Mcintosh 510 weirdness

slimecity

Super Member
Sorry the heading to this should be "5100 weirdness(!)" - I have bought a 5100 preamp/amplifer. It has had a little bit of shonkey work done on it, which I will need to ensure is OK, or can be reversed - it appears that someone has inserted a trimpot just before the transistor pre-drivers - only on one channel - i suspect perhaps to make the (usually fixed?) bias adjustable? I will add photos soon to show what I mean.

In the meantime I'd like help with finding a modern and obtainable replacement transistor for Q9-10 & 101-102 - this is listed by McIntosh as a BC168C - any equivalent that anyone recommends? There are four types that come up at the equivalent site that I use (BC238, BC182L, 2N3710, KT373A), however these arent obtainable from my usually NZ supplier, and no details are described of the equivalent's hFE grade. Can anyone recommend any other type? thanks!
 
I could take a guess but instead will look at the Mac cross reference when I open the shop Tuesday.
 
Thanks for that. One possible standard replacement I found for the BC168C is a BC550C. This may work? Its a shame cos I really want to upgrade the current transistors to a level beyond the "BC" series, however many of the current transistors in the 5100 cross reference to that series (and in any event, these replacements are easy to find). The ones I have found so far are:

BC169C - BC184L

BC107B - BC547C

2N4249 - BC556B

2N3569 - BC639

2N3645(c) - BC640.

cheers
 
Taking a quick look thru the mid 80s cross reference commercial column, non of the listed parts were used by Mac. That tells me that all those semiconductors were already obsolete by the mid 80s and replaced with new parts.

What are the Mac part numbers off of the transistors or the schematic?
 
I am stuck here for a bit waiting for a client to bring in his c35........

So many of these old parts are now so obsolete that the crosses for the crosses are now long gone.

BC169c is 132-057

obsoleted in 1975 replaced by

BC238/9 which are 132-092/93

obsoleted by mac in 1988

replaced by Toshiba 2sc2240BL (132-235) some still around but most are using

KSC1845 which are very available.
 
Hey thanks a lot for your detailed advice. Yes many of these trannies are very old and some are even the old "flying saucer" types, many similar trannies I just replaced in my sansui au999.

The weird modification some one made is shown on the attached photos - someone has inserted a trimpot in the pictures shown between the cases of two of the pre-driver transistors -Im not sure why - to control bias? Can I remove or replace this mod?

Also - the two large 5W white resistors at the top of each poweramp module board gets incredibly hot while the amp is running - is this normal? thanks!
 

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Hey thanks a lot for your detailed advice. Yes many of these trannies are very old and some are even the old "flying saucer" types, many similar trannies I just replaced in my sansui au999.

The weird modification some one made is shown on the attached photos - someone has inserted a trimpot in the pictures shown between the cases of two of the pre-driver transistors -Im not sure why - to control bias? Can I remove or replace this mod?

Also - the two large 5W white resistors at the top of each poweramp module board gets incredibly hot while the amp is running - is this normal? thanks!

You need to get the service manual and work through and make sure that you can get everything back to normal.
With the schematic you'll be able to find out what exactly that trimmer is doing....

I would say that is some kind of adjustable bias trimmer.....
Probably put there because one channel was running hotter than the other, so instead of finding out why, someone has put that trimmer in there.....

I'm not sure where you are located, but I could take a look at it for you if you want.....

There are always transistor substitutes available......I wouldn't worry too much about that....

I would get it going properly as designed first, and then worry about re-capping/transistor subs.....
 
Mcintosh MA5100 weirdness

Hello slimecity ,

1) you have the chance that one channel look like never touched so delete everything "strange" on the board with the "ajustable" and make a copy following the "never touched one"
2) once you have left and right identical make a measurement about each input DC voltages
3) be sure that none capacitors from input board is leaking and then everything will be in order
 
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