View Full Version : MA-6100 5A fuses burnt (2 of 4)


raco
06-06-2009, 01:52 PM
Hi all,

As I just finished my spring semester and get a taste of fulltime job (which is what I call summer vacation), I am trying my teeth again on some Mc flavor.

I am therefore attempting to bring back to life an MA-6100 which gets no sound out, except for a coupe of seconds after turn on (sort a fading away higher-pitched buzz, and then there's silence), and the same after turn off...

While I am still trying to rationally explain what could be wrong with it, I am performing further testing. Upon opening, I found out that two of the four fuses under the hood are burnt. There are four, one for each B, including + and - for each power amp section channel. The PS is simmetrical, so there's +, -, and GND for each channel. The fuses are on +, -, left, right = four. At this point, I am still tracking down what the burnt fuses are serving, but as there is no sound from either channel, I assume either both "+" arms fuses flipped, or both "-", if this makes clear sense. If both channel "left" or both channel "right" would have flipped, then the other channel should be up and running.

The voltages are all present for all stages within specifications. I measured them by taking all fuses out, including the good one, so I'd make sure the presumably bad amp stages are out of the circuit. The max voltage rating of any and all electrolytics has not been passed during this test, as far as I could figure out.

Therefore, I am assuming bad output transistors at this point. They're 132-080, 132-079. Anybody knows what transistors are these? The 1975 semiconductor list only states they are still under production. Same question stands for all transistors, including drivers - my experience shows they go with the finals when disaster occurs.

Any experience, tips, things learnt, parts numbers / names, etc. are very much welcomed. Thank you in advance for sharing your knowledge with me.

Best regards,
Radu.

RandyInDHS
06-06-2009, 01:59 PM
Hey, Raco... these links should help... good luck... Randall

http://www.pmillett.com/file_downloads/McIntosh/semiconductor_data.pdf

http://www.berners.ch/McIntosh/Downloads/Pricelist_parts_08-1977.pdf

skippy_ps
06-06-2009, 02:12 PM
Outputs are Mc pn 132-218, all npn. Onsemi MJ15003 should do the trick. I would disconnect the outputs, replace the fuses and see what you get.

Murray

raco
06-07-2009, 08:11 AM
Murray,

Thank you for your suggestion. What about the driver Ts? Namely, 132-079, 128-080? More often than not, the driver transistors flip when the finals do.

And the second thing: when you say "I'd disconnect the outputs," that's from where? The preamp section? The speakers? The power supply - probably not, as putting the fuses back effectively means connecting the power amp section to the PS.

Please clarify - much appreciated.

Thank you for pitching in.
Best,
Radu.

raco
06-07-2009, 08:30 AM
Randy - thank you for your suggestions... I couldn't find much clarification on those lists. They don't provide part names (universal names, like 2N3055) to all semiconductors on this component. It seems they would only do that for obsolete parts (relative to their availability from McIntosh), so techs would be able to get replacements from retailers...

All - would matching be essential? Which transistors out of all on the amp section would need it? And if any - do you have a parts source where you can find matched pairs? Reason I'm asking - I once looked for matched pairs of ubiquitous 2N3055 and couldn't find a place for the life in me. I ended up getting a bunch from Radioshack and matching them myself.

I am mostly a tube guy - I don't remember these things about semiconductors (I was a child last time I really built something based on junctions)...

Thank you again.
Best,
Radu.

skippy_ps
06-07-2009, 11:27 AM
My book doesn't have xref for 132-079/80. EM Terry as he will likely know or just phone McIntosh and get them there. Check them first?

I'd just pull the driver boards; that way it would be easier to check the stuff on the boards and disconnects the outputs. Anyway, I'd get in there and check the drivers and emitter resistors (don't remember where they are) R433,4,5,6. Also, check the 22 ohm resistors on the driver boards and replace them if they are out of spec.

Murray

dewickt
06-07-2009, 11:42 AM
Transistors 079 and 080 are still listed in the McIntosh parts book, no cross reference given. The outputs are MJ15003, this is the part you will get if ordered from the factory.

raco
06-07-2009, 12:30 PM
Terry,

Long time no talk! Thank you very much for pitching into this talk - you're as helpful and kind as always.

I wrote an email to Brian at McIntosh parts - I'll wait for him to reply. I was kind of hoping I could just use Mouser to get everything I need. I already have a pair of NTE transistor replacements for my latest Scott acquisition in my Mouser cart (for a 342 Receiver).

On a different note - from my explanation of what is happening, do you think I'm on the right track to assume the finals are busted? Shall I look elsewhere? What is your sense?

Thank you very much for all of your assistance!
Best regards,
Radu.