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Help! Yamaha M70 protektion problem

OskarBas1

OskarBas1

Member
Hello! I've recently recapped my m70 and changed trimpots and some other parts. I was going to adjust everything today and things worked out great but wen I was setting the meter lights i encountered a problem. I used a 8ohm 300w resistor as a load starting with the right chanel, I'm putting in a 1khz sine through the RCA. I'm measuring with the multimeter on the speaker pots and adjusting the gain until I reach 40v (200w) and then setting the meter light with the trimpot. On the right chanel everything worked out as intended but when i was about to do the same thing to the left chanel the amp went in to protection when reaching about 24 volts on the speaker terminals. Anyone here that have a clue about the problem or experienced something similar? I don't know if the issue was there before i started working on the amp since I haven't played it that loud. Pleas help me! Thank you!
 
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I just made it go up to 40v. When powering the amp on and slowly raising the gain it came in to protect mode at about 24v. When in protect mode i removed the RCA making it go out of protection mode. And then i lowered the gain slightly before connecting the RCA again and then it was possible to increase the gain up to 40v om the terminals without getting the amp into protection mode. But if i turn the amp of and on again it goes into protect at 24v. Repeating the same thing again i can make it go to 40v. Seams strange to me but something makes me feel that this ain't to complicated to fix.
Thanks for further help!
 
The APS circuit switches rails from +/- LB to +/- HB when needed.
 
OskarBas1 said:
I've resoldered TR151,155,159 163 and 167 and change the relays. Still the same problem
Click to expand...
Something in that circuit is not working.. Did you check the transistors ? Especially TR155... Carefully look over all the associated resistors in that circuit for signs of over heating .
 
Judging from your inability to achieve 40v . I would wager that it is not switching to +/- HB on the left channel.
 
avionic said:
Something in that circuit is not working.. Did you check the transistors ? Especially TR155... Carefully look over all the associated resistors in that circuit for signs of over heating .
Click to expand...
I only resoldered the transistors? I'm a complete beginner, how do I check the transistors?
 
Hello! Haven't done anything to the amp in some time but now I got back to it.
Desoldered and testet all the components mentioned and all looks good. I've compared the measurements with the corresponding components in the other channel and also switched out tr151, 155, 159, 163 and 167 with the corresponding components from the other channel. Left channel still good in protection when reaching 24v and right goes all the way up to 40 without problems.

avionic said:
Judging from your inability to achieve 40v . I would wager that it is not switching to +/- HB on the left channel.
Click to expand...
what does this mean? How can it be fixed?

What should I look for next. Troubleshooting this is beyond my comfort zone and knowledge regarding electronics. Pleas help!
 
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