Well at the risk of showing up in some more threads, I will explain my stupid move to maybe help someone else. I have banana jacks installed for the speaker out. They accept the standard plug with 3/4" spacing. Yesterday. I re-foamed a nold pair of Boston speakers and after I put them all back together I wanted to try them out. I was out of the two prong male plugs but I had some singles. Problem is that they have an all metal case. Not at problem as long as you are not trying to plug into an amp that is on. I should throw those away. Anyway, like the dope I seem to be, I plugged in the positive lead first and then proceeded to plug in the negative plug when I slipped and touched it to the other lead and the case of the amp. I saw the spark and said to myself, "that can't be good". So I shut it all down and got my DMM and checked. Sure enough 82Vdc on that speaker jack. So, I just finished puling the case apart and did a little testing. Kind of weird but maybe one of you have an idea what might be causing this so I don't have to wholesale replace everything. When I bring the amp up on my variac while monitoring the rails and the offset on the output, I see about -.5v up until about 40V rails, then it will gradually increase as the rail voltage goes up to around 1.5v. However, when the rails get to about 81V, the output will jump to around 79Vdc. If I shut down the power, the output voltage will sort of follow the rail voltage as it bleeds off but always stays within a couple volts. Once the rails are down to about 1-2Vdc, I can then bring it back up and the offset will be back to .5V. If I try to increase the voltage before it is completely down the offset will just follow it back up. All ideas are welcome.
Thanks, Terry
Thanks, Terry
Last edited:
No signal. I had turned on the amp first because I didn't like the the way is pumps the speakers when it comes on. Not a problem when I'm using two prong jacks. These stupid single metal jacks are another story. I won't try that again. Would have probably been OK if I would have hooked up the negative lead first.
Water under the bridge, I have the amp torn apart now. Going to have to find what I hurt. The outputs all measure OK with an ohm meter. Must be before them.
Water under the bridge, I have the amp torn apart now. Going to have to find what I hurt. The outputs all measure OK with an ohm meter. Must be before them.
At idle you would expect just a few mV and not much current even if shorted. Makes me feel like it may have gone south before you tried plugging in your speakers.
Check all pads for invisible bridges.
For documentation sake please report what you measured and the results when you say the outputs are fine.
Next time you get it up and running give it some burn in time into a dummy load.
Check all pads for invisible bridges.
For documentation sake please report what you measured and the results when you say the outputs are fine.
Next time you get it up and running give it some burn in time into a dummy load.
Hi Bob,
I posted a schematic in post #219 with the voltages on the working amp. I didn't include the outputs however.
One side is still working if you need more.
I did have a preamp hooked up but the volume was turned completely down. I'm just glad I saw the spark and didn't continue hooking up the speaker.
I posted a schematic in post #219 with the voltages on the working amp. I didn't include the outputs however.
One side is still working if you need more.
I did have a preamp hooked up but the volume was turned completely down. I'm just glad I saw the spark and didn't continue hooking up the speaker.
Hey Gajanan,
Yeah, I have to use a magnifying glass for checking the fine stuff. I am fairly sure there is no bridge at this point. I have been playing for 10 days now. I listen to it every day. I'm not diggin' the big turn-on thump though.
Do you guys have an idea what is causing it to swing negative when it reaches 80V. Its a little off to the negative side before that but when it gets to 81V it swings all the way negative.
Thanks, Terry
Yeah, I have to use a magnifying glass for checking the fine stuff. I am fairly sure there is no bridge at this point. I have been playing for 10 days now. I listen to it every day. I'm not diggin' the big turn-on thump though.
Do you guys have an idea what is causing it to swing negative when it reaches 80V. Its a little off to the negative side before that but when it gets to 81V it swings all the way negative.
Thanks, Terry
Hi Terry, it's not good to power up and then connect loads on amplifiers, I've seen folk's do this kinda thing many times before and stuffing amplifiers. damage happens so quick and it's no good retrying the amp again and it's just stress the components.
The components may check out ok how ever, when supply voltages are applied they will break down and give false readings.. one of the ways I work on faulty/damaged amps is use a bench psu made up of plus/minus 50vs with limiting resistors of 47 ohms on each rail to get things under way and once the unit is stable it's then reconnected back to it's psu for final load/testing. you could build a psu via using a spare transformer caps/bridge and fuse connections etc....the G RANDY SLONE book gives very good tips on this....
Regarding the big on thump this can be solve via inrush thermistor's or simple relay add on circuits/speaker protect will do aswel. Back to solving this new fault when things like this happen check your long tail pair transistors as these are in the direct feed back path
and can be damaged next is vas transistors/drivers and them s/c protect transistors etc.
use your working side as ref points...
Regards A.
The components may check out ok how ever, when supply voltages are applied they will break down and give false readings.. one of the ways I work on faulty/damaged amps is use a bench psu made up of plus/minus 50vs with limiting resistors of 47 ohms on each rail to get things under way and once the unit is stable it's then reconnected back to it's psu for final load/testing. you could build a psu via using a spare transformer caps/bridge and fuse connections etc....the G RANDY SLONE book gives very good tips on this....
Regarding the big on thump this can be solve via inrush thermistor's or simple relay add on circuits/speaker protect will do aswel. Back to solving this new fault when things like this happen check your long tail pair transistors as these are in the direct feed back path
and can be damaged next is vas transistors/drivers and them s/c protect transistors etc.
use your working side as ref points...
Regards A.
Last edited:
Amptech, I'm trying to understand the danger of how plugging speakers. Isn't the speaker relay doing the same thing as connecting the speakers to the terminals? Sure, the return connection is always made and the relay is in the hot lead, but my DH-500 had connected the speaker with the amp delivering a mid level signal almost every day for nearly 30 years. Problems could occur if the cables are shorted, though.
If, as in Terry's case, there is no signal and DC offset is in the tens of mV how much damage can it do to short that to ground. Isn't the bank of emitter resistors between each output device and the output enough to limit the current to a safe level when starting from mV? With 0R47s the current should have been ~0.1A per output device at a DC offset of 50 mV. In a PM Terry told me that this channel only had 7 mV DC offset, so the outputs should have survived shorting.
Anyway, If I understand your problem correctly, you get ~rail voltage on the outputs and the rail fuses are intact. This implies that the high current stages are OK.
If you somehow blew a rail fuse this will result in rail voltage offset. I've done this a couple of times on LowTIMs (errant probes and screwdriver) but simply replacing the fuse restored all to good graces. I presume that you have checked the rail fuses and still have a problem, though.
I'd set up the board like you did for checking your LowTIM amp before powering up. I don't have the schematic handy, but jumper the capacitor across the Vbe multiplier to keep the output stage turned off and jumper the to 1K that provide intermediate feedback to the main feedback trace.
Check for offset now. Likely you will still have the same offset. If not, check the predriver and drivers.
Check that one of the VAS protection transistors hasn't shut off one side, making the VAS go to the other rail. check the voltage across the emitter resistors on both sides of the VAS.
Check the voltages on the top of the zener strings. you should see 40V. What is the base voltage on the inputs? Measure inside the input capacitor, you should see 0V.
Measure the voltage across the resistors loading the input differentials. They should be symmetric.
Once you restore basic operation, my guess is the thump is caused by the CCSs or cascodes turning on at different times. Check that the proper or at least matching values are inserted on both positive and negative sides. The electrolytics in the front end could be at opposite ends of their tolerance bands or the resistors that you calculate based on rail voltage could be slightly different values. Either would change the time constant and affect front end turn on.
If, as in Terry's case, there is no signal and DC offset is in the tens of mV how much damage can it do to short that to ground. Isn't the bank of emitter resistors between each output device and the output enough to limit the current to a safe level when starting from mV? With 0R47s the current should have been ~0.1A per output device at a DC offset of 50 mV. In a PM Terry told me that this channel only had 7 mV DC offset, so the outputs should have survived shorting.
Anyway, If I understand your problem correctly, you get ~rail voltage on the outputs and the rail fuses are intact. This implies that the high current stages are OK.
If you somehow blew a rail fuse this will result in rail voltage offset. I've done this a couple of times on LowTIMs (errant probes and screwdriver) but simply replacing the fuse restored all to good graces. I presume that you have checked the rail fuses and still have a problem, though.
I'd set up the board like you did for checking your LowTIM amp before powering up. I don't have the schematic handy, but jumper the capacitor across the Vbe multiplier to keep the output stage turned off and jumper the to 1K that provide intermediate feedback to the main feedback trace.
Check for offset now. Likely you will still have the same offset. If not, check the predriver and drivers.
Check that one of the VAS protection transistors hasn't shut off one side, making the VAS go to the other rail. check the voltage across the emitter resistors on both sides of the VAS.
Check the voltages on the top of the zener strings. you should see 40V. What is the base voltage on the inputs? Measure inside the input capacitor, you should see 0V.
Measure the voltage across the resistors loading the input differentials. They should be symmetric.
Once you restore basic operation, my guess is the thump is caused by the CCSs or cascodes turning on at different times. Check that the proper or at least matching values are inserted on both positive and negative sides. The electrolytics in the front end could be at opposite ends of their tolerance bands or the resistors that you calculate based on rail voltage could be slightly different values. Either would change the time constant and affect front end turn on.
Last edited:
What I should of said was, I've seen many folks change and even add more speakers to amplifiers when they been full go at venues and some due to faulty leads have damaged amps plus I've had plenty of burnt offerings to solve over 20 plus yrs due to this kind of thing.
Even valve amplifiers have to be correctly loaded and not have cabs swapped out when in use due to voltage spikes...
Terry's amp might be lucky to have no damaged parts and just have rail fuse popped as this amplifier has short circuit built in that limit's and shunts drive voltage to ground via the steering diodes. There is also the implement of catch diodes across the o/p to each rail and this simply steers back emf spikes to the supply rails..
Those who have worked on the old peavey cs800 range would of noticed it to did the speaker pump at turn on/off and it used triac system that pulled the whole o/p stage to ground and blowing it's primary fuse, this type of dc trip circuit would take many good o/p's with it.. this type of circuit is still in may of there new amps even citronic use this kind of thing with mosfet o/p stage..I'm not a big fan of this method and wasting good o/ps.
Even valve amplifiers have to be correctly loaded and not have cabs swapped out when in use due to voltage spikes...
Terry's amp might be lucky to have no damaged parts and just have rail fuse popped as this amplifier has short circuit built in that limit's and shunts drive voltage to ground via the steering diodes. There is also the implement of catch diodes across the o/p to each rail and this simply steers back emf spikes to the supply rails..
Those who have worked on the old peavey cs800 range would of noticed it to did the speaker pump at turn on/off and it used triac system that pulled the whole o/p stage to ground and blowing it's primary fuse, this type of dc trip circuit would take many good o/p's with it.. this type of circuit is still in may of there new amps even citronic use this kind of thing with mosfet o/p stage..I'm not a big fan of this method and wasting good o/ps.
First of all, I swap speakers and amps all the time when I'm comparing them side by side. I'm not saying it is the best thing for them but I have been doing it for years now without a problem. I believe there was something wrong with this channel all along and I just didn't realize it because once it was up and running it sounded fine and even showed a clean wave on the scope. However, this was the channel that in the beginning was way off side to side. It is the one that finally started working after many scrapings around the tracks. Once it started working it actually had a lower offset than the other channel. Once I got the outputs all attached and the bias set, I let it play for hours while it was still all laid out on the table. I didn't really notice the big turn on thump until Anthony asked about it. I watched the speaker on the left channel as I turned it on and there was a pretty good hum and the woofer on the JBL 4425 came full forward for about two seconds then returned to normal and the hum would stop. I never looked at the right channel to see if it was doing the same but now I'm thinking that it wasn't because I hooked up my DMM to the right channel to see if I could see a big voltage spike and I didn't. Now I wonder if it was only the left channel and didn't check that one.
I'm going to try and do all the tests you just suggested as soon as I can figure out the terms you used so I can find the right components. I may remove the o/p section to make it easier.
I'm going to try and do all the tests you just suggested as soon as I can figure out the terms you used so I can find the right components. I may remove the o/p section to make it easier.
Last edited:
Evening Terry, the big thud and hum sound is normal due to the amp stabilizing it's self at power up. one thing, do you have a local music shop near you? if so pop in and ask them to power up a guitar combo amp on, I bet you hear thud/pop/hum..
Even H/H..Laneys..peavey's did the same thing.
I bet if you turn on any of your other amps they will do the same thing with speaker cone movement then normal operation, in short its the power caps taking change etc etc..
Use the noted schematic voltage readings and compare it to the faulty side also hook up the safety resistors in the fuse rails to limit current follow even use 47 ohm at 3w is fine or use your v/r transformer and lamp tester.
Regards A.
Even H/H..Laneys..peavey's did the same thing.
I bet if you turn on any of your other amps they will do the same thing with speaker cone movement then normal operation, in short its the power caps taking change etc etc..
Use the noted schematic voltage readings and compare it to the faulty side also hook up the safety resistors in the fuse rails to limit current follow even use 47 ohm at 3w is fine or use your v/r transformer and lamp tester.
Regards A.
I just tested all of my other amps, and not one moved the woofers on my speakers. I really think I was seeing some serious offset when this thing was starting up. It reminded me of a few years ago when I bought a Soundcraftsmen amp and just plugged it in with the speakers attached. I heard a similar sound and the speakers flexed. I shut it down and checked and had rail voltage on both sides. I considered myself very lucky that I didn't ruin the speakers. I never did that again. From that point forward, I always check for offset on the outputs before I hook up any speakers. The problem with this Superamp is it measured really good when up and running. It only did that for about 2 seconds and then stopped. You can be sure I will not hook any speakers up to this again until I'm sure that won't happen again. Also, I have about 6 guitar and bass amps so I checked them too. None of them moved the speakers that way this did.
The journey continues.......
The journey continues.......
Here is what I found today.
I ran the rails up to +-75V so it wouldn't go negative. I got the voltages shown in the attached drawing. After I took the readings, I lifted D7 & D8. That didn't change much but I was able to raise the rails to 87V before it would swing negative. When that happened, the offset would go from -1.5V to -50Vdc. Q12C went from +53V to +39V and Q13C from -46V to -53.
Attachments
God Morning Terry. Recheck the voltages placed near R66 and R65. Near R65 you have -2.0, but near R25 you have 2.5 and these points are (should be) connected by a trace. Or was that the measurement on the feedback trace?
Are these measurements with C12 jumpered?
Andrew brings up a good point. Is there an oscillation on the output?
Are these measurements with C12 jumpered?
Andrew brings up a good point. Is there an oscillation on the output?
Hi Bob,
Sorry I should have been more clear. The measurements are with a jumper across Q7E & C. Also with D7 & D8 lifted. I need to find a better program to use to post the voltages so I can gt them closer to where they go. The +2.5 is Q10E. The -2.0 is Q14B.
I will try to test for oscillation. How do I go about checking that? I can easily hook up the scope. Should I hook D7&D8 back up? What about the jumper on Q7? Would it be better to remove the outputs from the circuit for this?
Thanks, Terry
Sorry I should have been more clear. The measurements are with a jumper across Q7E & C. Also with D7 & D8 lifted. I need to find a better program to use to post the voltages so I can gt them closer to where they go. The +2.5 is Q10E. The -2.0 is Q14B.
I will try to test for oscillation. How do I go about checking that? I can easily hook up the scope. Should I hook D7&D8 back up? What about the jumper on Q7? Would it be better to remove the outputs from the circuit for this?
Thanks, Terry
To check for oscillation, just hook your scope to the output with AC input selected and look for anything.
That still is odd. You're saying that Q10 emitter is at +2.5, or +2.5 DC offset. Q14 and Q15 bases are tied together turning them off, yet their emitters are ~4.5 V more positive than the bases. To me this implies either Q14 is leaking or Q16 and 26 are. Check the voltage across R36. It should be 0 with Q7 jumpered.
Oops - that's the other part of the Leach check procedure I forgot. In addition to jumpering Q7
I'm going off the grid for a day or so, I'm not ignoring you. Hopefully someone else will chime in and help you sort it out in the meantime.
That still is odd. You're saying that Q10 emitter is at +2.5, or +2.5 DC offset. Q14 and Q15 bases are tied together turning them off, yet their emitters are ~4.5 V more positive than the bases. To me this implies either Q14 is leaking or Q16 and 26 are. Check the voltage across R36. It should be 0 with Q7 jumpered.
Oops - that's the other part of the Leach check procedure I forgot. In addition to jumpering Q7
- Temporarily tack solder two 100 ohm resistors to the back of each circuit board, one from the loudspeaker output to one side of R36 and the other from the loudspeaker output to the other side of R36.
I'm going off the grid for a day or so, I'm not ignoring you. Hopefully someone else will chime in and help you sort it out in the meantime.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.