McIntosh / Mac 1500 questions

[Chriswashere]

Hey all, it’s been a while since I’ve been on here.

A few months ago I was helping a coworker move and found this MAC 1500 which belonged to his brother. It was partially disassembled and hasn’t worked since at least before 1981. The rest of his setup was all broken as well, though I’ve taken care of that.

This Mac worries me though. I’ve had my hands on smaller tube units like Heathkit mono amps or my Dynaco ST70, but this one I’m a little anxious about.

So, it does seem to “work”. It had a bad 12au7 in it, and one of the can caps had a bad section. I have repacked the can caps (yeah, maybe a sin here and yes the ugly new caps are up top for all to see). So now on a DBT it’ll play for hours, but it sure is warm. If I put it on a variac I can slowly crank the voltage on it, but after running it for exactly 5 minutes (I timed this for the 10 min warmup for adjustment), the main fuse blew. After that it seems to hum a little through the speakers or maybe that’s my paranoia and anxiety with it.

It seems this thing has a history of running hot. The terminal strips have been cooked so I have new ones coming. Looks like replacing those may be an adventure... Also it has charred wires and someone has used hotglue on the 5w resistors so when I gave it full power glue started melting and running from the unit. There is also hot glue on top of some of the tubes which also melts under full power and smells...

Any tips or advice would be appreciated. I was thinking of replacing everything on
the terminal strips but wasn’t sure about finding the diodes listed (D11-D14) though I haven’t pulled them yet. Same goes for all the 5w resistors and the 2 33kohm ones. There’s also a huge .47UF cap I haven’t replaced yet so maybe that’s an issue.

My schematic is pretty garbage but I’ve been able to use it to poke around some. So I do have that. I do not have a chance to work on it more than a few times a week so it’s been a long term project already, and progress will be slow.

 

[gadget73]

Can you measure the total current draw at idle? Curious how that compares to the fuse rating. Normally fuses are 50% or so above what it idles at. I'm just assuming the fuse is the correct value too.

Have you checked the output tube current flow? Even if you can't run it the full 10 minutes, you should be able to get an idea if its going nuts and maybe crank the adjustment pots to drop it down some. I'm curious if its just way out of adjustment or if maybe there is something going on with the bias supply, bad coupling caps, bad tubes, something. The service manual is wonderfully clear except for the schematic but it looks like a fairly standard output stage with caps coupling the grid of the 7591 outputs to the plates of the 12au7 inverter. if those caps are bad it will draw way too much current. If the bias supply is weak, it will do the same thing.


related to the bias supply, caps there are positive to ground, fitting them backwards will cause you problems.

 

[ramjet]

They usually do not run warm if there are no problems. Having done this before, I suggest you first remove all of the tubes in the amplifier section (one 12AX7, two 12AU7 and four 7591) test them and set them aside (if good). Next, sort out/replace the charred power supply wiring, replace the resistors with hot glue on them (it will all never come off) and get ALL of the voltages correct. There is no point in moving forward until all the PS voltages are correct. The rectifier diodes are nothing special, use 1N4007 if they are in fact bad. Replace the "huge" .47uf cap. The coupling caps for the output tubes (.1uf) should be checked for leakage or replaced if you do not have the proper test equipment. Next, if you haven't already done so, clean the hot glue off of the tubes when cold (X-acto knife) and re-install and power up slowly on your variac while constantly monitoring the bias on all four output tubes; do not exceed .7vdc. For longer output tube life anything between .4vdc and .6vdc is fine.
The combined volume control/power switch is a common failure point. The damage done by a faulty power switched caused a small fire in the 1500 I have now. The end results were burned wires and open resistors all over the power supply. I suggest that if the power switch is working that you no longer use it to turn the unit on and off and use a power strip to do this instead. Be advised, there are other electrolytic and mylar caps in the 1500 that are not in the power supply section but need to be checked/replaced as well. I also suggest you ask a moderator to move this to the McIntosh forum. Good luck.......

 

[Chriswashere]

Can you measure the total current draw at idle? Curious how that compares to the fuse rating. Normally fuses are 50% or so above what it idles at. I'm just assuming the fuse is the correct value too.

Have you checked the output tube current flow? Even if you can't run it the full 10 minutes, you should be able to get an idea if its going nuts and maybe crank the adjustment pots to drop it down some. I'm curious if its just way out of adjustment or if maybe there is something going on with the bias supply, bad coupling caps, bad tubes, something. The service manual is wonderfully clear except for the schematic but it looks like a fairly standard output stage with caps coupling the grid of the 7591 outputs to the plates of the 12au7 inverter. if those caps are bad it will draw way too much current. If the bias supply is weak, it will do the same thing.


related to the bias supply, caps there are positive to ground, fitting them backwards will cause you problems.

Currently I’m having a little bit of an issue getting the forum to behave, but I appreciate your response greatly.

Amp draw at idle was .8 amps, maybe a bit low but that’s what I got.

BUT your mentioning the cap that has the positive to ground got me looking, and sure enough I had swapped it around the wrong way. I have not been able to test it since doing this though.

I work for my State’s park service and what I do is through the week I have it in the park with me. I use the maintenance shop at night to fabricate what I need physically on the chassis and to have some working space. On Saturday I take it to my folks where my former workshop was (with all my various electronics tools), and I test from there. It’s not an ideal system by any means so it sucks in regards giving timely replies.

I’ll keep updating though. I’ve got parts ordered including new 5W resistors and the terminal strips that have burnt away. I did check the bias before finding the turned cap and it was extremely high.

 

[Chriswashere]

They usually do not run warm if there are no problems. Having done this before, I suggest you first remove all of the tubes in the amplifier section (one 12AX7, two 12AU7 and four 7591) test them and set them aside (if good). Next, sort out/replace the charred power supply wiring, replace the resistors with hot glue on them (it will all never come off) and get ALL of the voltages correct. There is no point in moving forward until all the PS voltages are correct. The rectifier diodes are nothing special, use 1N4007 if they are in fact bad. Replace the "huge" .47uf cap. The coupling caps for the output tubes (.1uf) should be checked for leakage or replaced if you do not have the proper test equipment. Next, if you haven't already done so, clean the hot glue off of the tubes when cold (X-acto knife) and re-install and power up slowly on your variac while constantly monitoring the bias on all four output tubes; do not exceed .7vdc. For longer output tube life anything between .4vdc and .6vdc is fine.
The combined volume control/power switch is a common failure point. The damage done by a faulty power switched caused a small fire in the 1500 I have now. The end results were burned wires and open resistors all over the power supply. I suggest that if the power switch is working that you no longer use it to turn the unit on and off and use a power strip to do this instead. Be advised, there are other electrolytic and mylar caps in the 1500 that are not in the power supply section but need to be checked/replaced as well. I also suggest you ask a moderator to move this to the McIntosh forum. Good luck.......

Thank you. I have replaced most of the caps but have not ordered a replacement for the .47, though I’ve been eying it. I forgot to do this on good ole Justradios. My bad there. All tubes have been tested and will be tested again on two testers just because I’m weird that way and also like playing with both testers and comparing.

The bias was extremely high so I shut it off. On point TP6 it was 17.25v, and the rest were between 5v and 8v. So I won’t be doing that again. I found a cap that should have been positive to ground that I had swapped around so my bad there, hopefully that was the issue or at least a issue. As said in my last post I’ve got some parts on the way, but judging by the post service they’ll be here anywhere from next Monday to a year from now.

It’s good to know about the power switch. My dad does a similar thing with his Luxman r1070 after having the switch repaired (with difficulty) in the 80s. The damage it can cause worries me a bit though but that’s up to the owner to take care of it. It seems his goal is just to have it working so he can use it a few times like his brother did.

Anyways I’ll keep on it. It looks like I’ll have to drill out the rivets holding in the terminal strips so that’s not very fun sounding. I’ve never had to remove a transformer from a unit before to get at anything so that’s a bit worrying. Other than that it’ll be a steady and slow go at it when I can. Luckily the owner doesn’t care about time (or hasn’t so far).

Also, would you know if it’s true only 2600 of these were made? I read that online somewhere.

 

[gadget73]

hm, if current draw from the wall is only 0.8 amps it doesn't seem to make sense for the fuse to blow, unless its just not the right one. Guessing the fuse is probably closer to 3 amps?


5-8 volts at the test points represents an incredible amount of current though. I'd be a little concerned about those tubes. They're not meant to pass anywhere near that kind of current.

 

[62caddy]

Also, would you know if it’s true only 2600 of these were made? I read that online somewhere.

2,634 to be exact acc to Berners which was the second rarest Mc receiver. The rarest is the MAC4200 with only 1,189 produced.