Dannyx
New Member
Good day folks. I'm new here so hopefully I'm doing everything right and can get some help with this amp I'm trying to fix. I like to give plenty of details, so please bare with my long messages, since there's a lot to discuss.
So: it's a Technics SU-V550 - fairly old stuff - but the price was very low when I got it and it worked....to a degree. By that I mean it sounded fine in the left channel, but the right channel was all distorted (especially noticeable when setting the balance to the R channel).
Long story short, me poking around the inside trying to find the defect, removing components, testing them, RE-testing them, putting back in, etc resulted in this thing biting the dust...a couple of components went up in smoke and I couldn't get it going since, which is why I'm kindly asking for some help, since I read a similar thread for a similar amp (a SE-A70) over here and I was pleasantly surprised not only by the similarities between mine and that one, but also by the similarities of the defects. It was way back in 2010, so I hope it's ok if I link to it here.
The current state of the amp: main relay is not coming on, standby LED flashes normally for around 6 seconds, then begins flashing rapidly indicating a fault (when the relay was supposed to click).
Through my own wits and using the schematic I linked to here for your convenience (PDF is too large), I managed to isolate the problem to the R channel: the center bit of the current sense resistor Z302 read over 2v (measured with - probe of DMM on chassis GND), which according to the manual is interpreted as a fault by the "computer", so I began tracking down to see where it's coming from. By contrast, the L channel read nothing there.
ATM, to prevent further damage, Q309 thru Q316 are removed. During my latest "tests" today, I tried powering it on without D314 in place, since I was at my wits end trying to figure out why I got 4v on the sense resistor and why just about every transistor on that R is going to the full 50v rail. Q306 began smoking without D314 in place. I did the test with a series lightbulb between the amp and the wall socket - I imagine the reaction would've been more violent otherwise. I pulled the plug, removed Q306 and ironically it SEEMS to have survived: B-C and B-E give normal readings and C-E is "open" with DMM in diode mode. I put both D314 and Q306 back in and decided to have another go (YOLO)...this time Q72 started smoking, but same story: removed it, measured it, normal values. I'm worried it may have taken out IC71 as it gets rather warm to the touch. I already replaced it once. To make matters worse, the L channel now died as well - I can tell this because measuring the center bit of Z301 it never leaves -6v (with GND probe of DMM on chassis GND).
Starting to suspect it's not a transistor to blame, but one of those many diodes, since they're kinda "special" and hard to find: MA77W-A and 20A90 (=OA90A-R from what the other thread said).
I think that's enough for now. There's actually more info I could give, but I figure a wall of text will put people off. Please let me know what else you might need and thanks for everything. I'm sure we can get it going again. I DO have decent (if not above-average) know-how in electronics, but transistors have always eluded me
So: it's a Technics SU-V550 - fairly old stuff - but the price was very low when I got it and it worked....to a degree. By that I mean it sounded fine in the left channel, but the right channel was all distorted (especially noticeable when setting the balance to the R channel).
Long story short, me poking around the inside trying to find the defect, removing components, testing them, RE-testing them, putting back in, etc resulted in this thing biting the dust...a couple of components went up in smoke and I couldn't get it going since, which is why I'm kindly asking for some help, since I read a similar thread for a similar amp (a SE-A70) over here and I was pleasantly surprised not only by the similarities between mine and that one, but also by the similarities of the defects. It was way back in 2010, so I hope it's ok if I link to it here.
The current state of the amp: main relay is not coming on, standby LED flashes normally for around 6 seconds, then begins flashing rapidly indicating a fault (when the relay was supposed to click).
Through my own wits and using the schematic I linked to here for your convenience (PDF is too large), I managed to isolate the problem to the R channel: the center bit of the current sense resistor Z302 read over 2v (measured with - probe of DMM on chassis GND), which according to the manual is interpreted as a fault by the "computer", so I began tracking down to see where it's coming from. By contrast, the L channel read nothing there.
ATM, to prevent further damage, Q309 thru Q316 are removed. During my latest "tests" today, I tried powering it on without D314 in place, since I was at my wits end trying to figure out why I got 4v on the sense resistor and why just about every transistor on that R is going to the full 50v rail. Q306 began smoking without D314 in place. I did the test with a series lightbulb between the amp and the wall socket - I imagine the reaction would've been more violent otherwise. I pulled the plug, removed Q306 and ironically it SEEMS to have survived: B-C and B-E give normal readings and C-E is "open" with DMM in diode mode. I put both D314 and Q306 back in and decided to have another go (YOLO)...this time Q72 started smoking, but same story: removed it, measured it, normal values. I'm worried it may have taken out IC71 as it gets rather warm to the touch. I already replaced it once. To make matters worse, the L channel now died as well - I can tell this because measuring the center bit of Z301 it never leaves -6v (with GND probe of DMM on chassis GND).
Starting to suspect it's not a transistor to blame, but one of those many diodes, since they're kinda "special" and hard to find: MA77W-A and 20A90 (=OA90A-R from what the other thread said).
I think that's enough for now. There's actually more info I could give, but I figure a wall of text will put people off. Please let me know what else you might need and thanks for everything. I'm sure we can get it going again. I DO have decent (if not above-average) know-how in electronics, but transistors have always eluded me
Removing D314 was akin to putting bias to MAX and then some... that clamps bias.. without it allows pulldown to vs pullup without regulation... no wonder there was smoke
