Potential WOPL project

[wattsabundant]

I recently took a look at a couple Peavey CS 800's for a friend. I'm guessing he bought them for less than a $100 in need of repair. The electrolyic caps had a date code of 1978 indicating they're 40 years old. They've had a hard life and were pretty beat up. They're very heavy with a large transformer and about the same specs as a P/L 400. The design is modular with molex type connectors that allows a module to be changed out for a quick repair.

Given how cheap they can be had, I could see converting them with a White Oak driver board. The module is pretty much like the White Oak back plane design. I believe they had 10 outputs per channel which should equate to high reliability. This would no doubt be a challenging project for someone with a pretty good understanding of amplifier circuits. If it could be pulled off,the possibility of a very nice (sounding) amplifier with a low investment is out there.

 

[grapplesaw]

Don those could be very good unit when done. There are a few variants on this module. The oldest is what I think you are talking about

here is a link to some photos and schematic
https://www.diyaudio.com/forums/solid-state/169909-cs-800-repair-pics.html

looks like .33 ohm emitter resistors and bacillus the same design in the output end as a phase linear. Joes board would be good there.

Glen

 

[wattsabundant]

After sleeping on this idea it occurred to me that each channel/module has it's own driver board. Consequently to use one white oak board would get even more messy. The other option would be to use a driver board on each module with half of it unpopulated. Possible, but not very probable.

One interesting thing about the Peavey is the design assumes a significant DC offset means catastrophic failure (blown outputs). Rather than an output relay they use a crowbar circuit that fires a triac across the output. It protects the speaker but often does more damage to the amp. Every one of these amps that I have repaired had a blown triac which is not easy to get to. One of the amps I looked at recently had the ground wire on the black binding post blown off because of this.

 

[laatsch55]

Shouldn't be too hard to disable that circuit..

 

[oldphaser]

Jack Sondermeyer (formerly with RCA) worked on the Peavey CS-800 design.

https://peavey.com/news/article.cfm/action/view/id/798/cat/1

The following came from "How This All Came About by Hartley Peavey" (https://peavey.com/support/whitepapers/Chapter1.pdf)

"....In ‘67 or ‘68 (I don’t remember exactly when), RCA came out with a series of specialized audio transistors that were ideal for audio amplifiers. At the same time, RCA Labs in Somerville,New Jersey, developed a set of application notes for what they called “Quasi-Complimentary” solid-state power amps. These application notes were the “inspiration” for many companies seeking to get into high power solid-state audio amplifiers. Almost all the hi-fi companies, as well as Peavey, Crown, and others adopted the basic RCA format. I built a 120-watt amplifier that would more than hold its own against the competition, but I discovered that the “dual slope” protection system as innovated by RCA was virtually unusable with highly inductive loads such as bass speakers. With the “back EMF” voltage from bass speakers, the protection circuit would engage (thinking that the output was shorted), thus causing an unpleasant “snapping sound.” I couldn’t figure out what was causing this. In utter desperation, I called RCA’s applications group in New Jersey and got bounced around to a number of people that seemingly didn’t know any more than I did about the problem. After multiple phone calls to RCA, I finally got in contact with a guy named Jack Sondermeyer. When I described the problem, Jack immediately told me what to do about it since he had been on the original design team that developed the now-famous RCA circuit. I had experienced some reliability problems with our output stages and Jack helped me to stabilize my amps with speaker loads that were both inductive and capacitive at the same time. Jack and I conversed often, and I persuaded him to do some “moonlighting” for me. He formed a consulting company called Astro-Associates. He did engineering work for me on an hourly basis and sold me quite a few computer-grade capacitors.

Several years later, I drove up to New Jersey to actually meet Jack and his wife, and while there I made a very strong pitch for Jack to come South to become the Chief Engineer at Peavey… Several months went by and Jack agreed to bring his family down and take a“look-see.” By that time, Jack’s family had grown to where he and his wife Audrey had no less than five children. They all piled in Jack’s Ford station wagon and drove to Mississippi. I took great pleasure in showing them around Meridian and the surrounding area. In 1972, the Sondermeyers decided to make the change and accepted my offer. Jack and his family moved to Mississippi. A pivotal event for Peavey Electronics!

I will never forget that first day in 1972 when Jack came into work. He came in my office and announced that “we are going to throw out all your power amp designs and start all over from scratch!” I told him that he was crazy, that I’d worked for seven years and had finally gotten my solid-state amps where they were reasonably reliable… Much more so than my competition, in fact. I told him that what he was asking was not reasonable because it would be a huge amount of design work, circuit board layout, etc. In addition, I told him that I had very carefully followed the RCA TRANSISTOR MANUAL parameters precisely…He said, “Yes I know, that’s the trouble…..all that information is pure bullshit!” I asked him how did he know that. He replied, “I helped write it!” I could only mutter “well I’ll be damned.” Then and there, Jack and myself proceeded to redesign all our output stages.