McIntosh MC2300 repair

So I had my good and faithful MC2300 short yesterday and crisped up two amplifier cards (there are 6-8 I think). Anyways... I’ve done lots of power amp repairs, but they’ve all been modern amps with exact parts available. The output transistors part numbers are: 132-070 or 32-070 depending on where I look haha but I think they’re the same thing. Now the exact parts are obviously not available anymore. I even herd one guy ordered a new one from McIntosh and they just sent him an NTE equivalent part haha. I hear a: MJ15003 is a good equivalent.

But here’s my actually question. I’m absolutely in love with this amp, and I’m just curious, is that a good equivalent part? Will I most likely feel a large difference when I change parts? If I am going to hear a huge difference, is there another higher quality part that I could look into? Cause this thing is the last think in the chain haha I don’t mind it getting better, but I fear it getting worse.

I very well may be over reacting too. Purely from the death of my good friend (the amp) that’s track every project with me in the last 3 years

I have successfully used 2N5303 to replace the outputs.
YMMV
Best,
Bruno2000

bruno2000 said:

I have successfully used 2N5303 to replace the outputs.
YMMV
Best,
Bruno2000

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Awesome! Thanks for the help! Did you notice any difference in the sound? I feel like the Mcintosh version was just a rebrand to start with and not really that special.

> not really that special.

Looks like a jellybean design to me. BIG jellybeans, but not magic beans.

Looks like 6 pairs but is really a 1-to-2 Darlington. Hard to judge the Q115 to Q119+Q123 ratio.

300W/6pair is 50W per pair, a conservative value.

300W/4pair is 75W per pair, safe for large TO3.

80 Volts max swing.

The nominal 300W at <28Vrms means the loading is less than 2.6 Ohms. From other data I suspect <2r, but there's some resistance in the OT. 40V peak in 2r is 20 Amps peak. 20A/6 is 3.3A peak per device.

The dead-short protection limit looks like 4.5A, a fair match for 3.3A on nominal load. (So we wonder how you blew it up??)

The total output section is a triple (at high levels), so not-much gain needed in each device. It would be typical of Mac to not design for magic beans, but let the circuit define what devices will do.

_I_ think selected 2N3055 would work, at least for a while.

I do NOT endorse literal 2N3055 for this beast. You never know what a '3055 is today. And there are better choices now. >80Vceo, hFE >20@4A. I am thinking Ft >1Mhz. MJ15003 is "more than you need", but readily available ($4 at Mouser).

Deviants from "normal" amplifiers:

Output autotransformer to make 100V peaks (300W in 16r) from 40V swing on transistors

Input amp gain of 11 so power section only needs gain of 3.6 (more NFB in simple driver)

Small 100V rail for large driver swing without bootstrapping.

PRR said:

> not really that special.

The dead-short protection limit looks like 4.5A, a fair match for 3.3A on nominal load. (So we wonder how you blew it up??)

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Yeah not quite sure either haha my new engineer flipped it on while I was in the other room. Just herd crackles and ran in to turn it off haha. I’m gonna go ahead and just do a full rebuild while I’m at it. Actually on 2 of them. Cause I need to service my B amp anyways.

I’m curious if it’s party due to the fact that I’ve been running it for about 10 hours a day for the last 3 years. Also, when this particular one was shipped to us, ups decided to roll down a mountain ( from the looks of it). It bent the front panel (which is like a 1/4in thick!!). Plus broke all the plastic mounts the hold the output cards/heat sinks in place. So I had to rebuild all the mounts and what not. May have just finally decided it wanted a  rest. So I’ll rebuild her and see how she does. Not looking forward to having to snag so many massive filter caps haha “$$$”

Thanks for your input!! Always appreciate great info from knowledgeable people!! Make my job so great to have people for help!!

Knotty said:

Awesome! Thanks for the help! Did you notice any difference in the sound? I feel like the Mcintosh version was just a rebrand to start with and not really that special.

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They sound the same to me.  I have 2 amps that I rebuilt about 20 years ago.  Great amps if you don't mind the weight.
Best,
Bruno2000

Knotty said:

But here’s my actually question. I’m absolutely in love with this amp, and I’m just curious, is that a good equivalent part? Will I most likely feel a large difference when I change parts?

Click to expand...

There is so much NFB in this circuit, individual performance of components hardly count. Just about the only parameters that count are those related to reliability (voltage, current and second-breakdown).