McIntosh MC2300 repair help

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carlosV

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Joined 2015
#1
Hello
I have a mc2300 that needs repair, I already checked all the components and seem to be fine
The problem is that I`m getting a distorted and scratching sound at any volume level , the transistors get really hot in a few minutes, I even can hear music coming from the left channel transistors

any help would be appreciated Thank you
 
#2
If you have already checked all the semiconductors with a diode tester,
then measure and record all the voltages in the bad power amplifier channel.
Also measure the emitter resistor values (when off).
 
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#5
carlosV said:
Hello
I have a mc2300 that needs repair, I already checked all the components and seem to be fine
The problem is that I`m getting a distorted and scratching sound at any volume level , the transistors get really hot in a few minutes, I even can hear music coming from the left channel transistors

any help would be appreciated Thank you
Click to expand...


Seem to be, in what way?
Visual inspection? - actual values? - voltage readings?


Overheated transistors in that unit mean improper bias, which would indicate resistors out of spec, capacitors leaking, and the card connections being corroded.
After decades of aging, those units need a mandatory complete "going over" in order to be reliable.


That is a monster amp at 128 pounds, 300W/per, used mainly by bands, outdoor venues, and with multiple speaker systems.
It's insane to even use it in a home setting, nevertheless some "wattage freaks" would do such a thing, or perhaps to impress others with its size. 😱


I've restored an MC2300 last year, and now the owner uses it at home, at "normal listening levels", a total waste.
 
#6
thank you for the replys, i appraciate it
the mc2300 was working fine, but one day the elft channel broke
two output transistors broke, they were replaced, it worked

after that, the two C 5 capacitors in the input board were replaced by higher capacity ones as the plan was to recap the mc2300 . i turned it on then the sound coming from it was distorted and scratching both channels ,so i put back the original capacitors but the sound was stil distorted and scratching

i started to check the all the boards components and found that the Q 112 transistors in the channel boards were bad i replaced them with the suggested replacements these are the 2N5320
turn it on again and the same distorted, scratching sound and getting too hot only this time i could hear sound coming from the transistors of one channel
 
#13
carlosV said:
after that, the two C 5 capacitors in the input board were replaced by higher capacity ones as the plan was to recap the mc2300 . i turned it on then the sound coming from it was distorted and scratching both channels ,so i put back the original capacitors but the sound was stil distorted and scratching
Click to expand...
Original FILM & DISC capacitors are probably fine. These are usually damaged only when output transistors fail and allow high rail voltage to rampage through the system out the base line, blowing drivers, resistors, capacitors, semiconductors willy-nilly.
Original Electrolytic capacitors are likely trash. This includes aluminum and tantalum ones.
The electrolytics have a plus near one lead, or have "NP" for non-polar after the voltage.
These are aluminum bottles filled with water solution and sealed with rubber. After 2 to 20 years the rubber deteriorates & the water leaks out. If it was slow enough they don't show a pattern of leakage. Best test for them is a $99 ESR meter. I test them with a calender. Amp over 10 years old, sounds stupid, out they go 2 at a time. Followed by sound check after each two, to see if you injected a problem with bad solder joint, wrong design part, or whatever. If so, back up, you know where you caused the problem.
Buy long life (3000 hours service life up) capacitors for replacement. many shops use 500 or 1000 hour ones because they want you to pay them to do this again in 3 to 5 years. Digikey & newark have the service life in the selector table grid if you ask for it.
2nd the motion of old tech, dirty contacts, dirty switches, dirty connectors, can make this scratchy problem. Cleaning and or replacement help.
Overheating can be mis-adjustment of the idle current through the output transistors. Measure without sound the current across emitter resistor of 1 or more output transistors. 20-50 ma is a usual value. I=V/R. You actually measure the voltage & divide to find the current.
If you're this new, read and heed. Never touch metal in an electronic device with both hands. Current >25 ma across your heart can stop it. Never wear jewelry on hands or neck while doing electronics. 1 v through metal can burn your flesh to charcoal. Wear safety glasses, parts can explode. Solder can splash into your eye especially desoldering.
You can use a scope to probe through the circuit and see where the scratchyness originates. Look for spikey waveforms instead of regular waves 20-20000 hz.
 
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#14
thanks indianajo for all the information i really appreciate it

i'm not really expert but i have managed to fix some amplifiers that I`ve owned before like a pair of bryston st7 mono blocks : sony ta 77se sherwood the old ones and some others, but never a mcintosh, this one is really daunting
I have to say that I don't have that much testing equipment other than just some multimeters , a capsitor tester and a sodering station

Most of the time I've tested amps off, which have been enough to find the faulty components and fix them

But its always welcome some guidance as to how to test and see where the components are located like the emitter resistor as this time I'll be testing the mc2300 turned on

Any way I'll take my time as I never try to repair a mcintosh amp before specially this one
 
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