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Using non-matching caps in McIntosh 2125 repair? Or will it destroy amp/burn down house?

Hi all,

Please take 5 minutes to read and let me know if what I am going to attempt poses any physical danger (fire, broken McIntosh amp, etc.)


today is my last day off before I start an 8 week rotation of working 7 days a week from 4am-8pm (the joys of med school) so I am scrambling to replace the caps on my old McIntosh 2125 amp. I didn't consider it until I would hear an occasional crackle from time to time, and also BOTH meters have never worked (although the light up fine). 
I already cleaned all connections with DeOxit, replaced all 6 IC's on the meter PC board, as well as all of the electrolytic caps. Which offered no change. My plan today is to correct a few signal diodes on that same board which have their polarities reversed, according to the service manual at least, but they are reversed in a symmetric fashion so I don't even know if it matters since the culprits are in pairs, never solo or at a point of termination, so overall the phase output shouldnt be changed by their configuration (I'm guessing).

with that said, I'm working on to the Power Supply PC Board today while I still have the time. 
First major issue I discovered when looking closely between the big parallel axial caps were 2 signal diodes that looked obliterated. I have enough silicone signal diodes and zener diodes to do the whole board, as well as the caps, well almost all the caps. I DONT have exact replacements for the 2 big electrolytic caps (C307, C308) that are rated to "85c, 2200MFD-16v". They don't appear to be In bad shape, but I will have to take them off anyways to get to the busted diodes inbetween them. 

So option #1, remove them from the circuit board and test them with a digital multimeter (Klein Tools model MM2000) if that's possibly? I wouldn't know what readings would equate to "bad" though?

option #2: I have 2 Kimber Kaps .22+/-10% @ 600V. I'm assuming the .22 is micro (u) units, because it's not stated on the cap wrapper.....obviously this is pretty far off from the originals, but just so I'm prepared - when I remove/test the originals if I find they are bad will these Kimber Kaps suffice for a few months until I have time to install a proper replacement? Or would I run the risk of them blowing up/starting a fire/breaking my amp?

any input would be greatly apprefiated! I am damned and determined to get these meters working today...

thanks!)

hockey4496

Showing 10 responses by hockey4496

hockey4496 OP
104 posts
 
wow...I just typed for 20 minutes and got and error. So in an interest of time I'll simply post the photos for now and ask if anyone has any advice as to why there is a signal diode bridging the leads of a single capacitor on the back of the circuit board, and also can I use the same 16V Zener diodes I bought for D304-306 on D119? D119 is currently an empty slot, and it doesn't specify a voltage like the others. Finally youll notice all the red circle of the things I'm going to replace, but just for reference the ones that have a red Z means it should be a Zener diode, but currently has a standard signal diode.

http://s1096.photobucket.com/user/hockey4496/library/

let me know what you think or the parts I selected and if you have any feedback on this project in about to get started on.

thanks everyone!!
hockey4496 OP
104 posts
 
So far not so good, left the front end of amp sticking out of the housing about 5" so I could calibrate the meters if they worked. Unfortunately the power guard lit up right away and a white billow of smoke rolled out of one of those 2200uF....nothing looks fried, aside from that cap, and after turning it on 5 minutes later there was no smoke but no sound either. 

Is that cap the culprit or could there be more serve damage? I'm experienced at soldering, so I am sure it had nothing to do with a short.

thoughts?
hockey4496 OP
104 posts
 
Good eye Gs, I had that pic mislabeled, I just used the same signal diode as I did for D302, 303 (1N4004).  I did however use the 16V Zener at D119, which was entirely vacant before. I'm not sure why the manual specifies 16V zeners for d304-d306, and then for d119 it just says "Zener"

I changed everything marked in the photo and am about to put it all back together now, keep your fingers crossed
hockey4496 OP
104 posts
 
Thank you so much Kirkus for taking the time to explain that. Very helpful, and I'm sure people will continue to use that info for years to come when googling for help on old McIntosh meters!

if it makes any difference here is some other info:

-All lights work EXCEPT the meters. They were brightly illuminated prior to my "repair"

-The powergaurd red light comes on for the right channel if I turn the meter knob to "watts" or "Hold" (and its brighter when turned to the "hold" position. The light is not illuminated on "0db, -10db, -20db"

-No sound from either channel except an occasional crackle or whisper like it's trying to play but the signal isn't making it..., at all levels of gain, one speak off the other on, etc. etc, for all combinations of volume and channels.

-one positive note, the METERS MOVE! Although they never hit above -20watts, they were atleast flicking a tiny bit and were synchronized. It didn't seem to effect them when i adjusted the db/wattage dials on the meter board though.

does this new information help narrow down the source further? I pulled both boards and there is no physical evidence of a short, even on the capacitor I thought was smoking. Which it only did for 5 seconds the first time I started it up, never since.

thanks again for your help! If pics would help let me know and I'll snap a few of whatever you need.



hockey4496 OP
104 posts
 
Just discovered it was D303 that was smoking....not the capacitor. D302 looks fine...what's the deal??
hockey4496 OP
104 posts
 
http://s1096.photobucket.com/user/hockey4496/library/New%20discovery%20McIntosh

There is a tiny drop of solder on C307 that happens to be right above a pin hole on the circuit board, which on the other side (back) has solder over the hole. Does this mean anything? Should that C307 be tilted a little to make a connection??
hockey4496 OP
104 posts
 
I also just found a few of these transistors (2N3904-A06) seen here: https://www.fairchildsemi.com/datasheets/2N/2N3904.pdf

are these usuable able anywhere on the Power Supply PC board? 
hockey4496 OP
104 posts
 
Anyone??? Skip all the post and just read the last one
hockey4496 OP
104 posts
 
What does that mean?
hockey4496 OP
104 posts
 
Okay, I re-did everything. All new caps and diodes, even swapped out that D119 that was missing originally for a 6.2V Zener and removed that mystery diode on the back of the board as suggested.

After replacing everything except transistors and the C307/C308 caps I triple checked polarities plugged only the power supply board back in (leaving out the meter board)...turned on the power (preamp disconnected), and this time all the lights were brightly illuminated, no power guard warning, then 2 seconds later a plume of smoke....yep same thing. BOTH D302/303 cracked in half

either way I know I need to replace C307 and308 (recommendations??) but in addition to that, could the batch of rectifier diodes I’m using be bad? This is what I am using for d302/303: http://www.amazon.com/1N4004-Axial-Silicon-Rectifier-Diodes/dp/B008LTJOX6?ie=UTF8&psc=1&redi...


pleaseeeee help I just want my music back
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