Hello everyone,
First post here, hopefully someone can help me out diagnosing this SE-9021. The problem I'm having is an intermittent thump and crackling from the right channel. The problem sometimes goes away for a few hours but always comes back. All caps have been replaced prior and after being unable to bias the DC offset correctly, I started meausuring DC voltages across the board. All seem to be well except for the base of TR118, after each DC spike on the speaker terminals, here the voltage drops to +- 0.5V and lower. I'm also getting spikes on testpoint 104 and 106, which seems logical giving the circumstances. The problem seems to happen most in low cut mode instead of DC (not even sure what this means). My best guess would be a bad coupling cap or a transistor going bad but because all the cap's have been replaced, probably the second? I'm pretty new to hifi repair so any tips and tricks are welcome!
Thanks for the help!
Jesse.
First post here, hopefully someone can help me out diagnosing this SE-9021. The problem I'm having is an intermittent thump and crackling from the right channel. The problem sometimes goes away for a few hours but always comes back. All caps have been replaced prior and after being unable to bias the DC offset correctly, I started meausuring DC voltages across the board. All seem to be well except for the base of TR118, after each DC spike on the speaker terminals, here the voltage drops to +- 0.5V and lower. I'm also getting spikes on testpoint 104 and 106, which seems logical giving the circumstances. The problem seems to happen most in low cut mode instead of DC (not even sure what this means). My best guess would be a bad coupling cap or a transistor going bad but because all the cap's have been replaced, probably the second? I'm pretty new to hifi repair so any tips and tricks are welcome!
Thanks for the help!
Jesse.
Attachments
Welcome to the forum!
I would have attached the service manual for benefit of other members, but it’s too large. The link might work.
https://elektrotanya.com/cgi-bin/do...5iy3d&fid=320524&file=technics_se-9021_sm.pdf
Your problem might be a mechanical intermittent. Try poking/prodding with a wooden chopstick to see if you can provoke the problem. Be safe!
I’ll assume your problem is a defective component. This amp has flat gain down to DC; the “Low Cut” feature introduces a blocking cap C140 (about 2Hz).
Would you provide more detail about behavior when adjusting VR104? You seem to be describing transient glitches at the speaker, corresponding to step changes at base TR118. Am I interpreting correctly? You mention being unable to set offset to 0V. Using first step in adjustment procedure, with input shorted as prescribed? Please describe what you experience.
Thanks. Good luck!
I would have attached the service manual for benefit of other members, but it’s too large. The link might work.
https://elektrotanya.com/cgi-bin/do...5iy3d&fid=320524&file=technics_se-9021_sm.pdf
Your problem might be a mechanical intermittent. Try poking/prodding with a wooden chopstick to see if you can provoke the problem. Be safe!
I’ll assume your problem is a defective component. This amp has flat gain down to DC; the “Low Cut” feature introduces a blocking cap C140 (about 2Hz).
Would you provide more detail about behavior when adjusting VR104? You seem to be describing transient glitches at the speaker, corresponding to step changes at base TR118. Am I interpreting correctly? You mention being unable to set offset to 0V. Using first step in adjustment procedure, with input shorted as prescribed? Please describe what you experience.
Thanks. Good luck!
Hello and welcome,
Double transistor 2SA798A (TR101, 102 in your amp) is known to cause the intermitent/random misbehavior. Usually is replaced with a very well matched pair of KSA992's (thermally coupled together).
Not necessary related, but I think you should consider replacing variable resistors too.
Regards,
Cosmin
Double transistor 2SA798A (TR101, 102 in your amp) is known to cause the intermitent/random misbehavior. Usually is replaced with a very well matched pair of KSA992's (thermally coupled together).
Not necessary related, but I think you should consider replacing variable resistors too.
Regards,
Cosmin
Ok guys, thanks for the swift reply!
BSST, tried that myself too. Did not come to me the file might be too big. Thanks for providing the link! I tried the chopstick method by the way but to no avail, pretty sure it's a component... So first things first, I tried switching over TR101 and 102 to see if the problem moves to the left channel but this doesn't seem to be the case. I tried the alignment instructions from the start, shorting both 107,109 and 108,110 and I'm able to get the readings to approximately 0V. So adjusting VR104 gets me allmost exactly where I should be. Both channels seem to be drifting but the left one stays around +-5 mV while the right channel touches 10 mV and more. These readings are without the ouptut glitching. same goes for the second adjustment for the low cut operation.
So the amplifier is not longer misbehaving, hard to do any measurements. But I do recall the base of TR118 stepping down after each spike on the output.
I'm also gonna replace the variable resistors, adjustment is really a pain. Thanks for the tip Cosmin!
Thanks for the help,
Jesse
BSST, tried that myself too. Did not come to me the file might be too big. Thanks for providing the link! I tried the chopstick method by the way but to no avail, pretty sure it's a component... So first things first, I tried switching over TR101 and 102 to see if the problem moves to the left channel but this doesn't seem to be the case. I tried the alignment instructions from the start, shorting both 107,109 and 108,110 and I'm able to get the readings to approximately 0V. So adjusting VR104 gets me allmost exactly where I should be. Both channels seem to be drifting but the left one stays around +-5 mV while the right channel touches 10 mV and more. These readings are without the ouptut glitching. same goes for the second adjustment for the low cut operation.
So the amplifier is not longer misbehaving, hard to do any measurements. But I do recall the base of TR118 stepping down after each spike on the output.
I'm also gonna replace the variable resistors, adjustment is really a pain. Thanks for the tip Cosmin!
Thanks for the help,
Jesse
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Little update, after switching the transistors the problem DID move to the other channel. I only noticed after doing the adjustments again and testing for some time... So if I get this correctly Cosmin, you are suggesting I buy 4 KSA992 and heat shrink them together (or something like that) with some thermal paste in between. And just make sure the emitters are soldered together?
Regards,
Jesse
Regards,
Jesse
Hi, Jesse,
Yes, you're on the right path. Just make sure the replacement transistors are as close as possible regarding hFE parameter. For that, you should buy minimum 4 of them but make sure they are the same lot, in case you don't own some super-multitester (or better, a curve tracer) to match them. I bought KSA992FBU version from Mouser years ago, the entire lot is on paper reel. Make sure your supplier is legit.
Given their price, you could buy more of them to increase chance of matching, the unused ones in your actual project will be usefull on another.
Good luck.
Yes, you're on the right path. Just make sure the replacement transistors are as close as possible regarding hFE parameter. For that, you should buy minimum 4 of them but make sure they are the same lot, in case you don't own some super-multitester (or better, a curve tracer) to match them. I bought KSA992FBU version from Mouser years ago, the entire lot is on paper reel. Make sure your supplier is legit.
Given their price, you could buy more of them to increase chance of matching, the unused ones in your actual project will be usefull on another.
Good luck.