MC 502 Restoration

I bought a Mac MC 502 50 w/channel amp from a guy on Craiglist. I probably paid too much at $325, but hey, it's a Mac and I needed something in my garage.

I knew it needed new glass and several replacement lamps when I bought it. I got it home and popped the hood. I quickly realized there was PCB (p/n 045397) with the power & power guard lamps (four lamps total) soldered in rather than fuse type or wired lamps. No problem I thought - I ordered new bulbs off ebay and when they came in, I pulled the board and fired up my trusty soldering iron. Just then Murphy walked in with his damn law degree. The pads lifted from the board. DAMN! No problem, I'll call McIntosh parts and order a replacement board along with the replacement glass. Murphy was working that day and answered the phone, he told me they no longer make or sell the PCB. DAMN! I hate Murphy!

Anyway, to make a long story short, I used a CAD program to lay out a new PCB. Because the power lamps are on all the time and are first to blow, I made provisions to replace the two power indicator incandescents with LED's and a series resistor for each. If I want to use an incandescent, I just use a jumper rather than the resistor.

I ordered a dozen new boards and I can't wait to get them in and try it out. It'll be nice to see it lit up like a Christmas tree again. I'll post pics when I get them.

Mike

Mike,

You might have something there. You really do have to watch your heat on the iron when you replace those bulbs. I like the idea of migrating to an LED because the bulb will fail again when the amp is used regularly. I for one am anxious to see the finished product.

The boards came in! I'll post pics of the new glass and a populated PCB soon.

mikekohlmann said:

The boards came in! I'll post pics of the new glass and a populated PCB soon.

Click to expand...

Nice! :thmbsp:

Is it just me or do Macs sound better when all the lamps are working? :scratch2:

Anyways...I made and tried two different boards, one with all incandescents and a second with LEDs for the power lamps (red power guard lamps are still incandescents). Both look great, however, the power guard circuit is a little more complicated than just a simple 11.4VAC supply running the bulbs.

In normal operation, the power lamps (green) are illuminated and the red power guard indicators are off. When power guard is activated, the green lamps turn off momentarily when the red lamps come on.

Since LEDs don't present the load of an incandescent, they remain lit when the red power guard lamps come on. Not a huge problem and in fact, it's a livable compromise for me to know that the LEDs are running cool, drawing much less current, and will last longer than I will. (I guess I could install a dummy load across the LED's to simulate the load of an incandescent. Hmmmm)

I also swapped the two NE5534N opamps with Burr Brown OPA627B's. We'll see how these sound in the next few days.

This little amp really amazes me at how nice it sounds for only 50 watts and no autoformers!

Mike

BTW, if anyone is looking for a replacement PCB, I ordered 16 so I have a few extras. Just send me a PM.

How did the op amp replacement go?

The original NE5534 op amps are arguably a little slow for audio - low slew rate, which translates to limited high frequency range.

http://www.audioholics.com/audio-amplifier/amplifier-slew-rate)

But, they are quiet and have relatively low distortion. All in all, they weren't all that bad considering they are 35+ year-old technology. There are a LOT more modern op amps that will outperform them, but does it translate to better sound? Who knows.

Personally I think it's worth upgrading - the OPA627 is kind of old too, but has great specs, i.e., much higher slew rate, higher bandwidth, lower noise, and lower distortion. They definitely make the amp quieter, plus they're really expensive and I happened to have a pair just laying around.

I should test the amp's frequency range with both and see. Hmmm, maybe after I get the recap done I will. :scratch2:

mikekohlmann said:

The boards came in! I'll post pics of the new glass and a populated PCB soon.

Click to expand...

Hi. Not sure you’ll get this, but I’ve just destroyed my power guard pic and am wondering if you still have any of these beauties?

Thanks

Maybe @Mike Gibson can help you to message this member - not active here since 2015 ...

I cannot access his email to send him a message. I'll ask around and see if anyone knows it. If he hasn't been around in 5 years I doubt he's coming back anytime soon. I couldn't find anyone with that user name on Ivan's site either. Which doesn't mean he's not around. Just not under that name I can see.

Mike Gibson said:

I cannot access his email to send him a message. I'll ask around and see if anyone knows it. If he hasn't been around in 5 years I doubt he's coming back anytime soon. I couldn't find anyone with that user name on Ivan's site either. Which doesn't mean he's not around. Just not under that name I can see.

Click to expand...

Thanks so much.

MC 502 update:
Looks like I’ll be making my own Power guard pcb, and NOWHERE NEAR as sleek and beautiful as mikekohlmann’s.
Holy shite; several orders of magnitude nicer than the original.
Just an FYI: the original power guard pcb and flat wire in the 502 are pure trash.
Apologies to all the McIntosh devotees here, but the board and bulbs and flat wire feel like a total last minute afterthought.

I had a power guard bulb out on my MC502 recently and was kind of afraid that the board was going to be trash when I got in there to look at it. It turns out it was fairly easy to access (no need to remove front glass) and remove. I got a few factory bulbs from Absolute Sound Labs and replaced the one that was burned out. Works great now. The whole thing took about 20-30 minutes, which isn't bad, but kind of crazy they made changing a bulb so difficult....