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In Reply to: RE: Need MCS 3245 power supply repair help/advice posted by PJN on July 09, 2008 at 10:35:13
I see 4 caps that have boiled out, several "dry, overheated solder pads around some resistors, a chip off a high-temp power resistor, and a few other issues.. The devices you are wondering about are "transistors"... they are either NPN or PNP (pointless to go into a electronics lecture here..). They may be fine or not- you really need a "diode test" function on your DVM (I assume it's a digital tester), and the knowledge of how to interpret the readings. They are semi-conductor devices like diodes for testing purposes... In this portion of the circuit they will be common transistors, and very inexpensive (3904's or 3906's or something like that..)... you can replace everything if you wish, but something drew too much current from the power supply in the first place, and it may just smoke again... from your pic you have about $20 of parts to replace on this board (very approximately)... good luck...
Follow Ups:
Thanks for all the input. I've decided not to bother spending my time or my brother's money chasing repairs, there are to many unknowns and no guarantee of a good outcome. Besides as I recall I never was very impressed with the MCS, my old Kenwood and Sony recievers at the time sounded much nicer. I'll build a chip amp out of the parts instead.
Bad caps do not trip breakers or blow fuses except in the rare cases when they are shorted. That junk you see on the board is not cap crap, it is glue used during manufacturing. Sometimes when it turns color it becomes conductive but almost never enough to blow a breaker or fuse.
You don't want to give up yet ? Get a DVM and check the outputs. Doesn't matter what they read unless it is 0.000. That is a short when you use the -|> |- scale. You find that, remove them and the drivers. The other channel should work.
People always think caps, it ain't. Caps in the PS can cause hum and noise. Caps in the amp can cause loss of gain and response. I have only seen shorted big caps that break a breaker or blow a fuse in 40 years twice and one of those units was subjected to overvoltage, got 240 instead of 120.
I was the audio guy at three places I remember, and when I talked, they listened. We were always on a budget and that means not replacing $50 worth of parts when what it needs is fifty cents.
If it failed during a thunderstorm it could be the main rectifiers, get the DVM and I will tell you how to find out.
May I add? To spend ANY amount of money on this unit is of questionble merit, but I also understand FULLY the importance of "sentiment"- if it is THAT important, then spend as much as you can afford.... I suspect that you will smoke that same ps filter board instantly though... so get a variac and ramp up the 120 Vac slowly while monitoring the o/p voltage (speaker terminals) and stop the instant you see ANY increase in DC voltage- I'm betting the o/p transistors (one or both channels) or the driver transistors are shorted... so don't go too far down the trail unless it's that important....
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