AJ scratch noise issue
I'm having a issue with Aleph J after overheated death while I was doing stupid over bias testing. FET Q6 is cracked and shorted to the chassis. Alumina insulator is also cracked. (I guess this would be the cause.)
Q5, Q6 and Alumina are replaced and everything started working again, EXCEPT a noise issue. I tried different PSU, so it's not PSU issue.
The noise sounds like a very quiet vinyl surface scratch noise, and it gets louder after the amp is turned off (for about 5 seconds or so). Healthy board doesn't make any noise while it's on nor after it is turned off. (except usual thump)
I'm wondering which component is damaged. 😕
Please help.
I'm having a issue with Aleph J after overheated death while I was doing stupid over bias testing. FET Q6 is cracked and shorted to the chassis. Alumina insulator is also cracked. (I guess this would be the cause.)
Q5, Q6 and Alumina are replaced and everything started working again, EXCEPT a noise issue. I tried different PSU, so it's not PSU issue.
The noise sounds like a very quiet vinyl surface scratch noise, and it gets louder after the amp is turned off (for about 5 seconds or so). Healthy board doesn't make any noise while it's on nor after it is turned off. (except usual thump)
I'm wondering which component is damaged. 😕
Please help.
Only one channel. Actually, my setup is multiple amp, so it is a channel for one of the high frequency driver.
Plasnu.
Is it possible to post pictures of working+nonworking channel; both sides of pcb?
Otherwise It's a little hard to help!
Anyway, when one channel is up, You can always compare meassurements.
Is it possible to post pictures of working+nonworking channel; both sides of pcb?
Otherwise It's a little hard to help!
Anyway, when one channel is up, You can always compare meassurements.
Hi Jesper.
Thank you for your replay. Cosmetically, there is no difference between those boards as far as I can tell. I throughly checked them with my old eyes. Bias and offset is the only thing i measured and they are fine. Anything else that I should measure?
pull out mosfets , test them (matching jig)
pull out JFets , test them (matching jig)
replace small bjts
check all resistors , both visually and with meter
plasnu.
I would replace Q4 for a start... not to say its a qualified guess, but it's sitting at the "output" ... next is the 2sj109
Rgds; Jesper.
I would replace Q4 for a start... not to say its a qualified guess, but it's sitting at the "output" ... next is the 2sj109
Rgds; Jesper.
Thank you, ZM. (as always). 🙂
I'll do it next weekend. I'll order parts first.
BTW, ZM's PSU motor run cap mod with ICAR 80uF seems like it has brought a considerable improvement with my AJ. Background of the music is more "quiet" "black" than without them. Thanks!
Thanks Jasper, I'll replace all of the active component first. I replaced only 2 of them so far...
I did a mistake of not using the jumper from -IN to GND for the input of one of the boards. After powering it up within a minute the DC voltage increased to 19.8V and got smoked near the big caps around C5/6 but not sure where exactly or which part got damaged. After checking with 6L6 realized that I need to jumper the input negative to the GND but still unable to pin point the damaged component. I did change the Q4 (ZTX450) suspecting it would have smoked but this isn't an issue.
Need your input to pin point the issues to make it work as I am yet to connect and setup the second board. Will do that only after I test and make this board to work.
Thanks
Need your input to pin point the issues to make it work as I am yet to connect and setup the second board. Will do that only after I test and make this board to work.
Thanks
Hi...
Best if you can take some pictures of the PCB. Both from top and bottom.
The schematic i attached, is this the correct one for youre build ?
Jesper.
Attachments
replace bjts
pull jfets out and check with matching jig
check output mosfets at least with diode test ; lift one end of source resistors ,to isolate them if needed ........ or just pull them out and check with mosfet matching jig
recheck zeners (if any) , recheck resistors ; lift one end of each if needed
pull jfets out and check with matching jig
check output mosfets at least with diode test ; lift one end of source resistors ,to isolate them if needed ........ or just pull them out and check with mosfet matching jig
recheck zeners (if any) , recheck resistors ; lift one end of each if needed
Not having a jumper on the input did not cause your problem. Check and be sure one of your mosfets is not grounded. Not hard to do with an ohmmeter. It would be the first thing I would check. A raised hole or a piece of metal under the insulator piercing the insulator is a common problem. More than likely it was a resistor that smoked.
Here are some of the pictures that I took for your reference.
I will check the grounding of all the mosfets with the Ohmmeter. And I hope that I haven't smoked the mosfets as they were matched quads 🙁
The resistors near the Q5 and Q6 were pretty hot when it smoked.
I will check the grounding of all the mosfets with the Ohmmeter. And I hope that I haven't smoked the mosfets as they were matched quads 🙁
The resistors near the Q5 and Q6 were pretty hot when it smoked.
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Well nothing to see 🙂
You have checked the PSU, without any thing connected? Also for right voltages, and polarity ?
Jesper.
You have checked the PSU, without any thing connected? Also for right voltages, and polarity ?
Jesper.
Yes the PSU is fine and outputs +/- 26.6V and I checked this without any output connections to the Aleph boards. By the way I am using the diyaudio stores universal PSU with 0.47ohm 3W resistors with 18000UF 35V Panasonic caps total 8.
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I checked all the mosfets and they are not grounded to the heat sink. Nevertheless plan to remove the board from the heat sink and check it once again if any metal piece is piercing as I drilled the mounting holes myself 🙂