M-400 Service Manual?

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capzloc
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M-400 Service Manual?

Post by capzloc » Mon Jul 07, 2008 2:52 pm

Hello- I searched the forum and I couldn't find an answer, so I apologize if this topic is redundant.

I was recently given an M-400 amp that has some buzzing in both channels and has made some intermittent popping sounds. I didn't want to kill my speakers, so I disconnected it. Anyways, I was looking for a service manual for this amp so that I could possibly repair it. If anybody has a digital copy that would be ideal, otherwise I would be willing to pay a small amount to have a copy shipped. If anybody has this service manual, please let me know and we can work something out.

Thanks!
-Matt

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Re: M-400 Service Manual?

Post by engtaz » Mon Jul 07, 2008 5:11 pm

Welcome capzloc. Glad your here. Lots of knowledgeable people here.
engtaz


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OBI56
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Re: M-400 Service Manual?

Post by OBI56 » Mon Jul 07, 2008 6:55 pm

Welcome to the forum capzloc.

You had to go choose the most difficult Carver amp to work on, didn't you? O:) O:) O:)

First things first: do you have a scope, signal generator and prior knowledge of working with triac fired magnetic field coil amps? If not, I don't recommend you trying to play around in one of these puppies. Aside from the obvious danger of electrocution or frying the magnetic field coil (which is no longer obtainable) this amp is known as a marvel of packaging a 10 gallon amplifier into a quart sized box. I know, I have 2 of them and have disassembled them to do some exploration with the goal of upgrading them. I have plenty of experience with playing around in amps and this one definitely scared me.

The buzzing could be something as simple as adjusting the magnetic field mounding screws (the 4 on the bottom with rubber washers around them) a bit until the buzzing stops. Are your popping sounds really intermittent or at turn-on/turn-off? That one is also a relatively easy fix that does not require anything more than a small disk capacitor and a soldering iron. Anything beyond that will probably require a service call.

If you have the technical expertise and equipment needed to troubleshoot and repair one of these, then I'm sure that we can manage to find you a service manual. If you have a friend who repairs amps and has never repaired a magnetic field amp, don't let him touch it either. That is how most all of them get blown up; by people who do not know what they are getting into.

There are few reputable service shops around that do service these and do a good job. You can find a list of them in the repairs section.

Good luck!
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Re: M-400 Service Manual?

Post by BillD » Mon Jul 07, 2008 7:25 pm

OBI, I have an M-400 that buzzes in one channel only (right). I think that means I can rule out the power supply. The buzz isn't loud and barely discernible over music, but there. I've been meaning to get in there and looke around. Any suggestions?
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capzloc
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Re: M-400 Service Manual?

Post by capzloc » Tue Jul 08, 2008 9:11 am

Thanks for the replies, everyone!

OBI56-
I am a senior majoring in Electrical Engineering, so I have quite a bit of experience doing rework and using test equipment. I'm not too familiar with the M-400's specific topology, so I think you may be right that it's not worth the effort. Thanks for the tip about the mounting screws, I'll try to fiddle around with them. I think the popping may just be at power up/down, so I might try adding that cap as suggested in the thread about that problem.

Thanks for your help-- if those simple tweaks don't work, I'll just forget it. I always feel bad when there's an electronic device I can't fix up, but I guess I can let this one go. :)

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Re: M-400 Service Manual?

Post by BillD » Tue Jul 08, 2008 3:01 pm

I wouldn't give up. This is just the thing a budding EE needs to determine his mettle.
It should sound like it isn't there!
There is a difference between hearing and listening...
Making life enjoyable through expensive electronics.
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Carver: C-4000 & C-1 preamps, PSC-60 preamp/tuner, TX-11a tuner, M-400 (2), C-500, M-500, M-500t, M-500t Mk.II, A-500x, AL-III loudspeakers (2 pr.)
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Re: M-400 Service Manual?

Post by OBI56 » Tue Jul 08, 2008 3:42 pm

NP Capzloc. I was just asking these questions because I've seen firsthand what can happen to someone who accidentally shorts out the triac in there; this was about 20 years ago. A technician in one of the service centres I was supervising tried to short out the triac on an M-400 , just as any other technician without a service manual at the time would have done not knowing the dangers and specific design particularities of magnetic field coil amps. He actually blew clear off his stool onto his back and his heart stopped for a few seconds. You'd be amazed at how many people think that thye can repair anything with a screwdriver, a pair of plier and a soldering iron. I'm just very cautious about this particular model.

That being said, if you know what to look out for, it is one of the most intriguing amps you'll ever look at. PM me if you want to discuss this further.
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Re: M-400 Service Manual?

Post by BillD » Tue Jul 08, 2008 5:34 pm

Yeah, the real key is that the field coil can't take anything like full 120vAC. Think about it this way. Triacs are used in dimmer switches. Now say, you had a bulb that could only take 30 volts screwed into the socket. As you turned up the dimmer (in this case, as feedback is telling the power supply that the voltage rails are sagging), you'd get more light (more of the AC sine wave to the field coil = rails would go up, feedback would go down, and steady state would be maintained). So, you'd be playing with the dimmer switch to get just the right amount of light out to lighten the room, but not enough to blow the bulb. Shorting the triac is like turning the dimmer all the way up and putting 120v into that 30v bulb (or in this case, the field coil - both of which will blow).
It should sound like it isn't there!
There is a difference between hearing and listening...
Making life enjoyable through expensive electronics.
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Carver: C-4000 & C-1 preamps, PSC-60 preamp/tuner, TX-11a tuner, M-400 (2), C-500, M-500, M-500t, M-500t Mk.II, A-500x, AL-III loudspeakers (2 pr.)
Sunfire:Theater Grand III processor, Ultimate Receiver, Cinema Grand Signature 400 ~ seven, True Subwoofer Mk. II, D-10 Subwoofer

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