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McIntosh - Hints and Kinks

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i appreciate all the help - went to the electronics store today and got the resistors , he offered to install for a nominal charge.

after i got home - everything seemed fine - the original problem seems to be gone. I saw some light wisps of smoke , removed the board , there is no more damage to the board . It appears that the 150ohm 2% resistor has a slight brown band around it from getting hot??

Does this resistor need to be 150ohm 5% resistor- could this be an issue or would it be something else?

i hope i am close to resolving , but i may have to take it into a repair center.

thanks

gary
 
Any smoke = bad.

That resistor should be fine at 2% - you've probably got one of the outputs, emitter resistors or one of the transistors on that board misbehaving. That's a good place to start, anyway. Perhaps Mr. DeWick can offer additional guidance.

Murray
 
murray - no more smoke - i will check everything this weekend - getting a MM today

I am also looking for 2505 schematic and parts list for amp with serial number 33kXX

A local shop has had this for 4 months - nothing , i am ready to get the parts myself and fix it. He says he has had a hard time getting the transistors - i will probably pick up the unit monday
 
The last time I worked on one these (or your 2505) would have been in the early 90s but I still have my schematics and I am rusty.

As per Macs instuctions we would repalce Q111,112,113,114 with the factory kit.
Having been burned by a 50lb. callback I would replace the feedback transistors Q101,102,103,104 also. Lastly in your case I can not beleive that Q109 has not failed. Illregardless I would have replaced Q107,108,109,110.

There is a certain elegence in a pro technician finding the part that caused the problem, and then fixing it. Having worked many a clinic with Davey O'Brien though, he would find the problem, isolate the part, then do a wholesale update stating "We can't chance this happening again."

We now know the electrolytics are failing. These should be replaced. This was a timeless peice of Ameican audio back in 90, even more so now. It deserves a competent through update.

I have taken a quick look and have not been able to find the service bulletin for this driver update. It was not a formal bulletin like for the later low voltage relay issue but something that was done out of the clinic program.
 
c_dk said:
The last time I worked on one these (or your 2505) would have been in the early 90s but I still have my schematics and I am rusty.

As per Macs instuctions we would repalce Q111,112,113,114 with the factory kit.
Having been burned by a 50lb. callback I would replace the feedback transistors Q101,102,103,104 also. Lastly in your case I can not beleive that Q109 has not failed. Illregardless I would have replaced Q107,108,109,110.

There is a certain elegence in a pro technician finding the part that caused the problem, and then fixing it. Having worked many a clinic with Davey O'Brien though, he would find the problem, isolate the part, then do a wholesale update stating "We can't chance this happening again."

We now know the electrolytics are failing. These should be replaced. This was a timeless peice of Ameican audio back in 90, even more so now. It deserves a competent through update.

I have taken a quick look and have not been able to find the service bulletin for this driver update. It was not a formal bulletin like for the later low voltage relay issue but something that was done out of the clinic program.
Click to expand...

Thanks for the info - i swapped the card as terry suggested and the noise went to the other channel , intermittently - same as the previous channel

my circuit board is 043-899 as opposed to 043-805. all the part numbers you mentioned are for the 805 board.

I am going to order the same parts you mentioned but with the corresponding part numbers for the 899 board, even though they may be different part numbers from what you referenced. Correct??
 
I received your PM and tried to answer it in your forum post of noisy 2105. I did quick check of my schematiscs for 2100 and 2105. The 2105 shows board 043-899 with notes referencing earlier circuits.
 
I have ordered a bunch of parts from mcintosh , but i still need 132-029 and 132-530 for 2505

what have you guys been using to replace these 2 transistors

thanks

gary
 
skippy_ps said:
I'll check my x-ref tonite when I get home.

Murray
Click to expand...

thanks


after i posted i checked the parts book from 1975 and it referenced 132-096 as replacement for the 029 and i ordered 2 of them

mac told me there were no longer avail/no subs - but i found it in the semi-conductor book
 
Original listing gives 029 as 2N4249 and 096 as BC416C.
The -530 crosses to a -024 as 2N3741.
Now to find crosses to the generics or a reliable source..
 
Hi I recently acquired a mx-112 to pair with my mc-2125. I am experiencing some high frequency hissing out of the left channel even at low volume. everything is grounded and I just cleaned it out. Where should I start? any advice?
 
OK you guys finally made me do it...I went to storage and pulled out my old test gear and my schematics from late 80s 90s. (found a c32 and 2250 I forgot about also)

My semi data sheets Part #-039-684 shows:

mc # 132-236 com# 2SA970BL subing for a host of obsolete trans. including 056,096,031,114,116,176,029, and more.

Mc # 132-235 com# 2SC2240BL subing for more than a dozen including 185,215,089,091-095,112,113,501-504,539,540,195 and more.

Leads need to be addressed correctly, ususally Mc will note this for you if order by orig. #.

Still lookig for T066 case for my discovered Model 19, which the MC2505 needs also. If you cut thru the google bs central semi seems to have made them recently, bought some pre drivers from Mac for a badly hurt 2125 and they sent central tabs. Know anything about them??
 
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zthomase23 said:
Hi I recently acquired a mx-112 to pair with my mc-2125. I am experiencing some high frequency hissing out of the left channel even at low volume. everything is grounded and I just cleaned it out. Where should I start? any advice?
Click to expand...

When you swap L for R it switches? All input selections? Dial FM off station does it change in other inputs? Only Mc sevice note is for "birdies" while tuning some FM stations.
 
Yep when I swap the outputs it changes. It is in all input selections. I can move the tuner knob and there is no aparent change in the sound (consistent across all selections) So I don't think it's fm interference. But birdies sound cute. I'll digup the drawing and test the output voltages? Gotta be something cooked when I got it it had a 2amp buss fuse in it and spec is a .5amp. I changed it out.
 
Since I have been out for a while many more caps are drying out and causing problems and the schematic shows at least 5 in the preamp boards alone.

I dont remember how easy it is to swap preamp boards on a 112 to confrm problem to preamp board. They look to be swapable. Clean contacts while out. Could easily be transistors and I seem to remember a tone control module being noisy years ago
 
Thanks, I'll start digging into it. Are dried out caps easiely identified or tested? Tone contol makes sense, it goes away if the treble is turned down. Deoxit?
 
These general "my Mc gear does this ________" issues really belong in the general area of the Mc forum unless it specifically addresses an issue brought up by Mr. DeWick. And, you'll likely get more responses.

Otherwise, this section gets clogged up with general repair questions that don't belong here.

Murray
 
dewickt said:
Since I get to work on McIntosh equipment most every day I think a Helpfull Hints topic can benafit all of us in the McIntosh fourum. Feel free to post your hint or kink, each one of us may have a unique solution for a problem they have run into and others may need a solution for.
Click to expand...

sounds pretty open to me. but my apologies. :tears:
 
c_dk said:
Since I have been out for a while many more caps are drying out and causing problems and the schematic shows at least 5 in the preamp boards alone.

I dont remember how easy it is to swap preamp boards on a 112 to confrm problem to preamp board. They look to be swapable. Clean contacts while out. Could easily be transistors and I seem to remember a tone control module being noisy years ago
Click to expand...

So I took it all apart, the preamp boards are soldered wire connections. not to easy to swap, at least for me. I cleaned everything and put it all back together and... still noisy. just out of the left channel. I fiddled around and plugged into the "tape out" and then... crystal clear out of both channels! So I am guessing that the tape out bypasses the tone control module? this means that the preamp boards are good right? maybe?
 
This is where a scope and a probe would be nice to go look for noise...
There are 5 tranistors from the middle of the board to the front (of the unit)that are after the tape out and would be suspect. Could be checked with thermal spray. The module could be bypassed from connection 1 to 4. The usual suspected caps are before the tape out. If you find a noisy trans. won't you have someone go thru the whole thing?
 
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