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Old 23rd November 2012, 07:48 PM   #1
WILD1 is offline WILD1  United States
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Default ARC M100s

Well...
Rainy day here in Texas. Can't complain we needed it. So I just decided to stay in the office today. Problem is that one of my Audio Research M100s just crapped out on me! Everything was fine and then all of a sudden a loud hum started coming out of one speaker almost like if the speaker wires shorted out. Really unpleasant to the auditory system besides scaring the bejeebas out of me. First thing I did was to turn it off. Then I pulled the output tubes and their regulator from the power supply. Turned it on just for a millisecond and I hear a hum from the amp this time. To my ears it sure sounds like a short. Any input on the problem would be greatly appreciated .

Thank you,
Tom Wild
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Old 23rd November 2012, 09:06 PM   #2
limono is offline limono  United States
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Check rectifier bridge but when I had one on the bench with loud hum problem it was expired PSU capacitor bank. They are not cheap unfortunately Heavy, complicated for tube amp and not easy serviceable behemot. Good luck!
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Old 23rd November 2012, 09:32 PM   #3
WILD1 is offline WILD1  United States
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Thanks for the input. I"m thinking the same thing. Something to do with the power supply. I recently replaced the PSU caps so I'm leaning more to your suggestion of the rectifiers. Also this wasn't a gradual thing it just happened all at once. If it was an expired or old PSU capacitor bank wouldn't it happen more gradually? Half the time if it isn't about cows or plants I don't know what the heck I'm talkin about. So if that's the case let me know I want be offended. Again any help would be greatly appreciated. This isn't the first problem I've had with these so I know what you mean about them not being easily serviceable.
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Old 23rd November 2012, 10:36 PM   #4
limono is offline limono  United States
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The one humming M100 I repaired came from a guy who turned the system on one day and one channel was humming loudly. I checked all the rectifiers , transistors and power transformer before I arrived at simplest remedy -PSU caps. It is a hybrid so if it's not PSU related problem you will need signal generator and scope to find the fault. The problem is some of the parts are color coded and available from Audio research with appriopriate price tag. Take it slow , it is a frustrating amp to repair (at least it was for me ) if you don't deal with such things everyday. Sorry I can't be much of help . Good luck.
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Old 24th November 2012, 04:24 AM   #5
WILD1 is offline WILD1  United States
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Default Rode Hard And Put Up Wet

Well...
I checked the capacitors out with a sencore LC 75 for leakage,value and ESR. They all seem to be fine. Afterwards I left them disconnected and turned the amp on for just a millisecond and the buzz or hum is still there. Now this is with all four output tubes out and the three regulator tubes in the power supply out. I think pulling these tubes rules out it being in the amplification or the output transformer. This might sound like a dumb question but when a diode fails does it allow current to pass in both directions or not at all? I sure hope it isn't the Transformer. This amp is not what I would consider one of their hybrids like the Classics. It does have a few opamps used in the power supply but no transistors. I replaced all of the opamps shortly after I bought them. Kevin if your out there I sure would appreciate your opinion.

Thank you

Last edited by WILD1; 24th November 2012 at 04:31 AM.
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Old 24th November 2012, 01:29 PM   #6
limono is offline limono  United States
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Sorry , the amp I had on bench was M300 my bad. Common diode failure is short so it will pass in both directions. To be sure lift one leg of a diode when checking .
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Old 24th November 2012, 06:39 PM   #7
WILD1 is offline WILD1  United States
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Default Too pretty to be in the shop!

Well beautiful day here.Sunny and 75.Great day to be on the lake. Unfortunately I've been in the shop working on this amp. Whenever I have a problem like this it consumes me until I fix it. This is what I found out. Checked the diodes in the bridges and they all seem fine. I did not pull them out of the circuit since they are in series and there is nothing paralleling them. Am I right in that hypothesis? To do this I had to turn the amps over and take the bottom off. While checking the rectifier bridge for the negative current I noticed a .25 amp fuse that the DC- current must pass through. It was blown! There is an electrolytic cap underneath used to filter or condition this negative current from it's bridge .Took it out of circuit and checked it. I just knew it was going to be faulty. Tested fine. Since I had the cap out and the fuse out I decided to turn the amp on and check and see if the buzz was there. It is but it is not coming from the transformer. The switch that turns the amp on controls a double pole single throw relay. This solenoid in the relay is what is creating the buzz. When I work the switch manually no buzz. I knew I had heard that noise before.When I was young I worked on Juke boxes and marble tables or pinballs for my dad. A lot of times the solenoids would create this same sound. My question is whether this buzz from the relay can be propagated and appear in the amplification stage so you would hear it in the speakers when they are connected? And why that .25 amp fuse in the negative voltage supply circuit blew? By the way there is a copy of the schematic at this site ARCDB - M-100
Thanks

Last edited by WILD1; 24th November 2012 at 06:45 PM.
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Old 25th November 2012, 05:11 AM   #8
WILD1 is offline WILD1  United States
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Default Yes!

Okay I think I fixed it. To turn on the amp the switch uses 12 volts to energize a relay. but without the transformer being on the switch doesn't have the 12 volts so the switch also energizes a thyristor to power the transformer at least until the switch gets its 12 volts DC from the transformer to energize the relay. I had a capacitor fail in this 15 volt circuit that supplioes the 12 volts and at the relay I was only getting 8 volts thus the chattering or buzzing from the solenoid. This is a small electrolytic cap that I bought when I changed out the large Mallory caps, however it is a little more difficult to replace so I postponed changing it out. After changing it out the relay now has 12 volts and no chattering or buzzing. This 15 volt circuit services a lot more than just this relay. I am keeping my fingers crossed that this will solve the hum in my speakers as well. Me and Jose Cuervo have already started celebrating.

Take care and cheers,Nick Nick
Tom Wild [WILD1]
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Old 25th November 2012, 01:56 PM   #9
limono is offline limono  United States
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perfect! Doesn't sound like an agony I was going trough with M300 bull .My back hurt for a month just from turning that turd around Whatever they say though , going trough a few pieces of ARC it's hard not to respect company for build quality and workmanship. Expensive? yes but at least there is something to show for it unlike many equally expensive but not nearly as robust and impeccable build like Audio Research (some of them fraudulently shoddy crap) Happy hrs with Jose
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