[MCC]

I'll definitely look into it. I still haven't quite decided how much of the restoration I should do myself, but it looks like professional help might be a necessity. Final calibration is something I don't have the equipment for. There's also a local place (Audio Labs) that does repair and may be able to help, as I'm weary of shipping this anywhere.

Another picture:

Under the preamp hood (more of those pesky transistors-some of which are in the signal path btw)

 

[jamato8]

I have caps from the 1920's that are still good but who knows. Yes you are correct, the caps need to reform and leaving the unit on will do that. Caps can read very poor when not used for years and over a couple of hours can regain the capacity they should have. Everything needs to burn in and settle. Also run the volume control back and forth to clean the wipers. I would also pull the tubes one at a time and reinsert them just to clean the pins a little as there may be a little corrosion.

If the amp wasn't used very much the tubes may well be perfectly fine. There is no reason a sitting tube, unless it leaks/gets gassy, will go bad. Again I have tubes from the 1920's that I use and are just fine but then I do have testers.

 

[nelamvr6]

That's actually a pretty cool find!

First things first: that's not an amp. What you have there is a receiver, and it appears to be a MAC 1500, which was first introduced in 1965. Production of that model was discontinued in 1967, making that unit at least 40 years old.

 

[MCC]

V4, a tube that has to do with FM tuning, just died. Dead. Off. No heat. I don't care about FM, but is it safe to run my receiver without that 6AU6?

I was careful in removing and cleaning them as suggested, using removal techniques I googled and a disposable antistatic cloth (no finger contact). V4 worked for a few minutes before I saw it cool down.

Anyways, most tubes are indeed original, several were replaced with Telefunken tubes and one tiny metal one I overlooked before is Sylvania. I found this thread which names this as McIntosh's first receiver. I'm going to try what they specify there since I have pretty much all the same problems. I now realize that I'm missing 4 dummy plugs that should be on the phono inputs.

Tomorrow I'm going to get a better multimeter and test each and every resistor and capacitor that doesn't have to do with stuff I don't need (RIAA phono stage, FM). Now all I need to do is find where Q15-20 are hiding as they are in the direct signal path and I plan on replacing them. Those 6 transistors are quite elusive.

Edit: Just found this

http://www.audioasylum.com/scripts/d...ubeprimer.html
Quote:

One of the most common causes of problems in older amps (besides tubes)is the b+ filter capacitors. After 20 years or so, these things go BAD!It doesn't matter if the amp was never used since from the day it was built, since capacitors just deteriorate with the passage of time. Macintosh used very good quality caps in their amps, and there are many with original caps out there that are still running fine.

Very reassuring. Less work (and cost) for me!

Also, scratch the transistor replacement. I doubt a 5 cent fairchild 2n3391a will sound better than what's already in there. If I figure out which are which (they aren't labeled) I'm going to shuffle them around a bit to see if a bad transistor is causing the background hum in the left channel. If not, they stay unless someone corrects my assumption.

 

[nelamvr6]

Yeah, the boys at Mac were pretty particular about the components they chose, I doubt you'll gain much by replacing any semiconductors, unless they've actually failed.

If you can do most of the work yourself and you manage to get this into good shape it'll be quite the windfall!

 

[MCC]

I've noticed that there are four bias adjustments on this receiver, I'm assuming one for each output tube (7591). Are the other tubes self biasing, or am I completely misunderstanding the concept? I haven't made any measurements yet but the fact that I have a cracked resistor on one of these manually controlled tubes is a pretty solid indicator of incorrect bias, correct? The resistor connects to the cathode on pin5 if I'm looking at it right.


Pic of the section I'm referring to

 

[joelk]

Quote:

Originally Posted by MCC /img/forum/go_quote.gif I'll definitely look into it. I still haven't quite decided how much of the restoration I should do myself, but it looks like professional help might be a necessity. Final calibration is something I don't have the equipment for. There's also a local place (Audio Labs) that does repair and may be able to help, as I'm weary of shipping this anywhere. Another picture: Under the preamp hood (more of those pesky transistors-some of which are in the signal path btw)

Paul's in the western Chicago 'burbs, within a reasonable drive for you.

 

[tbonner1]

Those were the good old days....
Build quality was great and built in the USA by people who cared.


Notice this was not really a "true" Macintosh but labeled a MAC.

 

[twc5964]

Good lord....looking at the photos underneath

It most certainly will be a challenge but I would replace every wire,capacitor,and resistor.Clean the pots with good quality contact cleaner/enhancer.That will take time but probably worth it.I would need help with the tubes and transistors if they needed replacing not to mention aligning the tuner. It's a beautiful receiver and being in your family for so many years in my opinion would be worth the effort. I wonder how long it's been sitting in the basement ?

 

[MCC]

Well, it looks like I'm going to have to wait a bit to start testing. I can't track down a good multimeter nearby that doesn't cost $90. I also went to the nearest ratshack to find that mall stores don't sell resistors or any parts of any kind! Isn't that what ratshack is for?

 

[twc5964]

I know it would make things much easier to find all you need locally but Michael Percy prolly has everything you need for that reciever or close to it.Check out the link.At the top of his website you can download his catalog.

http://www.percyaudio.com/

 

[MCC]

I'll check into it. I can't find any good electronics supply stores in the area so I'm going to need to do everything online, which is usually my preferred way of shopping anyways. Michael Percy does in fact seem to have everything I need except for 7591s which are apparently rare. I'll wait until I'm done with everything else first though before replacing any tubes.

Without a multimeter or a resistor to short the caps, I can only safely work on the components on top. So far I've moved all transistors from one channel to the other to find the source of the hum, and found that they're all fine. My guess is that the broken resistor/incorrect bias on one of the right channel 7591s is causing the hum, since I recently found out that the channels are actually switched. Turning the balance knob seems to confirm this. I'm going to leave the receiver off until I can replace all four cathode resistors on the 7591s and bias them as per service manual specs.

 

[003]

Quote:

Originally Posted by twc5964 /img/forum/go_quote.gif Good lord....looking at the photos underneath It most certainly will be a challenge but I would replace every wire,capacitor,and resistor.Clean the pots with good quality contact cleaner/enhancer.That will take time but probably worth it.I would need help with the tubes and transistors if they needed replacing not to mention aligning the tuner. It's a beautiful receiver and being in your family for so many years in my opinion would be worth the effort. I wonder how long it's been sitting in the basement ?

Don't know about you but the radio shack where I live has drawers full of caps, resistors, transistors, whatever, etc...

Go to ebay and search for "digital multimeter", there are tons of really cheap ones. I have a $10 ebay digital multimeter and I find it works very well.

 

[MCC]

There used to be another radioshack that did have parts literally 2 blocks from the one in the nearest mall but they closed due to self-competition. The market is saturated with these stores, there's another just 5 minutes away but I've decided to get my preliminary order from mouser.

For now I'm just getting some vishay metal films to replace the broken resistor and the 3 other ones like it, a 25w 25k-ohm resistor and some alligator leads to drain the caps, and a extech ex330 multimeter w/ capacitance and a non-contact voltage detector. Once I get the preliminary repair and bias done I can finally burn it in a bit more and then decide what I need to order for the overhaul. The 3 filter caps seem to be pretty rare so I might need to find some alternatives. These are:

C119: 60-40MFD @ 500-500v
C120: 40-40-40MFD @ 450-300-150v
C124: 50-50-50-50MFD @ 200-200-200-200v (I found this one for ~$40, don't know the brand and that's more than I want to pay)

 

[MCC]

Quote:

Originally Posted by twc5964 /img/forum/go_quote.gif Good lord....looking at the photos underneath It's a beautiful receiver and being in your family for so many years in my opinion would be worth the effort. I wonder how long it's been sitting in the basement ?

The MAC 1500's history, as told by dad:

My grandpa had acquired it in an estate auction in the late 60s and gave it to him for college. He had a reel-to-reel as a source and paired it with some nice headphones. We actually have the R2R too but it's pretty much dead. As far as the headphones go, they're MIA and he has no idea what they were.

When I finish refurbishing this amp and I get a pro to adjust it back to factory specs, I'll be sure to post new pics of my modifications. It might be a month, a year... who knows. I'm really interested in doing a thorough job now but cost limitations might slow progress. Plus, we haven't quite worked out who pays for what yet. When it's finished, it'll sit in dad's hifi room but I'll probably use it about as much as he will.