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MAC 4300V Service Manual Mystery

ColonelKurtz

ColonelKurtz

Are my methods unsound?
I am trying to adjust the power amplifier bias and the service manual says "Connect voltmeter test leads to TP1 and TP2 on the 'Heat Sink PC board...' " NOWHERE in the manual do I see the term "Heat Sink PC board" anywhere else except that one time. Maybe I'm blind, but I combed the manual twice. If anyone can shed some light on where the "Heat Sink PC board" can be found, it would be greatly appreciated.
 
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I was setting the second (left) one like I had the right perfectly in the middle of their recommended specs ( I set both to 8.5), when I slipped with the multimeter on the left poles just now (didn't have my grabbers) and pretty sure I smoked the bottom transistor... smoke and a mini fire... ya win some you lose some :rflmao:, but thanks a ton for sending the pic!
 
@jlovda the 4300V biasing procedure is a bit different from other Mac amps. The procedure is to set the bias for 8-10 mV between the two test points. You really can't do it with a Kill-a-watt.

@ColonelKurtz well that really sucks. I did something similar on mine. I bought mine with an absolutely fried driver board, and made a new PCB for it. But doing testing on the board I found it hit the PG earlier than the original, undamaged channel. When trying to compare voltages my meter lead slipped and I fried the original board. Still in the process of getting that working. Some of that saga is here https://audiokarma.org/forums/index.php?threads/mac4300v-fried-driver-pcb-options.1020671/

Sometimes "better" is the enemy of "good enough". I've been in enough of these rodeos to know better.

Yell if you need help, I feel like I know that damn board like the back of my hand. I'd suspect at least an output transistor, the driver for it, and the current mirror transistors as well as the bias transistor that's mounted down on the heatsink. And a few 100 ohm resistors and 1N4148 diodes. Be careful of lifted traces also.....
 
Oh good Lord I hope you're wrong :rflmao:I looked down in there and it looks like just the output transistor (I know, this means nothing)... I saw a tiny bit of smoke, pulled the probes, then at least 5 seconds later is when the fire started. Not sure how or what kind of current was building up to kick that off. That's what I get for being lazy and not getting alligator clips. Hoping its just the output transistor. Thanks for your help, and will consult if needed!
 
Yeah, unfortunately I expect it's a chain reaction. There's also a Zoebel network on that board but I wouldn't think that would have fried. The bias driver is mounted directly to the heatsink, not the board. You'll see that three conductor lead going to it.

Thankfully pulling the whole module takes 30 seconds. Unscrew the 3 or 4 screws holding down the heatsink to the chassis, unplug the rightmost 6-conductor plug, and lift the whole assembly up and then pull off the two wires connected to the thermal sensor in the back. Then the whole shebang comes out. Loosen two screws on the bracket holding in the PC board and you can pop the PC board out, and remove the plug from the bias transistor and the 6-conductor ribbon cable that goes to the heatsink board.

Can you tell I've done this a few times?!?
 
w3jn said:
Yeah, unfortunately I expect it's a chain reaction. There's also a Zoebel network on that board but I wouldn't think that would have fried. The bias driver is mounted directly to the heatsink, not the board. You'll see that three conductor lead going to it.

Thankfully pulling the whole module takes 30 seconds. Unscrew the 3 or 4 screws holding down the heatsink to the chassis, unplug the rightmost 6-conductor plug, and lift the whole assembly up and then pull off the two wires connected to the thermal sensor in the back. Then the whole shebang comes out. Loosen two screws on the bracket holding in the PC board and you can pop the PC board out, and remove the plug from the bias transistor and the 6-conductor ribbon cable that goes to the heatsink board.

Can you tell I've done this a few times?!?
Click to expand...

Thanks so much for your help. I was hoping everything you told me you lost was actually on the board in your pic, and that you had made a mistake... I put the Mac away for now, gonna be a couple weeks before I can get down in there... Thanks Again!
 
I bought it with a completely fried PC board and made a new one that largely works although the PG came on sooner than the other channel. WHen I was probing around the original board I fried it by shorting a couple transistors, being clumsy with the test leads. That crapout wasn't nearly as dramatic as the completely charred PC board that's in that linked thread. One output transistor, one driver, the bias transistor, one or two other transistors, a couple of 100 ohm resistors, a 68 ohm resistor, and a few 1N4148 diodes. But I still don't have it 100% working, it'll only go to about 8 watts before the distortion starts getting out of hand and the PG kicks in. I shelved it for a while as it was getting a bit frustrating.

Pay close attention to the 100 ohm resistors, 28.7 ohm resistors, and 68 ohm resistors. I think I might have fried a 560 ohm resistor also. Those low value resistors act somewhat as fuses when a transistor shorts and prevent further damage.
 
I think I'm a little confused... On my unit the smoke was coming from the board in the diagram attached here... I'm pretty sure that one is the only one I damaged... (attached here). Is the one attached here the one you're speaking of?my board.png
 
Maybe you got lucky. The only thing that can really smoke on that board, apart from the circuit board traces, is R37. Depends, I guess, on what you shorted with your leads, but in my experience this almost always leads to a chain reaction backwards - thru the drivers and bias transistor.

upload_2023-3-12_8-21-3.png
 
w3jn said:
Maybe you got lucky. The only thing that can really smoke on that board, apart from the circuit board traces, is R37. Depends, I guess, on what you shorted with your leads, but in my experience this almost always leads to a chain reaction backwards - thru the drivers and bias transistor.

View attachment 2824626
Click to expand...

Its a bit tough to see down in there, as I don't have time to take it out, but R37 is in fact what looks like is charred. Crossing fingers instead of wires...
 
Yep, just looked at mine and mine is charred also. THat explains why it won't bias correctly. I didn't even focus on that resistor, so I owe you a beer for bringing this up. It's a 68.1 ohm, 1%, 1/4 watt.

Don't be surprised if the bias transistor, etc are also gone. A bit of probing with a DVOM will tell all.
 
w3jn said:
Yep, just looked at mine and mine is charred also. THat explains why it won't bias correctly. I didn't even focus on that resistor, so I owe you a beer for bringing this up. It's a 68.1 ohm, 1%, 1/4 watt.

Don't be surprised if the bias transistor, etc are also gone. A bit of probing with a DVOM will tell all.
Click to expand...

Here's the arsonist... This view is from the bottom looking up. I'm pretty sure only this bottom transistor and maybe R37 above it was taken out... Hoping if I say it enough times, it'll be true.IMG_4876.JPG
 
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You have to remove the whole module - the board and heatsink assembly - as I described above. Easy. Then remove the two TO-3 outputs, making sure you mark which one came from where, then that board just falls out. And indeed it looks like that bias transistor has a chunk missing from the side.

Hardest part of removing the assembly is pulling the wires off that thermal switch on the bottom of your pic.
 
w3jn said:
Bummer.... bet you'll find a few more components bad.
Click to expand...

I'm pretty sure this unit is done. When I shorted it, I'm guessing I caused back voltage through everything like a lightning strike, now none of the inputs are working, etc etc... Its just chaos in there, now. I got the amp boards in perfect working order, now. The IC is probably fried. Its that front board behind the faceplate that is completely alien to me, and impossible to obtain. I think I might start parting it out, unless someone comes along and can say "you need this and this". What a bummer because I liked this weird Mac. It sounded great and looked great too. Hope you have better luck with yours.
 
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