Sherwood S-7310 Volume Pot Woes

[spark1]

I've been trying to get a Sherwood S-7310 back into good working order. I've been able to resolve all of its other problems, but am having trouble with a replacement volume pot.

As background; someone before me removed the power switch assembly at the rear of the original volume pot. I considered making some sort of modification to replace it, but ended up simply tying the two power wires together and controlling power with a remote-controlled outlet. Works just fine.

Unfortunately, I found that the wiper for the left channel volume was badly damaged and had extremely uneven resistance and drop outs. I also found that neither channel had output until the pot was turned to about 10 o'clock.

I sourced an NOS Alps pot of the proper resistance, and with loudness taps. Smaller form factor, and had to cut down its shaft and connect it to the outer end of the old shaft (which I also cut off, obviously) with a metal sleeve. This allowed use of the original knob. So far so good.

Wired up the new pot. Left channel works just as it should...volume increases smoothly from the get-go. But the right channel still remains quiet until the knob gets past 10 o'clock. At that point, it jumps up to the same level as the left, and seems to work as it should from there.

I was careful to measure the new pot for proper operation before installing it, and have verified after installing it that both sides measure pretty much the same with ohmmeter (multimeter). Using a signal tracer, I confirmed that both the volume and balance don't seem to show any problems with the output level...but I may not be checking correctly. However, the problem does show up at the loudness switch, and at the tone board (which comes immediately after the balance pot/loudness switch/volume pot.

I am not very good at interpreting schematics at a detailed level, and initially concluded that if the loudness function is not active, it could not be the source of the problem. Now I'm questioning that conclusion. Can anyone help me to sort this problem out?

On a related note; could I simply disconnect the loudness taps and expect the pot to work normally (albeit without the loudness boost)? Or is it not that simple?

EDIT: I disconnected the wires to the taps, and now the volume control works as it should on both channels. This would seem to isolate the problem to either the loudness switch or the right channel loudness tap. Right?

If so, how would I further isolate the problem?

Thanks for any help or guidance. I've attached a not-very-good schematic of the pertinent section.

 

[Warren-Oates]

I would check that loudness switch next.

 

[Namlook]

Is there a relay in that unit?

 

[spark1]

I would check that loudness switch next.

Thanks for your reply.

Access to that switch is a bit difficult. Would just reversing the leads that go to the loudness taps isolate the switch as the problem? I'm thinking that if the problem moves to the other channel, this would confirm it's the switch - or, I guess, one of the caps in the loudness circuit? The resistors in that circuit have already been tested.

For my education; why/how does the switch cause no output from the volume pot (until it's turned up to a certain point)?

 

[spark1]

Is there a relay in that unit?

If you mean a protection relay, no.

 

[Warren-Oates]

Yes I think switching the wires could be a good test. I think at a certain point on the pot the loudness function drops out, I think they usually drop out at about 11am on the switches. Do you still have the old pot that is junk? If you open it up and examine it you should be able to see where the loudness function ends.

 

[spark1]

I had a few minutes, so switched the wires. As expected, the problem moved to the other channel.

So, I'm thinking it must be either the switch or the capacitor in the loudness circuit.

I'll tear apart the old pot this evening to see if it helps me understand how it works. The only thing I can guess is that when connected, the portion of the pot affected by the loudness tap somehow "supercedes" the operation of the pot at lower volumes. What really confuses me is how/why this happens regardless of whether the loudness function is activated or deactivated. The only way the pot works "normally" - albeit without the ability to use the loudness function - is to have the loudness taps disconnected.

I'm assuming that I should pull the cap in the loudness circuit on that channel to eliminate it, but if it's the switch and I can't figure out how to repair it, I guess I can just leave the loudness taps disconnected. I am working on this receiver for my neighbor, and he is trying to keep cost to a minimum (I'm only charging him for the cost of parts). While he would like to see it working as it did when new, he said he rarely uses the loudness function anyway.

 

[mrk229]

Do you have easy access to the loudness switch? All it does is short/bypass the cap.

You can get at one end of the switch with the wire on the loudness tap.

Receiver powered down check it with an ohm meter for proper on/off action.

Or if you can't easily access it then check from the loudness tap to ground.

It should show in the neighborhood of 8.2k when the switch is closed and something else when open.

 

[spark1]

The switch (S5) has three pairs of tabs. With wires to the loudness taps of the volume pot disconnected, they tested as follows:

1) .3 ohms with loudness activated (which in this receiver = pushbutton NOT depressed), and OL with loudness deactivated (pushbutton depressed)

2) .3 ohms, on or off

3) 16.3 Kohms in off/deactivated position, .3 ohms in on/activated position

The front pair is 1), the middle pair is 2), and 3) is rear...viewing from front of DUT.


Measuring from loudness taps to ground:

A. With wires to loudness taps connected, loudness activated

​

Left = 8.86 Kohms​

Right = .2 ohms​

B. With wires to loudness taps connected, loudness deactivated

Left = 4.3 Kohms​

Right = .2 ohms​

​

C. With wires to loudness taps disconnected, loudness activated

Left = 8.9 Kohms​

Right = 8.6 Kohms​

​

D. With wires to loudness taps disconnected, loudness deactivated

​

Left = 8.9 Kohms​

Right = 8.6 Kohms​

Even though I still can't grasp how the loudness taps work, it seems clear that the switch is the problem, right?

It's going to be a bit of work to get it removed, a task I would only undertake if there is a good chance it can be repaired or inexpensively replaced.

 

[Warren-Oates]

Well your test shows the right channel has an easy path to ground, that explains where your signal is going. Anything weird by the switch or the wires? Post a pic if you can. It seems like by what you wrote their could be some sort of problem at the pot too. However since the new one is doing the same as the old one it seems unlikely. Could you post a pic of the new pot wired in? Does the loudness switch feel irregular at all, something bent inside? Can you hold the switch or jiggle it in a way to stop the flow to ground?

A used Sherwood loudness switch is probably cheap if that is the cause.

 

[spark1]

Nothing unusual that I can see. Moving, jiggling the switch has no affect on the problem. Solder joints on the switch board all good.

As requested, pics of the volume pot (loudness taps disconnected).

The pot had to be oriented this way to fit the chassis, which in turn required that the shielded wires for the loudness taps be extended.

 

[mrk229]

Nothing unusual that I can see. Moving, jiggling the switch has no affect on the problem. Solder joints on the switch board all good.

As requested, pics of the volume pot (loudness taps disconnected).

The pot had to be oriented this way to fit the chassis, which in turn required that the shielded wires for the loudness taps be extended.

Check the area where you extended the loudness wires. I bet you the short in in the splice.

 

[spark1]

Nope. Already checked.

And was very careful with the splicing, so as to avoid contact with shielding.

 

[mrk229]

Nope. Already checked.

Damn. Is it possible the center of the shielded cable melted and shorted where you can't see it? Some of that stuff melts real easy.

Can you easily lift the center conductor at the switch end and then check the volume control end to ground.

 

[spark1]

Not easily, but it can be done. I'll try later today.

Keep in mind that this issue also occured with original pot, original wiring.

 

[mrk229]

Not easily, but it can be done. I'll try later today.

Keep in mind that this issue also occured with original pot, original wiring.

Good point if it was doing it before then the right channel is somehow getting to ground.

Previously you measured at the loudness taps. Disconnect the tap wires and measure from each wire to ground.

 

[spark1]

Good point if it was doing it before then the right channel is somehow getting to ground.

Previously you measured at the loudness taps. Disconnect the tap wires and measure from each wire to ground.

Thanks. Will do.

It's certainly possible that the cause with the original pot was due to it being damaged. Whoever removed the switch obviously messed with the wipers, for unknown reasons.

In other words, the problem may have been caused by the pot then, but by a fault in the wire now. Your point about the insulation easily melting is a good one.

 

[spark1]

Sorry for slow response.

Loudness not engaged:

Right tap wire to ground = .2ohms

Left tap wire to ground OL.

Loudness engaged:

Right tap wire to ground = .2 ohms

Left tap wire to ground = 8.2 Kohms

Going to cut off the extension and re-do it. Showing my ignorance; these wires are shielded, with both the inner conductor (signal) and shield connected at the switch board. But at the tap end, the shield was (and is) not. So does the shield not serve as a ground? Only for noise rejection?

 

[spark1]

New wire solved the problem. Yay!

My check of the wiring when I first added the extension showed no problems, but I would guess that one of the hair-thin shield wires got loose before I put the heat-shrink on and caused a short. Thought I tested them again before soldering to the taps, but apparently not.

Logically, when you have a problem after making changes, you should first assume that you did something wrong. What threw me off the scent was the fact that I had the same symptom with the original pot and wiring. Turns out it was same symptom, but different cause...bad pot initially, cockpit error the second time.

Thanks so much for the help!!

 

[mrk229]

Sorry for slow response.

Loudness not engaged:

Right tap wire to ground = .2ohms

Left tap wire to ground OL.

Loudness engaged:

Right tap wire to ground = .2 ohms

Left tap wire to ground = 8.2 Kohms

Going to cut off the extension and re-do it. Showing my ignorance; these wires are shielded, with both the inner conductor (signal) and shield connected at the switch board. But at the tap end, the shield was (and is) not. So does the shield not serve as a ground? Only for noise rejection?

Great news.

Yeah with only one end of the outer shield connected to ground it serves to well....shield the center conductor from noise pickup.

The volume control pot must have been the original problem.