Grateful Dead Music Forum

A place to talk about the music of the Grateful Dead 

 #105580  by hawk900
 Thu Nov 24, 2011 6:15 am
I just got my MC2100 last week. First thing I did was hook a dvd player up to it and fired it up for an hour and let it play in stereo mode through left and right channels. Now later on that night I planned to do and overhaul/inspection replacement and 1/4 jack mod. Lastly will be banana jacks for both left and right 8 and 16 ohm and coms only. The rest will be snipped inside.
First I replaced the original input coupling caps c310,302 which are mylar and swapped them out for Polypropylene .47uf which smoooths out the high end. I replaced c307,308 emitter bypass caps with exact and added a .1uf film bypass cap between poles to clean up any extra garbage on the lines. c309,310 output couplings with exact electrolytic 100uf 15v+.1uf bypass.
C303,304 are the dc power supply caps 470uf 25v replaced with exact +.1uf bypass. All electrolytics should be changed not because of previous use but length of life. C1,2 feedback loop caps 330uf at 3v with a higher voltage +.1uf bypass as well. C11,12 DC voltage gain 150uf at 50v+.1uf bypass. C201,202 are the big cans in the cage 39000uf at40v. Only need replacing if signs of leaks and hum. C203 is a multi cap that measure 80/80/150/50uf at 200v min.
Next I pulled out each heat sink panel(two for each channel and silver thermal pasted the transistor covers 3 on each plate and replaced out the crusty block type resistor with stock. Now the driver cards I did the sentry monitor mod on both and cleaned up some resistors on each that looked burnt at the solder joints plus silver thermal pasted down the transistor cover on each card.
I also installed cooling fans on the top of the cage.
Check pics.


 #105585  by hawk900
 Thu Nov 24, 2011 8:23 am
JonnyBoy wrote:I went to radio shack and bought a 1/4" to RCA cord to go from my SMS to my MC100. It works great. I changed my 250 and 50 to 1/4", but kept the 100 stock in case I sell it I guess (yhea right). I have broken 3 of those cords on the 100 so far, and one of those times the pin in the middle broke off in it and got stuck into the Mac end of the jack. It has enough room to push it all the way in, I guess you get one Gimme before your screwed and have to change the RCA plug out anyway. There is really no "sound" difference in using RCA or 1/4", just the ease of not needing a special cord.

I love my rig above and beyond many many different amps/rigs I have used. Anyone on the fence wanting to get one or think it makes little difference, I strongly urge them to give it a go. I think its about one of the only rigs you can sell off for close to what you paid for it in the end anyway. Not only Jerry lives in there, but many great tones. on the "Con" side of the coin, it is a bitch to move around. get wheels.
Yeah those RCA jacks on the front the insulating material is definately rotted on all of original jacks. Big factor and they need changing either to 1/4 jacks or new rca's. To remove em grab a pair off wire snippers and on the inside there's a retainer ring you just squeeze together and the jack pops out the front. Don't for get to snip the wires duh. Then you need to drill appropriate size hole if your planning the 1/4 jack route.
 #112241  by hawk900
 Sat Apr 07, 2012 1:36 am
Yeah I recapped it with the best and bigest and used .1 uf film caps bypass caps on crucial audio traces. I just did the resitors on the driver card and finito . Still concerned about the .1 film bypass caps. What's your guys take on the subject . I know they clean up the line and flatten out the electrical noise levels crossing through the main cap but do they take away range and headroom etc?
Thank You .
 #116981  by hawk900
 Sat Aug 04, 2012 5:48 am
Now some guru help is required. If youre lookin at the front of the amp the 2 rear heat sinks surrounding the rear driver card are pumping heat out the top. You can really feel it. The amp cutout really quick and smoke was billowing out of 100ohm 5watt res two away from the 3.6 cement looking resistor.
Could this be caused by a blown output transmitter on the nearby heat sink?
Thanks so much. Also a repair guide would be sweet or a link that show step by step diagnostics.
 #116993  by SarnoMusicSolutions
 Sat Aug 04, 2012 10:25 am
I've seen it a few times when people re-cap an amp, like these Mac's, that when they replace the main filter electrolytic cap for the negative bias supply, that they inadvertently have it reversed with the "negative" terminal to ground. Since it's a negative voltage supply for the bias, the cap needs to have the "+" side tied to ground and the "-" side to the negative supply voltage side. Having this cap reversed can cause a short to the bias supply and dangerously underbias the amp allowing way too much current to flow thru the output devices. I'd start there by making sure the bias supply filter cap is correctly oriented.

These amps shouldn't get all that hot or need a fan under normal use, but it doesn't hurt. At idle, these are pretty cool running amps. If you sense heat at idle, something's off.

Also, bypassing an electrolytic with a film cap will generally only improve its high freq performance. No problem there. Sounds like you took a pretty smart approach with the restoration. Just gotta track down the glitch.

 #118951  by NSP
 Sat Sep 22, 2012 12:59 pm
JonnyBoy wrote:I have been having issues now with my MC250. I play low through it and its fine, I start playing at a reasonable volume or pushed it starts going out and making my speaker flop and freak out. I change power amps and it is gone. no issue anywhere else, so it is the MC250. since it is all working, I am assuming it needs cap replacement and upgrading. I will play and things will seem fine, then I dig in and it starts going in and out on me, and making the speakers flutter. I turn down the volume and wait a second I can play and its fine, then add more power again and there it goes again. New caps?

Does anyone know the kind of signs or behaviors the system will have when caps need to be replaced?
Hey JB,

Digging up an old thread. Did you ever figure out this issue? I have the same situation going on with the left channel of my MC250.
 #118984  by JonnyBoy
 Sun Sep 23, 2012 5:41 pm
I never did figure it out. But, it has to be something with the "Old" circuit of the mac. I really don't know the reason though for the speaker dancing. I have been using the MC100 now and no issues...yet.
 #120257  by mijknahs
 Sun Oct 28, 2012 6:04 pm
mkaufman wrote:What did you use before the MC100 conversion? 1/4" <-> RCA adapter?

Sorry I'm a little late in replying to this (about a year). Not sure why I wasn't notified but yes, I had made special cords in the past with 1/4" on one side and RCA on the other. Also have used 1/4" to RCA conversion adapters. Love the 1/4" jacks though! Now any short guitar cable works fine. More durable too.
 #120258  by Smolder
 Sun Oct 28, 2012 6:19 pm
Is there a good source for the output transistors? I'd like to have a set for my mc50.