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Old 25th May 2009, 04:13 AM   #1
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Default Yamaha EM-300 mixer troubleshooting

My band recently obtained an old Yamaha EM-300 powered stereo mixer for use as a PA. It works fine except for the right channel, which does not output sound. I have yet to open it up, as I can't seem to find a schematic, but I'm lead to believe that the problem is in the right power amp. Does anyone here know what to look for, or have a schematic of this console?
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Old 25th May 2009, 05:30 AM   #2
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Also, upon opening the console, I discovered two burnt 7 amp fast-acting fuses on a power supply sub-board. I'm guessing I should check the output transistors for shorts?
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Old 25th May 2009, 06:43 AM   #3
mjf is offline mjf  Austria
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hello.
looks like there was a "great short".
power off and
try to measure the output stage (transistors and resistors) of the working channel and the bad one (with a dmm,ohmmeter......) and compare that values.
perhaps you remove the fuses of the working channel too.
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Old 25th May 2009, 07:37 AM   #4
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Be sure to check all transistors which are direct coupled to the bad output transistor(s).
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Old 25th May 2009, 06:42 PM   #5
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i've rebuilt a few of those. output devices, driver transistors and a few resistors are usually all that goes. once in a while i've seen a bad VAS transistor.
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Old 31st May 2009, 09:12 PM   #6
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It appears that the output transistors are indeed blown, and of course, they are an obsolete type and nearly unobtainium. Does anyone know of any equivalent transistors to a 2SA747A?
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Old 1st June 2009, 03:04 AM   #7
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i'm guessing you need the 2SC1116A as well......... a good functional device replacement for these would be MJ15022/23. they're in the same beta range, they are 15A devices, while the originals are 10A devices.
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Old 1st June 2009, 03:27 AM   #8
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Quote:
originally posted by unclejed613
i'm guessing you need the 2SC1116A as well......... a good functional device replacement for these would be MJ15022/23. they're in the same beta range, they are 15A devices, while the originals are 10A devices
Yes, actually, thank you! I can't seem to find a suitable replacement for the 2SA747A, though, any recommendations?
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Old 1st June 2009, 09:25 AM   #9
Enzo is offline Enzo  United States
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He just suggested the MJ15022/23 pair. What is wrong with that suggestion? I would use them in a heartbeat. The Yamaha won;t much care, and they should work fine. AMps like this are remarkably flexible as to substitutions.

If ther is only one 747, then it doesn't have a partner to match to. If there are two on a side, then whatever sub you chose won;t be a 747 anyway, and it would be better to replace both for proper current sharing and thermal tracking. SO either way it looks like you are putting something else in there.

Allied sells the MJ15022/23 for $3.86 each. That seems reasonable against whatever an old Japanese number transistor would cost.
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Old 1st June 2009, 09:37 PM   #10
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Sorry, wasn't thinking, and didn't realize he was talking about replacements for both types! Thanks for the advice, I completely agree.
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