while setting, both inputs grounded?
while feeding the signal - was it SE ? if yes, neg input must be grounded
post pic of your setup arrangement
while feeding the signal - was it SE ? if yes, neg input must be grounded
post pic of your setup arrangement
This does not sound far off from my experience with either the all IRFP or Semisouth down version.
I never maxed out the Iq.
I initially set Iq for about 700mV because I was working with a 4U/300 equivalent chassis with lower dissipation.
The setting changes with temp equilibrium of the amp. In my case it would settle at about 675 mV after an hour (about 1.35A).
That setting was not far at all from the pots being centered and the Iq adjustment is definitely slower and less responsive than the DC offset.
Later I added additional heat sinking and cooling fans. That allowed me to approach 800mV at start up (775mV at equilibrium).
This activity sounds similar to my experience. The offset adjustment was quicker/more responsive.
I never got offsets near 2V…except during shut down.
My offsets were always less than 200mV. Are you sure your scale wasn’t set to mV on the meter?
Dancing was always present and especially affected by temp fluctuations like the lid being on or off or even standing over the amp and breathing.
20 minutes was not long enough for my amps to reach equilibrium…even working in my furnace room where my bench is.
What you described regarding setting offset and then it climbing would indicate you hadn’t reached temp equilibrium…you end up “chasing” it to no avail.
You can make an adjustment at start up, but it will change as the amp comes up to temp.
You should make your final adjustment at equilibrium.
But then guess what? It won’t be the same again at the next startup.
I had a cold offset between 75-150mV in all my amp channels.
They would stabilize near “0” at equilibrium.
They continued to dance no matter what in my experience…about 10mV or so.
You will be very lucky to have a small offset difference between a cold start and full temp equilibrium (say 100-150mV).
Evidently this amount of offset is acceptable (based on what I was told by ZM and others).
Don’t know that your sound test would be valid if the amp wasn’t adjusted properly…ZM or others would have to comment.
I’ve pulled the board out of the chassis already. I’m testing it by replacing one of the boards in the M2X amp I use everyday.
I have a soldered jumper between IN- and GRD. For bias and offset adjustments I connected a test clip wire between GRD and IN+.
Photo shows those connections. When the board is in the chassis it’s also connected to GRD, U+, and U-
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Thanks @chromenuts
I will set bias at 700mV and let it "cook" for an hour before having another go at setting the offset. And I'll double check the voltmeter units but I'm pretty sure it was showing several volts (not mV). And the way it would dance +/- 50mV around zero after adjustment, then a few seconds later climb to +3V on its own was strange.
I will set bias at 700mV and let it "cook" for an hour before having another go at setting the offset. And I'll double check the voltmeter units but I'm pretty sure it was showing several volts (not mV). And the way it would dance +/- 50mV around zero after adjustment, then a few seconds later climb to +3V on its own was strange.
Here's a video of the wandering offset voltage. Sorry for the vertical format; I'll do better next time :-}
In the video I make a one turn adjust with the blue tool and then leave it alone (it's off camera, but I assure you the pots are untouched from 00:25 to the end of the video). The offset goes to -3V, climbs up to zero over ~10 seconds, then drifts back down to -3V all on its own. The amp had only been on for 10 minutes, so I wouldn't expect to be able to zero out the offset, but shouldn't it be more stable and not vary that much while the pots are left alone?
I'll let it cook for an hour and try again.
In the video I make a one turn adjust with the blue tool and then leave it alone (it's off camera, but I assure you the pots are untouched from 00:25 to the end of the video). The offset goes to -3V, climbs up to zero over ~10 seconds, then drifts back down to -3V all on its own. The amp had only been on for 10 minutes, so I wouldn't expect to be able to zero out the offset, but shouldn't it be more stable and not vary that much while the pots are left alone?
I'll let it cook for an hour and try again.
Another video after being powered on for over an hour with the cover closed. I adjusted the offset to around zero, closed the cover again, then waited a minute for it to settle down before starting to record the video. The offset is all over the place. What could cause that?
Hey slk23
Sorry to see you’re still struggling with the offset issue.
As I mentioned, I had “drifting” present in the DC offset that centered around where I had managed to tweak it to…which was approximately “0” at temp equilibrium.
However my “drift” was on a lower level of magnitude (maybe 10mV) and I never had any more than 150mV or so of offset at either cold start or full temp equilibrium.
You’ll have to wait for ZM or someone else to help answer the question as to why your offset is so high.
Sorry to see you’re still struggling with the offset issue.
As I mentioned, I had “drifting” present in the DC offset that centered around where I had managed to tweak it to…which was approximately “0” at temp equilibrium.
However my “drift” was on a lower level of magnitude (maybe 10mV) and I never had any more than 150mV or so of offset at either cold start or full temp equilibrium.
You’ll have to wait for ZM or someone else to help answer the question as to why your offset is so high.
gimme a picture of your amp inside, of course with top lid removed, just to grasp visually what we are dealing with
now, one thing - are all LEDs lit?
second - same behavior in both channels?
I mean, in this moment I have no clue what's going on, sorta being in mental loop trying to decipher what you did in first place when you had smoking parts, and what could happen afterward
you say all resistors are in proper place
same goes for all semis
same goes for all diodes
you have even additional caps, so no way that any particular combination of parts is resulting in oscillation
time for staring at goats
pics
now, one thing - are all LEDs lit?
second - same behavior in both channels?
I mean, in this moment I have no clue what's going on, sorta being in mental loop trying to decipher what you did in first place when you had smoking parts, and what could happen afterward
you say all resistors are in proper place
same goes for all semis
same goes for all diodes
you have even additional caps, so no way that any particular combination of parts is resulting in oscillation
time for staring at goats
pics
The LEDs do light and everything else seems normal except for the output offset voltage. I suspect there’s a bad component causing that issue I’ve ordered a better desoldering tool so I can remove transistors, etc. for proper testing.
I’ve been focusing on only the left channel for now since I’m waiting for parts to complete the right channel board.
Here’s the inside of the M2X amp I’ve been using to test the Babel J2 board. I’ve simply been removing one of the M2X boards and replacing it with the J2 board. The M2X amp works fine (listening to it now) so I’m 99.99% sure my problem is in the J2 board.
I’ve been focusing on only the left channel for now since I’m waiting for parts to complete the right channel board.
Here’s the inside of the M2X amp I’ve been using to test the Babel J2 board. I’ve simply been removing one of the M2X boards and replacing it with the J2 board. The M2X amp works fine (listening to it now) so I’m 99.99% sure my problem is in the J2 board.
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One of my SissySIT builds was all over the place like this (bad, even by SissySIT standards 🙂 where the blame lies in the Tokin parts, not the fault of SissySIT of course 
) and I replaced one of the potentiometers to fix it. Try tapping the pots when it's more stable and see if that induces any jumping all over the place. Best of luck and hope you conquer this one soon!
Cheers,
Stephen
) and I replaced one of the potentiometers to fix it. Try tapping the pots when it's more stable and see if that induces any jumping all over the place. Best of luck and hope you conquer this one soon!Cheers,
Stephen
Status report
Left channel board:
I've replaced T201-T210 and removed, tested, and resoldered P201. All 5 LEDs light when the board is powered up. I set the bias at 750mV, gave it an hour to warm up with the chassis cover closed, but can't get the output offset to stabilize. It wanders +/- 3V over a minute's time.
Right channel board:
Replaced T101-T110. Did diode tests on the LEDs. When powered up all 5 LEDs briefly flash on, then go out. I shut off power quickly. Nothing on the board feels hot; only M102, M103, R133, and R136 got warm.
At this point my suspicions are on JFETs J101-J104 since I accidentally soldered a couple of the transistors into places where thru-hole JFETs would go on the topside of the boards. I removed them and did diode tests on the SMD J101-J104; they appear to be okay but I'm not sure if it's a valid test while they are in-circuit.
Any ideas or suggestions? I'm not sure why this build has been so problematic; my M2X and WHAMMY HPA builds were almost trouble free.
Left channel board:
I've replaced T201-T210 and removed, tested, and resoldered P201. All 5 LEDs light when the board is powered up. I set the bias at 750mV, gave it an hour to warm up with the chassis cover closed, but can't get the output offset to stabilize. It wanders +/- 3V over a minute's time.
Right channel board:
Replaced T101-T110. Did diode tests on the LEDs. When powered up all 5 LEDs briefly flash on, then go out. I shut off power quickly. Nothing on the board feels hot; only M102, M103, R133, and R136 got warm.
At this point my suspicions are on JFETs J101-J104 since I accidentally soldered a couple of the transistors into places where thru-hole JFETs would go on the topside of the boards. I removed them and did diode tests on the SMD J101-J104; they appear to be okay but I'm not sure if it's a valid test while they are in-circuit.
Any ideas or suggestions? I'm not sure why this build has been so problematic; my M2X and WHAMMY HPA builds were almost trouble free.
send me an email and I'll send you set for Steeplejack
closest thing I can think to B. J2, while you still have option to Schade, if you want more sugar
more straightforward to build, damn fine amp
actually, I'm listening it every day in my Workshop, pretty much from posting of thread linked above
closest thing I can think to B. J2, while you still have option to Schade, if you want more sugar
more straightforward to build, damn fine amp
actually, I'm listening it every day in my Workshop, pretty much from posting of thread linked above
Came back from a motorcycle trip to visit my brother... family reported some fuzzy distortion in the right channel while I was away (noise in the tweeter was mentioned specifically)... seemed to perhaps be intermittent, so I was told...the right channel was non-functional when I powered everything up this am to see what was what.....I ruled out everything else in the chain—appears to be something in the right amp channel.... I don't know what's going on—but I thought I'd get some process feedback from the group, maybe some bullet-pointed "rules of engagement" for amp issue diagnosis when all was well for months then not....
Check PSU voltages...then....?
(BTW, all LEDs that should be lit on all components are lit—haven't looked for anything else visual yet.) Schematic in hand....
(Way fun to listen to M2Xs again though!)
Check PSU voltages...then....?
(BTW, all LEDs that should be lit on all components are lit—haven't looked for anything else visual yet.) Schematic in hand....
(Way fun to listen to M2Xs again though!)
ZM, is this the universe telling me to install my donated Semisouth critters?
....After I figure out what's wrong with the right channel of course.....
....After I figure out what's wrong with the right channel of course.....
Okay ZM, 28VDC on the outputs... so, DC blocker tripped. OV on Iq, stone cold....PSU voltages check out, all LEDs lit. No visual signs of trouble...
first thing - diode/short check of mosfets
if suspicious, first compare same test with proper channel
dismount if needed
same fingerpoking for all semis on channel - again - compare with other channel
if in doubt, desolder part, check, replace if needed
if removing mosfets, re-check for burs on hsinks and re-new thermal pads/goop if needed
of course, when mounting new mosfets, wise to test isolation - must be open circ between mid pin (drain) and heatsink
I have all rights to remind, I made all mistakes and then some, at least twice
if suspicious, first compare same test with proper channel
dismount if needed
same fingerpoking for all semis on channel - again - compare with other channel
if in doubt, desolder part, check, replace if needed
if removing mosfets, re-check for burs on hsinks and re-new thermal pads/goop if needed
of course, when mounting new mosfets, wise to test isolation - must be open circ between mid pin (drain) and heatsink
I have all rights to remind, I made all mistakes and then some, at least twice