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Author Topic: 1977 Peavey CS-800 Amplifier not outputting fully?  (Read 10614 times)

Steve M Smith

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #40 on: June 24, 2015, 02:57:35 am »

For the rest of the story about those broken sensistors, it was an old part that we had used in earlier versions of CS amps (to sense heat sink temp)...
A broken temp sensor reads like a very hot amp... The vendor finally got their act together but the early days of the CS800S were more stressful than they needed to be.

I don't know if it's connected, but I have a CS800x which exhibits a strange heating problem.

There is something just behind the front grille to the left which gets hot as evidenced by warm air coming out, even with little or no load.  About every ten seconds, the fan switches to high speed and cools it.  It only takes about a second to cool down, after which the fan slows again, so it's obviously not the main heatsink.

The amp works fine through this so I don't really know what the problem is.  Any ideas?


Steve.
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Dirk Bolle

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #41 on: June 24, 2015, 03:43:03 am »

BTW you need to use your real name...(its the rules). 

For the rest of the story about those broken sensistors, it was an old part that we had used in earlier versions of CS amps (to sense heat sink temp). Right around the same time as the CS800S rolled out, the vendor selling Peavey the sensistors changed their production over to China to reduce cost. Purchasing got a number of engineering samples that all tested out fine and didn't break... BUT the real parts coming from China were not very robust and broke if you looked at them sideways. A broken temp sensor reads like a very hot amp... The vendor finally got their act together but the early days of the CS800S were more stressful than they needed to be.

JR
added my name - sorry to get everyone in a lather. i was headed out of the office and threw in a quick reply. apparently, i misheard the part name. (or mis-remembered!) but the  description you give matched the person i talked to, may well have been you back in the day. Still impressed that someone at the factory would just tell someone on the phone how to fix things. that sure doesn't exist much any more, in this day of hyper - litigation.

And as for the symptom, i figured the same thing. heated up, opened up, shut the amp down, turned on the fans. (breakage would obviously do the same thing).
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Dirk Bolle

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Re: Posting Rules
« Reply #42 on: June 24, 2015, 03:46:39 am »

Please go to your profile and change the "Name" field to your real first and last name as required by the posting rules displayed in the header at the top of the section, and in the Site Rules and Suggestions in the Forum Announcements section, and on the registration page when you registered.

Mac

done, thanks.
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John Roberts {JR}

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #43 on: June 24, 2015, 05:51:30 am »


I don't know if it's connected, but I have a CS800x which exhibits a strange heating problem.

There is something just behind the front grille to the left which gets hot as evidenced by warm air coming out, even with little or no load.  About every ten seconds, the fan switches to high speed and cools it.  It only takes about a second to cool down, after which the fan slows again, so it's obviously not the main heatsink.

The amp works fine through this so I don't really know what the problem is.  Any ideas?


Steve.

The fan is only going to come on in response to heat sink temperature rising so heat is from power transistors getting hot. The likely suspect if getting hot with no load is a little too much class A bias current in the class A/B output stage. IIRC those amps had no bias trim pot, just used precision specified diodes and tightly specified Vbe motorola power transistors.

I wouldn't lose any sleep over this, if anything the hotter bias will result in less crossover distortion so amp will exhibit even lower distortion than typical CS800x, and they are pretty good already.

If it bothers you, you could tweak the bias string by adding a resistor in parallel with the double diode to drop the output stage base voltage a few mV, but good luck trying to do that without test equipment etc.

Relax...

JR
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Steve M Smith

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #44 on: June 24, 2015, 06:21:30 am »

Thanks.  I was trying to work out what else could get hot but couldn't think of anything.  I just thought it strange that it could cool down sufficiently in a second for the fan to deactivate.  I suppose it's called a heatsink for a reason!

I think I will just leave it as this amp is never worked hard.

I have a CS1000x and the CS800x in a rack with some graphic EQ and a reverb and a little 12 channel mixer which gets used for small folk/acoustic events.  As it's all in the same rack, I felt the warm air on my leg as I was sitting at it.  If the amps were in a separate rack, I don't think I would have noticed.  It ran all night last time with just that side running a single channel of monitors.

It's out again this weekend so I will use it as is but take a spare amp just in case.


Steve.
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John Roberts {JR}

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #45 on: June 24, 2015, 10:14:38 am »

Thanks.  I was trying to work out what else could get hot but couldn't think of anything.  I just thought it strange that it could cool down sufficiently in a second for the fan to deactivate.  I suppose it's called a heatsink for a reason!

I think I will just leave it as this amp is never worked hard.

I have a CS1000x and the CS800x in a rack with some graphic EQ and a reverb and a little 12 channel mixer which gets used for small folk/acoustic events.  As it's all in the same rack, I felt the warm air on my leg as I was sitting at it.  If the amps were in a separate rack, I don't think I would have noticed.  It ran all night last time with just that side running a single channel of monitors.

It's out again this weekend so I will use it as is but take a spare amp just in case.


Steve.

I can tell you a story about the CS1000 too... There never should have been a CS1000, Too close to the CS800 on one side and too close to the CS1200 in the other direction, an only 200W step is not a huge audible difference, BUT.... Peavey likes to constantly revisit popular models offering incremental improvements. Well one year they decided to bump the CS800 up to 900W so the CS900 was introduced... You don't remember the CS900? Probably because there were so few made. The dealers were usually happy with new, better, stronger, etc. but almost every dealer asked if they could still buy more CS800. The suggestion that the CS800 was going away made the dealers want them even more. So the death of the CS800 was premature, and the CS900 was nudged up to 1000W so we didn't have to explain an 800W and 900W amp... The CS1000 is arguably one amp too many, but it went on to be a very strong seller. Now you know the rest of the story.

JR

 
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Steve M Smith

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #46 on: June 24, 2015, 10:42:10 am »

I suppose the binary sequence  1, 2 4, 8, etc. appeals to people.

In the late eighties and early nineties we only used 400w and 800w amps from the CS series.

130 watts seems to be a common Peavey power rating too.  The XR500 and the Peavey monitor amp with just a graphic EQ and volume control on the front had that rating.  Probably the same amp module and heatsink.

I have a couple of XR500 amplifiers plus the amplifiers from an XR700.  Every time I look at them, thoughts go through my head about building them into some powered monitors.


Steve.
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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #46 on: June 24, 2015, 10:42:10 am »


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