Oh pots... why have you forsaken thee?! These amps are higher maintenance than a Friday afternoon production Ferrari. Here we go again...
If you find that a channel suddenly sounds weak, before you start freaking out and shopping for tubes, CHECK THE POTS!!
The pots in these amps are absolute rubbish! I've already replace the mids pot on channel 2 a while back, now the treble pot on channel 1 crapped out, and within a few weeks the gain pot on channel 3. The typical give away is that suddenly the channel sounds weak tone-wise, or if a gain pot goes, gain will go up (weakly) as you dial from zero to about 6, and then start to drop again going to max. Overall, either way, the channel sounds like crap.
The pots in these amps look like this (just for illustration, the actual pots are bargain basement Alpha's);
The issue is shorting within the pot, from trace (pin) to the body (chassis). Not dead shorts, but in the order of between 200 and 300 ohms to 200 to 300 kilo ohms, which is enough to seriously suck the circuit to ground. Which pin will short is a crap shoot, I've found any one of the three can go bad.
While testing the upper row (clean 1 and OD1) is easy with the PCB in place, you'll have to remove it to get at the lower row. A pain, I know, but there's enough slack in the harness to work it out. The lower PCB can stay in for testing, but will have to be pulled out if there's a bad pot that needs changing. Again, there's juuuuust enough room to pop the lower board out with everything connected.
Testing is easy; simply test for resistance between each pin and the threaded part of the pot. Ideally, you should see infinite resistance. If you don't, replace the pot. Even if resistance is in the megohms, replace it. Megohm reading only serves to tell you that there is some continuity, which will only get worse with time.
Here (below) is a new A1M Alpha pot. I've tested resistance between the shaft bushing to all three pins (pin 3 test shown), and all test infinite resistance (OL) on my DMM;
Here (below) is the A1M (gain) pot I removed from channel 3. Pin 3 test is shown here with about 230 ohms resistance, but I also read about 1Mohm resistance from pin 1, and about 800Kohm on pin 2. Small wonder why channel 3 sounded like a tin can.
The other pot in the picture above is the treble pot from channel 1, a B250K. To the spindle bush, it read 280 ohms on pin 1, and about 170kohms equally on pins 2 and 3. YIKES!! Needless to say, channel 1 was sounding a bit flat.
EDIT - PLEASE NOTE, THIS TIP MAY APPLY TO OTHER EGNATER AMPS AS WELL. IF YOUR AMP USES THE AFOREMENTIONED ALPHA POTS, AND TONE/VOLUME IS WEAK ON A CHANNEL, CHECK THE POTS FIRST!
At some point, my plan is to remove all the original pots from these PCB's all together, and wire new ones to the PCB's. The PCB's I'll mount to the chassis with standoff's. This will effectively get rid off all the remaining shit pots, as well as remove any stress from the PCB's from pots that may be soldered to them slightly out of alignment. Don't get me wrong... board mounted pots do have a place in some designs, but this ain't one of them places.
I've said it before, and I'll say it again, it's a great amp... when it's working. Regardless, it's a wonderful case study of how an amp should never be built. A well deserved low blow; I've bought Berhringer crap that's been less trouble than this amp. Want to make America great again? Build stuff at home, and stop contracting out to <insert name of shit-hole low wage country here> to have stuff built. Build quality is "good from far, but far from good". The parts must have been sourced from a sub-contractor to the sub-contractor's uncle's cousin, twice removed, so QC is probably as prevalent as a working fire extinguisher in a sweat shop... non-existent. I don't mind paying more for quality, as long as the "more" doesn't go to lining your pockets with cash (or me having to buy replacement parts to fix the crap you sold me).
If you find that a channel suddenly sounds weak, before you start freaking out and shopping for tubes, CHECK THE POTS!!
The pots in these amps are absolute rubbish! I've already replace the mids pot on channel 2 a while back, now the treble pot on channel 1 crapped out, and within a few weeks the gain pot on channel 3. The typical give away is that suddenly the channel sounds weak tone-wise, or if a gain pot goes, gain will go up (weakly) as you dial from zero to about 6, and then start to drop again going to max. Overall, either way, the channel sounds like crap.
The pots in these amps look like this (just for illustration, the actual pots are bargain basement Alpha's);
The issue is shorting within the pot, from trace (pin) to the body (chassis). Not dead shorts, but in the order of between 200 and 300 ohms to 200 to 300 kilo ohms, which is enough to seriously suck the circuit to ground. Which pin will short is a crap shoot, I've found any one of the three can go bad.
While testing the upper row (clean 1 and OD1) is easy with the PCB in place, you'll have to remove it to get at the lower row. A pain, I know, but there's enough slack in the harness to work it out. The lower PCB can stay in for testing, but will have to be pulled out if there's a bad pot that needs changing. Again, there's juuuuust enough room to pop the lower board out with everything connected.
Testing is easy; simply test for resistance between each pin and the threaded part of the pot. Ideally, you should see infinite resistance. If you don't, replace the pot. Even if resistance is in the megohms, replace it. Megohm reading only serves to tell you that there is some continuity, which will only get worse with time.
Here (below) is a new A1M Alpha pot. I've tested resistance between the shaft bushing to all three pins (pin 3 test shown), and all test infinite resistance (OL) on my DMM;
Here (below) is the A1M (gain) pot I removed from channel 3. Pin 3 test is shown here with about 230 ohms resistance, but I also read about 1Mohm resistance from pin 1, and about 800Kohm on pin 2. Small wonder why channel 3 sounded like a tin can.
The other pot in the picture above is the treble pot from channel 1, a B250K. To the spindle bush, it read 280 ohms on pin 1, and about 170kohms equally on pins 2 and 3. YIKES!! Needless to say, channel 1 was sounding a bit flat.
EDIT - PLEASE NOTE, THIS TIP MAY APPLY TO OTHER EGNATER AMPS AS WELL. IF YOUR AMP USES THE AFOREMENTIONED ALPHA POTS, AND TONE/VOLUME IS WEAK ON A CHANNEL, CHECK THE POTS FIRST!
At some point, my plan is to remove all the original pots from these PCB's all together, and wire new ones to the PCB's. The PCB's I'll mount to the chassis with standoff's. This will effectively get rid off all the remaining shit pots, as well as remove any stress from the PCB's from pots that may be soldered to them slightly out of alignment. Don't get me wrong... board mounted pots do have a place in some designs, but this ain't one of them places.
I've said it before, and I'll say it again, it's a great amp... when it's working. Regardless, it's a wonderful case study of how an amp should never be built. A well deserved low blow; I've bought Berhringer crap that's been less trouble than this amp. Want to make America great again? Build stuff at home, and stop contracting out to <insert name of shit-hole low wage country here> to have stuff built. Build quality is "good from far, but far from good". The parts must have been sourced from a sub-contractor to the sub-contractor's uncle's cousin, twice removed, so QC is probably as prevalent as a working fire extinguisher in a sweat shop... non-existent. I don't mind paying more for quality, as long as the "more" doesn't go to lining your pockets with cash (or me having to buy replacement parts to fix the crap you sold me).
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