Chances of repairing flush broke of pot shaft?

Old Guy8

My Jazz buddy's: Cliff (RIP) & Cleo
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New to attempting audio repairing. I acquired a Pioneer SA7500 mk2 amp for $20 with working tuner at garage sale. Came less broken off volume pot shaft and knob. Previous owner looking for missing parts. Shaft is broken just under flush with pots sleeve opening. I just temporarilly glued a screw on end for testing unit when it cures. I have cleaned all the other pots and switches. What a differance.
Question is. Might it be possible tocenter drill and drill an 1/8th. inch hole in pot shaft end for a repair pin. Do same for shaft going in knob? Use a roll pin or splined pin?
Anyone tried this? Or am I trying to make repair difficult? Is a new pot a possibility?
thanks
Old Guy
 

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Is it possible? Absolutely, I've done it in the past but I've never been 100% happy with it and ultimately replaced the pot.
Search around AK and you'll see various repairs.
If the screw seems to hold I'd cut off the head and epoxy a sleeve of the correct diameter over it. It'll likely not be centered but will work.
 
w1jim speaks true. Replace the pot, if possible, and if not, there are threads here about this very thing. I wish you luck.
 
w1jim speaks true the pot, if possible, and if not, there are threads here about this very thing. I wish you luck.
w1jim and onwardjames,
Thanks for replies. Maybe I should try to identify a replacement pot that could work. Try that first. Your right that not likely to get a repair pin/knob on centerline. This is the third time tried gluing screw on. So that's not a permanent solution.
thanks again. Like to get this classic vintage amp back in operation. Really like the looks of these Pioneers!
 
I did this recently with great success, drilled and tapped the remaining stub then wound In a bolt that had been ground to the correct shape to accept the knob.
 
I did this recently with great success, drilled and tapped the remaining stub then wound In a bolt that had been ground to the correct shape to accept the knob.
Maybe, I start with trying a repair off shaft. If that does not work, then look of pot replacement. I have reservations on tapping that small. Grabread, thanks for your idea.
 
replace the pot first choice ..second choice repair it with parts from another same make and series .. simply use your track wafer part in the doner one ..
or even try to call the company that made it and talk nicely to them and see if they will supply spare parts .i have had good success with this method for modern stuff . the girl on the phone generally says i will send a bag of 10 for free as its not worth the hassle with invoices etc .. i do have a way with talking on the phone though .
 
replace the pot first choice ..second choice repair it with parts from another same make and series .. simply use your track wafer part in the doner one ..
or even try to call the company that made it and talk nicely to them and see if they will supply spare parts .i have had good success with this method for modern stuff . the girl on the phone generally says i will send a bag of 10 for free as its not worth the hassle with invoices etc .. i do have a way with talking on the phone though .
Petehall,
I'm a novice on electronic repair. Have tool design background. l need to educate myself on what components makes up a pot. How to disassemble, etc. Check out the sticky threads in diy.
Thanks
 
I have been wadeing through that auction site and other suppliers in diy parts suppliers thread, for a knob replacement. I estimate the original was about 50mm OD with 6mm ID. Very large knob. Any sources to suggest for search? Found a couple on that auction site, but finish was blasted orr OD was too small.
Thanks again.
 

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I think the most important is the diameter of that bolt that hold it against the chassis. If you get a too large pot it wont fit. Anyway there are just 2 or 3 standard values for that diameter, and it should be associated to the shaft diameter.

Then, check the pot value, it will be something like 50K "B". and the pins. 3Pins potentiometers are regular pots (stereo have 6 pins since they are 2 potentiometers together). 4 Pins potentiometers are harder to find, they have a "derivation " at some point, and difficult to find in the same values.

If you get a shorter shaft, I think it will hold the knob anyway.
 
I have been wadeing through that auction site and other suppliers in diy parts suppliers thread, for a knob replacement. I estimate the original was about 50mm OD with 6mm ID. Very large knob. Any sources to suggest for search? Found a couple on that auction site, but finish was blasted orr OD was too small.
Thanks again.

I used a set screw tuner knob in place of the splined volume knob on my ka-5700 integrated, and it looks good. A lot of the receiver tuner knobs from the mid/late 70s were solid aluminum, unsplined and held to the shaft with one or two small set screws. I'm guessing that most of the pot shafts from this era were 1/4" diameter whether splined or smooth. A cheap digital caliper from HF or similar will confirm http://www.harborfreight.com/6-inch-composite-digital-caliper-93293.html. You might not find one that is as big a 50mm, but 40+mm(ish) is pretty common.

 
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It's a massive amount of work, and the original pot is worn and bad, anyway. Definitely a fool's errand. A new pot costs $1 and will be trouble-free for years. Replace.
 
It's a massive amount of work, and the original pot is worn and bad, anyway. Definitely a fool's errand. A new pot costs $1 and will be trouble-free for years. Replace.
Everyone brings their experience and skills to the table. If this was a Hydraulicly clamped milling fixture, mechanical vs electrical, would have fixed it and not started thread.
I may end up replacing pot, but not comfortable in electronics repair world.
thanks again for all ideas.
 
I used a set screw tuner knob in place of the splined volume knob on my ka-5700 integrated, and it looks good. A lot of the receiver tuner knobs from the mid/late 70s were solid aluminum, unsplined and held to the shaft with one or two small set screws. I'm guessing that most of the pot shafts from this era were 1/4" diameter whether splined or smooth. A cheap digital caliper from HF or similar will confirm http://www.harborfreight.com/6-inch-composite-digital-caliper-93293.html. You might not find one that is as big a 50mm, but 40+mm(ish) is pretty common.

I used a set screw tuner knob in place of the splined volume knob on my ka-5700 integrated, and it looks good. A lot of the receiver tuner knobs from the mid/late 70s were solid aluminum, unsplined and held to the shaft with one or two small set screws. I'm guessing that most of the pot shafts from this era were 1/4" diameter whether splined or smooth. A cheap digital caliper from HF or similar will confirm http://www.harborfreight.com/6-inch-composite-digital-caliper-93293.html. You might not find one that is as big a 50mm, but 40+mm(ish) is pretty common.

From pictures of this SA7500 ll amp I am guessing original was near 44mm OD, because faceplate cavity is 47mm ID. Have found some solid aluminum knobs on that auction site this size with 1/4" ID for 1/4" or 6mm shaft. Just looking for the best visually matching knob short of one from same model amp. Suppose can wait for one to show up from a parted out unit.
thanks.
 
FWIW, I've repaired things like that when no other option was possible. Basically, you remove the pot and cut a metal or plastic disk or cylinder slightly larger than the outside dimensions of the pot. Mount the pot in the disk and put that in the lathe. Center drill to spot the shaft on center. Drill a short 1/8" hole keeping the torque to a minimum- you're relying on relatively fragile internal stops. Clean up the end to make it square if possible. Make a new shaft with a 1/8" pilot. Loctite with #271 or stronger, taking extreme care not to Loctite the OD of the shaft!
 
May try finding a pot replacement. There is a Jtronics Supply here as a source. Going to get out of my comfort zone into world of electronic repair again.
Thanks again to all,
Old Guy9
 
I think the most important is the diameter of that bolt that hold it against the chassis. If you get a too large pot it wont fit. Anyway there are just 2 or 3 standard values for that diameter, and it should be associated to the shaft diameter.

Then, check the pot value, it will be something like 50K "B". and the pins. 3Pins potentiometers are regular pots (stereo have 6 pins since they are 2 potentiometers together). 4 Pins potentiometers are harder to find, they have a "derivation " at some point, and difficult to find in the same values.

If you get a shorter shaft, I think it will hold the knob anyway.

So I believe have identified the variable resistor in the SA7500 mk2 service manual. Description: Variable 100k 32 step (volume) part # ACV-162. Markings on pot square case reads: 162A then, 192Y100k (ohm symbol) X2AD
The volume pot is the one with the square sheet metal case. Looks to have two lines of 4 pins each soldered to the board above it. In picture of board you ca see the 8 soldered pins. Don't know if pics show that cover and other components are close by. I found similar looking pot with same part #ACV-162, said to be from another Pioneer unit, on that auction site.
I apologize for showing this information piecemeal, and for wavering what to do. I know pitfalls of drilling and tapping shaft and possibility of screwing it up. A lathe would be helplul to drill hole on centerline.Don't have access to one.Current pot works with slight static when moved, that I might cure with deoxit. Do you think I can find a current production replacement pot for this or is used market it?
Thanks again for help.
 

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The temporary screw I super glued on the pot has held a few days of use. A a previous suggestion, going to try Dremel trim head off 1/4" screw. If it holds and does not melt glue from grind, will mount new knob. I ordered a knob from similar family amp, looks identical. See if this works.
Thanks again
 
If your super glue fails, then you may try first removing screw/bolt head with hacksaw/whatever, then re-gluing the setup with epoxy (JBWeld for example) instead of the super glue. Take a sheet of thin shim stock and wrap it around shaft down in the bushing. This should keep epoxy from gluing shaft to inside of bushing. You could insert a pin by drilling small pilot in pot shaft with a Dremel in a plunge router attachment. Use a milling cutter to get a non-wandering hole. Tape a locating jig to the front of the amp to accurately position the end mill, and keep the plunge router mechanism as tight as you can for accuracy' sake. Replicate the jig for drilling the end of replacement shaft, and use a looser-fit pin to allow final alignment as guided by the shim stock. Ultimately you can leave the shim stock in place, or peel it away if you prepped it with some release agent like vaseline (which I'm guessing would work with epoxy.) A lot of fine work, but should survive in the hands of a careful owner.

Complete pot replacement is likely easier, of course.

Chip
 
If your super glue fails, then you may try first removing screw/bolt head with hacksaw/whatever, then re-gluing the setup with epoxy (JBWeld for example) instead of the super glue. Take a sheet of thin shim stock and wrap it around shaft down in the bushing. This should keep epoxy from gluing shaft to inside of bushing. You could insert a pin by drilling small pilot in pot shaft with a Dremel in a plunge router attachment. Use a milling cutter to get a non-wandering hole. Tape a locating jig to the front of the amp to accurately position the end mill, and keep the plunge router mechanism as tight as you can for accuracy' sake. Replicate the jig for drilling the end of replacement shaft, and use a looser-fit pin to allow final alignment as guided by the shim stock. Ultimately you can leave the shim stock in place, or peel it away if you prepped it with some release agent like vaseline (which I'm guessing would work with epoxy.) A lot of fine work, but should survive in the hands of a careful owner.

Complete pot replacement is likely easier, of course.

Chip
Chip,
Some good ideas. Especially useing Dremel.
After super glue, if fails will try the epoxy. These fixes are reversable if they fail. Saw a used pot, that was not cheap. Best solution if in good shape.
Thanks
Old Guy
 
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