MA6200

Matt Mincer

Active Member
Hey All,

I have a MA6200 on the bench and the Power Guard LED on one channel is turning on a fairly low volume. It's definitely something the driver board because I can pull both boards and switch left for right and the LED follows the board. I have checked the transistors in circuit and they appear okay. Any ideas?

Also, does anyone know the bias adjustment procedure? Hifiengine has the schematic and the owners manual but neither shows bias unless I'm missing something.

Thanks
Matt
 
Okay, I'm just getting back to working with this amp. The board is a 045-398. All or the transistors test okay in circuit but we all know how accurate that is. I also switched the power transistors between the 2 boards so it's not those. Nothing looks burnt, bloated, or broken. The voltages I have tested seem to be what the schematic says and compare to the good channel. I may not have tested them all though. Take a look at the o-scope screenshots in the link below. I have a 1 kHz signal going into the aux input and an 8-ohm load on the output. The volume is about at 1/4. The right channel sweep looks clean and gets up to 23VAC before it thinks about turning on the power guard. The left channel has this weird cutoff top and the voltage never gets above 1 VAC. Also, it turns on the power guard with almost no volume. Any thoughts as to what's causing my problem? hate to just start swapping parts but that may be the solution.

Thanks
Matt

https://flic.kr/s/aHsmncmWcJ
 
Okay, I think I got lucky. While poking around, I noticed that R847 was a little darker than it's twin R845. It should be a 0.15 ohm but measured 6K. I don't have a 0.15 but put a 5 ohm in to test. It's not fixed but way better. Off to Mouser!

M
 
You need to determine why so much current was drawn thru the resistor. It is surprising it did not go open.....
 
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