Peavey XXL repair!

[ohmy!!]

If no luck then Go with Enzo's thoughts,, You need to establish a Real connection to the power amp output.
if it's like the one I worked on then several switches/ connections happen between poweramp out and speaker.
Phil.

The effects loop is working fine. How would I go about establishing a real connection to the power amp output??

[J M Fahey]

Hi ohmy.
We still have a communication breakdown here (No, not Zepp's song)
You state that 

It doesn't output any sound through the speakers.

Yet you claim that

The effects loop is working fine

How wouild you know?
Usually the loop out is the preamp out and the loop in is the power amp in.
If that is not true, coupled to the fact that

there's a little board that has the output jacks as well as a few switches to change up the Watts, tightness, and ohm selection for the speakers.

, all of this very unusual on an SS amp, it all show us that it's quite different to most other amplifiers .
To be able to offer you further suggestions I guess you'll have to provide a readable schematic.
Good luck.
PS: call Peavey.

[bry melvin]

from what I have read...this thing has..pushpull transitors to a transformer mimicking tubes. Sounds like early SS amps sort of...

Until these posts I had thought this thing was an hybrid.

Interesting.

And the "breakdown" seems more like Mellissa Etheridges.

I would really like to see that Schematic too...

[Enzo]

COmpletely solid state amp.

You can't just unplug the output transformer and get sound, what I mean is that the transformer is not necessary to the circuit, you can connect a speaker direct to the output.  You would have to do that inside though, using clip wires or something.


The FX loop might work fine, but the point is not if the loop works as a loop.  The point is that the loop jacks have to carrry the signal past themselves, and if the contacts on them that do that get dirty or oxidized, then the signal can't pass.  Thus we suggest plugging a cord from FX send to FX return to see.

[ohmy!!]

Yeah, I tried plugging a wire between the effects send and return.  It doesn't do anything different.  I'm going to get a hold of Peavey later today to get a schematic from them. I know the effects loop was working properly because I can plug the preamp out on the XXL into the poweramp in on my VTX and play it and ran some effects in the effects loop on the XXL.  This amp has effects send/return jacks as well as preamp out/power amp in jacks. Sorry if I'm not being clear enough.  

[phatt]

Yeah, I tried plugging a wire between the effects send and return.  It doesn't do anything different.  I'm going to get a hold of Peavey later today to get a schematic from them. I know the effects loop was working properly

*****because I can plug the preamp out on the XXL into the poweramp in on my VTX
and play it and ran some effects in the effects loop on the XXL.***** 

Q? DID You Get Sound from Speaker in XXL Amp?

IF you did get sound,,,, then highly likely one of those 2 loops (yeah the ones you Forgot to mention) Is broken/malfunctioning.
Then try direct (Working) Cable in "BOTH LOOPS"

ps.. Please please try and stick to the Amp in need of repair
Otherwise it can get extremely confuzzing for those trying to help you.
Remember we don't have your Amp/Amps in front of us. 8|
Phil.

[ohmy!!]

No, no sound comes out of the speaker output on the XXL at all ever.  I was saying that I can use the preamp on the XXL when I run the preamp output on the XXL into a different amp's poweramp input and sound comes out of the other amp's connected speakers (speakers connected to the other amp not the XXL).  During which, I had some effects pedals looped on the XXL's effects send/return and the effects played out fine.  The only way I've been able to get sound is running the preamp into a different amp's poweramp with the speakers connected to the different amp.    Like you said you can't see them both so it's difficult to explain.



If I'm talking straight about the XXL head itself with a set of known working speakers plugged into it I get no sound.  When I run cables into each of the effects loops (preamp out/poweramp or effects send/return)  I still have no sound from the speakers.  I have run cables into the send/return as well as the preamp out/ poweramp in at the same time and separately. I hope that clears things up   .  I'm going to call Peavey for the schematic now so I'll have that shortly.  The only sounds that are being made from this amp are coming from the output transformer, not the speakers.

Thanks for all the input and help so far.

[ohmy!!]

Just got the schematic from Peavey, here they are.

https://docs.google.com/fileview?id=0B5zWkfJSuEm8ODViMTJkMGMtNzYxNC00MTMzLTg3NmQtMmZmYmEyMGUxYWM4&hl=en

for whatever it's worth here's a few photos of it now that it's out of the head cabinet:



Uploaded with ImageShack.us



Uploaded with ImageShack.us



Uploaded with ImageShack.us

[J M Fahey]

Hi ohmy.
Well, we have advanced a lot, but still no cigar.
Please re-write Peavey, thank them for the nice Power Board schematic they sent you, that it's something but not enough, because you do not have a fuse-burning amplifier, but one which does not, yet produces not sound at all on a known good speaker, so you want to check related wiring.
This schematic does not show the output transformer, the impedance switch, the damping switch or the jack wiring so would they please be so kind as to send the "output board" schematic as well as the amplifier wiring.
Please don't send that piece by piece but whatever they feel will be necessary to fix it. Thanks.
Back to what we have: this is a conventional amplifier (sort of) with some extras:
1) A patented  "Tube emulator" front end (U1b, 2a, 3a+b) driving
2) the conventional amplifier (U2b + Q2 to Q7) which just to be special  is connected with its output grounded and the audio signal taken from the Power Supply centerpoint. Oh well
3) a slow turn-on mute (FET Q10) which might be muting your signal.
Ask Peavey for what's missing , yet we'll start by checking that the conventional power amp works (I think it does) so to go on we need that extra info.
It is a complex amp, hopefully it does capture some of the Tube vibe.
Good luck and Patience.
PS: nice pictures.
If somebody does want to see them full size but imageshack refuses because "it's not a premium ($$$$) account), just click:
http://img69.imageshack.us/img69/3271/img0687a.jpg
http://img194.imageshack.us/img194/6011/img0688ev.jpg
http://img706.imageshack.us/img706/9724/img0685s.jpg

[teemuk]

While you wait for rest of the schemas from Peavey...

Transformer's primary looks like to be wired in between J10 and J11. There's four secondary wires coming out of the transformer, most likely common and three impedance taps. The board houses a selector switch. The condition of that circuit should be rather easy to test.

Note that this power amp is a grounded emitter circuit. It actually grounds the power transistor's emitters, causing output devices modulate the supply rail voltage according to output signal. The output signal is then taken from the center tap of the mains transformer, instead of using that as the common point like in most amps. It's not any more complex setup than a conventional power amp with ordinary emitter output but you just got to understand that you will be measuring things sort of backwards and conventional techniques used with conventional amps (e.g. measuring rail voltage, measuring emitter to common rresistance for shorts etc.) may give "strange" readings that make you scratch your head for hours.

The other half of the output board houses the additional controls. I don't know what the "damping" does (likely it's just a feedback-magnitude control) but basically this is just the plain T-Dynamics power amp circuit discussed in my book (there's a link for free-to-download .pdf on this site). It's shown in a much simplified form in that too. The first versions of that circuit used potentiometers for "power" control. Since then Peavey simply replaced them with fixed resistor values selected with switch. So, the power control is a two-pole switch for alternative resistance values that control gain of amps U2A and U2B. Again something of which's overall condition should be pretty easy to verify, even without complete schematics available at the moment. They either work or they don't.

Read the section of my book that discusses Peavey's tube emulating circuits. It deals with these circuits and may be helpful.

Addtionally, this site hosts some schematics of the first gen. TransTube amps with T-Dynamics. It will likely fill most of the blanks missing from the XXL schematic. Not exactly but still in a manner that gives you the overall picture.
http://www.about-guitar-amps.com/free_guitar_amplifier_schematics.html
You need to register but it's quick and free and you won't receive any spam from them.

[Enzo]

OK, if the output transformer sings, that means the amplifier is driving signal into it.  SO if nothing gets to the speakers, I'd be thinking the wiring and impedance switch.

Try this.  Powr off.  plug a cord into a speaker out jack.  Set the impedance switch to 16 ohms.  Now at the free end of the cord measure resistance from tip to sleeve.  You are measuring the resistance of the 16 ohm output auto-transformer winding.  If you get open indication, then the switch, wiring, or transformer is open.  If you get an ohm or two, then it might be OK.  If you get a dead short, it may be a problem, but the low resistance of a good winding is hard to differentiate from a short.

Inside the amp, check this:  there is a small board for the output jacks and impedance switch.  On the end of this board is a small 4-pin connector to the auto-transformer, and right next to the speaker jacks is a small 3-pin connector to the power amp.  The power from the amp comes through a coil with parallel resistor on its way.  The amp output is wired to the 4 ohm tap.  Select 4 ohms on the switch, and you could unplug the transformer and still get full output.   SO) look at that coil of wire and the 5.6 ohm 5w resistor next to it at the end of the little board.  Is the solder to them intact?  Tug up on the coil a little, is either end broken free?   Measure the 5.6 ohm resistor.  Since it has a coil of wire in parallel, it should measure shorted.  If it reads 5.6 ohms, then the coil is open.

And dismount the little board and look closely at the switch.  ANy chance someone bumped the switch knob inwards and pushed the back out of the switch itself?

[ohmy!!]

OK, if the output transformer sings, that means the amplifier is driving signal into it.  SO if nothing gets to the speakers, I'd be thinking the wiring and impedance switch.

Try this.  Powr off.  plug a cord into a speaker out jack.  Set the impedance switch to 16 ohms.  Now at the free end of the cord measure resistance from tip to sleeve.  You are measuring the resistance of the 16 ohm output auto-transformer winding.  If you get open indication, then the switch, wiring, or transformer is open.  If you get an ohm or two, then it might be OK.  If you get a dead short, it may be a problem, but the low resistance of a good winding is hard to differentiate from a short.

I don't know if this makes a difference but I'm using an analog meter.  Whether I have it set to 10x, 100x or 1Kx and I measure this the needle doesn't make a move so I guess something is open. 

[ohmy!!]

Inside the amp, check this:  there is a small board for the output jacks and impedance switch.  On the end of this board is a small 4-pin connector to the auto-transformer, and right next to the speaker jacks is a small 3-pin connector to the power amp.  The power from the amp comes through a coil with parallel resistor on its way.  The amp output is wired to the 4 ohm tap.  Select 4 ohms on the switch, and you could unplug the transformer and still get full output.   SO) look at that coil of wire and the 5.6 ohm 5w resistor next to it at the end of the little board.  Is the solder to them intact?  Tug up on the coil a little, is either end broken free?   Measure the 5.6 ohm resistor.  Since it has a coil of wire in parallel, it should measure shorted.  If it reads 5.6 ohms, then the coil is open.

The solder is intact, it's pretty tight in there also.  The resistor does measure shorted.

[ohmy!!]

While you wait for rest of the schemas from Peavey...

Transformer's primary looks like to be wired in between J10 and J11. There's four secondary wires coming out of the transformer, most likely common and three impedance taps. The board houses a selector switch. The condition of that circuit should be rather easy to test.

Note that this power amp is a grounded emitter circuit. It actually grounds the power transistor's emitters, causing output devices modulate the supply rail voltage according to output signal. The output signal is then taken from the center tap of the mains transformer, instead of using that as the common point like in most amps. It's not any more complex setup than a conventional power amp with ordinary emitter output but you just got to understand that you will be measuring things sort of backwards and conventional techniques used with conventional amps (e.g. measuring rail voltage, measuring emitter to common rresistance for shorts etc.) may give "strange" readings that make you scratch your head for hours.

The other half of the output board houses the additional controls. I don't know what the "damping" does (likely it's just a feedback-magnitude control) but basically this is just the plain T-Dynamics power amp circuit discussed in my book (there's a link for free-to-download .pdf on this site). It's shown in a much simplified form in that too. The first versions of that circuit used potentiometers for "power" control. Since then Peavey simply replaced them with fixed resistor values selected with switch. So, the power control is a two-pole switch for alternative resistance values that control gain of amps U2A and U2B. Again something of which's overall condition should be pretty easy to verify, even without complete schematics available at the moment. They either work or they don't.

Read the section of my book that discusses Peavey's tube emulating circuits. It deals with these circuits and may be helpful.

Addtionally, this site hosts some schematics of the first gen. TransTube amps with T-Dynamics. It will likely fill most of the blanks missing from the XXL schematic. Not exactly but still in a manner that gives you the overall picture.
http://www.about-guitar-amps.com/free_guitar_amplifier_schematics.html
You need to register but it's quick and free and you won't receive any spam from them.

Thanks, I'll register and read up on it!

[J M Fahey]

Good guess.
Follow the wiring backwards until you find where you lose continuity.
Don't skip the obvious, as in: you get to a switch, it is "on", you assume there is a path for electrons there, you skip it and go on ... maybe that switch is dirty or broken. And so on.