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volume control doesn't reduce one channel to zero. In a few seconds it almost does...

haskellbob

Apr 13, 2020
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Hello!

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

If I turn the unit on right after turning it off, there is sound there at a pretty high volume, which drops after the "warm-up"), the volume hits a plateau and will not go below that.

There is a “null” spot in the rotation of the volume control potentiometer where the volume is almost zero. But that spot is not where the potentiometer is at the zero point; it’s at a place where some volume would be expected. At that point it seems like the proper volume control gradient takes over; from there on up the control regulates volume correctly.

Again:
The volume control does work once the volume in both channels is higher than the "volume that cannot be removed"... and from there on up.

I thought the problem was the volume control itself so I replaced it. To my dismay, the problem is still there. If the volume control is the culprit it means the new one is as defective as the one it replaced. That's hard to believe, although obviously it could happen.

I’m in need of a knowledgeable opinion.
Can someone help me out? I’ve drawn circles around the volume control on the first attached file. The second file is the entire schematic for the AR-15.

Thanks in advance for any help!

bob
 

Attachments

  • volume control problem.pdf
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  • 0 AR-15 whole schematic.pdf
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Harald Kapp

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This is the same spot where we've discussed loudness control and the 4th pin on the potentiometer before. It seems that the "null spot" is likely where the tap on the potentiometer is for loudness control.
Since volume control works from this point upward (towards louder), it seems that the connection to the ground pin of the potentimetrer may be broken:
upload_2020-5-24_11-28-58.png
As the issue persists with a new potentiometer, the break is likely to be in a trace on the pcb. To test this assumption, connect the ground pin of the potentiometer to a known ground point near the potentiometer with a piece of wire. Keep the wire as short as possible to minimize noise.
 

haskellbob

Apr 13, 2020
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That makes good sense and would seem to be the answer. But I tried connecting the ground pins to nearby ground points, and to the chassis itself, with no change. I disconnected the two wires going to the center tap, and that made no difference.
I also measured resistances. At the low volume, counterclockwise, point, resistance from the middle pins to the ground pins is zero, and resistance to the signal pins is about 12.5 k. That seems odd, since it is supposedly a 25k ohm pot. Also, resistance from ground pins on the pot to chassis ground and pcb ground is zero.

Is there any other possible cause to this?

Thanks.
 

Harald Kapp

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At the low volume, counterclockwise, point, resistance from the middle pins to the ground pins is zero
That is to be expected.
and resistance to the signal pins is about 12.5 k
pins? Do you mean both the center tap and the upper pin? Did you measure the pot while soldered into the circuit? If so, remove it from the circuit and measure again. In-circuit measurements cannot be easily done because the other components and the wiring will influence your measurements.
Is there any other possible cause to this?
A short circuit between the ground pin and the center tap would give these readings too - assuming the center tap is truly at the center.
 

73's de Edd

Aug 21, 2015
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Sir Bob Haskell . . . .
Is this volume post spread out betweeen different posts ?
Since I never received an answer to an earlier post as to where you would come up with such an exotic replacement pot.
So that we might reference to its data sheet or supplier source.
Dual ganged-180 degree spaced terminals on a DUAL 25K with an audio taper(s). 1/4 in long shaft with twist tab mount.
Taking your referencing to one channels pot portion and enhancing it with an INSET and labeling the pot terminal as to the Heath terminal assignments . . . the wiper is at about 2/3 volume UP position.
NOW . . . .have you made connectuions in agreement with this as is being shown ?

REFERENCING . . . . .

upload_2020-5-26_6-29-40.png











73's de Edd . . . . .

Most people are shocked when they find out how really incompetent that I am as an electrician.


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