I bought a USA400 on flee bay with a partially damaged channel a few years ago. Operates fine at normal volume levels, but when drive hard channel 1 craps out. The issue was intermittent and could be cleared with a smack to the front panel, but now fault is always present (at high power).
Issue description:
1) Channel 1 (or 'A') illuminates clip LED prematurely and output is clipped/distorted. As signal amplitude rises, clip LED becomes steady on and output collapses.
2) The turn on delay doesn't function for ch 1/A.
I have a copy of USA series 'Technical service manual'. I have measured all components that manual calls out for these 2 issues. Resistors all measure OK, Diodes OK and DC voltage measurements on relevant nodes seem fine compared to channel 2/B
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/SM_scena/QSC/qsc_usa-400_900_1310.pdf
I did find one of the big electrolytic caps loose and re-soldered it. No change in operation as expected (in parallel with others)
Didn't find any loose nuts, burns or other visual problems.
I also tried experimenting with current limit pots on ch 1/A under a partially clipped condition (speaker as load). Lowering limit makes the clip LED stay on, so the limit circuit appears to work. Given this and DC/voltage measurements, the problem seems to be elsewhere...
Next step is to trace input path with a sine wave/scope and hopefully find where things fall apart compared to the other channel.
Any other clues/recommendations welcome.
Thanks
Issue description:
1) Channel 1 (or 'A') illuminates clip LED prematurely and output is clipped/distorted. As signal amplitude rises, clip LED becomes steady on and output collapses.
2) The turn on delay doesn't function for ch 1/A.
I have a copy of USA series 'Technical service manual'. I have measured all components that manual calls out for these 2 issues. Resistors all measure OK, Diodes OK and DC voltage measurements on relevant nodes seem fine compared to channel 2/B
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/SM_scena/QSC/qsc_usa-400_900_1310.pdf
I did find one of the big electrolytic caps loose and re-soldered it. No change in operation as expected (in parallel with others)
Didn't find any loose nuts, burns or other visual problems.
I also tried experimenting with current limit pots on ch 1/A under a partially clipped condition (speaker as load). Lowering limit makes the clip LED stay on, so the limit circuit appears to work. Given this and DC/voltage measurements, the problem seems to be elsewhere...
Next step is to trace input path with a sine wave/scope and hopefully find where things fall apart compared to the other channel.
Any other clues/recommendations welcome.
Thanks
There are two 100 or 220 uF bootstrap caps for the op amp power supply on each channel. If they go bad the op amp power supplies collapse if the output swing goes above about 13 volts. Both of my 1310's did that (eventually) and new caps fixed it. As the caps age and lose capacity, the amp will start to limit earlier than it did when it was new but usually you don't notice till the caps are just plain gone.
Given a bang corrects it then I would go for bad connector or bad soldered joint.
A good close visual inspection might throw something up.
A good close visual inspection might throw something up.
Thanks for the info. The board is stuffed with 220u 35v as opposed to 100u on schematic. I have new caps to install will try channel 1/A first and if that works then swap out channel 2/B as well
When it was intermittent, I inspected, poked, pushed and wiggled everything to find the sensitive area, but without success.
It seemed to change when I swapped racks or even move the rack around, so chassis flex used to have some effect.
Will take another closer look.
Manuals are usually written by pro writers who don't have a clue. Usually before the product is even prototyped. All of those components are the least likely to change or blow at 10 years, unless the amp was run into a short and blew the output transistors. Then all kinds of driving resistors diodes and transistors will be blown by the rail voltage.
If the amp is over 10 years of age, I would do the last thing you said and see how far the two channels act the same. If a bad solder joint, or oxidized connector, the signal will stop at some point. Clean or resolder the blocking point.
Actually, I use a pocket radio for a signal source, tuned to a rock station so I can see the beats of the music on the AC VOM pointer, or the scope (none of my scopes work).
Components to check at over 10 years of age, #1 rail caps for capacitance and ESR. However, ESR/capacitance meters for 3000-20000 uf are $119. Personally, if volume is down, I just change the **** electrolytic caps based on the calender. The ones for the +-15 supply are also likely, plus any other electrolytic cap. I've put about 300 of them in various amps and organs since I quite working. Other aging components, volume pots, or switch contacts that affect the input. Trim pots anywhere can lose contact on the wiper. However those will show up on the follow the music with the scope test.
Best of luck.
Fixed!
Thanks for the info. I agree the manual is not for distant future diagnosis of component degradation.
'Output collapses under load' does point to E2/3. When I measured voltages on them I had no input signal so they appeared stable.
I started by swapping out the +/-15V rail caps and that solved the issue with channel 1/A.
😀
It appeared that channel 2/B then clipped before 1/A, so I replaced those caps as well. The amp seems to drive noticeably harder but perhaps a placebo effect....
Since I was in there I replaced the mute R/C cap and that fixed the short turn on delay issue. There is only one circuit that clamps both channels.
The other electrolytic cap in signal path is 'E1', it AC couples input signal. It is a 4.7u non polar. Perhaps this one could be replaced with a ceramic
I also have a USA series 1310 so I suspect that it's caps are in similar condition. I wouldn't say it is lacking power but I plan to replace at least the 15V rail caps in it too.
The cost of replacing all the big electrolytic is approaching what paid for the amps so going to just be happy with them for now.
Thanks again for everyone's help. I appreciate that the folk here are respectful and helpful. Not all forums are.
Thanks for the info. I agree the manual is not for distant future diagnosis of component degradation.
'Output collapses under load' does point to E2/3. When I measured voltages on them I had no input signal so they appeared stable.
I started by swapping out the +/-15V rail caps and that solved the issue with channel 1/A.
😀
It appeared that channel 2/B then clipped before 1/A, so I replaced those caps as well. The amp seems to drive noticeably harder but perhaps a placebo effect....
Since I was in there I replaced the mute R/C cap and that fixed the short turn on delay issue. There is only one circuit that clamps both channels.
The other electrolytic cap in signal path is 'E1', it AC couples input signal. It is a 4.7u non polar. Perhaps this one could be replaced with a ceramic
I also have a USA series 1310 so I suspect that it's caps are in similar condition. I wouldn't say it is lacking power but I plan to replace at least the 15V rail caps in it too.
The cost of replacing all the big electrolytic is approaching what paid for the amps so going to just be happy with them for now.
Thanks again for everyone's help. I appreciate that the folk here are respectful and helpful. Not all forums are.
Congratulations on an inexpensive repair.
I usually spend 4 to 10 times what I purchase a system for on e-caps. For the organs donated to me to keep them of the dump, infinity. A free H112 organ cost more than a convertible Mustang in 1968: took about $100 to make it sound better than a wet kazoo. With palladium plated key contacts rather than the rubber contacts in a modern keyboard, it should without further repair outlast a Nord Stage 2.
I usually spend 4 to 10 times what I purchase a system for on e-caps. For the organs donated to me to keep them of the dump, infinity. A free H112 organ cost more than a convertible Mustang in 1968: took about $100 to make it sound better than a wet kazoo. With palladium plated key contacts rather than the rubber contacts in a modern keyboard, it should without further repair outlast a Nord Stage 2.
I did same cap replacement on the 1310. Got it back together and tested it out, no change in performance. All was well for a short time.
I had claimed victory too soon. Channel 1 on the USA 400 exhibited the same behavior again after cap replacement. I assume just opening up the unit and poking around caused it to appear fixed. I bumped the front panel and signal is clean for now.
Then the 1310 went into shut down. I found a wire harness stuck in the cooling fan so got that fixed.
I also noticed that the low pass filter was on and set to 50hz, which is not what I want given my subs are tuned to <20Hz. I switched off the filter and re-tuned the gains. Had to take out ~8db on the subs vs mains. The low end is even more impressive now.
Will have to wait for the 400 to fail again and then try to troubleshoot signal path.
I had claimed victory too soon. Channel 1 on the USA 400 exhibited the same behavior again after cap replacement. I assume just opening up the unit and poking around caused it to appear fixed. I bumped the front panel and signal is clean for now.
Then the 1310 went into shut down. I found a wire harness stuck in the cooling fan so got that fixed.
I also noticed that the low pass filter was on and set to 50hz, which is not what I want given my subs are tuned to <20Hz. I switched off the filter and re-tuned the gains. Had to take out ~8db on the subs vs mains. The low end is even more impressive now.
Will have to wait for the 400 to fail again and then try to troubleshoot signal path.
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