GrandDesigns
New Member
Ah yes, this makes perfect sense. Thanks for the simple explanation.
Bogen AP-60 found in the dust
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- ap-60 bogen bogen ap-60 tube amp
Ah yes, this makes perfect sense. Thanks for the simple explanation.
Resistor dividers are easy, whats not intuitive is the ratios relative to pot rotation. For a 100k log pot, at half travel it will be about 10k from wiper to ground, and about 90k from wiper to signal input.
Just as a rough place to start maybe try a 22k to ground and a 75k from signal input on the section being bypassed and see how that works out. Can always play with the values so long as it comes out to nearly 100k total.
Just as a rough place to start maybe try a 22k to ground and a 75k from signal input on the section being bypassed and see how that works out. Can always play with the values so long as it comes out to nearly 100k total.
GrandDesigns
New Member
I knew this potentiometer would be small, and yep, it's tiny. The PCB mount contacts will surely be a challenge to solder to cleanly. I ordered a pack of small assorted fiberglass proto boards that should have the same 2.5mm hole spacing as this thing. That should give me more room to solder up the factory connections without shorting the contacts at the pot, or melting the pot from over soldering the tiny terminals.
The shaft is also too short, as I figured, and the size is a tad bit smaller than the Bogen ones. So I'll have to work up an adapter or get an extender that will work with the factory knob.
As suggested, I was able to measure the resistance and get a low to high reading with the correct 2 terminals on each stage with a clockwise turn.
I diagramed all of the factory wires on paper as they are now and put matching color paint dots on each of the 4 different wire groups just to be 100% sure when I make the swap.
The shaft is also too short, as I figured, and the size is a tad bit smaller than the Bogen ones. So I'll have to work up an adapter or get an extender that will work with the factory knob.
As suggested, I was able to measure the resistance and get a low to high reading with the correct 2 terminals on each stage with a clockwise turn.
I diagramed all of the factory wires on paper as they are now and put matching color paint dots on each of the 4 different wire groups just to be 100% sure when I make the swap.
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GrandDesigns
New Member
I got the old pot out. Here are some photos of it for those interested. I measured the resistance of the terminals and yes it's flakey. On most of the terminals I was able to get reliable low (~ 8 or 10 ohm) resistance readings when the thing is turned 100% the way up or town, but the readings in the middle are either nothing, or jumpy. This seems to correspond with the uneven audio I heard a while back.
I wonder if there's anyone out there who rebuilds these things?
I wonder if there's anyone out there who rebuilds these things?
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You might be able to salvage some single or double pots to get the board with the trace and install them into your 4 banger. I assume you have doused the sections well with Deoxit fader lube at this point. The carbon traces ultimately will wear through and not work well. I'd bet you could find some inexpensive single pots of the correct style and do the rebuild yourself.
no chance you could take it apart for a deeper cleaning? How about IDing the manufacturer to get some donor parts to rebuild it? I've done that using single pots of the correct value from the same brand, with some care you can un-stack the whole works, replace the wafers, and stack it all back together.
I didn't realize this was a concentric pot
I didn't realize this was a concentric pot
GrandDesigns
New Member
Please confirm this is correct: The resistance should increase as the pot turns clockwise.
GrandDesigns
New Member
Thanks for the continued input. Yes I could try to rebuild the original 4 gang pot. That might be ideal here. I would need to educate myself on the makeup of the thing and then get some parts. I did work it over with deoxit and CRC contact cleaner orignally, however with all of the wires soldered to the thing it was difficult to get into it. Being removed from the chassis I can try to hit it again, however I suspect this was the most used knob on the unit and the internals are probably just worn thin and a cleaning won't help. But a rebuild could be in the cards. I will sideline that for now.
I did get the new 4 gang volume pot installed. It works perfectly as far as I can tell, giving a noise-free and correctly gradual up/down change to the volume as expected.
The bass control works great. The trebble control works but the high end frequencies are not as prominent as they should be, I don't yet know if that's the fault of the trebble circuit or somewhere else. I'm also getting some hum and there was a prior suggestion to add an additional ground and I haven't investigated that yet. So these problems can wait for now.
Now I have confirmed that I am only getting one channel of audio to the speakers, so I need to troubleshoot that. I know that the problem doesn't start at the inputs, as I can connect my audio source to AUX or MPX and still only get 1 channel at the speakers. If I swap my L and R input cables, I still get a good signal, although only to the one speaker.
One clue is on the 'dead' channel, there is a continual hum that is not changed by the volume knob or switching inputs.
So, what are some useful techniques to pinpoint the location of the problem? I don't have an oscilliscope to show me the signal at any given point, so that's not an option. I could use an external audio amp and probe the line level signal at various points to check for audio. I could feed a known good signal into the front of the amplifier circuit to rule out the parts that lead up to that point. I could swap pairs tubes since I have one known good channel of tubes.
I did get the new 4 gang volume pot installed. It works perfectly as far as I can tell, giving a noise-free and correctly gradual up/down change to the volume as expected.
The bass control works great. The trebble control works but the high end frequencies are not as prominent as they should be, I don't yet know if that's the fault of the trebble circuit or somewhere else. I'm also getting some hum and there was a prior suggestion to add an additional ground and I haven't investigated that yet. So these problems can wait for now.
Now I have confirmed that I am only getting one channel of audio to the speakers, so I need to troubleshoot that. I know that the problem doesn't start at the inputs, as I can connect my audio source to AUX or MPX and still only get 1 channel at the speakers. If I swap my L and R input cables, I still get a good signal, although only to the one speaker.
One clue is on the 'dead' channel, there is a continual hum that is not changed by the volume knob or switching inputs.
So, what are some useful techniques to pinpoint the location of the problem? I don't have an oscilliscope to show me the signal at any given point, so that's not an option. I could use an external audio amp and probe the line level signal at various points to check for audio. I could feed a known good signal into the front of the amplifier circuit to rule out the parts that lead up to that point. I could swap pairs tubes since I have one known good channel of tubes.
