Yamaha EMX amp blows main fuse

[Harald Kapp]

On behalf of Joe:


Hi, I have been going around in circles with my latest project a yamaha emx 660 mixer amp. I sorted out the obvious problems to start with some diodes, a couple of output trannys. I also replaced the voltage regulators 7815 & 7915 which should, I believe, work as a pair. All screwed back together onto the heatsink and main fuse blew again. Replaced fuse and using my trusty 100 watt light-bulb short-tester got a healthy glow. Unscrewed the output trannys from the 'sink and still the bulb glowed. When i unscrewed 1 of the VR's the bulb went out. Replaced both again and screwed back down, bulb glows. So set about tracing diodes backwards to power supply - ALL ok. Powered the amp up again, leaving VR's unscrewed from heatsink and works a treat. I can screw either down to the heatsink and not get a short but with both a short occurs. Both are reading -ve from the case and normal from respective pins.
What am I missing or should I just insulate 1 before screwing down.
I am a hobbyist with some knowledge of this particular amp board but the first of about 15 I have rescued with this fault.
HELP !, and Thanks in advance.
Joe

 

[Harald Kapp]

Chances are that you're lteraly screwing things up (pun intended) by mounting the 7915 an 7815 onto the same heatsink without insulation.
On the 7815 the tab on the back of the chip is GND, on the 7915 it is input.
You need an insulating pad and an insulating bushing (e.g. like this kit) for at least one of the two components, I recommend for both of them in case something else is screwed to the same heatsink.

 

[shrtrnd]

Maybe, one of the Voltage Regulators itself is defective. Just because they're new, doesn't mean one doesn't have a defect.
Otherwise, I'd go with electrically isolating them from the chassis. Mica washes with heat sink grease.
Harald is right, that a positive and negative voltage regulator are physically constructed to BE positive and negative.

 

[jbmdisco]

First, thanks Harald for diverting me to the correct place.
Next thanks Shrtrnd for the advice. I have now insulated both and given them their own little heatsinks. A bit more poking about revealed that the 2 op-amps had been affected, these replaced and bench tested the amp came to life. However after re-assembly and switching on the test lamp glowed at it's full 100w. Another strip down and discovery of an output transistor casing shorting to earth. A mica film sorted this out.
I am realising some problems with the efficiency of D2650 and B1647 mosfet output t's. I have noticed as they are getting harder to get the quality of the casings is getting poorer. I have not, as yet, managed to source a good replacement by part number or specification but hope to do so quite soon as I have managed to aquire another 2 EMX amps using these parts. I am thinking that it may have been a part designed exclusively for Yamaha.

Thanks again lads for the help.
Joe