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Old 28th Aug 2006, 10:18 pm   #1
tivo
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Default DAC90A - no low volume

I have a DAC90A that plays well but I can't turn the volume right down. The lowest setting is still a reasonable listening volume.

Some components have already been changed on this receiver by a previous owner - C20 and C21 (HT smoothing), C15 and C18 (audio coupling) all with high quality modern replacements.

A couple of unusual changes include R18 (scale lamp shunt - should be 75ohms) had been replaced with 250ohms (coincidentally the same value as R16). Side effect of this was a blown scale lamp (3.5v item) - I have replaced both lamps with 12v 100ma items and these now appear to function OK.

C17 had been replaced with 0.0003uf (should be 0.003uf) - this is tone corrector so I guess the sound is a bit "brighter" than it was designed to be.

C16 is original but now 0.14uf (should be 0.05uf) - I don't believe a capacitance increase here could cause the problem (this was measured in circuit with one end of R6 disconnected, close to 330ohms, so the increase could be explained by wiring etc.)

R7 volume control reads OK the full length of it's track so the problem is not here.

I have now measured pretty much all I can without serious desoldering.

Has anyone experienced this fault? Any suggestions as to what might be causing this?

Last edited by Darren-UK; 15th Jul 2011 at 2:04 pm.
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Old 28th Aug 2006, 10:26 pm   #2
stephanie
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Default Re: DAC90A - no low volume

Are the earth connections all fine?

Try subbing the volume pot....just for kicks....that's usually what I do first. I've had to replace lots of bum pots.
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Old 28th Aug 2006, 10:31 pm   #3
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Default Re: DAC90A - no low volume

Have you checked the UF41 cathode resistor, to which the bottom end of the volume control is connected?

I've also known a bad UBC41 have odd effects on volume control operation, though not in a DAC90A.

Paul
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Old 28th Aug 2006, 10:38 pm   #4
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Default Re: DAC90A - no low volume

UF41 cathode resistor (R6) was 336 ohms, just 6 ohms over what it should be. Not bad for what looks like the orginal but, unfortunately, not the problem here.
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Old 28th Aug 2006, 10:40 pm   #5
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Default Re: DAC90A - no low volume

The subject of the value of R18 comes up again and again. The set I restored recently had a 75 ohm resistor fitted and it was obviously original. As I understand it the Service Sheet wrongly shows the value as 250 ohms.

The correct wattage 3.5V bulbs are difficult to obtain, but sometimes crop up on eBay. They are very dim.

As it happens my set suffered from just the fault you mention, ie can't turn down the volume completely. I cured it by changing one of the valves. I can't remember which, but it wasn't the output valve.

All the valves in my set had/have inter-electrode shorts, but the only fault this gave rise to was the volume control problem.
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Old 28th Aug 2006, 11:38 pm   #6
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Default Re: DAC90A - no low volume

Hi Tivo, I would be inclined to change C16 irrespective. Also suspect the UBC41 and check by substitution if poss. Andy
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Old 29th Aug 2006, 5:45 pm   #7
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Default Re: DAC90A - no low volume

Have you tried short circuiting the wiper, and the earthy end of the volume control ?

It may be that the wiper is not quite reaching the end of the track.

I have had this in quite a few sets. It doesn't need much resistance here to prevent the volume from reaching zero.

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Old 30th Aug 2006, 11:00 pm   #8
tivo
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Default Re: DAC90A - no low volume

I've now replaced C16, no difference.

Earth connections all apear OK.

I've re-checked the pot on a meter and wiper to each end goes all the way down to zero.

The lowest volume I get is actually pretty loud - probably about where I would set it for normal listening! (I would guess it runs at around 1/3rd volume as a minimum).

I have located and ordered a full NOS valve lineup so I should be able to check these by substitution shortly although the existing UBC41 and UF41 look new to me. However, with valve on bench, I am measuring 11Mohm between grid and anode on UBC41 with digital multimeter - that can't be right can it?
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Old 31st Aug 2006, 1:41 am   #9
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Default Re: DAC90A - no low volume

Hi Tivo, well I can confirm that the dial lamp shunt resistor was changed as a modification from 250 Ohms to 75 Ohms, 4W +/10%. You should be able to get away with a 68 or 82 Ohm here. The anode resistor for the UY41 was also increased from 150 Ohms to 250 Ohms +/20%, again a 4 Watt component.

The volume control does not go to earth at the bottom end, it is wired to the cathodes of both the UF41 and the UBC41 so a fault in either of these valves could upset the circuitry here. I do not have any voltages for this point but the DAC90 using E series valves is very similar and has a voltage listed of 1.7V.

The wiper of the pot is connected to two resistors via a 0.01uF cap (C14). These resistors are R7 (100k) feeding the grid of the UBC41 (pin 3) the other resistor is a 2M2 (R6) going to chassis, worth checking this one as old high value resistors have a habit of going much higher! The shared cathode resistor is 330 Ohms (R8) with decoupling provided by a 0.05uF cap (C15) wired in parallel across it. There is another 2M2 (R5) that might be worth checking, it connects the grid of the UF41 (pin 6) to the AGC line via the secondary winding of the 1st IF transformer.

In this respect, also check R10 1M which connects the AGC line to chassis. If all these points are found to be ok then the next option will be to try some new bottles which is probably the root cause anyway but at least you can say that you tried to save some cash.

Regards,
Les.

Last edited by Darren-UK; 27th Jul 2007 at 10:11 am. Reason: General tidying.
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Old 2nd Sep 2006, 9:22 pm   #10
tivo
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Default Re: DAC90A - no low volume

Well, I was hoping my new bottles would be here by now but no such luck. I then remembered I had a Ferranti 545 somewhere and as a longshot I checked out the valve lineup - it's the same as the DAC90A.

After a short search through the loft I found the radio, pulled out the UBC41, placed it in the DAC90A and switched on.

The volume now works perfectly - FIXED!

The substitute UBC41 is a Mullard all glass, the problem item (also a Mullard) has a metal skirt.

Does anyone know which type the receiver would originally have had (skirted or all glass)?

What are the differences between the two types (besides the obvious)?

What could be causing the 11MOHM short between anode and grid of the faulty tube and is this likely to be fixable (e.g. could it be akin to the AF117 tin whisker problem that could be "blown" clean)?

Is it likely that the 11MOHM "short" was causing this problem or is it likely the UBC41 has an additional problem?
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Old 2nd Sep 2006, 9:36 pm   #11
G0PKH - Pete
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Default Re: DAC90A - no low volume

Hi tivo.

Glad you have found the problem.

Regarding the "skirted" valves, I think these are the original types that would have been fitted from manufacture, I have found a number of these in DAC90A's that I have restored. I am pretty sure that electrically the valves are identical, not quite sure why the skirt was used though.

Regarding "blowing" the 11M "short" away, well I guess anything's possible but you would certainly need an extremely high voltage to do it .

Some people have tried this with UL41' s and have been mildly succesful.
ISTR a design for a power supply using an EHT tripler somewhere.

Pete

Last edited by Darren-UK; 27th Jul 2007 at 10:08 am. Reason: Edited to conclude thread.
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