McIntosh MA 6100

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McIntosh MA 6100

Postby DeathRex » Fri Apr 29, 2016 11:20 am

I got me a McIntosh MA 6100 off of ebay in a non working condition. I was thinking of putting in tubes or just fixing it. The power transformer is good, the output transistors are shorted. Haven't worked on transistors for a while, so I thought I would give it a go. The schematic is freely downloadable, but parts of it are very blurry, and some parts are just wrong; C137 labeled as R137, electrolytic capacitors labeled as reverse polarity. The last thing I found is a zener diode and resistor swapped side by side, and I guess it still worked.
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Re: McIntosh MA 6100

Postby cdmawolf » Sat Apr 30, 2016 7:59 pm

Go to Berners.ch for good service manual. Also there is a Negative rail so the electrolytics would be wired as such.
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Re: McIntosh MA 6100

Postby DeathRex » Sun Jan 22, 2017 3:46 pm

I replaced almost every component on the 5 boards, at least all the capacitors and resistors. Replaced the output transistors, fuses, and the 2 big power supply capacitors. Fired it up, I found out the card edge connectors were iffy, so I replaced them. Fired it up again, and it worked, except one side was lower and noisy, found one of the transistors they used as a diode, and a diode that they used as a diode was shorted and replaced them. Then a 0.33ohm 5W resistor was bad, and with that replacement it worked, except one side was lower than the other. Traced it to the balance pot. Now the balance pot has a presence switch attached, so I took it apart and one side is 500Kohms and the other is 1.2Mohm, with the middle about 0ohms on the 1.2Mohm side, and 316ohms on the 500Kohm side, and that is where the audio is low.

I'd like to keep the balance control, and I would like the balance to be in the middle. any way I can accomplish that?
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Re: McIntosh MA 6100

Postby Geek » Sun Jan 22, 2017 4:57 pm

Did a couple of those recently - both with shot volume pots.

As for that loudness tap, just replace the volume with a 500K dual log and place a 100K resistor in series. Where the resistor meets the pot, run your lead to the compensation.

It won't be a fun job, unless you have fingers like a small (and I mean small) child =:o
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Re: McIntosh MA 6100

Postby DeathRex » Sun Jan 22, 2017 5:39 pm

Not the loudness, I think that works. The balance control is dicky.
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Re: McIntosh MA 6100

Postby DeathRex » Sun Jan 22, 2017 7:16 pm

OK, using 2 resistors as a voltage divider I lowered the voltage on the high pot, but as I was checking between the preamp and output I noticed that my output still had more volume on one channel than another, even with the preamp at an equal state. As I lowered the volume, while checking the output, the 2 channels became equal then slightly reversed. so I checked my volume control, and it is very dicky, with one channel at 9Kohms, the other is at 30Kohms. I probably need to replace it, but it is the power switch too.
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Re: McIntosh MA 6100

Postby Geek » Sun Jan 22, 2017 8:01 pm

DeathRex wrote:Not the loudness, I think that works. The balance control is dicky.


Mouser 652-PDB182GTRB1504MN should fit the bill.

It's knurled, split shaft. Break half off and pad it to fit the McIntosh D-shaft.

The other option is 652-PDB182GTRB2504MN and dremel the shaft into a "D".

I checked my volume control, and it is very dicky.... I probably need to replace it, but it is the power switch too.


This is the hard one, as to find an appropriate pot WITH a 120V switch is difficult. Maybe from a small supplier, but in the store, I need parts in 24H, so it's Mouser or Digikey or nothing.

Mouser 652-PTR902-2020KA504 is the one I used, but you have to use a relay system to switch the mains.

In the old days, you could pop off the case of a regular 24mm pot and add a SPST switch. Even Radio Shack carried the addon. Alas, "progress" didctates anything higher than the 3.3 volt like is "extremely high tension" :/
/cynic
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Re: McIntosh MA 6100

Postby DeathRex » Wed Jan 25, 2017 7:43 pm

Does anyone have a speaker delay circuit? This McIntosh thumps when I turn it on and farts when I turn it off.
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Re: McIntosh MA 6100

Postby cdmawolf » Wed Jan 25, 2017 10:03 pm

They are on ePAy for less than $20 from Chiinnna
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Re: McIntosh MA 6100

Postby Geek » Wed Jan 25, 2017 10:41 pm

DeathRex wrote:Does anyone have a speaker delay circuit? This McIntosh thumps when I turn it on and farts when I turn it off.


C`mon, you`re not THAT new to the hobby ;)

For instant turn-off, you need to make a separate protector supply (1-diode, 1-smallish capacitor)
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Re: McIntosh MA 6100

Postby DeathRex » Thu Jan 26, 2017 9:35 am

Geek wrote:
DeathRex wrote:Does anyone have a speaker delay circuit? This McIntosh thumps when I turn it on and farts when I turn it off.


C`mon, you`re not THAT new to the hobby ;)

Why yes, yes I am. I've fixed close to 100 radios, and made many amps, but they are all tubed. I have 2 Sansui amps that I bought in the 80s, they both have relays that you can hear click at 3-4 seconds. I don't remember my Dynaco ST-120 having a thump, only thing I did to it was replace the capacitors. R1051 and URT-23 didn't have a thump either.
This mac has a loud fart when turning it off. I replaced the 2 big ass caps, and even put the old ones back in, and it didn't make a difference.



Geek wrote:For instant turn-off, you need to make a separate protector supply (1-diode, 1-smallish capacitor)

Whaaaaattt? (???)
I have some 555s, played with them in the past, and even have the radio shack engineering book. I'm going to rectify the 6.3V lamp voltage to power the 555, but couldn't figure out how to remove power from the 555 fast enough and have good enough filtering for the VCC, to click the relay off.
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Re: McIntosh MA 6100

Postby DeathRex » Thu Jan 26, 2017 9:42 am

Come to think, this one does have a speaker protection circuit, I wonder if it is supposed to do something when turning on or off, or if it is causing the fart? R433 was mostly open (26K), I wonder if the triac or diac is open?
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protection_zpsibevi3mt.png
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Re: McIntosh MA 6100

Postby Geek » Thu Jan 26, 2017 1:14 pm

Heya,

That's a speaker protector in case the output line gets DC on it. It's not a mute circuit.

OK, sorry for the tease, I thought you had SS experience as well :$

The farting is not normal and should be fixed than worked-around. That's usually caused by bad power decoupling somewhere, or a leaky coupling cap. You recapped all of it, right? Or just the big PSU caps?

I'm on my way into the store, so I can show you a nice power supply for the delay when I get home tonight (I work until 2100 PST), unless someone else does in the meantime.

Cheers!
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Re: McIntosh MA 6100

Postby DeathRex » Thu Jan 26, 2017 2:33 pm

Geek wrote:That's usually caused by bad power decoupling somewhere, or a leaky coupling cap. You recapped all of it, right? Or just the big PSU caps?

Well Trump said the multi cap can was good and would last another 1.3 million years, so I left it. [:) He wouldn't lie to me, would he? *)

Anyway I replaced all of the film caps and the other electrolytics on the cards. Do have the electrolytics on order from mouser with the pots and other stuff. I plan on restuffing the can. When running, the amp is pretty quiet, just a bit of hiss.
On SS I found that I don't like how one transistor can bias another, and then another even 3 or 4 down the line. Makes it harder to troubleshoot.
This thing had 7 bad transistors, 2-3 dozen bad caps (I measured them), 2 bad pots from the factory, 4 blown lamps, 4 blown fuses, and 4 bad resistors. It looked and sounded dead when I got it. Now with help from Gregg and yall, (and a huge pile of money) it's almost finished. I'll have to take a after pic.
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Re: McIntosh MA 6100

Postby Geek » Fri Jan 27, 2017 1:08 am

Okay, I have a simple suggestion that runs of the 6100's bulb line. :))

Omron relays IME are fast, so they can chatter with too much ripple on the power line. They're reliable and inexpensive and the one mentioned doesn't have the polarized coil headache.

Cheers!
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