Hello guys...
Some time ago I posted a thread about my SU-3600 having noise... it was fixed thanks to helpful people as always here.
It has worked without issues for some time, until it started emitting pops and clacks and then I finally decided to stop using it and start finding the issue.
Last test I made burned the fuse with a 1kHz test signal and volume near first quarter, on 8 ohm dummy load, several times.
I replaced the capacitors with panasonic 105C as some of them had started leaking or were reading large esr, cleaned the boards with isopropyl as they had a lot of old flux (especially the power amp boards), and replaced trimmers as they were really oxidized...
Right channel adjusts correctly and I'm able to adjust ICQ and DC imbalance as stated by the manual, but this is not the case for left channel.
I found that D302 was shorted and TR304 dead (tested open, reads like a capacitor between C and E) - replaced TR302 and TR304 with paired 2SC733 (closest match I had at hand and same Hfe as TR302) and D302/D304 with 1N4007 I had at hand. ICQ adjusts fine, but VR302 has no effect on the imbalance voltage, it stays at 2mV or so, and I'm pretty sure this is the main reason the fuse blows (the amp is not stable anymore I would say)
Looking at the schematic and trying to find the reason VR302 has no effect, without success...
Maybe some people here would be able to help me direct my efforts?
I have all the equipment needed, but I'm not familiar enough with amplifier design to find the issue without some guidance... I know this place is filled with a lot of nice people ready to help, so I take the chance 🙂 Thanks in advance!
Some time ago I posted a thread about my SU-3600 having noise... it was fixed thanks to helpful people as always here.
It has worked without issues for some time, until it started emitting pops and clacks and then I finally decided to stop using it and start finding the issue.
Last test I made burned the fuse with a 1kHz test signal and volume near first quarter, on 8 ohm dummy load, several times.
I replaced the capacitors with panasonic 105C as some of them had started leaking or were reading large esr, cleaned the boards with isopropyl as they had a lot of old flux (especially the power amp boards), and replaced trimmers as they were really oxidized...
Right channel adjusts correctly and I'm able to adjust ICQ and DC imbalance as stated by the manual, but this is not the case for left channel.
I found that D302 was shorted and TR304 dead (tested open, reads like a capacitor between C and E) - replaced TR302 and TR304 with paired 2SC733 (closest match I had at hand and same Hfe as TR302) and D302/D304 with 1N4007 I had at hand. ICQ adjusts fine, but VR302 has no effect on the imbalance voltage, it stays at 2mV or so, and I'm pretty sure this is the main reason the fuse blows (the amp is not stable anymore I would say)
Looking at the schematic and trying to find the reason VR302 has no effect, without success...
Maybe some people here would be able to help me direct my efforts?
I have all the equipment needed, but I'm not familiar enough with amplifier design to find the issue without some guidance... I know this place is filled with a lot of nice people ready to help, so I take the chance 🙂 Thanks in advance!
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I continued to try wrapping my mind around this but cannot see why the trimmer would have no effect on the DC balance... Could it be that TR306 is dead also? I unsoldered it and tested it good on my component tester...
JC, this bring back memories. It was my first real amplifier like 47 years ago. Where did my life go?? May I please ask for a nice picture of it?
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2mV offset is pretty good if measured at the output with input shorted. The problem is likely somewhere else.
That's not about the 2mV DC output, that's about the fact that the trimmer variation does NOT vary the DC output on this channel...
I fear that for some reason the amp will not behave correctly if there is no control on the DC balance... it blows the fuse when volume gets upped to 20-25% and there is no reason I see except for the current being high enough to burn the fuse at a level where it should not do so, and I would say it has something to do with the input section... since there was an issue in this section, precisely the diodes and transistor connected to TR306, and VR302 has no effect, I cannot think of something elsewhere...
I fear that for some reason the amp will not behave correctly if there is no control on the DC balance... it blows the fuse when volume gets upped to 20-25% and there is no reason I see except for the current being high enough to burn the fuse at a level where it should not do so, and I would say it has something to do with the input section... since there was an issue in this section, precisely the diodes and transistor connected to TR306, and VR302 has no effect, I cannot think of something elsewhere...
I checked the voltages they are just off by a few hundred mV but in sync with the main voltages, and resistors are good, but I will recheck tomorrow
So... after checking and double checking, I finally found the issues
The 33 and 100 ohm resistors had suffered the years, one 33 was a bit toasted and one 100 had a leg fall off when unsoldering for replacement. Result, all 33 and 100 ohm resistors were replaces by 2W metal film
The wire from connection point (302) was also not soldered anymore, and for good measure I also replaced TR306 and the diff pair by 2SA564A and a pair of matched 2SA733
Also, the main reason I had no reading on the meter, to my utter shame, the fuse had blown again...
All this gave me full functionality again, dead silent at low volume and perfect sound, followed the alignment procedure and this channel is working again!
The right channel is starting to behave a bit also, with a flutterboating noise being present... I think I will give it the same treatment as left channel.
I have another issue though, is that the phono input preamp is reacting to adjustment only if it has a very high input signal level... not that an issue as the turntable I have has a line output level, but still, I would like to have it working fully... that will be for a next episode
The 33 and 100 ohm resistors had suffered the years, one 33 was a bit toasted and one 100 had a leg fall off when unsoldering for replacement. Result, all 33 and 100 ohm resistors were replaces by 2W metal film
The wire from connection point (302) was also not soldered anymore, and for good measure I also replaced TR306 and the diff pair by 2SA564A and a pair of matched 2SA733
Also, the main reason I had no reading on the meter, to my utter shame, the fuse had blown again...
All this gave me full functionality again, dead silent at low volume and perfect sound, followed the alignment procedure and this channel is working again!
The right channel is starting to behave a bit also, with a flutterboating noise being present... I think I will give it the same treatment as left channel.
I have another issue though, is that the phono input preamp is reacting to adjustment only if it has a very high input signal level... not that an issue as the turntable I have has a line output level, but still, I would like to have it working fully... that will be for a next episode