Bose L1 Volume pot replacement

[DrGonz78]

So I got this Bose L1 on the bench with a bad volume pot on the main Mic channel #1. It is vertical pcb mounted 9mm type Alpha brand pot and it is a dual ganged 10k c type. So it is a reverse log taper which really makes finding the exact replacement part very hard. I have come across a 10k linear pot that will fit...
https://www.mouser.com/ProductDetail...24DBuU%2f00%3d

Do you think it will work to replace with a linear type pot instead of type C reverse log? Or if anyone can point me in the right direction to find the exact part that would be amazing! I called Bose and NO they will not provide part (of course right?) and it is not under warranty anyway. I have attached the schematic and the part is located on the first page VR100A and VR100B 10k c. Thanks.

BTW I did find a 10k reverse log panel mount (side mount rather than vertical) on ebay. My thought was to put wires going to the board and mount it to the panel, but that idea simply won't work as it's threads would not clear the mount hole. So now I am thinking the 10k linear might be my best option.

Edit: Also I did find one that is reverse taper and it would fit in place, but it is 100k value...

[The Dude]

If the shaft is broken or the contacts of the shaft, you could order any pot from the same series and remove the shaft for use with your existing pot. If it's not a broken shaft, I would carefully take it apart and determine what exactly the problem is. Sometimes you can give the contacts a little tweak and make it work again. If the wafer itself is bad, I guess plan B is in order. Of course it's best to just replace a bad pot, but sometimes desperate times call for desperate measures.

[DrGonz78]

If the shaft is broken or the contacts of the shaft, you could order any pot from the same series and remove the shaft for use with your existing pot. If it's not a broken shaft, I would carefully take it apart and determine what exactly the problem is. Sometimes you can give the contacts a little tweak and make it work again. If the wafer itself is bad, I guess plan B is in order. Of course it's best to just replace a bad pot, but sometimes desperate times call for desperate measures.

Oh yeah Dude this thing is a goner. I always check what went wrong with the pot and see if there is any hope. Unfortunately there was no hope in resurrecting the dead for this poor thing.

I have been searching even the most obscure resources of the internet for a replacement and nothing found yet. If I was going to panel mount something and rig it up then the shaft length would need to be longer. Also, if I did that then there is the spacing between panel and pcb to contend with too.

[DrGonz78]

Actually I don't even think I have successfully found a source for even a 10k linear pot yet. Looking over the data sheet the correct mount for top adjust is P09x5N --- in which it is P09 = model series, x = number of gangs, and 5N = Style. It seems that I could only source P09xN types which would only have side adjust styles. I have to look over another option on Mouser but it might prove to be a similar problem.

So I did find a B20k that is definitely the right mounting style, whilst I will double check my shaft length too. So would a 20k linear pot work okay if this is the only thing I can find??
https://www.ebay.com/itm/2-x-9mm-Alp...j-34FeJen0otHQ

[MarkusBass]

Of course it will work. But you need to solder the R171 resistor = 22k (it's open now). Otherwise, you would change the gain of the preamp. You may also consider changing R169. The value has to be calculated.

Mark

[DrGonz78]

Of course it will work. But you need to solder the R171 resistor = 22k (it's open now). Otherwise, you would change the gain of the preamp. You may also consider changing R169. The value has to be calculated.

Mark

I figured that it might take some other tinkering to get it right but that aint a problem. Ok so around 22k on R171 and the pot is a 20% tolerance at it's highest value. Lots of times you see a 10k pot measuring 9k or just above that really. Well possibly I could even look at lowering the value on R190 to bleed just a bit of signal to ground or is that overkill? What would be involved to calculate a fitting value for R169?

[MarkusBass]

I figured that it might take some other tinkering to get it right but that aint a problem. Ok so around 22k on R171 and the pot is a 20% tolerance at it's highest value. Lots of times you see a 10k pot measuring 9k or just above that really. Well possibly I could even look at lowering the value on R190 to bleed just a bit of signal to ground or is that overkill? What would be involved to calculate a fitting value for R169?

Don't "over-think" the problem. Of course, the pot may not have accurate value. Also the resistor may have some tolerance. If you have access to precise resistors, use 20k there. If not, use 22k and it will also be OK.
Changing the value of the R190 is not a good idea. You only need to calculate the value of the R169 resistor. I would expect to have the same voltage divider when the pot is in the middle position. I would also assume that the resistance of the 20k pot in the middle position is twice of the 10k pot. In other positions it will be just approximated.

EDIT: you could calculate the value of the R169 resistor (because you have half of the pot in parallel with additional resistor and then in series with the other half of the pot - they form a voltage divider) but I think it's not needed. By replacing the pot with twice higher value, you increase the resistance of each half of the pot (assuming they have the same tapper). And, in order to keep the same ratio of the voltage divider, you need to increase the resistance of the additional resistor twice as well. So the answer is 6k. I would use the nearest available value.

Mark

[DrGonz78]

EDIT: you could calculate the value of the R169 resistor (because you have half of the pot in parallel with additional resistor and then in series with the other half of the pot - they form a voltage divider) but I think it's not needed. By replacing the pot with twice higher value, you increase the resistance of each half of the pot (assuming they have the same tapper). And, in order to keep the same ratio of the voltage divider, you need to increase the resistance of the additional resistor twice as well. So the answer is 6k. I would use the nearest available value.

Mark

So funny that I almost posted 6k as the value for R169 since we doubled the pot resistance. It was more lizard brain in context as it seemed logical but only cuz my gut was saying so. Now I am thinking I should have gone with the gut feeling after all. Thanks again Mark and I will post back with some results later.

[DrGonz78]

So I got looking on the board to make sure that it matches up nicely with the schematic. However, on the schematic it does state R171 is open but on the board it actually does have a 4.7k resistor in place. At first I was concerned that maybe the schematic does not match up, but the good news is everything else does match. So does that mean I can simply add 4.7k to the value I wish to put into R171 and install a 26.7k resistor?

[DrGonz78]

So I replaced the C10k pot with a B20k and R171 now has 27k resistor. R169 got a 6.2k resistor and the mic settings seem pretty good on the amp. Mostly the amp does not get very loud until about half way up on the volume. Still waiting to here how the thing gets along at a gig.

[MarkusBass]

I thought that you planned 20k or 22k for R171.

Mark

[DrGonz78]

I thought that you planned 20k or 22k for R171.

Mark

Yes that was the plan and I did order a 22k resistor just in case. The schematic showed open connection at R171 but in fact there was a 4.7k on the board. So my thinking was to add that value and 27k was the closest I could find. Now I am waiting to hear back if the PA is working fine. It is a close personal friend and I have another piece of gear here that I am working on. So it won't be a big deal to just pop off the 27k and add 22k instead. Do you think that will be better?

[MarkusBass]

Yes that was the plan and I did order a 22k resistor just in case. The schematic showed open connection at R171 but in fact there was a 4.7k on the board. So my thinking was to add that value and 27k was the closest I could find. Now I am waiting to hear back if the PA is working fine. It is a close personal friend and I have another piece of gear here that I am working on. So it won't be a big deal to just pop off the 27k and add 22k instead. Do you think that will be better?

I think that you forgot to tell us that R171 is assembled on the board and its value is 4.7k . Without this information I suggested to use 20k or 22k to keep resulting resistance on the same level. Now, the information about 4k7 resistor changes everything. How did you come up with an idea that 27k is close enough to 4.7k? By using incorrect resistor you change the gain of the circuit. You have to calculate initial resistance 10k||4k7 and do the same calculation for the new pot. This is very basic calculation so I'll leave it as an exercise .

Mark

[DrGonz78]

I think that you forgot to tell us that R171 is assembled on the board and its value is 4.7k

No big deal but I did mention it in post #9. Did not hear back on that post so I kinda jumped the gun apparently. Oh well the thing is working pretty well for now but yes I want to get this just right.

I am figuring that 4.7k in parallel with 10k (original value) that it was getting max resistance of 3.1k or so. So now I have way too much resistance and see the error in my thinking. Now with 20k pot in place I need to match that max resistance of 3.1k right? So my simple math is telling that 3.7k resistor will get the job done. Does that make sense or did I get it wrong?

[MarkusBass]

I'm sorry - I didn't notice it . With 4.7 you get 3k2. To get the same value with 20k you need 3k8. The nearest value is 3k9. 27k seems way to high.

Mark