MC250 Repair: Transistors, etc....

USMC Spike

Well-Known Member
Hi all,

Havent been here in a while. Figured I'd do the amp first.

How do you use the T03 parts in place of the
TO-66 parts with out drilling and screwing up
the heat sinks?

In my case this old McIntosh MC250 is so prestine
I really don't have to try to drill it for the T03 parts.

I'm sure it's in here somewhere but I just didn't see it.

Thanks.
 
What transistors are you considering for subs. I have to say I am not completely happy with my test case MC2505. Had to set it aside to get at the backlog and hope I will not have the time to get back into it for a while!
 
I can't see how this can be done without using different heat sinks. I don't even have any good workaround suggestions, too.

Cheers,

David
 
^^^ This is true - and without lots of metal filings flying around at that.

Cheers,

David
 
The MJ2250 and 2251 are the TO66 case also. My objective was to find to220 case devices that might be available in 10 or 15 years.
 
The MJ2250 and 2251 are the TO66 case also. My objective was to find to220 case devices that might be available in 10 or 15 years.

At the rate the semiconductor industry is moving, one might as well try to predict the weather for then. There's no way to know for sure whether or not
a particular device is going to still be around that far out. Might as well not bother to restrict your selection under those circumstances. About all you can really do to be sure is horde some extras.
 
At the rate the semiconductor industry is moving, one might as well try to predict the weather for then. There's no way to know for sure whether or not
a particular device is going to still be around that far out. Might as well not bother to restrict your selection under those circumstances. About all you can really do to be sure is horde some extras.
Buy a full set of semiconductors, put them in a baggie, and tape it inside the chassis.
 
Transistors

Probably not so good, unless we use antistatic bag.
Maybe good to wrap in aluminum foil first.

TO66 PARTS
I'd like to use the MJ21194s NPN for the outputs, they are T03.
I think Fred mentioned these were the all star parts that
on semi makes.

For the smaller T066 on the head sinks two types
the NPN and PNP...MJ2250/51 compatible with them?

Now, if we use the 2N3767, 2N3741 in the TO220
do I add small heat sinks for them?

Then the next question is what about the TO66 component
heat sinks on the driver boards?


OTHER TRANSISTORS
On the other boards I'm getting almost intermittant results testing
the other transistors. I'm not sure if it is due to caps being too old
like the 10uf 100uf caps.

or

If the problem is the small little mounting tubes for the EBC that
the transistors are soldered to?

The problem is when using the diode check mode of meter, some
transistors measurement changes from OL to 3 volts with pressure
on the leads. I wasn't sure if because of the physical tube or that
because of the old transistors a little pressure on them would cause
a small shift on the internals of the transistor itself?

How many of y'all have actually removed the tubes and soldered the
transistors directly to the boards?

Has anyone identified the good replacements of these transistors?

I'm sure this isn't the first time this came up,m I've spend 10s of hours
reading through many of the posts that I could find. I wish there was
a summary somewhere, but it is what it is.

Also, does anyone have a complete listing of all the parts in a MC250
instead of just a replacement parts listing? Weren't there some
maintenance or updates published for the MC250 amp that
exists but might not be on the net?

I have to go through the outputs again. I have a lot of measurements
of the three board transistors....I keep mixing up the right and left
channels with the fore and aft arrangement of paper layout of the boards.

The front board is the right channel while the rear board is the left channel.

Sometimes it helps just to state the obviousness just to clarify it for myself.

AK is a good resource...thanks for your assistance.
 
The correct sub for the output transistors are MJ15003G, the MJ21194 is not correct.

What version of the MC250 is it? Do you have the class A driver (TO66) on the Chassis or on the circuit board?

You can mount TO220 transistors into the existing TO66 socket, don't waste you time trying to find a TO66 replacement. If it is working I wouldn't replace it....
 
The correct sub for the output transistors are MJ15003G, the MJ21194 is not correct.

What version of the MC250 is it? Do you have the class A driver (TO66) on the Chassis or on the circuit board?

You can mount TO220 transistors into the existing TO66 socket, don't waste you time trying to find a TO66 replacement. If it is working I wouldn't replace it....

Yes, not sure if class A or not...Q11, Q12, are TO66, on the driver boards

Hopefully tomorrow I can check the transistors again now that the
inpurt board is removed. I was thinking I should pull the wires off it,
but one, then the second broke off. So, now the board is removed,
I get to redo a couple of wire ends.

I'm trying to fit some Panasonic ECQs in there. That and bpyassing
the 100uf electrolytics with some very good films followed by making
room for the 10uf ECQs films I'm installing.

How do these compare: MJ15003G, the MJ21194 I think Fred (the L.A.
Tech) was saying here that the MJ21194s were the best IS just about made and sound great. and have the same general specs as the originals.
 
What transistors are you considering for subs. I have to say I am not completely happy with my test case MC2505. Had to set it aside to get at the backlog and hope I will not have the time to get back into it for a while!

c_dk, what is going on with your MC2505? How were you refurbing it?
What don't you like about the test case? Didn't you just update one
side of it and not the other?

Care to share pics about what you've got and what you'd like different
or done differently.
 
How do these compare: MJ15003G, the MJ21194 I think Fred (the L.A.
Tech) was saying here that the MJ21194s were the best IS just about made and sound great. and have the same general specs as the originals.

It's not a case of comparing them, the MJ15003G are the McIntosh factory recommended replacements.
 
While all looked good on the scope and soundtech it started triggering the amps breaker.

Have had to set it aside to take care of other units.
 
Transistors

Kevzep,

Yes, MJ15003G is the factory recommendation.

Two other people with larger brains than I and
bigger ears than I have recommended other
outputs for this device.

These include:

MJ15024 has better bandwidth
and
MJ21194 has same bandwidth but much better speed.

Now are we concerned about melting down the xfrms?
Shouldn't be a problem McIntosh has fitted ceramic disc
caps back there so it shouldn't experience HF oscillation.

What about the drivers? While the TIP41c/42c are close
to spec, why not just used the MJ1502x as a faster driver?
It can drive the Hily acclaimed MJ21194, MJ15024
and the MJE15003G

I think the 24 has wider gain-bandwidth product.
Nelson Pass thought the 24 would be better.

What say you, Others?

I''ve already bypassed the new caps and have them installed
on the Input board. It's drying at the moment
There are a few problem transistors on this board....sadly.

So I have to hunt down replacements.

Yes we can go stock, better, best. I always like to hang with the best.

I'm open for discussion.
 
When you are playing with matches in a room full of gasoline cans it really does not matter what type of matches you use or the octane of the gas.

Without careful re engineering and research, since you are basically trying to re engineer this amp, I am quite sure it will go into oscillation and blowup with the parts you are tossing into the ring.

Please post your research and results; learning new things is always good.
 
C_dk, I'm not sure the match/gasoline simile works.
As long as the gas is in sealed gas cans...no problems.
Vapor, spills, NG leaks definitely ARE problems and matches.

I understand what you and Kev are saying, appreciate the
heads up. I'm wondering now, who might have tried it and what was the result?

Now, isn't using T0-220 in place of TO-66 parts also re engineering?

As long as I have to replace parts, I might as well replace with best possible
that will sound good.

The whole amp is subject to the cheesy little slide switch
mono/stereo...arrrgh.

Not to even mention all the 55 year old CC resistors and the
carbon pots the small signals have to wade through.

being amplified by the noisy first gen SS devices.

It's just difficult for me to pull apart, put together, pull apart, put together.

Hopefully it can be limited to twice.
1. Once to get it going,
2. Then once to upgrade parts.

A lot of crap will be cleaned up by using carbon film resistors.
I'm don't want the amp to have the sterile metal film sound.

We'll see how it goes.

Thank you for your care and concern and taking the time to warn
me about it.

The oracle of Spike tells me she thinks an oscillation sniffer is in my
future.
 
*** Please disregard all prior posts about upgrading this amp ***

The challenge is to get it working.

Update unless I'm reading this incorrectly me thinks the input and
driver board transistors are toast. Maybe it isn't as bad as it seems.
Using European decimal comma not period, period is for space.
These are the measurements:

input boar d
120V in, red 40.7 Green -40.7
P/S has been eliminated as a source
of the problem. All voltages in spec.

Problems are with the Transistors.


Qx.....B....E......C
Q1 -,16 18,3 -,6
Q3 -,16 -,62. 28,3
Q5 19,0.. 19 .-1,6

Qx...B.....E.....C
Q7..12,5mv....0,6..-38.3
Q9..-0,6 0,6 -39
Q15 ,01 ,01 1,1
Q17.02 12mv -0,66
Q11 -38 -38 -0,7


Base of Q15 2.0 mV (15mV spec)
Base of Q17 2.7mV (-25mV spec)
Rear Left Channel Power Output



Qx....B.....E.....C
Q2 -,16 -0,66 18,36
Q4 -,16 -0,66 20
Q6 18,32 19 -1,8


Qx...B.....E.....C
Q8 7,7mv 0,59 -38,32
Q10 ,01 . ,59 -39,7
Q16 ,01 ,006 1,1
Q18 0,0 ,006 -0,63
Q12 -38...-38..-0,7

Base of Q16 10.5mv (15mV spec)
base of Q18 1.5 mV (-25mv spec)
Front Right channel Power output

Both output driver boards have low ORG wire 6
Front wire 6, -,7mV (-25mV spec)
Rear board wire6 4.7mV (-25mV spec)

So how much will fried outputs pull these down?
Or is problem with the devices themself?

I don't want to risk sending a signal through it in this state.
But it is stable...its dead.
 
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