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Author Topic: 1977 Peavey CS-800 Amplifier not outputting fully?  (Read 11452 times)

Ivan Beaver

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #10 on: June 14, 2015, 10:16:08 pm »

I assumed the power output was lower because the speakers are not nearly as loud as they once were even a few months ago. I can understand speakers not performing as well as they once did when they were newer, but for BOTH of the speakers to all of a sudden stop being loud and for the amplifier to be driven into overload WAY before the volume even gets half way up leads me to believe the amplifier is the culprit.


Have you ever thought the problem might be upstream from the amp?

If you have 1 speaker hooked to each side of the amp (I am guessing that is the way it is hooked up), then for BOTH sides of the amp to lose level (power or gain etc) would be VERY VERY rare.

The only thing that is common is the power supply, and that does not affect the gain.

And it would be extremely rare that the transformer would  go bad in a way such as to lose output voltage.

Something else is missing here.
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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 #11 on: June 14, 2015, 11:15:41 pm »

Have you ever thought the problem might be upstream from the amp?

If you have 1 speaker hooked to each side of the amp (I am guessing that is the way it is hooked up), then for BOTH sides of the amp to lose level (power or gain etc) would be VERY VERY rare.

The only thing that is common is the power supply, and that does not affect the gain.

And it would be extremely rare that the transformer would  go bad in a way such as to lose output voltage.

Something else is missing here.

At first we thought it was the mixer (Peavey MD-8) because the mixer gave us some trouble in the past, but I did a test and bypassed the mixer, limiter, and everything else and went directly into the amplifier and the same issue was present. I've very confident the problem is narrowed down to the amplifier.
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John Roberts {JR}

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #12 on: June 14, 2015, 11:23:15 pm »

Good questions! I'm sorry I didn't elaborate more in my post.

1. The amp output resistance is 6 ohms. The speaker the amplifier is usually connected to is also 6 ohms.
This does not make me confident of the other measurements. The amplifier's output impedance will generally be milli-Ohms, hundredths of one ohm. Speakers are generally called 4 ohm or 8 ohm and will exhibit different impedances at different frequencies.
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2. I did not use the multimeter to measure the amperage, I used an old analog ammeter I had laying around to measure the AC current output.
If you know the ballpark impedance of the loudspeaker, a reliable voltage measurement of the amplifier output while driving the speaker should be adequate.  This is difficult to do with sine waves since even the CS800 and most speakers will not be happy making continuous sine wave power for more than a few seconds.
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3. The 35v was first observed whilst playing some pink noise. I wanted to find what frequency was giving me the most power from the amplifier and after some testing, found out it was around 57-60hz.
What is the crest factor (peak to average ratio) of that noise source. If high it may be clipping at only modest average power.
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Hopefully this helps you.
Not really.

You said both channels exhibit low output, indicate overload, and both act the same. I won't bore everybody with a long list of possible internal or external faults since I don't feel like we have a solid power measurement yet.

Just in case, have you measured the mains voltage where the amp is plugged in? While to drop to half power like reported the mains voltage would have to sag a full 30% which is not likely either.

Good luck.

JR
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Mitch Philips

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #13 on: June 14, 2015, 11:36:47 pm »

Just in case, have you measured the mains voltage where the amp is plugged in? While to drop to half power like reported the mains voltage would have to sag a full 30% which is not likely either.

The outlet measured a low-but-reasonable line voltage of 109 volts with all of the equipment on.

EDIT: Remeasured the the line voltage and it was roughly 70 volts at the amplifier's outlet. I'm actually starting to believe that our neighbors who run an industrial kitchen (who moved in roughly around the same time as the issue) may be causing the issue. The space next door sat vacant ever since we moved in. I'm wondering if some of their equipment is sharing the same circuit as our sound equipment. Though unlikely, this is the only thing I can think of causing this issue. Other than some abnormal issue within the amplifier itself.
« Last Edit: June 15, 2015, 01:08:23 am by Mitch Mockary »
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Douglas R. Allen

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #14 on: June 15, 2015, 05:03:07 am »

Although I doubt both speakers and jacks have a problem if the DDT circuit see a short somewhere it will limit pretty fast. Did someone move the amp just enough to unplug the 1/4 inch jacks in the back just a little? ( if so used )  Or problems at the speaker/cable end? Check the DCR at the speakers and just before the cable plugs into the amp.

Douglas R. Allen
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Ivan Beaver

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #15 on: June 15, 2015, 07:25:07 am »

The outlet measured a low-but-reasonable line voltage of 109 volts with all of the equipment on.

EDIT: Remeasured the the line voltage and it was roughly 70 volts at the amplifier's outlet.
What is the voltage at OTHER AC outlets in the facility?

If it is that low there-I am REALLY surprised you don't have a lot of hum in the system.
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John Roberts {JR}

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #16 on: June 15, 2015, 10:27:28 am »

What is the voltage at OTHER AC outlets in the facility?

If it is that low there-I am REALLY surprised you don't have a lot of hum in the system.

The old CS800 would run down to crazy low mains voltages precisely because so many were sold into 3rd world countries. Of course when the mains voltage drops the power output does too...

Most modern amps would probably shut down at 70VAC. Other gear in the path might drop out of regulation and hum.

JR
« Last Edit: June 15, 2015, 12:03:50 pm by John Roberts {JR} »
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Brian Jojade

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #17 on: June 15, 2015, 11:40:38 am »

70v mains is CRAZY low and indicates a serious power issue somewhere.  When I see a voltage that low, it's generally because of a lost neutral somewhere. While your amplifier is seeing low voltage, devices on the other leg are going to be seeing higher voltage, which can cause catastrophic results.

At this point, it's strongly recommended to hire an electrician to come assess the situation.  There's a good chance a fire is waiting to happen somewhere.  This is serious.
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Brian Jojade

Mitch Philips

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #18 on: June 15, 2015, 12:23:06 pm »

At this point, it's strongly recommended to hire an electrician to come assess the situation.  There's a good chance a fire is waiting to happen somewhere.  This is serious.

Thanks for the heads up. I already have one showing up today for a separate issue. I'll ask him to take a look and I'll get back to you with what he finds.

Thanks!
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John Heinz

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #19 on: June 15, 2015, 08:02:45 pm »

I have an old Peavey CS-800 amplifier that is currently installed in an old nightclub in Los Angeles. It was installed when the club opened in 1978 and it's still used to this day. However, one complaint that was brought up to me was that it was not getting loud enough anymore and the "Overload" lights were going on much sooner than previously, even with the same set of speakers, which were installed in 1995.

I measured the output of each channel on the amplifier and it measured 200w which is exactly half of it's original output per channel.

What would cause an amplifier to start doing this (besides it's age of course)? The amplifier has been properly maintained, never overdriven for long periods of time, dusted regularly, kept in a well-ventilated rack, etc. Because it still works (sort of) fine, could whatever would be causing the issue be replaced or fixed?

Ultimately, if the amplifier could not be repaired, the club would just invest in a new one. But, I wouldn't mind trying to get it fixed.

Hopefully you can help!
Thanks!
-Mitch

I'd carefully inspect the crossover in the KF300's. You might have a capacitor that took a DC hit and physically blew open which could cause a short or a capacitor where leads broke off. Passive crossovers in excess of 12 db work by essentially shorting out the bandpass that is not supposed to get to the speaker. This is why passive speakers have an insertion loss of 3db over biamping. If I remember correctly I think EAW has a complicated 18db filter along with emphasis for the compression driver. As for the CS800 power supply, if the filter caps were going bad, the ripple would show up as hum in the audio or you would blow fuses hard. Those amps are tractors. Re-cap the power supply and the few electrolytic capacitors on the amp modules and it will run another 28 years.

John Heinz
Concert Quality
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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #19 on: June 15, 2015, 08:02:45 pm »


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