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    EPI M201 8" woofers

    I recently acquired a pair of EPI M201's, I noticed when I got them home that one of the top 8" woofers was completely seized :( After removing it from the cabinet, it appears the magnet became disconnected from the basket and tore the voice coil to shreds. I'm aware of the Human Speakers...
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    Sansui 5000A strange amp boards

    Working on a friends newly acquired 5000A and it appears to have aftermarket amp boards with totally different topologies than the F-1040 or F-6013 boards normally found in a 5000A. They use MJ4035 and MJ4032 darlington output transistors (the right channel OT's are fried). There was a tiny...
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    Yamaha M80 repair

    Finally got it! Figuring my base voltages were okay I wired in C-E resistors to simulate the bias current, yet again my base voltages were all out of whack :( I double checked the PNP emitter resistors and sure enough, one was completely open but looked perfectly normal. I installed a new...
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    Yamaha M80 repair

    Yes it is, I pulled it from the circuit yesterday and it measured 216 ohms. I guess my auto-range DMM might be throwing me off a bit, 400 mV is still 0.4 V. Still a difference from the PNP's of the same channel; is it close enough?
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    Yamaha M80 repair

    Prepare yourselves for a long-winded post. So as I was probing around trying to trace the low base voltages throughout the VAS stage I heard a POP, and watched my dim bulb tester light up like a Christmas tree. I can't say for certain whether I bridged a collector to emitter junction with my...
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    Yamaha M80 repair

    Thanks for the reply! I did readjust bias to 15 mv immediately after powering on with output transistors installed. It adjusts accurately and remains stable after warm up. The OTOvoltages above are with both channels adjusted to 15 mv after warm up.
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    Yamaha M80 repair

    Finally got around to working on this again. I installed the base/emitter and collector/emitter resistors in place of the output transistors. Powered on DBT and adjusted bias, no shorts! I re-installed the output transistors, powered on DBT, no shorts, relay clicks, and it doesn't go into...
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    Yamaha M80 repair

    No, from my understanding it was not in protect mode and could play from the right channel. I have new mica caps on order, hoping this does the trick! I also ordered some 1K/5W resistors to solder between the collector and the emitter of the power transistor pads to simulate the bias current...
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    Yamaha M80 repair

    My mistake! I think the light is reflecting funny off the top of that transistor in this picture, it is actually a TO-126. It does! It looks like he did re-did the binding posts too, very clean work. I did remove +/- pairs at a time, sorry for the confusion.
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    Yamaha M80 repair

    Thanks EngineerNate! To clarify, the 0.6V forward/2.35V rearward is the scent of something in-circuit bypassing the diode?
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    Yamaha M80 repair

    I also checked D129/D131, and D130/D132 (in circuit). D129/D131 measured ~0.6 volts forward, and ~2.35 volts rearward. Interestingly enough, D130/D131 measure ~0.6 volts forward, and nothing rearward? Not sure what the correct measurement should be while these are in circuit, I will pull them...
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    Yamaha M80 repair

    Finally back to this after the holidays and got to checking all VAS transistors and fusible resistors on the base leads... All checked okay! I did notice a few of the small stacked film capacitors around the 2x large caps in the amp section looked "blackened", I pulled C129 and C160 and both...
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    Yamaha M80 repair

    Thanks avionic! This is on my to-do list tonight!
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    Yamaha M80 repair

    Working on a friends M-80 that was re-capped and had the infamous glue completely cleaned and all local corroded components replaced. It was working fine until recently, when the left channel went out. First thing I did was unsolder all 8x output transistors to test out of circuit, all...
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    Sansui 9090db cycling in/out of protection

    I misspoke before, I meant to say I was measuring between the emitter and chassis when I noticed the slight intermittent shorting. That's when I started tracing the output and isolated the protection board. Speaking of... I finally got some light on this protection board. I took a closer...
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    Sansui 9090db cycling in/out of protection

    But if I take the protection board out of the left channel circuit, the short from collector to chassis goes away?
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    Sansui 9090db cycling in/out of protection

    Strange. I was checking for continuity from the collector to the chassis in the left channel last night and noticed it would intermittently show a slight connection. I pulled all the output transistors and checked the mica, I replaced one that had a crack but all the others were fine...
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    Sansui 9090db cycling in/out of protection

    I put the unit on mains voltage and it still powers up nicely, except I can't adjust bias any higher than 24 mA without it falling back into protection cycling. DC offset remains a stable +/- 10 mV unless it goes into protection (if I set bias any higher than 24 mA). Playing music (directly to...
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    Sansui 9090db cycling in/out of protection

    It's still on DBT with a 100w light bulb. I'm also noticing the caps on the left channel are slightly warm to the touch, compared to the right channel which feel cold.
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    Sansui 9090db cycling in/out of protection

    Aha! It took looking at with at with a magnifying glass, but I found C05 and C06 both had broken traces at their connections to R19/R20. I fashioned some direct connections with some clipped leads, soldered them in, powered on with the DBT, relay click, DIM BULB, and we have a stable amp! I...
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    Sansui 9090db cycling in/out of protection

    Thanks again Tom! With the amp board unplugged and pulled from the unit, I get a rock solid +/- ~60V (and the unit comes out of protection and remains stable). When I plug the amp board in, I do indeed get huge voltage swings at the +/- 60V power rails (also measured at the collectors on the...
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    Sansui 9090db cycling in/out of protection

    Thank you DougBrewster! According to this I was wrong about the polarity change, back to the drawing board :(
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    Sansui 9090db cycling in/out of protection

    Alright I changed the diode. It still cycled as it did before, bias was still steady at 30 mA on both channels, but there were still huge voltage swings on both channels. As I was poking around more, I noticed C09 and C10 appeared to have their polarity reversed (referencing the schematic, the...
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    Sansui 9090db cycling in/out of protection

    Thanks for your help Tom! I did monitor DC offset while it was cycling in and out of protection and it does spike up to >1.3V right as it kicks into protection. I pulled TR1 and TR3 from the circuit, both passed the 6 point check. While poking around I decided to re-check the diodes and found...
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    Sansui 9090db cycling in/out of protection

    Hello, I've got a lovely 9090db I'm working on that is showing some odd signs I'm hoping someone has dealt with before. To start, the unit had no output in the left channel, I found 2 defective output transistors and 1 bad transistor on F-2624 (TR05). It's worth noting I tested all transistors...
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