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Sourcing 1980's Yamaha G100 replacement pots

Started by gastric, April 11, 2023, 01:16:13 PM

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gastric

I picked up a Yamaha G100 212ii and need to source a couple of replacement PCB mounted pots. The schematic has part numbers but I've come up empty trying to reference those. Does anyone know what the current production replacement units would be?

* Channel 1 bass potentiometer A25k for bass pot on PA2 (channel 1) #401000HS310760
* Channel 2 gain push/pull A25k with switch for gain pot for PULL FAT-B (channel 2) #401000HS310770


joecool85

With a few pictures and some measurements (diameter of the potentiometer body as well as distance between pins), and then also if it is a knurled, smooth, or "flat d" post, and we should be able to help you find a potentiometer for you.
Life is what you make it.
Still rockin' the Dean Markley K-20X
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Tassieviking

#2
The footprint on the PCB looks like a standard size pot, PCB mount straight legs.
It appears that they are 16mm with legs 5mm apart from the PCB picture in the service manual.
You really need to get some pictures and measurements to get the right one.
Is the thread length standard, what type of shaft do they have etc.

Something like this might fit for gain: https://www.digikey.com.au/en/products/detail/bourns-inc/PDB183-GTR03-253A2/4699138

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There are no stupid questions.
There are only stupid mistakes.

phatt

Pot replacement can be a nightmare depending on the layout.
If it was me I'd try a can of cleaner spray inside the pots just to see if they come good.

I can't recall the name of the cleaner but any electronic supplier will have several types.
I know some will frown on me  :-X but I use WD40 or RP7 to clean scratchy pots with great results.

My Hifi amp had a really scratchy Volume pot and just a squirt of RP7,, been running fine now for the last ~5 years.
Phil.

Tassieviking

Are you thinking Deoxit Phil ?
I used WD-40 in industrial situations, especially if there was acid in the air, but that is frowned upon in amplifiers by many people.

I think Deoxit does not wash away the special lube in pots or something like that while WD-40 and Electraclean type products might, there used to be a "Lectraclean" that used to melt most plastics on contact, nasty stuff if you are not careful.

One can of Deoxit will last most people a lifetime at home if you only use it for pots.
There are no stupid questions.
There are only stupid mistakes.

phatt

Quote from: Tassieviking on April 12, 2023, 09:57:45 PMAre you thinking Deoxit Phil ?
I used WD-40 in industrial situations, especially if there was acid in the air, but that is frowned upon in amplifiers by many people.

I think Deoxit does not wash away the special lube in pots or something like that while WD-40 and Electraclean type products might, there used to be a "Lectraclean" that used to melt most plastics on contact, nasty stuff if you are not careful.

One can of Deoxit will last most people a lifetime at home if you only use it for pots.
Thanks TassieV, yes Deoxit.
I did purchase a can of Cleaner from Jaycar years back (probably not Deoxit) but found that WD40 worked just as well. I've used it many times to resolve scratchy pots.
Of course if the wiper has worn through the track then replacement is the only option.
Getting back to this amp the level pot on the ParaEQ section might be the hardest to source as it's a tapped pot. :'( 
cheers, Phil.

Tassieviking

Tayda has W50k pots but the wrong type of legs, nothing a side cutter cant fix followed by some copper strands out of some 4mm building wire.
Or just use the legs from some 4004 diodes and solder them on the pot.

At least I think W taper is the same as G taper, but if you want that center detent(click) you will have a harder time finding it.
There are no stupid questions.
There are only stupid mistakes.

gastric

#7
Already ran them all through with DeOxit. The bass control pot has something crunchy inside in the first 1/3 of the travel and doesn't work quite right. The gain pot is missing 1/2 of the split shaft but is otherwise functional. Hmmm, might be less effort to epoxy on half a shaft from a sacrificial pot if I can find a match. :)

Will bust out the camera and my cheap digital micrometer and post the info soon.

Note that Yamaha was of no help, as expected.

QuoteYou are correct in that the parts for that amp are obsolete. We can only really reference parts to a specific model the are associated with, as we have so many models and part numbers. So, I'm not able to cross-reference  those parts to another amp, unfortunately.

gastric

#8
Some photos, including my attempt to measure using a cheap digital caliper.

As best I can tell the non-switched pots are approx 16.5mm in diameter with 4mm leg spacing.








phatt

As it's unlikely you will ever find original switching pots there is a simple way to resolve the switching for Fat and Bright.

If you note, although the switches have 6 pins they only use 2 pins so it's a simple On/Off switch.

I would simply mount 2 small toggle switches on the front panel and run short wires from the pcb pads.
You can then replace the Volume and Treble pots with what looks like a standard 16mm 25kA pots,, well that is what I would do. ;)
Phil.

Tassieviking

I think it might be best if you remove the bad pots and clean the solder up with solder wick so you can get an accurate measurement of the hole spacing on the PCB.
I have not come across any 16mm or 5/8" pots with 4mm leg spacing before, but I have seen where someone has squashed the outer legs inward so the pots didn't fall out before they soldered them in place.(Not good practice)

I think you would most likely be able to fit a new pot with 5mm spacing in with no problem.
That looks like a single sided PCB with copper tracks on the bottom only, it should be easy to make the holes a bit larger to make them fit if you needed to.

The switched pot will be the hardest to get, I can't see if there is any brand on them at all, sometimes its a symbol stamped into the metal on the back of the pot.

It might be possible to dismantle the switched pot and just replace the disc with the carbon track taken from another brand new pot without a switch.


Here are the standard sizes for Alpha pots for reference, but it will differ with brands.


There are no stupid questions.
There are only stupid mistakes.

gastric

Onto my next adventure.

QuoteI think it might be best if you remove the bad pots and clean the solder up with solder wick so you can get an accurate measurement of the hole spacing on the PCB.
I have not come across any 16mm or 5/8" pots with 4mm leg spacing before

Another alternative I thought of was to just purchase a panel mount pot with the correct shaft length and diameter so everything looks normal from the outside, rotate the pot 90 degrees, then run wire to the PCB.

That might have been what @Tassieviking was suggesting.

phatt

Those pots are more than likely standard 16mm pots with 5mm pin spacing. Alpha and Bourns make em, an likely others also.
The only thing you need to watch out for when ordering is the thread and shaft lengths and of course the taper of the pot.
If you can find them with Conductive Plastic wafers, even better as they will last longer than you. The carbon units do tend to wear out prematurely.
Phil.

Tassieviking

I have no idea where I got the idea that one of the pots was a G50k taper but that's what I thought it was, most likely confusing this with a post on another forum.
A25k pots with a push pull switch should not be hard to source, the pots on the pictures reminds me of an old type of Noble pots from the 80's.
The Noble pots were a bit "chunky" around the shaft base where the thread starts.

I don't know where "gastric" lives but if in the states then these guys have a switched pot that could be made to fit : https://www.bestbassgear.com/alpha-25k-volume-potentiometer-audio-taper-push-pull-6mm-solid-shaft.htm

You would have to solder some stiff wire on the pot lugs to poke through the PCB,just some leads cut of some 1N4004 diodes would be good enough.
Standard A25k pots are easy to source, I get mine from Tayda mainly and I find them good.
Check Ebay for "A25k push pull"

I would be 99% sure that the pots are standard 16mm with 5mm leg spacing, but they might 5.08mm as well.

I personally would remove the PCB and then mount it from the outside of the case by tightening all the pot nuts before soldering the new pots in, that way the pots are aligned before the solder locks them in position. You really want the pots tight against the panel before soldering, if you solder them first and then tighten up the pots you could put strain on the pots and solder joints.
There are no stupid questions.
There are only stupid mistakes.

gastric

Good tip. On these amps the boards are mounted upside down in the chassis with all of the solder pads oriented upward.

QuoteI personally would remove the PCB and then mount it from the outside of the case by tightening all the pot nuts before soldering the new pots in, that way the pots are aligned before the solder locks them in position. You really want the pots tight against the panel before soldering, if you solder them first and then tighten up the pots you could put strain on the pots and solder joints.