Peavey CS-800 no power

[nosaj]

So i have a Peavey CS-800 Early model no power on. Get power at the fuseblock but power switch has no effect. The device in the pic is a fastpak triac. Looking at the 76 vs 79 schematics included. it doesnt really look like its required What benefit does it offer? Is it related to Fan speeds?
nosaj

[g1]

In the 79 schematic, the 'power switch' is just a low current trigger for the triac, which is the real power switch.

[nosaj]

So I should be fine bypassing it to run through rest of amp?
nosaj

[g1]

Yes, if it matches the schematic you can jumper across for testing.

[nosaj]

Ok its powering on now. I was wondering if someone could give me a teachable moment on the schematic. I would like to see how the signal flows though this schematic i see theres a positive and a negative side but not seeing how it splits like a PI. If maybe someone could draw a line for signal flow or give a website or source that shows how to determine it.

Thanks,
nosaj

[Helmholtz]

I would like to see how the signal flows though this schematic i see theres a positive and a negative side but not seeing how it splits like a PI.

I'm not an expert on SS power amps.
Just so much:
SS power amps typically use complementary or (quasi-complementary darlington connected) output devices which are driven from same phase signals having a DC offset for bias.
So no need for a PI (or an OT).
In class B operation the upper side conducts during the positive half-cycle and the lower side during the negative half-cycle.

[Jon Snell]

Ok its powering on now. I was wondering if someone could give me a teachable moment on the schematic. I would like to see how the signal flows though this schematic i see theres a positive and a negative side but not seeing how it splits like a PI. If maybe someone could draw a line for signal flow or give a website or source that shows how to determine it.

Thanks,
nosaj

U1 is the input voltage amplifier that feeds the comparater U2B. U2B feeds the signal to the voltage amplifier Q2 which inverts it at the same time. Q2 feeds the inverted signal to the pre driver that also inverts the signal on the collector but does not invert the signal on its emitter.
Q6 is held as positive as it is able, by the constant current source from Q5. Q5 supplies the constant current to Q9amplifiing the current and driving Q1 and in turn the top half of the power transistors.
Q5 is stopped from going possitive, called for by the constant current source and using Q6 as the pre driver and invertor feeds Q10, Q2 and the lower half of output transistors.
U2B samples the DC voltage on the output line and adjusts it to acheive 0volts, or thereabouts. Q7 & Q9 are current lclamps in case of over load.

What a complicated amplifier for you to cut your teeth on!

Schematic drawn with arrows. Red arrows non inverting path.
Green arrows path gets inverted at each transistor.
Brown is constant current path.
Hope you find this useful. It makes perfect sense to me but I am dyslexic and hope you understand my notes.
Peavey-CS-800-C-Series-7-79-Schematic.pdf

[g1]

Just to add that there is lots of compressor stuff there at the front that just makes it more difficult to figure out. If there is a schematic version without the comp. it might help you.
But you can just ignore that stuff if you get the gist of what Jon has drawn up.

[nosaj]

Thanks guys I appreciate it. The PDF helps quite a bit.
nosaj

[nosaj]

Right side of amp good Left side over loads Found Q7 bad Mps6530
Peavey cross references show 2n3642 MPS6531

Curious if there might be any other options that would work in this position. So I can look through what i have on hand. If not I will pull q7 from another driver board and complete at least this amp . i have 3 of them one complete, this one half working and a 3rd no output.

Thanks,
nosaj

[Jon Snell]

Almost any low power NPN will work. It is only to reduce drive current under extreme load conditions.
I have hsed 2N4401 and 2N4403 as its compliment.
It only sees a couple of volts so nothing special.

[nosaj]

2n3904 or MPSA 92( i have several hundred of the 92's)??
nosaj

[Jon Snell]

MPSA92 is the wrong sex.
You need NPN not PNP.
MPSA42 will work.

[nosaj]

ok got a chance today first thing is I tested transistor out of circuit this time and it test good but not in circuit .. Also when the volume is turned up you can start to hear the test signal(I am using a Dummy load) almost like a singing OT. I took measurements and when I was done shut it down. The heatsink was to hot to touch.

These are the pins on the driver board that connect to the Power amp board. The TO-3 transistors were tested out of circuit last week. Any suggestions I am almost of the idea the driver board is ok because I've swapped in another from a salvage amp and I still get the singing.
Thanks,
nosaj

Pin 1 -.316
Pin2 -67
Pin3 -66
Pin4 0
Pin5 0 GND
Pin6 1.0
Pin7 67
Pin8 .25

[The Dude]

Have you checked the triac in the crowbar circuit on the output jack board? It might be leaky if the amp once put out DC. You can temporarily bypass/unhook the jack board for testing, but check for DC before you hook up a speaker. Also, measuring resistance across the banana jacks might tell you if the triac is leaky.