Rebuilding the Power Supply on a Vintage Amplifier
Prologue: I inherited a McIntosh 1700 amplifier from the 70′s from my grandfather. It’s an amazing piece of equipment, assembled by hand and weighing in at around 40 pounds. A few years ago, it started making a horrible buzzing noise, which from research on the internets I identified as ripple voltage from failed power capacitors. In the process of fixing the problem, I learned more than I ever wanted to know about capacitors, vintage electronics repair, and why drinking and soldering don’t mix.
Sadder yet wiser from the experience, I wanted to write up a really thorough walkthrough of how to fix it if you’re having the same problem, because it might be helpful to someone and overall it was actually kind of an amazing experience for me. Drop me a line if it was helpful or you have any questions, comments, corrections, additions, rude gestures, barnyard animal noises, etc. Lezgo!
But First: You will be dealing with mains voltage and large capacitors, both of which can injure or kill, the latter even after the power has been disconnected. Obviously, I am not responsible for your safety. I am not even an expert, but I will try to remember to highlight really crucial things like this. If after reading through you think this is not for you, may I suggest this guy? I decided to do the job myself but he was very helpful and knowledgeable when I talked to him on the phone. Okay, continuing!
You Will Need:
- Soldering iron and solder. I got a Weller WES51 for Christmas and it’s the cat’s fucking pajamas, y’all. But I did this project with my $15 Radio Shack model, so it can be done with that. If you’re a masochist.
- Desoldering Iron. Spend the $20 for a cheap one, it’s worth it. Also desoldering braid is sometimes useful.
- Surgical forceps (2). For use as heatsinks so that you don’t damage parts while soldering, and generally to hold things in place while you’re working on them. There are other soldering clips designs but nothing else works as well.
- Service manual. This shows the electrical schematics and lists specifications for the parts. I got mine from this dude, by e-mailing him and asking nicely. He is awesome and if I’m ever in Switzerland I will buy him a beer. Update: I recently got permission to post it online, you can download it here.
- Replacement parts. Mouser and Digikey are both great, my parts list for the MAC1700 is here.
- Multimeter with, at a minimum, AC Volts, DC Volts, and resistance measurement.
- A dremel or hacksaw, for the multisection cans.
- Beer and Johnny Cash’s Live at Folsom Prison album.
Symptoms: Ripple voltage sounds like someone saying “zzzzzzzzzz…”. It’s a constant sound that will be present as soon as you turn your amplifier on, remain constant whether you are playing music or not, but increase or decrease with the amount of volume. It will definitely sound unpleasant, and if not fixed immediately may damage your equipment. Unplug your amp and do not power it up until they have been replaced.
Before we get hot and heavy with the soldering iron, you may want some explanation of what all that crap underneath the “No User Serviceable Parts Inside” does. Here’s a quick overview of how power rectification works and how to choose the right replacement capacitors.
April 21st, 2009 at 3:05 pm
just found this and want to first say thanks. a lot of guys don’t appreciate what a deal it is not only to fix something, but then to write about it so that others may benefit. i’ve got a 1700 which has been giving trouble on one channel-scratchiness and breaking down at volume-so i figure it’s time to look into it. can’t find any schematics so far and as i don’t have the manual, i can’t verify the tubes and look for new ones(i have an old tube tester and it gives all but one tube the thumbs up). this article should be a big help as i try to fix what is a very nice piece of equipment-thanks again-any further info on this receiver would be appreciated.
April 21st, 2009 at 3:23 pm
Glad you find it helpful! Note that the signal only passes through the tube section when you’re using the FM tuner. So if your problem only occurs only when using the radio, it’s definitely a tube problem, otherwise it’s definitely a solid state problem. If you want to send me your e-mail address, I’ll send you my copy of the 1700 service manual (which includes schematics and parts lists).
May 6th, 2009 at 4:56 pm
i would really appreciate a copy of the service manual as i suspect that i may need more than a recapping with this unit. i used it for about a year and it was great-then i started to get a choppiness in the left channel, so i put it away for later attention. i was using some jbl’s with it, but now i have found some wharfedales which i want to try w/it. it’s a piece that gives one a nice sense of satisfaction to have in one’s system, so i’m gonna have to do what i can. i downloaded your parts list, but it is a .xsl(which i believe is a windows format) and i am a linux and mac osx guy, so i’ll have to see what i can do-if the manual is pdf, that would be great, but if not, i’d still be more than grateful to get it and i will do what is necessary to access it’s info-i have an old drive w/windows2000 which might do.
thanx again, over the years, i have looked up a lot of technical stuff and i can tell you that persons who know their stuff are rare, but people who know their stuff and can relate that knowledge to others clearly and effectively are a real find. thanx.
May 13th, 2009 at 3:36 pm
@dave: I sent a copy of the manual to the e-mail address you used to leave the comment. Send me an e-mail if you don’t get it.
August 15th, 2009 at 1:01 am
hope your leed thing turned out for you. i’m gradually working my way into the 1700 project(after some mixups with part shipments). i want to replace the diodes but can’t find replacements-if you did the ones on your unit, what did you use? the #’s appear to be westinghouse 3988′s and 1N4820′s. my keyboard seems to be messing up, so i hope you get this, and thank you for taking the time to respond. dave
August 20th, 2009 at 9:09 pm
Unfortunately, I can’t seem to find the part numbers that I used–I might have bought them at an Electronics Part Outlet, rather than online. But they’re just power rectification diodes, so as long as you equal or better the specs you’ll be fine.
The 1N4820 is easy because it turns up a datasheet with a Google search.
For the 3988, I’d just work from first principles. AC (I assume you’re in the US) is 120V, so there’s your reverse voltage. From the owner’s manual, you know that the max power draw is 270W = ~2.25A.
So if you find a ‘power’ or ‘rectification’ (not ‘signal’) diode rated for, say, 300V max reverse voltage and 3A forward current, you’re good. I think.
January 13th, 2010 at 11:21 pm
hi Max,
Great blog on DIY MAC1700 recap. I just bought one. I will do the recap using your guide. I would really appreciate a copy of your service manual, as a back up in case I ran into issues.
thanks in advance. Ken
January 16th, 2010 at 10:55 pm
Best of luck Ken, I just sent a copy of the owners and service manual to the address you used to leave a comment.
Hey for future people, if you want a the service or owner’s manual e-mail is probably a little better way to contact me, I don’t always see the comments right away. My e-mail address is in the “About Me” section.
April 23rd, 2010 at 8:19 am
Hi Max,
It was very kind of you to create and publish this website. It has been very educational for me.
I have a Mac 1700 textbook case of loud buzz at the speaker terminals and headphone output and about 3vDC as well. It just spontaniously happened yesterday after the unit had been on for about 8 hours. I have the schematic but not the service manual. If you would please email it to me I would really appreciate it. Also, I have a question. Would it be ok to disconnect the wires to the old cans and then put replacement caps in the chassis area underneath the old caps, rather than disassemble and rebuild the cans? Do you Think there would be room?
Thank you for all your help!
Kip
March 8th, 2011 at 8:25 am
Dear Max,
Great job here. I found this in a search looking for hints on repairing a MAC 1700. It belonged to my father in law. he gave it to my sister in law when he bought a MAC 4100 years ago. The 1700 got pretty well trashed by kids over the years (broken face plate, grime). Anyhow, the 4100 came to my family when Dad died and now the 1700 has come this way as well. This 1700 has a good working amp and preamp but the FM sounds horrible. My suspicion is tubes. I would really appreciate if you could send a copy of the service manual my way, can’t seem to find it anywhere else.
Thanks.
Dale
March 12th, 2011 at 12:46 pm
The manual is now posted here:
http://www.maxwellrosspierson.com/wp-content/uploads/2009/03/MAC1700_ser.zip
Best of luck!