What happens if you unplug the auto-transformer completely, set the amp to 4 ohms.  Still no sound?

There should be a yellow and a blue wire from the main board over to the output jack board.  That is the main output.  Follow the yellow wire back, I think it should go to the main board, then from there back to a thermal breaker on the heatsink.  Check that thermal breaker, it should measure continuity between its terminals.  It opens when it gets real hot.  If it is stuck open, you get no sound.

Can you get clip wires on those yellow and blue wires?   APply a test signal to the power amp, doesn;t need a lot, just enough to hear when it works.  Now clip some speaker on your bench to the yellow and blue wires where they come off the main board.  Got sound?

Right next to where those wires connect are two power resistors - 0.33 ohm 5w.  Make sure they are not open.  They should check like a short.

For testing, you can connect one side of a test speaker to ground, and with a clip wire on the remaining speaker terminal, probe the yellow wire, then either end of the thermal breaker, then back to J12 from the second terminal of the thermal.

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?

Like any component, I test it by the way it does - or does not do - its job in the circuit.   Does a filter cap leave too much ripple?  Does it not hold up under load?  Does a coupling cap let through DC?  Does a coupler seem to not pass signal?   Does a tone shaping cap not shape tone as it should?

In the repair business, someone is paying me a dollar a minute to work on his stuff.   SOme shops charge twice that even.   How many minutes should I spend testing a 38 cent capacitor?  If I think a cap is bad, I just replace it.   Parts are a lot cheaper than labor.

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.

Any "tech" who dismisses entire brands of gear out of hand like that doesn't deserve the title "tech" in my book.  Drive a little further and find someone worth a damn.   Just because he doesn't understand the Heritage doesn't mean it is not a fine amp that thousands of people enjoy and rely upon.   If the heritage design is crackpot, then so are most of the Music Man amps that Leo Fender made.

I didn;t realize the XXL had a transformer on the output.  it is a solid state amp, and doesn't need the transformer.  The transformer may contribute something to tone, or might do nothing more than offer full power out at different impedance loads, but nonetheless, we can remove it and connect a speaker directly to the amp output for testing.  it will be the last thing I would suspect either way.  The connector to it is far more likely to be trouble than the transformer itself.

For a tech problems like "dead" and "zero sound" are actually usually easier thing to fix than "smoke" or "distorted."

What is this autoformer you refer to?   As far as I know, this amp has a plain old power transformer.

This could be a million things.  A bad thermal sensor, a loose wire, and bad impedance switch, etc.


You might not be able to send the amp to the same tech as the guy had, but there are good competent techs all over the country.

Actually on this one there is an extra stage that switches in.

Here is the schematic.

Hi.

It would be very helpful to know what amp you have, so we know what circuitry is involved.

If the schematics are not posted in the schematic collection on the Fender web site, CONTACT them and request them.   There is a note right there about schematics not in the listing.

Thermal problems are not necessarily in parts that get HOT.  CAn be, but often is not.  As the entire thing warms up, the temperature of everything climbs, and the silicon in transistors warms up.   Just as you can have a 500v cap that is leaky at 450v but not at 350v, you can get a small part that is fine at 28C but not at 30C.  SO don't limit your search to things that get really hot.  Check them too of course, but check everything else too.

I don't know that model or have its papers, but you might call Yamaha in Buena Park and see if they do.  They will likely want to sell you a copy of the manual for it if they have it in their files, but really, $10-15 is worth it to not have to screw around.   Their manuals tend to be very complete, with board layouts and trace art, schematics, mechanical blowups, parts lists, and any service procedures like bias adjustment.

You have two power transistors, but I bet there are a bunch of small transistors as well.  Did you freeze everything on the board, or just the parts you felt were getting hot?

Oh poop.   It might have been even clearer if I had typed that the grid is 75v more negative than the cathode instead of 785v.  Oh well.

Well, when you are designing something it is only complex once.  Once it is done it is done.   Peavey had a number of amps using that basic power amp design, just as Music Man did.   I couldn't tell you who thought of it first.

I am no expert on this, but for the user, the design complexity is not an issue.  But out of it he gets a pretty efficient amp that is strong and won't really have any bias issues.   These darn things ar LOUD.

Hartley Peavey has written a number of white papers on various aspects of his product designs.  This is just the sort of thing he might write about, though I haven't looked.

Look at the Music Man amps, they are of similar circuitry.


When the cathode is grounded like most fixed bias amps, then the grid is indeed generally at negative voltage.   But cathode biased amps generally will not have a negatiove vokltage on the grid, because the cathode is made positive instead.

The whole point of bias is the relationship between the cathode and the grid.  What the voltages actually are is less important.   Having a tube grid at -25v and the cathode grounded is exactly the same as having the cathode at +25 and the grid grounded, as far as the tube is concerned.

SO having a cathode at +90 and a grid at +15, is the same to the tube as having the cathode grounded and the grid at -75v.   Either way the grid is 785v more negative than the cathode.   And in the case of the Heritage here, if you think about it, that is pretty cool bias.  These amps are working close to class B.


Obviously since the voltage across the tube plate to cathode is important, B+ would be different in each example.

Start at the start.  The power supply.  The thing runs on +/-27VDC for the amplifier, adn +/-12VDC for the preamp.   Look at teh schematic.   Do you have something like 25-30VDC both positive and negative at the main filter caps?  And ther are two 5 watt resistors of 270 ohms that drop the 27v down to the 12v zeners, so is ther 12 volts of each polarity?   Here is a tip - find the two 270 ohm 5 watt resistors.  Measure voltage at each end.  One end should be 27v and the other end 12v.  The 27v wil be approximate, the 12v should be pretty close.

Headphones hum, but do they pass guitar signal?  DO any of the controls affect this hum?

Is there ANY background noise in the speaker or is it utterly silent?

Common problems with small amps are broken solder under the input jack, or broken input jack.

+15v on the control grid is perfectly reasonable when the cathode sits at +90v.

Here, explore the power amp circuit.
http://www.schematicheaven.com/newamps/peavey_heritage_vtx.pdf