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Author Topic: 1977 Peavey CS-800 Amplifier not outputting fully?  (Read 11453 times)

Mitch Philips

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #20 on: June 15, 2015, 09:41:43 pm »

So when the electrician came, he discovered that circuit the amplifier was plugged into was also being shared with the neighbors. That issue was solved. But, the amplifier was still having the low power issue so I asked if he could measure the output at the speaker terminals and it read 4.4 volts at 2.6 amps at full power. My multimeter read the roughly the same output. This is even with the proper input voltage and mains voltage. Yet, the amplifier still outputs considerable volume, a lot more than 4.4 volts could supply.

This amplifier is beginning to be more trouble than it's worth. I thought for sure fixing the mains voltage would solve the issue. But, something still seems to be wrong with the amplifier.

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John Roberts {JR}

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #21 on: June 15, 2015, 10:48:01 pm »

If the amplifier was only making 4.4V and 2.6A the load would be around 2 ohms (unlikely) and the volume would be a lot less than 1/2 power (also unlikely number).

Can you try a rental or loaner amp in that location, to see how does it act? 

JR
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John Heinz

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #22 on: June 15, 2015, 11:03:05 pm »



There is a slim chance that one of the zener diodes used to regulate the +/- 15 volt rails is shorted and a low voltage rail is collapsed. I say slim because the op amps that get fed the voltage would tend to distort the sound, if pass any signal at all. With the way that amp is built, a good bench tech can pull the amp modules and verify the working voltages is a few minutes. There's a +/- 80 volt rail and the +/- 15 volt rail. If those voltages exist, I seriously doubt the problem is with the amp. There is a clamping triac that goes bad on the speaker terminal boards, but generally they just short out and the amp blows fuses upon powering up. Speakers use the term nominal impedance because the impedance varies with frequency. Digital meters have too slow of a response time to give any real picture of voltage as it relates the amp output, unless the amp is connected to a dummy load that maintains constant impedance and pink noise is used as a signal source. Amplifiers are nothing more than a variable frequency switch and a power supply that isolates the line voltage from the output voltage. Hope you find the problem. I think this case is a great educational feature for everyone following the thread. Good luck.

John Heinz
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Jim McKeveny

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #23 on: June 16, 2015, 07:24:04 am »


EDIT: Remeasured the the line voltage and it was roughly 70 volts at the amplifier's outlet.

This is the issue.
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Ivan Beaver

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #24 on: June 16, 2015, 09:40:37 pm »

So when the electrician came, he discovered that circuit the amplifier was plugged into was also being shared with the neighbors. That issue was solved. But, the amplifier was still having the low power issue so I asked if he could measure the output at the speaker terminals and it read 4.4 volts at 2.6 amps at full power. My multimeter read the roughly the same output. This is even with the proper input voltage and mains voltage. Yet, the amplifier still outputs considerable volume, a lot more than 4.4 volts could supply.

This amplifier is beginning to be more trouble than it's worth. I thought for sure fixing the mains voltage would solve the issue. But, something still seems to be wrong with the amplifier.
What was your "measurement of"??

Sine wave-noise other?

When you say 4.4V, was that at clipping?  How did you know?

Just because something is "measured" does not mean it was measured PROPERLY.

Sorry-but there have been a number of statements made that make us unsure about your "measurement" technique.

With or without a load?

What was the input source?

Without supporting data-a simple number is pretty much meaningless.

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Ivan Beaver
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Mitch Philips

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #25 on: June 16, 2015, 10:02:37 pm »

What was your "measurement of"??

Sine wave-noise other?

When you say 4.4V, was that at clipping?  How did you know?

Just because something is "measured" does not mean it was measured PROPERLY.

Sorry-but there have been a number of statements made that make us unsure about your "measurement" technique.

With or without a load?

What was the input source?

Without supporting data-a simple number is pretty much meaningless.

Sorry about that, Ivan.

Tested using a single 60hz sine wave both loaded and unloaded with a 400w subwoofer. Both during loaded and unloaded testing, the amplifier went into overload. During both loaded and unloaded operation, the voltage read was ~4.4V at both of the amplifier's speaker terminals. That was the highest voltage I could get the amplifier to output after trying using many different frequencies.

The input source was an XLR feed from the Peavey MD-8 to the CS-800. We supplied the sine waves from our control booth computer which is a Lenovo ThinkCenter from 2011.

Sorry about the confusion
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Ivan Beaver

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #26 on: June 17, 2015, 06:38:23 am »

Sorry about that, Ivan.

Tested using a single 60hz sine wave both loaded and unloaded with a 400w subwoofer. Both during loaded and unloaded testing, the amplifier went into overload. During both loaded and unloaded operation, the voltage read was ~4.4V at both of the amplifier's speaker terminals. That was the highest voltage I could get the amplifier to output after trying using many different frequencies.

The input source was an XLR feed from the Peavey MD-8 to the CS-800. We supplied the sine waves from our control booth computer which is a Lenovo ThinkCenter from 2011.

Sorry about the confusion
It just sounds like it is broken.  Since it is "basically working" (ie not blowing the fuse and still passes audio), the parts should not be expensive.

Take it to a repair center and get it fixed. Since it is "basically working" (ie not blowing the fuse and still passes audio), the parts should not be expensive.

Bur FIRST I would make sure your speaker cable is fine (not shorted).
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John Roberts {JR}

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #27 on: June 17, 2015, 11:23:07 am »

I don't like repeating myself, but it is hard to have much faith in several of those measurements. A perhaps easier test is to borrow or rent a similar power amp and see how that behaves in the same spot with the same power, signals, and load.

It is certainly possible for any old amp to fail, but generally they fail completely, not just operating with lower output power.

JR
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Mitch Philips

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #28 on: June 22, 2015, 03:22:51 pm »

Sorry for resurrecting this thread but I found out what the issue was and I solved the problem. It turns out both of the speaker output terminals on the amplifier were shorting out on the chassis of the amplifier itself. After isolating the issue, I discovered the amplifier's protection circuit was kicking in right around half way volume.

I replaced the screw terminals on the amplifier and now the amplifier is running as it used to. I don't know if having the amplifier going to short for that long had a long-term effects on the amplifier, but so far, it does not seem like there is.

Thanks for everyone's help in this thread!
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John Roberts {JR}

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #29 on: June 22, 2015, 04:31:49 pm »

Sorry for resurrecting this thread but I found out what the issue was and I solved the problem. It turns out both of the speaker output terminals on the amplifier were shorting out on the chassis of the amplifier itself. After isolating the issue, I discovered the amplifier's protection circuit was kicking in right around half way volume.

I replaced the screw terminals on the amplifier and now the amplifier is running as it used to. I don't know if having the amplifier going to short for that long had a long-term effects on the amplifier, but so far, it does not seem like there is.

Thanks for everyone's help in this thread!
Those amplifiers were designed so even Peavey customers couldn't blow them up very easily.

If the output was a dead short you would have gotten no sound at all... So it was only a partial short, perhaps an ohm or two ohms. It is not good for the amp but they are designed to take a licking and keep on ticking, so no worries.  Just stop doing that. A lesser amp would release it's smoke.

JR
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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #29 on: June 22, 2015, 04:31:49 pm »


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