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Carver Reciever 900 left channel very low.

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homerjsimpson

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Post Tue Oct 28, 2008 9:16 pm

Re: Carver Reciever 900 left channel very low.

Here are the pics. Notice the two missing resistors.

You can see where the leads were cut, I am not thinking this was done at the factory...

Image

Image
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Robert R

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Post Tue Oct 28, 2008 11:07 pm

Re: Carver Reciever 900 left channel very low.

OK, the pictures, though blurry (you were too close!), reveal that the two missing resistors are the same part, one in each channel. I forgot that in Carver stuff the alternate channels are odd and even numbers, not whole different series of a hundred higher or lower. So R413 was the same as R414, but in the other channel. Notice how everything that has odd numbers has a duplicate with even ones.
If its in the tone control section their removal was likely some modification to change the curve of the controls. The change may or may not have been factory authorized. No idea what the values were, but when the unit is fixed, we can find the value for you if you want to replace the resistors and observe the difference. Someone has offered to open up his and look for it.

Clearly, since one channel works with that particular resistor missing, the other should too. You have some other problem.

We need to trace the signal through the dead channel and see where it disappears. And for that, working this way, I'd need at least a block diagram; preferably a schematic. Now, if it was on a bench in front of me, that's another story - a much shorter and easier thing altogether.

My potential information source has not gotten back to me, for some reason. Maybe he found that it isn't in the database, but I haven't even heard that. Let's give it another day or so, then I'll try again.

I have no idea how adept you are at this sort of work, so forgive a few impertinent but important questions. I assume you have basic tools. The fact that you know there is a tone control board is encouraging, if there really is one; I've never seen one of these 900s myself. More pictures, taken from farther back, would get me oriented, hopefully to the point that I could recognize important features in the layout.

Have you any test equipment? A volt-ohmmeter is essential for most anything. Soldering iron and some experience soldering? Can you tell an IC from a resistor from a capacitor? For the rest we can maybe fake it. A cheapo amp and speaker could be useful for audible circuit tracing, eventually, but I'm getting too far ahead, and this could take a while.

But maybe we'll get lucky. I'm due for some...

Robert R
"You know what folks? I'm happy."
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gdb

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Post Wed Oct 29, 2008 6:12 am

Re: Carver Reciever 900 left channel very low.

Try this place, they list the manuals for the 900. Good luck ! Geoff :)
http://www.vintageaudiomanuals.com/-C-.htm
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homerjsimpson

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Post Wed Oct 29, 2008 9:11 am

Re: Carver Reciever 900 left channel very low.

I have used a block diagram to find the lost signal in a preamp section of a reciever befor, using a good amp and an RCA cable with a cap on it. We founf the problem to!

I have a VM and a soldering iron too. I do know all the different components too. And how to test them, except caps.
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Robert R

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Post Wed Oct 29, 2008 12:13 pm

Re: Carver Reciever 900 left channel very low.

As to the medical history of this unit, did you get it knowing it had a problem? Did the previous owner know? This matters to me only as it concerns the possibility that the problem may have arisen from a working unit's having sat on a shelf (or worse) a long time, and come out lacking a channel. The emphasis on repair procedure would be somewhat different than if it was playing along fine and quit, and then got shelved or sold. If it died suddenly, in use, suspicion falls more on semiconductors. If it simply quit from long disuse, connections and capacitors become more likely culprits.

Everything that follows is just general information, since I have nothing specific on your model, at least not yet.

First thing to do is figure out what in there is tuner, what is preamp, what is power amp, and what is power supply. For that, picture posts would be great. I assume the tuner works? You don't want to get messing with the parts that work, just the preamp. Remember, almost everything in there is perfectly OK.


ICs are a good place to start, if they are they in sockets. If not, leave them alone for now. But if they are in sockets, try lifting them, gently, a little bit at each end (small screwdriver) and then reinserting and retrying the unit. No need to pull all the way out, just move them a bit. Sometimes time and the devil make for poor contacts, and a little wiggle works wonders.
I still don't trust switch contacts. Move them all a good number of times, while the unit is on and playing; low volume is safest. Work the Balance control back and forth about 10 times, for luck. If something cuts in, you've got it.

If that sort of thing fails, we can go to tests with the power on. But note well: shocks are possible (though not likely if you work only in the preamp section). It's always best to be ungrounded, and reach with one hand only. And even more possible is the possibility of making the whole situation worse if you short together the wrong two connections, even for an instant. When doing live tests, you must be accurate with your probes.

Best way to check caps is with a cap tester, of course, but to see if they're open, you can simply bridge another known good cap in parallel and listen to the results, if any. You don't even have to solder anything. Keep the volume level low for this kind of test - no need to blow a speaker. Not a bad idea to hook up junky, expendable speakers for this kind of work. (I'm afraid we might hear some suggestions from the Forum as to what these would be!). If the sound is suddenly there when you add a second cap, the one you bridged is open - smile. You don't have to use the exact same cap value for this crude type of test, just general ballpark. Bridge electrolytics with, say, a 22ufd at 25 volt (more volts OK) electrolytic; with these you must match polarity with the one under test. About a 0.047 ufd for the smaller, non-electrolytic ones. Try to find a couple of caps like that -there's always Radio Shaft, or even some junk electronics in the trash. Don't mess with any of your largest capacitors; they won't be in the preamp.

Anyhow, wait on this live-test stuff until I see the layout.

Robert R
"You know what folks? I'm happy."
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TNRabbit

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Post Wed Oct 29, 2008 12:18 pm

Re: Carver Reciever 900 left channel very low.

Get some rubber gloves to use while you're in there under power; makes a helluva difference~!
TNRabbit
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homerjsimpson

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Post Mon Jan 19, 2009 4:58 pm

Re: Carver Reciever 900 left channel very low.

OK. We finally got the service manual =D>

Maybe together we can isolate the problem.

Here are the schems for this unit.

Where to start??? :-k
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homerjsimpson

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Post Mon Jan 19, 2009 5:16 pm

Re: Carver Reciever 900 left channel very low.

more pics
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