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23rd Apr 2020, 5:47 pm | #1 |
Heptode
Join Date: Feb 2014
Location: Nottingham, UK.
Posts: 649
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Yamaha DSP-A1000. LH o/p stage fault.
I've had this on the 'to-do' list for months... It had the front LH channel's mid point at 13v+. After around 15secs the supply relay dropped out. At first I thought output transistors etc, but after a few basic checks, all the power transistors all appeared OK....Hmm.
OK, after a careful look and check of the diagram, the base of Q329 was at 14v+ it should be 1.6v+. Hence the offset on the midpoint. Whilst access to the print side of the PCB was excellent, it wasn't marked and the layout from a scanned manual was difficult to read. Next step, check Q313/311. Q313 base was at 12V+ it's emitter was at 0.55v+ as on the manual, so it was completely turned off, and should turn on Q311. So the next thing I thought would be that Q319 should be fully on - it's b-e voltage was 0.7v+ forward bias. To that I thought that the long tailed pair (correct term?) was OK. After more going round in circles I thought I'd measure the voltage across R371 which was 0.4V, and was the same as the RH channel.... All I could think was the whole o/p stage had shifted positive. the current through the emitter resistors was the same as the good RH stage. Then after yet more coffee, despite my first checks, I'll go back to Q311/313. Both these transistors are connected to the 50v+ by two 27k resistors in series. So R351 and R353 should around 25v across each of them. R353 was totally o/c. when will I ever learn... After replacing this resistor the amp worked. The voltage at Q311/Q313 emitters is still 0.56+, but with the new 27k in place there is base current to properly turn on Q319. I've found that Japanese amps of this age rarely break down due to transistor failure. it's often misuse overload, or quite often liquid ingress that causes problems There's a small bit of the cct below. Cheers everyone SJM |
27th May 2020, 10:57 pm | #2 |
Dekatron
Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
Posts: 4,748
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Re: Yamaha DSP-A1000. LH o/p stage fault.
Good write-up! Well done
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