Unfortunately, most people do not consider quadratic equations (parabolic function describing soft saturation) a layman's explanation. The section on symmetric/asymmetric transfer functions and their related harmonics is nice and they didn't involve Fourier transforms in the discussion. 

Postscript

Replaced the +5V regulator and the DSP chip is still loading down the output. I have to assume the DSP is frapped. I'm going to part out the amp and enclosure at this point.

Thanks JM and Enzo for the alternative solutions.

I'm going to order the Alesis module from PCM63.COM since I have several other projects that this might work in - interesting site.

I will call the service dept and see what the cost is to repair the EFX board.

I have the regulator and I'm waiting on the inductor and capacitor - still want to pursue fixing the board I have in the amp.

A peavey 258 EFX has been on the local craigslist and I'm trying to get in touch with the seller since the amp appears to have issues (not FX related). Might be a good parts donner.

I'll post results as soon as I have something worth reporting.

Here is the update of what Peavey has in stock (they always amaze me):
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Hello George,

99099570----EFX DIGITAL CBD ASSM    Retail $259.99 each (in stock)

70418304----CS4826 DSP/CODEC 2I/6O    Retail $64.81 each (in stock)

The original part number 70300079 has been discontinued. Below is the same with the ROHS number, meaning lead free.
30300079----47UF 35V 20% EL SMT6       Retail $0.49 each (in stock)

Part number 70800037 has been discontinued.

Thanks,
April

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I'm going to replace the regulator/capacitor and test the inductor - too bad it's EOL. Hopefully the issue was the cap drifted and the regulator went unstable.

If that isn't the issue then it would cost +$65 to change out the DSP. I do have access to someone who could rework the 128 pin package reliably and assume you can still host & boot via serial interface (U101 EEPROM - also discontinued). That's more work than I want to do but we'll see.

I found, besides the Cirrus Logic DSP chip, U 300 / LM2594 DC/DC buck converter is also getting hot. I desoldered U 300 and will replace it and see what happens. I'm tempted to also replace C18 though it seems to be okay.

Hopefully it"s the converter since the DSP is not replaceable - Cirrus Logic EOL'ed the CS4826 in 2005.....

If there is no hum at send (J3) then the issue is not in the preamp if I read the schematic correctly.

Recheck voltages at A,B,C,D and +- VCC. Make sure center tap is grounded....make sure your orientation (polarity) of the replaced filter capacitors C55/56/57/58 is correct.

It's the DSP Package that's getting hot _ I didn't have time to carefully check if any other components (DC converters) are getting hot. Also need to see what other voltages are present on J301....

If you put your DMM on AC volts and connect it to the output of the amp (turned on) what reading do you get?
Using the send and return try and see if the hum is in the preamp or power amp or both.
Do you hear the hum in the headphone output?

Measured AC voltage to ground for J18 = 19V and J22 = 19V - no limiter connected. The voltage across J18/22 is 38VAC.

When I remove J28 from the reverb board (DSP) the +-25V rails come back to life. Something is frapped on the reverb board - dang.

I pulled the secondary connectors and get 70VRMS across J6-8 and 35VRMS across J18-22. Didn't check to see what the voltages are refereced to ground. Will check this out.

Why drive U2B off the 25V rails with a zener diode/cap regulator when there is a seperate +- 15V rail already used on the other OP Amps?

Spring reverb pan can fail after years of vibration.......

Thanks for getting back to me.

With LBL removed there is a slight improvement in rail volatge but the +-25V rails are down to 7VDC so the 15 volt rail is even lower. Seems odd that I'm not drawing lots of current - the amp doesn't blow the fuse.

I can only guess that it's Q3/6 though they pass DMM test. Should check actual voltages around EBC. C14/28/15/29?

Looking at the circuit again, I'm wondering if the regulators are damaged? Also is this PA class G - where the 47 and 25V rails are switched depending on output demand?

Found one these on CL with a "for parts" status. With the amp connected to a LB limiter I get a soft orange glow and the channel select button does nothing. FX controls appear to work- LEDs light. No signal from the preamp and when driving the power amp input I get a very weak distorted signal at the output.

I pulled the secondary connectors and get 70VRMS across J6-8 and 35VRMS across J18-22. Reconnect secondary and get +-35VDC instead of +-47V rails. Quick DMM check of filter caps, Q3 and Q6 show no issue. What's loading down the 47V rails?

Did you check the power supply? I have seen the filter capacitor fail on these amplifiers - had one so dried up you could shake the capacitor and hear a rattle. There is a 1000uF/50V? aluminum electrolytic on the output that should be checked.

Early 90's stereo chorus which needed U8 replaced to get the chorus working again. Now I find that I only get chorus effect at one output amp - I believe it is a "stereo" effect   

I get the correct outputs at the preamp 1/2 (dry/mix). Where or how is the stereo chorus generated or routed?

BTW - why do people dis these amps? I have nothing but praise for Peavey designs and support.