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-   -   Peavey CS800 Doorstop to Decent (https://www.diyaudio.com/forums/pa-systems/330682-peavey-cs800-doorstop-decent.html)

Peter S 1st December 2018 07:20 PM

Peavey CS800 Doorstop to Decent
 
1 Attachment(s)
Hi Folks;
I am back to my revival of this old Peavey CS800. It has had a tough life but just seems like a worthwhile project. I was surprised that the functioning channel developed 330 watts in 8 ohms. The specs are very under-rated 280 watts. I have broached this subject in the tail end of my last thread (304403-pet-peeve-peavey-gps-3500).

I have found a couple of simple upgrades, here at diyaudio, for this amp. Additionally, I hope to eventually install a temperature sensing fan controller (robbed from a deceased Beta800 amp, possibly) and have the fan switch between variable and full.

I have some ON Semi MJ21196 outputs on order from Hong Kong ( Yikes?!!)
I have installed these in my Phase Linear 700B, so in the interest of uniformity, I thought I would use them here, I am hoping they are a good replacement for the MJ15024, if not even a little better?

I appreciate any input regarding this rebuild, as I am only a hobbiest, but hope to gain some knowledge. I suspect that this is a good candidate for beginners, as it does not seem nearly as complex as many amps in this power range. I understand that there are a lot of these amps around, but they had to be popular for a reason!

JonSnell Electronic 1st December 2018 08:44 PM

The output transistors rarely fail. If they have, it will be obvious!
Contacts and electrolytics are all that sometimes need attention.
Personally, they make good door stops!

rayma 1st December 2018 08:48 PM

Quote:

Originally Posted by Peter S (https://www.diyaudio.com/forums/pa-systems/330682-peavey-cs800-doorstop-decent-post5618854.html#post5618854)
I am back to my revival of this old Peavey CS800.

This is often just as effective as replacing parts with contacts, like pots and switches.
CAIG DeoxIT D5S-6 Spray 5 oz. with Perfect Straw

wg_ski 2nd December 2018 03:51 AM

They were popular *because* the output transistors rarely failed (unlike the PL). They would also put out 280 watts out of both channels driven at the same time at 8 ohms, which is why you could get 330 out of just one. And it could do that with sine waves for hours at a time, cheaper amps would thermal after a few minutes. You could run about 50-60% duty cycle at 4 ohms, which is still a heavier load than most hip hop. They claim to not be 2 ohm capable, but by today’s standards they are more rugged than amps rated to 2.

Similarly rated amplifiers today would be good for 50 watts of average power per channel at 4 ohms and no more, with the peaks of the waveform hitting 56 volts. And still be called 400 RMS per channel at 4 ohm.

Michael Chua 2nd December 2018 01:03 PM

It's not difficult to repair this workhorse.

I even modified the output to full complementary with MJ15024/25.

After all the hard work, I found that the CS800 transformer hums. Not an issue when on stage but for the home, it may be too loud for some listeners.

Burnedfingers 2nd December 2018 01:10 PM

Quote:

Originally Posted by JonSnell Electronic (https://www.diyaudio.com/forums/pa-systems/330682-peavey-cs800-doorstop-decent-post5618915.html#post5618915)
The output transistors rarely fail. If they have, it will be obvious!
Contacts and electrolytics are all that sometimes need attention.
Personally, they make good door stops!


I'll have to agree.... a door stop at best.

Not a commercial piece or a Pro Sound piece.

Something suited for the wanabe JD or weekend musician.

indianajo 2nd December 2018 01:37 PM

So many golden eared posters listening to 300 W amps with earphones. With real speakers at >20 W the difference between .0003% HD and .5% HD is IMHO inaudible. And most American males don't have hearing above 7000 hz anyway. Mine goes to 14khz. I have my doubts about the revision A with single op amp (5478?) and 8 predriver transistors, but later versions with the 4558 should sound as good as my PV1.3k at lower volumes. and the 4558 could easily be swapped out for a 33078 or RC4580. I used MJ21193/94 from Newark for my rebuild. People that don't believe blown outputs are common on the resale market have never set up a band in the dark on stage with 1/4 phone plugs for speakers, mixer to amp cable, guitar amp outputs and pickups, mike outputs, which cord is this? Whoops the vocalist tripped! SHORT! Speakons weren't invented til later and no amateur band ever bought them. Shorted speaker drivers aren't that unusual in stage bands, either. Swapping out the whole driver board is not impossible either. The KKR connectors are still available. But one thing at a time, get it working with the original drivers IMHO. Small transistors are not picky, I used MPS8099 npn and MPSA56 in the pnp TO92 ones, MJE15032-33 in the TO220's. For less work for great sound, CS800x hulks are down to $100 these days. These should sound as great as my CS800s after the road damage is repaired. X versus S is the elimination of a 30 lb transformer in the S, a big disavantage on the road for the X.

Peter S 2nd December 2018 06:24 PM

1 Attachment(s)
Hi Folks!
I seem to have generated some attention with what I thought was a mundane project. I consider this amp to be like a '57 chevy, not a Ferrari or Cadillac, but too nice for the scrap heap. I do have a Backyard system with 4 Cerwin-Vega 18" cabinets of about the same vintage, which the amp would be perfect for.

Hi Jon and Burnedfingers; At least at the end of this, I might have a shiny, multi-purpose doorstop!

Hi Rayma; I saw "Deoxit" at a shop in Barrie, Ontario. It was about $30 for a 5 oz can. I found a cheaper product, which so far, has also proven to be effective...I thought it would be cheaper to replace the pot, than spray it which a $30 can of contact cleaner! (kidding--maybe).

Hi wg-ski; Thanks for the words of encouragement, all that aluminum heat sink, power trans, the giant filter cans, and it's BGW 750 layout; I think it's a keeper!

Hi Michael; Is converting an amp to full complimentary, an expensive mod? Would it require completely new driver boards? I would like to know more about that conversion, are there after market or Peavey boards, if required? Also, thanks for the warning re: transformer buzz. I am using a Crest CA18 (1000 W/ch into 8 ohms) to drive my LF cabinets in my home theatre. I wanted that explosive punch, but after replacing the fans with slower, quieter units, the noisy torroid was a huge set-back. 20 dips in electrical varnish, 5 days over a heat lamp between dips and now this beast is dead quiet.

Hi Indianajo; Thanks again for your extensive response! I am looking forward to taking full advantage of your experience, once I get into the "thick of it". I remember holding up my nose at a CS800, in the late 70's, because of the hiss and noise. Of course, I was listening to this amp at a shop that was repairing my Phase Linear 700. My Phase was particularly quiet at that moment because it had 3 shorted outputs, smoked one of my JBL woofers and just cooked one of the Service-tech's test speakers!

I'm hoping this thread will be of interest to many, because so many of these amps are still kicking around. I will try to avoid repetition, but I really appreciate the contribution of all the relevant CS800 mods.

Attached photo; I was just 'doodling' with the cosmetics, while waiting for my outputs to arrive. Never-mind matching! the old outputs weren't even the same types.

indianajo 2nd December 2018 06:53 PM

Quote:

Originally Posted by Peter S (https://www.diyaudio.com/forums/pa-systems/330682-peavey-cs800-doorstop-decent-post5619632.html#post5619632)
Hi Folks!

I remember holding up my nose at a CS800, in the late 70's, because of the hiss and noise. Of course, I was listening to this amp at a shop that was repairing my Phase Linear 700. My Phase was particularly quiet at that moment because it had 3 shorted outputs, smoked one of my JBL woofers and just cooked one of the Service-tech's test speakers!
while waiting for my outputs to arrive. Never-mind matching! the old outputs weren't even the same types.

Wow, non part # match outputs. What a repair pro! I tried that once.
Those old 70's-80's op amps hissed like a steam engine, and the
4558 used in later cs800 is not great. I knocked 10 db hiss, maybe more, out of a disco mixer by changing from 4558 to 33078, plus a 22 pf bypass cap around the feedback resistor and a .1 uf ps bypass cap 2" from the IC. (both IC's actually, one .1 cap two op amp IC's.) Otherwise the fast 33078 oscillated about 1 mhz.
If you have the sc5278 op amp the pinout is wrong for modern IC's, but a flying IC socket is not improbable in a amp with as much room as the cs800. Then transition wires can modernize the pinout. Best of luck.

wg_ski 2nd December 2018 07:46 PM

Converting an original CS800 to full complementary is possible, but quite a bit of work. The main PCBs need major surgery, the driver boards just a minor mod. Modifying a PL is comparatively easy - since the relevant output transistor rewiring is all point to point. Since the PL is more likely to be used in hi-fi service it’s more value added than on the PV. If you were converting it to class H you would need the full complementary mod, but just rebuilding maybe not. Definitely fix the hiss - upgrading the op amps and cleaning up the input stage goes a long way.

As far as the amp only being suitable for wannabe DJ’s - that’s exactly who needs them. To properly do something simple like a school dance without overloading anything, keeping it out of the red, requires about a 6kW system. Can’t afford such a proper set up? You could crank up an old CS800 to the same kind of levels (with lots of distortion) and it won’t hurt anything (except possibly the tweeters - which can be oversized to take it). Try the same thing with say a GX3, and it will thermal before you get through the first set. Keep it up and you blow the output transistors. Even an old PL 400 would fail eventually - ask me how I know:)


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