Rebuilding the Power Supply on a Vintage Amplifier
Part I: ProloguePart II: Let's Solder!Part III: Multican Capacitors
Okay, so you’ve ordered all your parts and they’re sitting on your dining room table in little plastic baggies, your soldering iron is warming up, and you’ve just put on your Johnny Cash and opened a beer, right? Then let’s get to it!
In the Mac 1700, there are two large can type filter capacitors, a number of smaller signal capacitors scattered around the chassis, and three large multi-can filter capacitors.
Screw Terminal Capacitors: I subbed the two large 4000μF can capacitors with 4800μF ones. They’re an exact physical match, but came with hospital blue vinyl wrapping, which I removed. This part is pretty straightforward, as they have screw terminals, so you just unscrew the old ones and pop the new ones in, being careful to note the polarity. I put a piece of tape with an arrow on each set, pointing towards the front of the case, to make it completely idiot proof.
The two large screw-terminal type filter capacitors (new parts installed, old parts with terminals shown).
This is probably the point where I should talk about electrical safety when working with large capacitors. Capacitors store electrons, and if you place yourself between those electrons and a ground, they will shock you, and it will hurt. What makes this particularly easy to do is the fact that the entire metal chassis of the amp is the ground, much the way your car is wired. So here’s what you should do after you unplug the amplifier and before you start rooting around in there:
Check the positive terminals of all your large filter capacitors with a multimeter, on DC voltage. If it reads less than 12V, it won’t shock you, but you still might fry another component by shorting it to the capacitor with your metal tools.
If you have a cap that still has some juice in it, connect a 1Kohm (1000ohm), 5W resistor across the leads and let it discharge until the voltage reads zero.
Signal Capacitors: These are scattered around the 1700, mostly on PCBs.
Make sure you replace the capacitors on the pre-amp boards, as these will give the most improvement in audio quality.
Below are pictures of caps that you might easily miss, the two bipolar caps on the power protection board should definitely be done, as well as the single electrolytic in the tube area of the amp.
There are two easy-to-miss bipolar capacitors on the power protection board.
There's one electrolytic in the tube section of the amp. Try and match the lead length when you replace these, it matters in the tuner section.
Some Helpful Tips: I’m not going to cover the basics of soldering, but here are a few tricks I found helpful:
- Since these things were assembled by hand, it’s pretty easy to work on. I definitely found it helpful to unscrew the boards from the chassis, which allows you to push them around depending on what you need to get at
- If you get the surgical forceps mentioned at the beginning in the “You Will Need” section, you can clip them onto the wire for the component that you’re soldering, and the forceps will act as a heatsink, absorbing the heat of the soldering iron before it can travel up the lead and damage the component. Put the clip between the part you want to protect and the part you want to solder.
- As this is a stereo amp, there’s generally two of everything, so if you do the boards one-at-a-time you can always refer to the other board if you get mixed up.
- If you tear off about a foot of tinfoil, fold it over once or twice, you can use it to wrap up cables and other heat sensitive components before you start working in the area. That way, when-not-if your iron slips off what you’re working on, you don’t leave burn marks all over the place.
- Take lots of digital pictures of everything before you start messing with it, so you can refer to them if you need to.
- As you check/replace parts, highlight them on your service manual schematic. It will ensure that you don’t miss anything, and form a reference for you later.
- Take care to completely remove the solder from a part before you start bending the pin around, or you might start pulling the traces off the board. That is, the little copper conductive strips that are printed on the back of the board between the pins are fragile and easy to pull off.
Go to Section:
- Part I: Prologue
- Part II: Let's Solder!
- Part III: Multican Capacitors
- View All
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