Hi all. I’ve been restoring a Bogen AP-60 integrated amp. Only issue now is that the volume suddenly goes from quiet to overkill when turning the volume knob. Was mainly doing it on 1 channel. So I cleaned the pot and now both channels seem to be doing it, but maybe at a different volume setting than before (like at 5 instead of 8). Same thing happens if I connect to either the local or remote speaker connections. Does this just sound like a bad pot? Just checking if anyone had another idea in troubleshooting before replacing the pot. It’s one of those double knobs where the front and rear control the different channels. Lots of connections so don’t want to take the pot out unless it is the most likely culprit. It sounds great just the volume seems low and then spikes suddenly at a certain volume setting. Thanks
Volume spike issue
- Thread starter Spint
- Start date
Register to hide this ad
Also- as you can see from the pic I have the impendence selector at position 2 which I thought was 8 ohm but I’m not for certain. Here is a pic of that part of schematic but I’m really not sure how to tell what is what on the impedance selector. I’m running into some Heresy 2’s- which say nominal impedance 4 ohm on the back. Any way this could be an impedance mismatch and I’m on the wrong impedance selection?
Attachments
a quad pot? what the heck? Must be preamp in and preamp out sections for each channel. Bogen was somewhat fond of less conventional engineering, and that would be something I could believe they would have done.
What are you using for a source? If its something with a lot of output, it may just be that its hot enough to make the volume excessively touchy. An attentuator between the source and the amp may be a workable fix here. The chances of finding a replacement for that volume control are pretty darn low.
fair guess #2 is 8 ohms but without the owner's manual its hard to be absolutely sure. My assumption is 4/8/16/32 ohms, and thats in large part because I think those may be the same output transformers my DS-225 uses, and thats what I get for options. If you can confirm the transformer part number on the service manual I can verify that. P/N for the DS-225's iron is 83-308-000 and 83-308-010. Only difference is wire length.
What are you using for a source? If its something with a lot of output, it may just be that its hot enough to make the volume excessively touchy. An attentuator between the source and the amp may be a workable fix here. The chances of finding a replacement for that volume control are pretty darn low.
fair guess #2 is 8 ohms but without the owner's manual its hard to be absolutely sure. My assumption is 4/8/16/32 ohms, and thats in large part because I think those may be the same output transformers my DS-225 uses, and thats what I get for options. If you can confirm the transformer part number on the service manual I can verify that. P/N for the DS-225's iron is 83-308-000 and 83-308-010. Only difference is wire length.
any option to drop the gain on the Cambridge ? Often the very low part of the pots are where they get flaky. If you can knock it down a bit to allow the volume control to turn up a little more it may get less flukey.
and yeah if its LOMC this won't have enough gain. HOMC it probably does. This would have been built for standard mag cartridges in the 5 mv range. Could probably get by with as little as 2mv but cartridges tended to be fairly hot output back then.
What did you clean the pots with anyway?
and yeah if its LOMC this won't have enough gain. HOMC it probably does. This would have been built for standard mag cartridges in the 5 mv range. Could probably get by with as little as 2mv but cartridges tended to be fairly hot output back then.
What did you clean the pots with anyway?
Bill Ferris
Lunatic Member
, he probably didn`t use Caig Fader cleaner/lube spray, and used Deoxit instead and could have trashed the ganged pot`s carbon tracts. ! 
, 
, 
I first used contact cleaner- then I used some tuner cleaner for lube (both radio shack brand I think). I see no obvious physical defect with the pot- the inside of it is turning in sync with the knob.
It is very low gain (almost no volume at lower positions on volume knob) until you get to around 8 or 9- then suddenly it goes way too loud and distorted with only a slight movement of the knob. However it sounds great at the spot right before it "jumps/boosts"
The Cambridge doesn't have a gain control except for headphones as far as I can tell but I will double check the back of the unit when I get home today.
By the way- the unit has 2 phono inputs (magnet and crystal). I'm running the preamp into the aux input right now but the same thing happens when I tried a different input.
I'll try swapping to a MM cartridge or try a different source to see if the same thing happens.
If the pot is bad, then that can at least be replaced- just doesn't seem like its the pot since it happens on both channels now at about the same spot (even though the same pot controls both channels seperately).
It is very low gain (almost no volume at lower positions on volume knob) until you get to around 8 or 9- then suddenly it goes way too loud and distorted with only a slight movement of the knob. However it sounds great at the spot right before it "jumps/boosts"
The Cambridge doesn't have a gain control except for headphones as far as I can tell but I will double check the back of the unit when I get home today.
By the way- the unit has 2 phono inputs (magnet and crystal). I'm running the preamp into the aux input right now but the same thing happens when I tried a different input.
I'll try swapping to a MM cartridge or try a different source to see if the same thing happens.
If the pot is bad, then that can at least be replaced- just doesn't seem like its the pot since it happens on both channels now at about the same spot (even though the same pot controls both channels seperately).
mechanically they don't usually break, but the carbon track can dissolve with some types of cleaners. I've had no issues with CRC QD cleaner doing that but thats the only thing I specifically have experience with.
you can ohm check the pot sections and see what they do. I agree it seems less likely that both channels would fail in the same spot but without some testing we're all sort of guessing here. I've also never messed with this specific amp so I don't know if its just a thing that it does but its not something I'd expect it to do.
any chance you have a full schematic for this thing?
you can ohm check the pot sections and see what they do. I agree it seems less likely that both channels would fail in the same spot but without some testing we're all sort of guessing here. I've also never messed with this specific amp so I don't know if its just a thing that it does but its not something I'd expect it to do.
any chance you have a full schematic for this thing?
Okay- so I took some ohm readings.
I'll call the pots 1-4 from from front to back.
When testing the ends of 1, 3 and 4- nothing happens, no reading. Except on 3 it suddenly gives a way off reading when I get close to 10 on the volume (which is about where the issue happens when in use).
Wiper testing is inconsistent on these 3 wipers as well.
Only the 2nd pot seems to be working right.
Readings were taken without disconnecting anything (still in circuit)- does this matter?
So does this mean the pot is bad? Or do I need to disconnect it in order to properly test it?
I'm thinking I should just replace the pot.
From the schematic it looks like its 100K; realize I need to get a quad pot. And I'm guessing it has a "double" shaft to control both channels.
Just want to make sure I order the right replacement.
Thanks in advance!
I'll call the pots 1-4 from from front to back.
When testing the ends of 1, 3 and 4- nothing happens, no reading. Except on 3 it suddenly gives a way off reading when I get close to 10 on the volume (which is about where the issue happens when in use).
Wiper testing is inconsistent on these 3 wipers as well.
Only the 2nd pot seems to be working right.
Readings were taken without disconnecting anything (still in circuit)- does this matter?
So does this mean the pot is bad? Or do I need to disconnect it in order to properly test it?
I'm thinking I should just replace the pot.
From the schematic it looks like its 100K; realize I need to get a quad pot. And I'm guessing it has a "double" shaft to control both channels.
Just want to make sure I order the right replacement.
Thanks in advance!
yep, those mad bastards did it, level in and level out of the preamp circuits. Buffer immediately after the selector to drive it too. The only other thing I've run into with similar volume controls is a Pilot SP-216. I think maybe the Bogen PR-2 preamp does it as well.
Power supply has similarities to the Bogen DS-225 power amp too. Solid state, but wired in the same config with two HV supplies stacked on top of each other to get the 410 and 260v supplies both at low impedance for minimal voltage drop on the screens under load. Output stage is extremely similar also.
yeah I'm strange, I appreciate the uniqueness in designs like this. This is just not what you typically find in consumer gear. It works very well, but its more expensive to build it that way so mostly things were not.
anyway, exactly which terminal to what terminal were you testing on the pots? One end should be grounded on all of them. If you can read from the grounded lead to the opposite terminal it ought to give you somewhere in the ballpark of 100k on each with the volume all the way down. The reading from grounded end to wiper will depend where it is but the two that connect over to the treble pot should read an value, and the two that tie to the blend pot should read an other value. No idea what exactly, but they should be similar to one another. Looks like A and C go to the treble pot, and B and D will go to the blend pot. Not sure what order that physically represents but it shouldn't be too hard to work out. I'd guess A is closest to the panel but maybe not.
Power supply has similarities to the Bogen DS-225 power amp too. Solid state, but wired in the same config with two HV supplies stacked on top of each other to get the 410 and 260v supplies both at low impedance for minimal voltage drop on the screens under load. Output stage is extremely similar also.
yeah I'm strange, I appreciate the uniqueness in designs like this. This is just not what you typically find in consumer gear. It works very well, but its more expensive to build it that way so mostly things were not.
anyway, exactly which terminal to what terminal were you testing on the pots? One end should be grounded on all of them. If you can read from the grounded lead to the opposite terminal it ought to give you somewhere in the ballpark of 100k on each with the volume all the way down. The reading from grounded end to wiper will depend where it is but the two that connect over to the treble pot should read an value, and the two that tie to the blend pot should read an other value. No idea what exactly, but they should be similar to one another. Looks like A and C go to the treble pot, and B and D will go to the blend pot. Not sure what order that physically represents but it shouldn't be too hard to work out. I'd guess A is closest to the panel but maybe not.
something else I should mention from a closer look at the pics, it seems you've got a lot of old caps in this. They probably aren't doing you favors, and may actually be causing a fair bit of these issues. bad caps that leak DC voltage play hell with pots.
Looks like the section closest to the panel and the third one back are the B and D sections, so A must be the rear-most section.
Looks like the section closest to the panel and the third one back are the B and D sections, so A must be the rear-most section.
The wiper is the middle terminal on these pots (I confirmed this) so I tested the outer terminals and basically got no reading on 3 out of 4 pots. But wasn't sure if this method works without disconnecting the terminals- I can disconnect if needed just want to wait until I have more time to sit down with it.
All of the electrolytic caps have been replaced. But I left most of the molded caps in since didn't think these fail too much. I'm more used to guitar amps where vintage molded caps are typically left alone unless they are clearly bad. Maybe this isn't the case for home audio amps.
All of the electrolytic caps have been replaced. But I left most of the molded caps in since didn't think these fail too much. I'm more used to guitar amps where vintage molded caps are typically left alone unless they are clearly bad. Maybe this isn't the case for home audio amps.
if you're seeing open circuit, the pots are dead. You should see the resistance of the carbon track less whatever happens to be across them in the circuit, so not more than about 100k. Open means the carbon is fried or the wafer is cracked. Prob stuck finding same make and style pots to steal wafers from to fix this unless you can come up with a quad log pot and adapt it to work with the stock shaft and knobs.
if those are paper caps, those go bad all the time. If they are film caps, those tend to be reliable but still worth confirming no voltage leaks. If they leak DC it'll cook the pots. Instrument amps and the thought process with them is a whole thing that I don't mess with.
if those are paper caps, those go bad all the time. If they are film caps, those tend to be reliable but still worth confirming no voltage leaks. If they leak DC it'll cook the pots. Instrument amps and the thought process with them is a whole thing that I don't mess with.