tony

Last edited by Darren-UK; 26th Nov 2007 at 1:06 am. Reason: Harmonising posts.
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