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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
kirkus
502 posts
 
I see several problems here:

- You cannot replace the 2200uF capacitors with 0.22uF capacitors - the amplifier certainly will NOT appreciate a change of value (four orders of magnitude!) to this degree.
- The diodes near these caps (D302, D303) are NOT small-signal types (i.e. 1N4148) . . . they're rectifier types.  If they're 1A sized (about 2-3mm diameter), you can use i.e. a 1N4004.
- If you're referring to the diodes on the outputs of IC101/102 that are connected in parallel . . . it indeed doesn't matter which way which diode is placed, as long as they are placed opposite to each other.  I'd leave well enough alone, especially if the soldering for them looks original.  My paper copy of the service manual has conflicting reference numbers for these diodes - it's completely conceivable that they're merely documentation errors, not a production or service mistake.

In general, keep in mind that when substituting parts . . . it's far, far more important that they be the correct type and value for the circuit, then they be of "higher quality" in an audiophile sense.
gs5556
1,235 posts
 
You can find the schematic on line. The 2200uF capacitors C307 and C308 are there to provide transient suppression. In that circuit, there is about a 2 second charging time to bring the voltage up to where it can saturate transistor Q301 to energize the relay to pull in the contacts on the output transformer when the unit is turned on or the speaker switch is selected on.

If you put in that 0.22uF cap you will be defeating the transient suppression. The time constant will be almost instantaneous. 

Chances are cap 307 is shorted, putting D302 in parallel with the 5.6VAC source, causing it to burn. Replace the cap only with an electrolytic of the same value.
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!!
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