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haskellbob 23rd May 2020 01:03 PM

Volume control malfunction - probably NOT the potentiometer
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I have a Heathkit AR-15 receiver. I recently began having this problem: The volume control does not reduce the volume to zero, on one side only. The right side.
This started after I had the control way up high, because of a Passive Volume Control I inserted after the “Tape Monitor” output and the “Tape Monitor” input. I was using an external volume control to control volume, so the actual volume control on the receiver was turned way up – to the maximum level I figured I’d ever need it. Long story – with a passive volume control I was able to control other devices that were being fed by the Tape Monitor output as well as the receiver itself.
I can’t imagine why that would cause the problem I’m having but it seems like it did. I don’t enough to zero in on what happened when I skewed the volume in that way. I’m stuck at the cause-and-effect level, without the knowledge to go any further.
So the actual problem is, that while the volume in the left channel is reduced to zero by the control, the volume in the right channel hits a plateau and will not go below that. Not even the “balance” control can reduce it.
(NOTE: the Passive Volume Control has been removed and the only volume control being used now, and described here, is the one proper to the AR-15.)
After maybe 20 seconds or so, there is a “null” spot in the rotation of the volume control potentiometer where the volume in the right channel – as well as the volume in the left channel - is almost zero. But that spot is not where the potentiometer is at the zero spot; it’s a little bit after. In other words, at a place where some minimal volume would be expected.
I was willing to let it go at that, since I’d never really need the volume to be that low. So if the receiver powered up at that volume level it wouldn’t be a problem.
The control starts to work as it should at that point and from there on up.
(to re-cap: below a certain point in the Volume Control potentiometer’s trajectory, the right channel sticks at a certain volume and there’s no way to cut it out. Even the balance control has no effect on it. Above that point the volume goes up and down as it should, following the volume control.
BUT if I set the volume control at the above-mentioned “null” spot where both channels are almost zero, and then turn the unit off and back on again, the right channel comes on quite loud. After a short time, well under a minute, the volume begins to drop – back to the almost-zero spot again.
I thought it was the volume control itself so I replaced it. To my dismay, the problem is still there.
In my naďve way of seeing things, I have isolated the 10mfd capacitor that comes after the volume control and before the final two amplifying transistors in the pre-amp and/or the 10 mfd capacitor that intervenes between the pre-amp and the amplifier as possible culprits. My reasoning is that, since capacitors charge, one of those two, or both, capacitors might restrict the volume while charged but, after the unit is turned off it (or they) would discharge and no longer restrict the volume.
I don’t know if this theory has any validity. What I don’t know is how or why a capacitor would be used to restrict volume. So I’m in need of a more knowledgeable opinion. There may well be some other culprit in the current problem, and I am curious to know what it is.
Can someone help me out? I’ve drawn circles around the volume control and the two capacitors I mentioned on the first attached file. The second one is the entire schematic for the AR-15.
Well, that was my plan anyway. Maybe you can help me figure out why this website wouldn't upload my files. I guess th

I’d appreciate someone’s more knowledgeable analysis. Thanks in advance!


analog_sa 23rd May 2020 01:40 PM

My first thought is c129 (c130) having become leaky. Easy enough to just replace. Is the problem at all dependent upon the loudness switch position?

haskellbob 23rd May 2020 01:54 PM

I'll check. No, the loudness switch makes no difference. I think. I'll check that too.
Thanks for the help. I'll get back to you.

mikeAtx 23rd May 2020 04:46 PM

I'd try this. Short the wiper of the control to a good ground. If that sets the volume to zero, even on power on, I'd suspect a cold solder joint on the ground of the pot. If you have mini-clips, probably pretty easy to put one on with the unit off, and then turn it on to test. It seems very odd that it could be anything else, as the input to the amp section is right after the pot, and the pot wiper minimum position is ground.

haskellbob 23rd May 2020 06:15 PM

Actually, connecting the wiper to a good ground - the chassis - made a minimal difference. The volume at the "off" end of the potentiometer got a bit quieter but persisted; there was no way to get it down to zero.

I think I'm going to leave this to an expert. I have a friend who is an electronic technician. He will know what to look for and what to do. I have learned that if I push in beyond the level where I have understanding - which is pretty much anything beyond the most basic theorty - just leads to frustration and even anger at myself! Which makes no logical sense.

But anyway, thanks for the suggestion. I see what you were driving at. Another detail is: that I replaced the 10 mfd capacitor right there after the volume control, and now both channels are misbehaving.

As I said, I'm going to take it to my friend.

Thanks though!


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