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MC2125 with bad Drivers

Kingbleu

Kingbleu

Well-Known Member
When gain was increased the right channel would produce audible distortion, also the red limit light would come on. Swapped driver boards and the problem followed the board. Found a bad transistor on right driver board Q224 132-136 replaced with MPSA42 and that seemed to have fixed the problem. I had the amp running listening to music at a medium level for about 20 minutes and it blew the main 8amp fuse. Put the amp on a dim bulb and the bulb is bright. Disconnect right driver board bulb is dim, swap boards still right side disconnected bulb is dim.

Took out the heat sink all outputs RCA 132189, 132188 test good. Rockwood emmiters test good. Diodes test good. Driver transistors RCA132152, and F 905151 blue dot are bad. Im going to order some new drivers MJ15024, MJ15025.

I think when I replaced Q224 132-136 with MPSA42 I should have also replaced its partner Q20 132-150 with MPSA55. Is it possible that just replacing the one transistor drove the right channel into oscillation killing the driver transistors?
 
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Hello Kingblue

You schould know that the drivers in this amplifier are 132-151 blue point and 132-152 red point these transistors are no more available at the factory if the informations that I have are good . There is no substitutes. You can replace the power transistors by MJ15003 and MJ15004 for the output power transistors ONLY (132-188 and 132-189 ) not for the drivers.
Regarding the emitor resistors (5WL. ) one is red (good sign) no overheating there but the next one is not red it is dark red like if this component get heated once . Check his value who schould be precisely 0,15 ohm if it is not the case replace it by original resistor available at the factory
Be carefull these resistors are 139-125 non inductive type .
 
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clinic-audio said:
Where is the inductive and the "non" inductive ?

nobody find ?

Non inductive is 139125 the one above (top of the picture)

The inductive is the one with an L on it after the W ! :banana:
Click to expand...
So which is it. In one post you say its inductive. Now you say its non inductive.
 
After much reading I understand that the replacement drivers MJ15024, MJ15025 have worked for some but are not correct. They only have a ft of 4mhz while the replacements direct from Mcintosh MJ15021, MJ15020 have a ft of 20mhz. Looking through my parts I found one driver transistor MJ15021 that I ordered from Mcintosh awhile ago. This was for a Mc2200. Now all I need is the MJ15020
132-151 PNP MJ15021

I found a RCA410 in my parts bin. This looks like it might work.
132-152 NPN RCA410
 

Attachments

Can you adapt a TO220 part like the MJE15032/MJE15033? They are 250V 8A parts with a transition frequency of 30MHz. Not too different from the MJ15020/MJ15021.
Unless the original drivers are running at their limit of 100W in this amplifier (would not be a wise design if they are). Alternatively, some TO-3P like the Sanken 2SA1294/2SC3263.
 
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This brand is very secretive about the components they used, but i doubt that there was any unavoidable black magic there. If you strictly want to stick to the manufacturer rules then you have an unsalvageable piece of audio. Nobody has the original parts anymore, and the best that can be done is to find a substitute part. Some would not try this heresy, and it is of course up to them. The MJ15020/21 were acceptable, and others have used them. The closest you can get are TO-3P parts of the sort i suggested, that have similar Vce, Ic and ft to the MJ15020/21 and quite likely to the original parts. Good luck, and have fun with your repair!
 
Some parts are screened, curve-traced versions of the otherwise generic part either for performance or to avoid bad things, so yes there is something to the original parts, sometimes. Not “black magic”, but unless you have knowledge of why, it’s not always a simple cross-reference substitute.

@clinic-audio audio has some of this knowledge and experience and I appreciate his input.
 
Hello everybody

When I was the technical manager of MCINTOSH in France 35 years ago , I decide to buy the drivers transistor used in MC2200/2205/2255/7270 in a quantity large enough for many years . Today when I need to repair one of these amplifiers , to find an original TO3 driver transistor is not a problem .
I do not understand why some specialist did not think to preserve their stock and this way be able to repair units that the factory do not repair today .
 
There is an individual selling the RCA and Fairchild drivers on eBay currently along with the original output devices. If these drivers are such 'golden devices', he is selling them at a reasonable price.
 
clinic-audio said:
Hello everybody

When I was the technical manager of MCINTOSH in France 35 years ago , I decide to buy the drivers transistor used in MC2200/2205/2255/7270 in a quantity large enough for many years . Today when I need to repair one of these amplifiers , to find an original TO3 driver transistor is not a problem .
I do not understand why some specialist did not think to preserve their stock and this way be able to repair units that the factory do not repair today .
Click to expand...
Is this a nice way of saying no I will not sale my hoard of golden era, black magic.
 
Sorry Kingbleu
I have many request but nobody does confirm
since the time I did not remember your name or pseudo
PATRICE
 
As I have said in previous posts, the NJW3281G(NPN) and NJW1302G (PNP) are much better devices suited for this purpose than the 50-year-old stuff. The only reason I can think of that someone would not use them is that they want the convenience of the old TO-3 case to just pop in. If you look at the data sheet for these devices, you will see why.

Yes, I use them for service and have some applications in my personal equipment. It is not what we have and feeling that you matter to others that has value, it is what works and is good for today (gee, almost Biblical!)

Just remember, "Every man has a vice and nostalgia is her name."
 
Kingbleu said:
After much reading I understand that the replacement drivers MJ15024, MJ15025 have worked for some but are not correct. They only have a ft of 4mhz while the replacements direct from Mcintosh MJ15021, MJ15020 have a ft of 20mhz. Looking through my parts I found one driver transistor MJ15021 that I ordered from Mcintosh awhile ago. This was for a Mc2200. Now all I need is the MJ15020
132-151 PNP MJ15021

I found a RCA410 in my parts bin. This looks like it might work.
132-152 NPN RCA410
Click to expand...
Years ago, C_DK and I experimented with using the MJ15024 & MJ15025 as replacement drivers for the original drivers for this series, 132-151 / 132-152. These were slower devices than the replacements that followed.

I believe the sub order was as follows:

132-151 (2N6248) -> 132-204 (MJ15021) -> 132-230 (MJ15019)
132-152 (RCA1B09) -> 132-203 (MJ15020) -> 132-229 (MJ15018)

I've not used the 15024/15025 in some time as there are better alternatives...
Jose Jimenez said:
As I have said in previous posts, the NJW3281G(NPN) and NJW1302G (PNP) are much better devices suited for this purpose than the 50-year-old stuff. The only reason I can think of that someone would not use them is that they want the convenience of the old TO-3 case to just pop in. If you look at the data sheet for these devices, you will see why.

Yes, I use them for service and have some applications in my personal equipment. It is not what we have and feeling that you matter to others that has value, it is what works and is good for today (gee, almost Biblical!)

Just remember, "Every man has a vice and nostalgia is her name."
Click to expand...

I've had success with the 2STA1694 and 2STC4467, but they require some modification.
 
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