Peavey SC400 PA brain (No Output question)

[freeformfx]

Hey I just had to re register since I couldn't remember my password but I guess that's the way it goes as the memory goes
Anyway I have an SC 400 with ddt brain here on my bench that the owner claims had a run in with lightning in his church. Everything powers up fine pre and output voltages look good (-/+ 15v....-/+56v) no shorted outputs. I can hear a slight hum through the speakers when turned on.

before I remove the entire preamp side to check all voltages and signal is getting through to the output section I'd like to rule anything out in the power / output. I have found a TO220 device "MAC12M" that is shorted on the main output. I am not familiar with this part or its purpose but as it's mounted on the actual speaker output jack board I'm wondering if it's some sort of thermal protection device.

There are two... one with the Main outputs and one with the Monitor Outputs. The second one's junctions test fine(are not shorted)
If someone is familiar with this and can save me some Troubleshooting i really appreciate it
I have no schematic for this yet sorry

[Jazz P Bass]

MAC12M is a triac (crowbar)
Peavey uses them alot.
Link: http://www.datasheetcatalog.org/data...ypslpxqcwy.pdf
If it reads shorted, then it did it's job. (as it gave it's life)
Check out the output section real good.

[freeformfx]

ok then that's what I wanna hear. Far as the output section , w/o removing them or even using a scope ( which mine is f'd up) the trannies seem fine and nothing else shorted or open that I can find. And this also is w/o a schematic either.
So is it very possible based on the design and it's purpose that one of those MAC12M's going out would be all that I'm dealing with here? It's what I'm hoping at least.
Just wish I had one of these lying around

[Jazz P Bass]

Is the amp a Peavey Stereo Chorus 400?
I cannot seem to find Triacs on the output section on the schematic.
(may be a slip from Peavey)
If the output does have triacs, they usually give themselves up (to protect the speaker) when the output section is shorted.
Then again the triac may have simply failed.
I would check, check & double check the output transistors & there driver transistors.
If any are bad then start checking the associated resistors.

[freeformfx]

thanks for your replies. No it's not a stereo chorus (which i assume is a guitar amp) that's all I can find schems of though matching an SC400. It's a 1997 pa head
I know there are peavey CS400,etc... but this is SC'
I've not pulled the fan cover off the outputs but checking there junctions all looks good and the voltages are good (like 56v .430v 0v ) i think but i'd love a schematic. There is a lot of small stuff on this power/output board including a few ic's.
Yes I'm hoping the Triac gave out alone. I am wondering though if only one gave out wouldn't the other set of outputs be working?

[Jazz P Bass]

Call Peavey & ask for a schematic.
Wopnderful peaople to work eith!

[freeformfx]

yeah they're always the best IMO too. I checked this morning and they already got the schem to me so here it is i'll try to add it if I can. At least I think this is the schem that closest matches the unit. They sent me several models , the others were '98 and newer
I checked and it's the CR338 that is shorted
anymore help based on this schem is greatly appreciated

[52 Bill]

Just like JPB said, CR338 is a triac/scr that is designed to protect the speakers from damage when the power amp fails and puts large amounts of dc voltage on the output. I usually see these short only when there are shorted output transistors in the power amp. This also means usually blown fuses as well. Remove it for now as the amp will work without it in circuit.

Power up the amp and check all of the power supply rails. If both power amps are working and there is no significant dc on the outputs then plug in the speakers and see if both channels pass clean signal. Interrupt the preamp signal by plugging in a cord in the power amp in jack. Does the hum go away? If it does then the problem may be in the preamp. If it's still there, it's in the power amp or power supply.

[freeformfx]

thanks Bill , that's what I was also wondering. If the amp would function w/o the shorted triac. That maybe i'd fire it under protection and check for dc on the outs. Maybe I get lucky and it just works. Yeah right
as I said no shorted output trannies, there are no blown fuses. Haven't checked the drivers but will now with the schem

[freeformfx]

oh damn do I feel stupid... I just realized I was only checking the Triacs in crkt with a speaker cable inserted and they both read shorted between one junction with a cable plugged in. Removed they're fine. So again Output vltgs look good and there is no dcv on the output. Is this most likely an issue within the preamp or where it enters the power section then?
I guess I'll check the ribbon cable a little closer and recheck the -/+ 15v. I can clearly hear the power output kick on when powering up with that healthy sounding slight 'hum' just no signal getting through. This is why I assumed the output section was healthy on first check

[freeformfx]

Oh also the signal from the preamp "main out" is testing fine feeding another amp. So I am now thinking if the outputs themselves are fine that I should start looking from the front of the output section. there are a few IC's there which I have no replacements on hand.

[Jazz P Bass]

Is there a "Power Amp In" jack?
If That hums, than the problem is mosr likely in the power amp.
You really need to step back & give the amp a health check.
Check the power rails . The + & - high voltage rails.
You can test the rails with your DVM set on volts dc then flip it to read volts ac.
The Vdc should remain steady at idle.
The Vac ripple should be small. (at the least both rails should have the same ripple voltage)
Then check the + & - 15 volt rails.
Same thing : steady voltage.
But on the Vac measurement you do not want to see any ripple.
If the opamps are fed ripple they will not be happy campers.

[freeformfx]

Ok , first as always thanks for your attn. on this. I find no significant ac ripple on the low voltage rails to the preamp, starts reading about .4v and rapidly drops down to hundreths/vlt. I will have to pull the back again and measure on the high rails , should've done that originally though ( rusty on solid state mistake) I don't anticipate finding any though. I'll post this when I get it back open
here's where I'm at , I had checked the direct to Power amp via "power in" before as well as the Main out which does have a strong signal. But this time I noticed feeding a line level signal to the Power In I am getting an ever so faint signal through the speaker.
doing a simple audio probe at PA13N (Pin11) of the interconnect ribbon cable which is the preamp signal entering the power amp section (at R108) and the signal there sounds about what I'd expect to hear, a bit distorted but good signal
(Edited this post cause first test I had a bad ground at the probe...sorry)
so this leads me back to starting at the front of the pa section...Jive?

[freeformfx]

Ok more results, sorry to keep posting so quickly but I guess that's the whole point

The audio signal is entering the power amp fine just right up to r109 before the U100/101 opamps ( pins 3 and 1/6 respectively )and then nothing! problem is here I'm hoping. So I checked reference to ground at this point and get 23 ohms with or without power so obviously audio is not gonna go through there like that.

I am pondering an issue with the Q105 FET (Q205) . I don't see any -22v control voltage ("x" on schem) entering their gates. The source and gate read almost short as well , the same 23ohms. Which may be normal for this fet when no source voltage is put on it. so maybe i need to sort out whatever develops this control voltage.
hope i'm getting somewhere rather than spinning wheels

[52 Bill]

Remember that FETs will show low resistance from source to drain until there is a voltage applied to the gate. So yes look into the missing control voltage.