Hi all,
The Spectrol 100 volume control in my KSP-7B has gone funky. It looks like the shaft is able to slide in and out almost 1/8" (3-4 mm), as though it had a missing or broken thrust bearing.
When the shaft is pushed in, it can rotate continuously past 360 degrees, in other words it glides right past the stop pegs. The rear gang measures randomly in a way that is not consistent with logic. But when the shaft is pulled out, the pot behaves more or less as expected, although the two gangs still don't measure quite the same.
I found a datasheet online for the modern Spectrol model 100 as manufactured by Vishay, but none from the pre-Vishay era to which mine belongs. My pot has 5 pins per gang, but new ones only have three. I was hoping to find out how pins 4 and 5 are connected internally.
The 11258 ohm resistance rating that's printed on my pot seems kind of unusual.
My pot was once a fine, well-made piece of kit (pics follow) and I hope to refurbish it rather than replace it. I searched for info on vintage Spectrol restoration but came up empty. If anyone here has done this kind of work, any information will be greatly appreciated. Thank you!
PS - This guy wrote a piece on refurbishing a PAM-7, a Krell preamp similar to my KSP-7B that also has a Spectrol volume control, although his has model number 100-5528 whereas mine is 100-5529. His shows a 10 kOhm rating, which seems a lot more normal than the 11.528 kOhm on mine. He states that he has serviced the pot, without further details.
The Spectrol 100 volume control in my KSP-7B has gone funky. It looks like the shaft is able to slide in and out almost 1/8" (3-4 mm), as though it had a missing or broken thrust bearing.
When the shaft is pushed in, it can rotate continuously past 360 degrees, in other words it glides right past the stop pegs. The rear gang measures randomly in a way that is not consistent with logic. But when the shaft is pulled out, the pot behaves more or less as expected, although the two gangs still don't measure quite the same.
I found a datasheet online for the modern Spectrol model 100 as manufactured by Vishay, but none from the pre-Vishay era to which mine belongs. My pot has 5 pins per gang, but new ones only have three. I was hoping to find out how pins 4 and 5 are connected internally.
The 11258 ohm resistance rating that's printed on my pot seems kind of unusual.
My pot was once a fine, well-made piece of kit (pics follow) and I hope to refurbish it rather than replace it. I searched for info on vintage Spectrol restoration but came up empty. If anyone here has done this kind of work, any information will be greatly appreciated. Thank you!
PS - This guy wrote a piece on refurbishing a PAM-7, a Krell preamp similar to my KSP-7B that also has a Spectrol volume control, although his has model number 100-5528 whereas mine is 100-5529. His shows a 10 kOhm rating, which seems a lot more normal than the 11.528 kOhm on mine. He states that he has serviced the pot, without further details.
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looks like he makes a living doing repair and refurbishing work, so it's not too surprising he might not share tips.
but did you ask him directly? the worst that will happen is he says "no"
but did you ask him directly? the worst that will happen is he says "no"
Before you open the vintage pot I would suggest finding a similar pot on eBay to dismatle and learn how its assembled. Those are single turn pots maybe with conductive plastic or wirewound. The multiturn pots will be quite different in construction. Your description suggests the connection between the gangs has slipped or failed. The odd resistance may relate to a balance control or similar to get it all working right or the result of matching sections to close tolerance. When you open it share what you find. It may be oil filled which seemd to have been a buzzword at one point.
You're right, it is worth trying 🙂
My first thought was to look for a working vintage replacement, but those old Spectrols are extremely thin on the ground it seems.
While I would like to give restoring the original pot a try, I'm also prepared to replace it with a modern one if need be.