Need Schematic for Peavey HKS 12

[gary rabbitt]

Hi guys, looking for a diagram for the HKS 12 keyboard amp. The amp works, but there is a fuse (one of 3) blown and not sure what the purpose of that fuse is.
A schematic would be great to keep with the unit, just in case something fails later.
Thanks in advance!
Gary

[The Dude]

I'd call Peavey. They've been more than willing to help me in the past.

[gary rabbitt]

Thanks I will give them a call.
I looked at the amp today and found that a diode was burned, right near the phono jack input. Two diodes were right in that spot, I have the numbers so replacement will be easy. Both are 1N4744A.

Also found some fractures on the solder pads, two were on the output transistor pads!
What really got me, and i know it happens sometimes I found a capacitor that was never soldered at the factory. Odd how the amp still worked all these years.
I believe this box has had the main speaker replaced, horn, made in Mexico?, Tweeters made by Motorola?, burned diode area, factory missed solder job. I don't know if Peavey sourced parts from Motorola, or if these are after market.
Thanks!

[g1]

Peavey used those horns and tweeters in many models, so I'd think they are original. Not sure about the woofers but have no reason to suspect they were replaced.

[gary rabbitt]

Peavey used those horns and tweeters in many models, so I'd think they are original. Not sure about the woofers but have no reason to suspect they were replaced.

Thanks g1. You're right about the tweeter it did look very stock, was just surprised at the Motorola logo on them.

I think the woofers were replaced, as I found pieces of a torn cone, no doubt from the original woofer. Also the mounting holes were new, speaker held in with drywall screws. The original threaded screw mount anchors were not used. I have two of these units, and the second one also had the Zener diode area browned, but could still read the part number. (The Zeners are 15W 1A parts).
Unknown why they were burned, maybe the device they had in the 1/4" input sent some harmful voltage.

[The Dude]

Those Motorola piezo tweeters are used in lots of things- not just Peavey. It's a cheap way to add a tweeter because you don't need a crossover. I suspect the original speakers were blown because of DC on the output. You may want to unhook them until you verify that the amp is not putting out DC voltage.

[doctor]

Here is the HKS-12 schematic.

[gary rabbitt]

Thank you so much Doctor, I really appreciate the diagram.
As far as Dude's thought about the amp putting out DC voltage to the speaker, what is the easiest way to check that?
I have an old scope, but is there an easier way to check? Maybe just the VOM at a no signal state?

I don't know the history or use/abuse of the amp otherwise. Sounds great as it is. I am going to order the Zeners from Mouser, in addition to my regular cap order in a few days.
Thanks once again!
Gary.

[J M Fahey]

Schematic says this is a 90īs product.
If so, Motorola piezos would be standard (way back then stuff was still made in the USA) but woofer "should" be some kind of Eminence.

This one is clearly Asian and way newer, so itīs a replacement.

As of the burnt diode: those are added at the input tio protect Preamp from very high voltage fed to it, typically (doubly so because itīs a floor monitor) a speaker output.

Think 20 to 30V RMS, any current you want, compared to expected 1V RMS and less than 1 mA .... deadly.

So that diode died doing its job.
Replace both and simply donīt feed a speaker out into that *Preamp* input.

With such a strong signal this power amp would be driven into balls to the wall squarewave distortion, I can easily picture a very dead speaker.

But if it works now, fine, you got lucky.

I think gross overdrive more probable than DC at the output, which if present today woud be unmistakeable: turn amp ON > strong THUMP > speaker cone goes fully forward or backwards and stays there, strong hum heard through it, no sound.
Itīs not a subtle failure.

[gary rabbitt]

Thank you Juan.
Both speakers cabinets have the same thing done to each. Replacement speaker, and the diode area slightly toasted.
Someone did something wrong obviously to make that happen. I will replace both diodes in each amp. Mouser order going out this week.

I too think the speaker (woofer) failure was simply overdriving them. For my purposes, the replacement woofers are fine.
On powerup, I do hear the usual thump but absolutely no hum. I just checked, thump, but cone stayed in one place.
I was contemplating replacing the large filter caps, (I regularly replace all caps on the vintage radios I restore) but for the moment may leave them in place. No signs of bulging or physical leakage seen on those, or any cap on the board.
Thanks for all your help
Gary.

[J M Fahey]

I bet somebody had a powered mixer and one night decided these were good "speakers" .

Worst thing is that they must have worked .... sort of .... at least for some time.

Very old amps, say last Tubed ones or early SS ones in the late 60īs and early 70īs had a dedicated "line out" or "external amp out" which was NOT a Preamp Out (the correct solution) but a simple 10:1 or 20:1 (say 10k:1k) resistive attenuator straight across speaker out.

Goofy solution but very inexpensive way to "daisy chain" amplifiers for those large open air "hippy shows" such as Wood ... er .... something.

So that hypothetical powered mixer *might* have properly driven these powered monitors ... if somebody had cared to add the proper attenuated outputs.

A few modern powered monitors have a switch to select "line level" or "speaker level" inputs.

[gary rabbitt]

Thanks Juan, I bet you are right.
I plan on using the speakers only for feeding as a safe line level of recorded music through them. Small events where a lower level of sound is needed. I do try and take it easy on the amps I have. No daisy chaining here, lol
Thanks again.