Ok guys time to get to work xP
First order of business attempt to reach this little guy
Oops need some background info: customer (me) claims left channel spins 360. Amp has been tested seems to pass DBT and passes sound from guitar 🎸 (not the plan just testing)
So I figured the best way was to disconnect the front face plate. That did not leave me much room because the wires are tightly bundled and I do NOT want to open that Pandora's box unless I have to. So I laid it upside down and access was better.
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Then I saw I could unscrew the potentiometer bracket so I went at it with a sideways screwdriver but that was tedious I remembered a little ratchet set I bought a while back :tu:
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This has been educational because I learned there is a song by Eric Johnson called "zap"
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And no I won't be attempting that :lmao:
Now my volume pot is almost liberated
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I need to find a suitable replacement it's A250K
Also would you clip it off or desolder :loco
From you pics in the other thread it looks like it's an Alpha brand A50K (log) pot.
First before you even disconnect it I would pop the back cover off it and check if maybe it has just come apart. If the plastic piece with the wiper and stop has popped off the shaft you may be able to just press it on again.
Otherwise, if you have to replace it, Alpha pots are easy to come by, but you will want to match up the shaft so the know will fit right.
Maybe this: https://www.ebay.ca/itm/304482856805
Yes sir thanks
I might be up the creek trying to find a pot with the splined split long shaft. I think I looked at every one on eBay and Mouser last night :grr
None are even close to this length I need to measure it
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I know
I still need to attempt the repair
I guess I'm getting a taste of what the big boys have to deal with when doing this professionally :'(
"Hey man I fixed your amp. If you need to adjust the left channel volume just use a little flat head screwdriver 🪛"
:dbtu:
But I'm really excited about this amp :dbtu:
One thing I need to learn about is what happens if a different value k ohm potentiometer is substituted xP
If I need a 50k ohm
-what happens if I put a 10k ohm in
-What happens if I put a 100k ohm in
xP
What happens with different values depends on the circuit. It can affect the loading on the previous stage, change the knob settings, or make even bigger differences if it is in the feedback loop of an op-amp.
Sometimes what is done is rebuild with parts from a similar pot that is the correct value. You might be able to swap in the 'wafer' from an Alpha pot that has the correct value but the wrong shaft.
What is the length of the shaft ?
You will want to measure the splined shaft and the threaded shaft length and also add them together to be able to find a replacement.
The other option is replace both pots and also the knobs and you get a lot more choice to finding the pots, you can look at solid shaft pots or D shaft pots if you replace the knobs as well.
What is the diameter of the knobs ?
You can get "universal" solid shaft pots that have a long "cut to length" shaft.
Requires new set screw type knobs.
Thanks guys I got some measurements
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Also wow this thing looks tiny since I saw it last
I know G1 said to take it apart I don't know if I can handle it :grr
I found an old pot but it's 100k and looks to be PCB mount (?)
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Well I had to overcome my fear and not defy an order from my mentors
So I opened it up and it's not looking good
This black piece was loose
But now I see where the hickey is for the "stop" (I presume)
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Beyond that things get ugly(ier) quick
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EDIT. I seem have found a reasonable substitute except it is a stereo pot with two sets of terminals
The whole black plastic part is just pressed on to the shaft. See if you can pry/pop it off without damaging the wafer or carbon track.
Are there numbers/letters on the back of the metal housing?
Take a pic or post the numbers/letters.
Thanks guys I did manage to pry the black (wiper?) off
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I thought my fingernails would let go first I knew I couldn't pry against the black stuff and mess it up
:grr
Yes sir thanks it is an alpha A50K
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:tu:
Man that would RULE if I could just stick another plastic disc on there
Ok I ordered some pots xP
POT 50K OHM 1/10W CARBON LOG
DigiKey part number
118-PDB181-E420K-503A2-ND
Manufacturer part number
PDB181-E420K-503A2
EDIT
I figured this component was a gonner so I went ahead and desoldered it
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That pot PDB181-E420K-503A2 has a 20mm long shaft.
Is that long enough?
Maybe the PDB181-E430K-503A2 might be a better fit?
How long is the original?
Yes sir thanks no it might not be long enough idk xP
Quote from: saturated on October 19, 2025, 08:55:19 PMMan that would RULE if I could just stick another plastic disc on there
Any pot from that same Alpha series should have the same plastic part and you should be able to 'transplant' it over to your old pot.
Save the old parts in case you have trouble making the Bourns pot fit.
I got my replacement pot in today it's a little shorter but seems to be long enough
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I wigged out noticing the locater tab was 180 off but realized I put the old one back on wrong
Got everything soldered up and installed
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The first thing I did was hook up signal and scope but I couldn't get it to work :grr
I wanted to watch the amplitude change as I turned the volume control. I did manage to get input and output signal but seemed to be backwards to my horror :grr :'(
Moving right along I had to unhook my present jams and attach some speakers and a cassette player
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And I'm glad it's working :dbtu:
I'm glad the meters work and the volume is not backwards
I'm glad that both channels work.
I'm glad that surprisingly it's dead quiet when nothing is playing
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The thing that bothers me and that is for another day....is this thing is at like 3/4 volume and either the cops should be here or my wife trying to kill me.
So yeah it's not the loudest.
I have it at (for me) decent listening volume. If we were talking you might have to speak up or occasionally ask me to repeat something so that's cool.
But I thought like at half volume speakers would be melting :grr
And I know yeah there's probably many other factors xP
I'm noticing now the power light doesn't work.
But hey I'm loving it right now xP :dbtu: :lmao:
Been a crazy busy week and now I'm sitting here at 0430 rockin some tunes :dbtu:
Have you checked the spec for input sensitivity of this power amp? If it's designed for PA use it may need more signal than you are used to with hifi components.
Yes sir thanks
It did occur to me later that probably was kinda silly to just connect a cassette deck to it xP
I need to learn about all this stuff :grr
Also later I noticed the "power light" flickering on then realized that is actually a "clip indicator"
:loco
Also I probably need to set this amp to the side and not jack with it since I really like it and then go to work on the single channel version I picked up.