• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

MC2125 with bad Drivers

9 days ago I had this amp dialed in and set it aside. Today I plugged it in. Immediately after I turned the power switch on, Flames starting shooting out from underneath. Both driver boards have charr marks. The left amp module on the schematic (right module looking from front) went chernobyl. The driver transistor Q227 PNP (132-151) that I replaced with MJ15021 is toast, its reading diode or diode junction. All the other output transistors and the other driver transistor test good. The thermistor R279 (144-074) is what caught fire. I havent pulled the other amp module yet. Here are some pics. Also the rivet on the autoformer blew off.

So strange that it was working perfect one day and boom shakalaka the next.
 
Pics
 

Attachments

  • 20260514_215144.jpg
    20260514_215144.jpg
    61.2 KB · Views: 55
  • 20260514_213425.jpg
    20260514_213425.jpg
    75.5 KB · Views: 50
  • 20260514_213400.jpg
    20260514_213400.jpg
    65.5 KB · Views: 51
  • 20260514_213415.jpg
    20260514_213415.jpg
    64.1 KB · Views: 48
  • 20260514_213335.jpg
    20260514_213335.jpg
    105.9 KB · Views: 48
  • 20260514_213331.jpg
    20260514_213331.jpg
    109.2 KB · Views: 40
  • 20260514_213323.jpg
    20260514_213323.jpg
    108 KB · Views: 40
  • 20260514_213027.jpg
    20260514_213027.jpg
    79.1 KB · Views: 39
  • 20260514_213018.jpg
    20260514_213018.jpg
    76.9 KB · Views: 36
  • 20260514_213014.jpg
    20260514_213014.jpg
    79.1 KB · Views: 46
Pics
 

Attachments

  • 20260514_213227.jpg
    20260514_213227.jpg
    71 KB · Views: 47
  • 20260514_213303.jpg
    20260514_213303.jpg
    72.4 KB · Views: 46
  • 20260514_213458.jpg
    20260514_213458.jpg
    81.1 KB · Views: 47
  • 20260514_213513.jpg
    20260514_213513.jpg
    75.9 KB · Views: 42
  • 20260514_213452.jpg
    20260514_213452.jpg
    76.1 KB · Views: 42
  • 20260514_213448.jpg
    20260514_213448.jpg
    81.9 KB · Views: 44
ouch ! what a big short here !
your 500 ohm thermistor look completely dammaged !
I don't know if these component still available at the factory !
your driver transistors need to be replaced for sure this time
 
Where did you obtain the MJ15021 PNP driver transistor? The last made from ON Semi have not been available for quite some time. If it was a counterfeit...
 
Sorry, I read that after I replied. Those pc boards came with that amplifier? They are marked 128-355. What is the Serial Number of the amp? These pc boards appear to have been modified with some electrolytics, but have the later MC2205 traces and mods.
 
Last edited:
Contacted Mcintosh this morning. Ordered two more drivers MJ150121, and some emmiter resistors. They do not have the thermistor. The only information they have on the thermistor is its a 500 ohm beta-1900

The other amp module checked out good. I pulled all the transistors on the driver boards they all test good. I found one fried resistor 220 ohm on the driver board. I thought there would be a lot more damage.
 
Jose Jimenez said:
Sorry, I read that after I replied. Those pc boards came with that amplifier? They are marked 128-355. What is the Serial Number of the amp?
Click to expand...
Those are the boards that were in the amp when I got it. Im not original owner. Serial number is unknown.
 
Do you see any indication that the +drive wire lead or circuit touched ground? Also do you have a service manual with that actual pc board in it?
 
Last edited:
Hello

you can get the serial number if you read the chassis number and give this chassis number to the factory
this is only possible if the person who work on the "book" still present today at the factory and if it is not , this is over and lost for ever
The factory did that years and years ago to be able to rescover any serial number if somebody erase it .
if you need driver transistors let me know , I do not buy conterfeit components
for power transisotrs you can use MJ15003 and MJ15004 without any problem
What are the numbers stanped on your drivers boards if they are from original MC2125 ?
 
Jose Jimenez said:
Do you see any indication that the +drive wire lead or circuit touched ground? Also do you have a service manual with that actual pc board in it?
Click to expand...
Yes possibly. Directly under the driver board is the autoformer. It has a metal plate covering it. I see evidence that something may have made contact with this. Look at pic. And right above is the rivet that came off during meltdown.

I don't have the correct schematic.
 
I bet theese driver boards(128-355) are the original that came with the amp. The amp looked very factory and original when I opened it up for a recap. I did not see any signs of anyone being in here before.
 
OK. Good. We are getting there then. Do you feel that you can clean up the pc boards and get them looking good and working? Private message me if you need any components. I will check on the thermistor. Keystone Carbon may have been the original source.
 
Jose Jimenez said:
OK. Good. We are getting there then. Do you feel that you can clean up the pc boards and get them looking good and working? Private message me if you need any components. I will check on the thermistor. Keystone Carbon may have been the original source.dad
Click to expand...

Jose Jimenez said:
OK. Good. We are getting there then. Do you feel that you can clean up the pc boards and get them looking good and working? Private message me if you need any components. I will check on the thermistor. Keystone Carbon may have been the original source.
Click to expand...
Yeah PC boards have been cleaned and one resistor replaced. All the other components test fine. I only pulled the ones near the damaged areas. I might as well pull them all and check them.
 
From the photos it looked like the circuit lands survived. Did the thermistor check out ok? What was the secret behind cleaning the board?
 
You must log in or register to reply here.
Back
Top Bottom