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MC250 differential pair question

mnmmt

mnmmt

Active Member
I've been lurking on this forum for several months now, and have been working through my restoration of a recently-acquired MC250 with all of the help I've found searching through the forums. Thank you all for your posts over the years--they have been an immense help! I have restored several other vintage amps, and decided to take the plunge into McIntosh gear when I found an MX114 and an MC250 for a price I couldn't refuse.

The MX114 is in great condition, but the MC250 has had a failure of one channel--the output transistor shorted, which took out an emitter resistor, several resistors on the driver board, and a couple of transistors on the driver board. I am doing a full restoration, and have been replacing everything that is not to original spec. It's been fun and will hopefully be a rewarding project.

I've tried to research this but haven't found anything on point in the forums: One of the differential pair transistors on the driver board blew out (it looks like it exploded--there's a crack through the top, and a couple small chunks of plastic missing). I found a number of NOS transistors at a local electronics shop, and am wondering how I should match the transistors and how closely I need to get the transistors matched for this differential pair? I've got a pair that I have matched HFE-wise to within about 3, but am wondering if I need to get some more so that I can make an exact match. Should I also match other measurable specs, or is HFE sufficient?

Thanks for your help!
 
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[Edited: I was mixing up the diff. pair with the sentry monitor transistors]

The electronics store had tons of NOS stuff--I even bought out their supply of genuine 2n5303 Motorola output transistors.
 
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Do you have the cross reference ?
Mac had there own #s.
There's a cross reference for commercial #s floating around. do a search . if you don't have luck , I'll see if I can find it for you
 
I do have the cross reference documents from McIntosh. The differential transistor in my unit is Mc part 132-056, which Mc lists in its cross ref documents as 2n5087. I picked up a handful of these, and they are in the neighborhood HFE-wise of the identical transistor on the other output board, but I haven't found any that are identical in gain.

The issue I am looking at is matching--taking 5 or 10 of the same replacement transistors, there is a wide variation in how they measure in terms of HFE and I want to know if I need to dead-on match the hfe gain or other spec, or if the pair I have that is within 5% of each other is okay to use. These transistors are pretty rare, so I'd like to know if I need to locate more so that I can match the pair perfectly.
 
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I wish I knew. I have a Mac 1900 going on the bench soon. I also was wondering how closely the outputs should match
 
That is something I am also curious about. I have read that the newer MJ15003g have much tighter tolerances and therefore don't necessarily require matching to each other, so long as they are all the new replacement models. But I have older transistors that may not have as tight of tolerance.
 
If you are going to use NOS transistor from the 60s you should invest in a curve tracer. There are more parameters than HFE to match, that is what a curve tracer allows you to do. Mac happily discontinued supplying these old transistors in the 80s.

Pickings were slim for transistors in the mid 60s and while Mac started designing these amps knowing the hand writing was on the wall for quality tube availability they continued to build and offer for sale the MC240 tube amp concurrently with these transistor amps.
 
Thanks for the replies. If I were to use modern replacements, would the newer build tolerances be tight enough that I do not need a curve tracer, or should I look to get matched sets from Mc?
 
Mac would send you the same ones you can get at Bdent, which, according to Echowars, is one of the few places to get non counterfeit obsolete Japanese transistors. I have purchased devices from them and they so far have matched well with those same devices I have left over from Mac in the 80s.

Fairchild ksc1845 and ksa992 have also worked very well.
 
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