MC250 Repair: Transistors, etc....

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.

I would and do use the factory recommended parts, it really that simple, you could be opening up a can of worms. Not that I really care what you do, but hot rodding amps is not something I am into personally, so I will always recommend the factory parts.
Using factory recommended replacements is what will get your amp going.
Messing around with "better" parts, well, you take the risk.

MJ15024 and MJ21194 are exactly the same transistor, one is a Motorola part number the other is an ONSemi Part number.

If you want to listen to these "experts" and want to use different output devices, make sure you test the amp for stability and oscillation.
These early Mcintosh amplifiers are sensitive to what devices are used in the output section and they do go unstable and oscillate if you use the wrong parts, so be careful.
 
*** 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.

Have you checked the emitter resistors?

Why don't you just take the transistors out and test them?
Diode test will tell you....
 
I would and do use the factory recommended parts, it really that simple, you could be opening up a can of worms. Not that I really care what you do, but hot rodding amps is not something I am into personally, so I will always recommend the factory parts.
Using factory recommended replacements is what will get your amp going.
Messing around with "better" parts, well, you take the risk.

MJ15024 and MJ21194 are exactly the same transistor, one is a Motorola part number the other is an ONSemi Part number.

If you want to listen to these "experts" and want to use different output devices, make sure you test the amp for stability and oscillation.
These early Mcintosh amplifiers are sensitive to what devices are used in the output section and they do go unstable and oscillate if you use the wrong parts, so be careful.

Kev, good points and well noted by me.. Yes, as I mentioned...
I do listen and hear ya, and c_dk. My goal is to get it working
and sound good.

Probably better to get it going and enjoy.

Checked the larger emitter resistors that they were all good, will have
to check the little emitter resistors on the driver and input boards

Will post the diode checks of B-E and C-E. IN circuit...
my concern was frying the transistors trying to pull them
off the board/little tubes.
 
Here are the emitter resistor measurements:

Input section:
R101 19.62v
R107 -1.3v
R102 19.57
R108 -1.3v

Left channel rear:
R85 -.006
R87 .003
R39 .512
R35 -96.8
R43 82.5
R57 0.0
R59 0.0

Right channel front:
R84 -.004
R86 .002
R40 .503
R36 -96.2
R44 82.5
R58 .014
R60 0

This doesn't seem bad, but I don't know,
not having worked with much solid state gear.

Surely there is a maintenance manual or some type of trouble shooting
guide and build sheets for these amps. Where are they to be found?
 
From what I've gone back threw and looked at the
schematics....it appears to be alright and these readings
are within spec.

So accounting for individual differences of components
they balance out pretty well between channels.

For example, the input board emitter reading match.
from -1.3v to .05V for the differences between channels
in r101, r102.

The output emitter resistors pretty well match at 82.5 V
and biased at -96.8 V and -96.2V now if I could just find
my connectors I'd run a signal through it and check it.
 
Just because the 'factory recommended' replacement is a 40 year old ancient 2MHz ft transistor, doesn't mean you shouldn't use a superior device like the MJ21194 instead. Bear in mind, that 'recommendation' was probably made 30 years ago, when the MJ21194 was either really expensive or didn't exist.

There is nothing in that topology that would indicate it wouldn't be perfectly happy with a better output transistor. I bet you'd see an improvement in HF THD at high power as well. The MJ21194 is way more linear.
 
Hi Cec, thank you, I have all the basic doco for the
MC250. I was hoping for something more substantial.

Especially as transistors were relatively new I would have thought
Mc had full parts breakdowns not just for the the replaceable parts.


First is to get her going and let her run for a bit.


Somewhere around here someone recommended a TO66 device
for this as a driver. I"ll need 2 x NpN and 2 x PnP.

Next question when replacing outputs, if there are stability problems,
then what do we adjust? the Emitter resistors Make them a bit larger?

What about some more feedback? collector resistors a bit smaller?
 
Somewhere around here someone recommended a TO66 device
for this as a driver. I"ll need 2 x NpN and 2 x PnP.

Next question when replacing outputs, if there are stability problems,
then what do we adjust? the Emitter resistors Make them a bit larger?

What about some more feedback? collector resistors a bit smaller?


Emitter resistors:

These emitters resistors Mac selected work hand in hand with the VI limiting circuitry. So any tampering with their values and you will need to re-engineer the VI limiting so it behaves properly, And you will have to re-calculate the forward bias of the outputs to compensate for the different emitter values also.
Now this is not a impossible task but its an impediment to your getting the amp running like originally designed by Mac, which seems to be your first order of business from what I have been reading and following in this thread.

Feedback:
Feedback while solving some issues can cause others also. Unless your set to deal with all the possible negative issues along with the positive ones then I would hold off on this until absolutely needed...Again this will be impeding your original focus of just getting the amp to run as built by Mac first....

Stability issues:
These sorts of problems come in all colors and types, and they must be addressed as they appear.
I would be poorly guessing to suggest that you do about anything at this time because I do not know of what issues that may befall your new parts endeavor with this amp.
You may get by without any major issues at all. Then again you might just be creating a very troublesome set of issues, that will be very hard for any qualified person to de-construct and resolve on a open forum.
Usually instability issues require hands on, eyes on and equipment on to properly diagnose and then compensate for.
I like to think I am good at what I do, but even I feel very uncomfortable trying to take on such problems in a online situation. Hands on is the only way I would even try to be of professional help to you.

I have seen over on the DIY where several learned people try to debug a amps problems. It can get to be a bloody mess as engineers argue amongst one another debating whats the best avenue of resolution. While everyone's ideas have their own merits, trying to get two or more engineers to agree is like trying to get a jury of 12 to convict...lol...

TO-66 devices:
I have a very old source of parts, and they have some TO-66 devices on hand. The dust on the parts plus their original dates codes with verify their authenticity. < and the fact the business has been their for almost 40 years collecting these devices as surplus >
I will thumb back thru the thread and pick up the 2N numbers and make a call and let you know if I can get you the four devices you say you need. Count on getting a PM from my no later then tomorrow concerning this matter.

And not to add fire to any debate already looming in this thread. More modern TO-220 devices would properly be the best way to go, even if the old parts can still be had. The newer parts would most likely be more robust, and they could be made to fit the TO-66 pattern on the sinks with some ingenuity and resourcefulness on their install.

Nothing will be easy on any Mac gear this old IMHO, and any original parts will be stupidly priced due their short supply and of course the speculators that may be holding on to these devices for just such a sale situation.

I tried to find a 250K gain pot recently and sent the owner back to Mac to get one at the ridiculous price of ~$70.00 shipped for a $2.95 2 watt carbon gain pot. But if he wants it to fit the glass faceplate he has no other source possible that I could find in two hours of searching world wide. There were aftermarket replacements available at $129.00 a copy, and they were specifically listed as mac upgrade replacements. So you see Mac isn't the worst at parts, the aftermarket is simply stupidly over-priced, But they believe they got you by the balls, so their is no mercy...

Mac wants almost half of what it costs to re-cap the whole amp brand new from Mouser for one gain pot...Aftermarket wanted ~$8.00 less then the whole re-cap costs for one gain pot.
I would stock NOS parts forever too, if I made margins like these on just parts lol lol lol...My conscience won't let me do such things...and I don't sell parts, I just do service....Boy did I pick the wrong end of this business...lol...

Hope some of my efforts are helpful to you....
 
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Will the SQ be worth all this, or is it basically nostalgia for early gen Mac S-S?
I'm considering making a bud an offer on an original 2100, but these threads on rebuilding these things make me question the effort over getting something less needy and costly tho much more modern tech. In other words, is it going to sound better than an Adcom GFA-535 (with all six of the elyticaps replaced), or just 'different' ?

Sent from my SM-N900V using Tapatalk
 
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Just curious where still TO66 I looked on the internet:

Mouser still has NPN Central Semiconductor 2N3583 and 2N3584.

At Newark they have PNP 2N6420 manufacturer unknown on order minimum 25 pieces to buy.
 
Just curious where still TO66 I looked on the internet:

Mouser still has NPN Central Semiconductor 2N3583 and 2N3584.

At Newark they have PNP 2N6420 manufacturer unknown on order minimum 25 pieces to buy.


While this company is on the net they are listed as a broker only so They will not come up on a normal search. They have a $50.00 minimum order billing system so your not likely going to want to deal with them on a onesy twosy situation.

I have known and done business with these folks for several decades and two sets of owners, so I get to go behind the counter so to speak.... Perhaps I can help USMC Spike, maybe not, but I will look see.....
 
btw I was not telling those transistors to be suitable for the specific amp since I do not know, but just mentioning the one and only "probably suitable" any TO66 still to be found. I have no McIntosh knowledge at all, some old defective McIntosh is still on my wishlist but they are not to be found over here....
(I also do not know which suitable other types existed so maybe transistors alike this were used?).

The fact is, I have seen using them in high power video deflection amps, some output stage used 2 pairs of 2N5631/2N6031 and the drivers were 2N3583/2N6240
 
Cec, Resistor John, GS & Pio,
Kev, C_dk.

Thank you for the thoughtful reply's.

Goal
So, I set out with a lofty goal, and had
a dose of reality. I have to make sure
I get the amp going and stay focused.

Part of a three piece set that I've had that
doesn't work. Eventually it will go to my
little girl who just turned two years old
last week. She's got time, I don't.


Known and non Obvious Challenges

I think we're all in agreement these amps have
seen there better years...and time is ticking.
They've done well for first gen solid state.

They are noisy...the parts, transistors,
capacitors, and resistors we are talking
old carbon comps here and many will
crumble on desoldering tugging etc.

I understand this and understand it is
also the nature of the beast. Summarized
we can just all agree:

It Is What It Is.

I used to hate that saying and thought it was
such a cop out. Over the years it's groan on me.

Transformers
Also have to consider transformer windings
are at the end of their useful life. Enamel's
design life from what I under stand is about 40
years. The odd's are the McIntosh pres & amps probably
weren't as abused as say Music Instrument amps.

Going Forward
Continue to gather parts to get it working.

I'm not interested in changing out parts for part sake.
I am interested changing parts that will "clarify" the
sound; that is, lower the distortion and lower the noise
while providing high signal levels through out the amp.

Current Goals
1. Get it working properly.

2. Document and measure that baseline.

3. Enjoy it.

4. Upgrade parts that will make a difference.

5. Document and measure those.

That my plan any way. I'll also try to post pics and information
as I get to them and go through them.
 
I think a labor of love and legacy is a worthy objective, I could never bear to see one of these things discarded regardless of other more practical considerations.

Sent from my SM-N900V using Tapatalk
 
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PM sent to Cec,
Attempted PM sent to Restorer-John.

Taking the toddler out for ice cream.
 
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Getting closer.

Measureing DC Offset
Can anyone describe the best method to measure
the DC offset in this amp?

And if it is off, the procedure to adjust it hi or lo and
which side or both sides to adjust? and Which components.


Square Wave Testing
I've completed a preliminary square wave testing as the amp
sits in the prior photo. Testing into 5.1 ohm 75W dale which
is approx equal to 8 ohm speaker.

Measure clipping the highest voltage w/o clipping = 0dB.
Set input down -2dB and the make square wave and measure.

Onset of Clipping
Right channel clips the top first at greater than 46.2 V p-p/22Vrms
Left channel clips bottom sine first as greater than 46.1 V p-p/17.07 Vrms

Full power measured:
Right channel 41Vp-p, 14.52 Vrms
Left channel 41.4Vp-p, 14.65 Vrms

Right Channel

Results were okay. No ringing etc.
Measured at -2 dB @ 50Hz,1kHz, 10kHz, 20kHz, 50kHz, 100kHz.

at 20kHz slight rounding at forward edge, 50kHz more pronounced
rounding maybe about 1/4 to 1/2 the hight on scope. I have pics
but camera battery died after one channel of pics...can't find the
charger, it's an odd ball Sony with a non standard interface.
 
Measuring DC Offset
Can anyone describe the best method to measure
the DC offset in this amp?

And if it is off, the procedure to adjust it hi or lo and
which side or both sides to adjust? and Which components.
.

You would need to disconnect the auto transformer from the output of the amp and then measure the DC voltage present on the lead going to the auto transformer and your black probe connected to RCA or main rail supply ground.

Vintage amps like this can have oddly high DC offsets because of dissimilar semiconductor devices mostly, and possibly due to poor design or application. Since your amp has auto transformers your speaker will never see any DC offset as it gets shunted to ground in the auto transformer.
A rather neat way of hiding this test parameter but the auto transformer protects your speakers instantaneously from output stage failure also so depending on your technical point of view its a wash, or a win win.. all sort of depends on how you view each of those design features...

What you need to worry about is if that DC fundamental grows with signal drive input. IF it does then your amp has real issues and it will run hot like hell on the output stage most likely since all of the DC will shunt to ground in the transformer the only clue you have DC offset issues is over heated sink temps. Seen that issue posted here before, but its a easy one to figure out if you understand the Mac design concepts used in this era of amplifier.

There is NO adjustment to speak of. An adjustment could be designed and added but its not something I would want to describe on open forum since it requires injecting DC voltages in the front end of the amps feedback to compensate for any errant DC offsets and if your DC offsets float or grow with signal drive it wont fix a darn thing just make things worse then they already are..So no telling on open forum how to make things possibly worse then they already could be, sorry....my personal rules of help engagement.

I hope some of this helps you out.
I use DC offset voltage measurements in many ways, but most of all its tells me the overall health of the amp before repair and after since a proper repair will lower the DC offsets to like new conditions in most cases. If it does not then something is not right with the workmanship or the semiconductors installed.
My preferences for reasonably acceptable DC offsets and I do mean MY preferences are 0.0 to 5 MVDC is like new condition. 5.0 to 15.0 MVDC is typical rebuild passable spec from most reputable manufactures I have rebuild specs for. I don't care for this spec but I'm an grail chaser just ask anyone here. I like my spec to be as close as possible to like new, or I consider that I missed something somewhere...

Anything above those numbers and I would be looking around seriously for leaky transistors , bad caps, off spec resistors etc....

Now this line of thinking applies mostly to newer balanced complimentary class AB and B amp designs and newer designs have better more tightly manufactured semiconductors in them then 40 year old designs do so take it with a grain of salt older designs might just be way off from what modern semiconductor designs are capable of and expected to do with current modern semiconductors in them... This is after-all an antique amplifier design so balance your options when taking into account the age of your amp your dealing with and my personal engineering expectations above...:thmbsp:
 
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