Volume Control not working

[hpleung]

Hi,

I went and performed some resoldering of wires after not using it for a while. The idea was to remove some of the thin wires and replace with bigger one. Of course, in the process I must have messed up the re-connections.

The observation is that - there is sound but a bit distorted because the volume is very loud (probably maximum) and the volume control is not working at all. I did the voltage check before I turned it on and they were within range.

Are there any pointers on where I should look or if there is resistance reading/voltage reading I could perform that could provide more hint on where the problem is?

Thanks.

[Paul Birkeland]

I would post some photos of what you have wired. 

[hpleung]

Here are a few pictures. Much appreciated. Thanks.

[Paul Birkeland]

I would set the volume pot to the middle of its rotation, then measure resistance (with the amp off) between the upper middle, lower middle, upper outside, and lower outside pot lugs on the volume pot.

If you do a lot of soldering on the volume pot, it's possible to damage the pot, and it's completely possible that the ground lugs have been cooked enough that they are no longer contacting the carbon track.

[hpleung]

If i am measuring this correctly,

Upper Left Lug <---> Upper Middle Lug =  81.6 Kohm
Upper Middle Lug <--> Upper Right Lug = 14.5 Kohm

Lower Left Lug <--> Lower Middle Lug = 78.8 Kohm
Lower Middle Lug <--> Lower Right Lug = 13.7 Kohm

[Paul Birkeland]

Your pot is working and should be able to control the volume.  You may have a loose connection elsewhere that's not allowing things to work properly.  It could absolutely be a loose black wire feeding the pot or leaving the pot to the rest of the circuit.

[hpleung]

Ok. One question - near those solder point on the pcb such as “OA” and “B+”, there are 2 nearby holes. One of them was used the last time and today, after i took out the old wire, i soldered into the 2nd holes.

I am assuming those 2 holes are connected and the 2nd hole is provided for soldering convenience.

I wanted to check that my assumption is correct.

Thanks.

[Paul Birkeland]

Yes, those holes are connected.

I suspect the black wire that leaves the RCA jacks may not be making continuity with terminal 3.

[hpleung]

I noticed my red RCA center pin has 90.7 kohm but my white RCA center pin has OL resistance. I believe this is also supposed to be 90.7 kohm. Wanted to confirm this - note this has the speedball upgrade.

[Paul Birkeland]

That is wired to one of the pot terminals that you reported as having the correct resistance, so either the white wire is broken or perhaps not well connected (or maybe a measuring issue).

[hpleung]

When I measure the resistance from the Center of the RCA Plug (Where the White Wire is attached) to the Upper Middle Potentiometer Lug (Where the White wire is attached to), the resistance is about 95.8kohm.

However, when I measure the resistance between the RCA Plug (Where the White Wire is attached) to the Potentiometer Lug (Where the Black wire is attached), the resistance is 0L.

This does not happen for the Red Wire. Something is not well connected that only affects the white wire.

[Doc B.]

It may be that the white wire is broken inside the insulation. Try measuring the resistance from one end of the wire to the other, in other words make sure the meter probes are touching the actual copper wire and not the terminals that it is attached to. If you still get an OL reading that is probably the issue.

[hpleung]

I have checked and its not the white wire that is broken.

Since the resistance I am getting between 12U and the Center of the RCA plug is 0L, it must mean there is a break in the chains of connections between 12U and the Center of the RCA plug (White Wire).

I believe that if 2 points are connected, then the resistance between them should be near 0 ohm.

I would like to measure the resistance between every 2 points starting from the center of the RCA Center Plug (White Wire) all the way to 12U. I can trace the white wire, but i start to get lost once we reach the Tubes Sockets; I cannot tell which one is connected to which (input vs output).

If the above is a good idea, can someone point me on what 2 points to measure specially around the Tube Sockets?

[Paul Birkeland]

Since the resistance I am getting between 12U and the Center of the RCA plug is 0L, it must mean there is a break in the chains of connections between 12U and the Center of the RCA plug (White Wire).

12U is not directly connected to the center of the RCA plug.  It would be connected to the shell, what is that resistance?

If the resistance between the center pin of the RCA jack and the upper outer lug of the volume pot is not 0 ohms (or at least close to it), the white wire is broken or not well connected.

[hpleung]

The resistance between 12U and the outer RCA Plug (Not the Center) is near 0 ohm. But the Resistance between 12U and Center Pin where the white wire is connected is 0L.

Note this problem does not exist for the Red Wire.

Also, the resistance between the Center Pin RCA Plug (of the White Wire) and the Upper Left Outer Lug of the volume Pot (Where the white wire is attached) is 0 ohm. I also tried mounting a different wire which did not help.

I mentioned this before, but the resistance between the Center Pin RCA Plug (Of the White Wire) and the Middle upper Lug of the Potentiometer is 96.4 kohm. I believe, this means that the connection between the RCA Plug all the way to the potentionmeter is ok.