Found an old McIntosh amp *56k*

Update 1/25/12, for those getting here via search

My 1500's still going strong since I created this thread a few years back. Since then I've replaced the two burnt out resistors in the power supply with Dale CW-5 12k 6.5w resistors. I've also bypassed the rectifier diodes and larger capacitors with Vishay MKP 630v 0.01uF capacitors. This noticeably quieted the noise floor with headphones.

 

The RCA jacks were starting to go bad for me so I replaced 3 sets with gold plated ones from Ebay. This isn't a reversible operation- the existing jacks need to be smashed with a pair of pliers and eased out of their home. I also had to widen the holes a bit with a circular file to get the new ones to fit. Since this change I've added a turntable to my rig so I'll need to change those next- I have a channel that keeps going out due to the old jacks.

One word of caution: by design, the 1500 drives the 7591s very hot- beyond, in fact, the specs of modern replacements. I kept burning them up (red glow) until I made a few adjustments. I now run the 1500 with a 114V wall voltage and I keep the test voltage for the bias on the 7591s at less than 0.6v. I haven't had one die on me since I made these changes almost a year ago. I went through more than I care to count beforehand.

 

 

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Update: 5mp pics of the insides in post 8

 

A few months ago I dug up an old Mac 1500 amp in my grandparents' basement and forgot all about it when I found the electrical condition to be, well... not good.

Today I dug it up again and played around with it, nothings changed but this thing weighs a TON!

Front panel, off (it has a nice blue glow when on)


Back panel


Interior


Closeup of audio section


So, how does it sound? In its current state, pretty bad. For the first few minutes, it doesn't sound that bad at all. But, turning any knob on the front makes loud staticy noises and sometimes causes the left channel to go out. It can take a lot of effort but I can usually get it working again for a minute or two before it starts to fade away. After it warms up, everything gets muddy and pretty much turns into one big blur. Also, the left channel develops a rather annoying hum after some time.

So, it's obvious that at least one tube is bad, but this thing has 13 of 'em and I don't have a tester. Many have the McIntosh logo on them and may be original! I've heard that old McIntosh amps like this are pretty good, but how much real potential do you think this guy has? It's rated @180w but I have no idea when it was made. I don't know how much I should invest in it for repair or if at all.

Finally, what would be the resale value of this amp in its current state vs. repaired?

I have a feeling I have something with good potential on my hands, but I don't really know to what extent as I hail from the digital generation.

Edit:
I just tried it again after a few cycles spread out over the past several hours. It still hums but I think it just woke up! The sound has greatly improved after probably decades of unuse, and I don't have any problems with the stability of the audio anymore. It sounds pretty decent now, but I can tell that it's still not back to prime.

Edit 2: I think I just found the stability problem. Some of the switches on the front don't like being touched. I'll have to take a look if I can stay away from it long enough to let it cool down.

Edited by MCC - 1/25/12 at 6:59pm

I just opened up the bottom. WOW, there is MUCH more to this thing than I thought! Oh, and Sprague caps! I'm uploading new pics now, this time full-res 10mp.

Edit:

Excellent feedback, thanks!

Edit 2: Here they are, sorry for the huge size but this receiver is so complex I thought it warranted it. I did halve the size though since bayimg has problems showing 10mp pictures, but 5mp is still huge.

Entire inside
Top left
Top right
Bottom right
Bottom left

As you can see in the last picture, I also have a broken resistor... now to find where it came from.
Edit 3: Oh, duh... right above the shell.
Edit 4: The broken resistor appears to be wirewound, leading me to believe all of them are.

Looks like it's time to buy a tube tester, dig up my multimeter, and get to work!

At the time I turned it off for cooldown before I opened it up, it had already greatly improved. It's going through a sort of hyper speed re-burn-in. I'm only trusting it with KSC-75s but the sound is already blowing me away. I'm hooked. To think I actually let this receiver sit for more than a day.

Yes, I do. Thanks for the concern, these caps probably can indeed give quite a shock.

Thanks for the service manual link. I actually just bid on one but someone else has a higher max, so no wasted cash.