Volume Control VC4 repair / cleaning question

[Mark]

Hey Y'all, 

Restoring my 1979 Seeburg SMC1 Disco.  It had only one volume which was "loud".  The VC4 Volume Control could only be turned with pliers and made loud static noises when turned but no change in volume.   I learned that if I just "pushed upward" instead of turning it, then it dropped to a low volume which was ok.   

I removed the VC4 and sprayed the inside contact plates with "safe" circuit board / contact cleaner while turning the volume control up and down until it moved easily by hand (no pliers required).  I reinstalled the VC4 and now it does adjust the volume however it goes from "Loud to Much Louder" with no static crackling sounds.   Turning it all the way down where it should be "off" is "Loud".  All the wires attached to the volume control look fine and nothing appears damaged.  

I am stumped as I don't fully understand the inner workings of the volume control but figured cleaning the internals by spraying contact cleaner would do no harm. 

This Seeburg is located in my home office so the lowest it goes is a bit loud for this size room.    I ended up changing the connections on the SHP3 amp from 16 watts to 8 to 4 then down to the lowest 1 watt so the volume is now "ok" however that is not a good "fix".

Wondering what to do going forward and looking for your input.    

Thanks ~ Mark

[Ron Rich]

Hi Mark,
Which volume control do you have ? Just replace it-- do you have the motor on it ? You should NEVER need pliers to turn it !
Ron Rich

[Mark]

Hi Ron! This is the basic V4 manually rotated shaft attached to the potentiometer. Not a PRVC powered unit. Per the schematic, it is all "mechanical innards". All I did was sprayed inside using the contact cleaner can "straw" whilst turning the shaft back and forth to aid in the cleaning process until it turned freely.

Before cleaning it was "Loud" and no change in volume / just static while turning / difficult to turn. After cleaning, turning the VC4, the volume went from Loud to Louder, no static and normal pressure to turn.  

So cleaning did some good but still cannot lower the decibels to what I need for the size of my room.  

I am going to swap out the SHP3 amp and see if that makes a difference.    

If all else fails, I can just order an Audio Potentiometer from Mouser (or Amazon) but unsure what part to order.  The VC4 schematic shows 25K for left channel and 25K for right channel.    So when looking at  replacement "2 gang audio potentiometers" they offer 20K ohm and 50K ohm.    Would I go with 25K + 25K = 50K or 25K so closest is the 20K?

www.mouser.com/c/passive-components/potentiometers-trimmers-rheostats/potentiometers/?number%20of%20gangs=2%20Gang&product=Audio%20Potentiometers&resistance=50%20kOhms

Thanks again


Regards ~ Mark

[Ron Rich]

Hi Mark,
Nope-- you do not "go" with either Mouser offers-- You order the correct one from one of the suppliers listed in our
FAQ's section --"parts--books" post ! Ron Rich

[Mark]

Thanks Ron.  The one listed there is Tisdale and their site lists older seeburg volume controls so will email Jason to see if he has any NOS.  
The majority of the dual audio potentiometers I find in parts catalogs have the 3 prongs for each channel: 1 Line in, 2 wiper and 3 ground thus Six wires. however the Seeburg VC3 and VC4 have the 3 prongs / wires but also an additional wire coming from the back side of the pot thus a total of 8 versus 6 wires.   So what are these 2 wires for?

I have searched all my various Seeburg materials and cannot find good written info on the volume control explaining the other 2 wires. The best schematic was the last page (40) of the Service Manual for the combo all STD models STD160 thru STD4 yet I cannot determine what these 2 wires "do".   Would you please let me know?

[Ron Rich]

Hi Mark,
They were for "Loudness control--ie. bass boost"-- Your SHP-3 may have eliminated this "feature" due to "cost cutting"-- Look at a schematic for any Seeburg amp from the SHP-1 back to the TSA-1---Ron Rich

[Mark]

Thanks for imparting your knowledge once again Ron.

I searched other "vintage audio" type forums and learned a bit. The fourth terminal is commonly called a "4th center tap". It is connected in series with a resistor and capacitor to ground to compensate for slight changes in frequency response as the pot is rotated.

I traced the actual wiring and schematics. These right and left center taps in the SHP3 chassis schematic are shown coming in at connector J5111 as pin 3 "Rt. HI. Tap" and pin 5 "Lt. HI. Tap". They go to inputs 20 and 5 on the A5101 on the Preamp Board with a resistor and a ceramic cap before going to the Bass "max med min" switch.

After much searching I found a suitable replacement and ordered it.   Will post here once installed to see if it functions as intended. If anyone wants to find the seller google: New 100k ohm Stereo Potentiometer w/ Loudness Taps Receiver.  

For those like me who are novice about "Pots"  there are many people on the internet who explain how it works however they all use the basic 3 taps type potentiometer to explain.   This 4th tap is no longer used in current electronics.   


To answer my original question I will use an example of a windshield wiper on my vehicle.  Over time and use, the part of the wiper that contacts the windshield gets worn down until it streaks with less wiper touching the glass.   Cleaning the glass and the rubber wiper makes it better but still needs to be replaced.   When you turn the pot you turn the inside wiper which wears against the surface and over time wears away thus changing the resistance aka volume. Cleaning them got rid of the gunk thus the static and the volume worked from Loud to Louder.  Not exactly the same but hey. 

[Ron Rich]

I would use the exact--correct, replacement ( and have--many times) rather then searching all over for one the "might work". Ron Rich

[Mark]

Thanks Ron. Following up on this forum post. I was not able to locate a forum supplier with this center tap potentiometer for sale. If you mention the supplier who has this exact correct replacement, I will gladly purchase one to have on hand for the next jukebox that has this issue. The 4 terminal center tap NOS pot I obtained from an audio website works great. regards Mark

[Ron Rich]

Hi Mark,
If you have found "--something that works great", how about sharing it with us ? Please post it in the parts post, in our FAQ's section--also post it here-- Thanks, Ron Rich

[Mark]

Sure Here is the link. www.ebay.com/itm/124038410905

[Ron Rich]

Hi Mark,
How did this work out ?
Ron Rich

[Mark]

Hi Ron! Thanks for asking. Works better than any seeburg volume control I have, even those that I cleaned with contact cleaner. long enough shaft, turns smoothly and volume changes nicely. Soooo much better than the "hard to turn" with static and jumps in volume of the older pots. Studying the schematics I think the 4th tap is important for people who "crank it up" above half way on the wiper however I have this jukebox in my office thus cant shake the walls too much.

[Ron Rich]

Hi Mark,
Is the shaft on it close enough, to hold the Seeburg Clutch disc, for the PRVC-4's ?  Ron Rich

[Mark]

Hi Ron, I honestly doubt it would work for any "powered remote control" type units as it is a tad smaller in size. This is an adequate replacement for the VC4 and VC3 pot. Unsure for any earlier VCs as I have not attempted to replace. Regards, Mark