Fender 400 PS, now on the bench. Know anything about this?

[pompeiisneaks]

Wow, after following this thread I was curious about these beasts, and there's literally only one available on reverb.com and none on ebay going for 5k? OOF not something I'M going to be looking for soon hahah

~Phil

[Matthews Guitars]

For those of you who may be interested in acquiring one of these beasts, know that when I'm done having fun with it, I will eventually want to sell it,
because while it's tons of fun to figure it out and work on it and restore it, it's not the kind of amp my heart lusts for. And no way am I going to be asking five grand for it!

Realistically...less than half that. I'm just not THAT guy who insists that it's top dollar or nothing!

And if you're going to be in central Florida, maybe you feel a huge need to feed the Evil Disney Rat a big chunk of your paycheck because your kids need entertaining, you could drop by and check this thing out and maybe save the freight costs and take it home with you.

[pompeiisneaks]

For those of you who may be interested in acquiring one of these beasts, know that when I'm done having fun with it, I will eventually want to sell it,
because while it's tons of fun to figure it out and work on it and restore it, it's not the kind of amp my heart lusts for. And no way am I going to be asking five grand for it!

Realistically...less than half that. I'm just not THAT guy who insists that it's top dollar or nothing!

And if you're going to be in central Florida, maybe you feel a huge need to feed the Evil Disney Rat a big chunk of your paycheck because your kids need entertaining, you could drop by and check this thing out and maybe save the freight costs and take it home with you.

Hey now, be nice, I work for the magic rat

~Phil

[TUBEDUDE]

For those of you who may be interested in acquiring one of these beasts, know that when I'm done having fun with it, I will eventually want to sell it,
because while it's tons of fun to figure it out and work on it and restore it, it's not the kind of amp my heart lusts for. And no way am I going to be asking five grand for it!

Realistically...less than half that. I'm just not THAT guy who insists that it's top dollar or nothing!

And if you're going to be in central Florida, maybe you feel a huge need to feed the Evil Disney Rat a big chunk of your paycheck because your kids need entertaining, you could drop by and check this thing out and maybe save the freight costs and take it home with you.

The shipping on that will bring it up to $5k.

[Matthews Guitars]

When it comes to that time I'll whip up a nice crate for it on my tablesaw and it'll ship very well protected via fedex ground service. Two inches of foam padding on all sides of the amp inside the crate. Shipping costs (within the USA) will be quite reasonable.

[Matthews Guitars]

For the time being I'm keeping this beast. When driving it with a Tube Screamer set to max clean boost, it's a monster. It compares well with any of my vintage Marshalls. One hell of a rock and roll tone comes out of it.

I have managed to work out the reverb circuit problem. Next I'll fix the tremolo. And I'm going to make a replacement front panel for it. I am well qualified to do that.

[Stevem]

If your only running it on one output then that's a heck of back braking amp to haul to a gig!

[Matthews Guitars]

I'm not a gigging musician. It's of no consequence to me.

[Matthews Guitars]

After all this time I still haven't sorted out this amp's problem with the reverb circuit. Seems simple enough, right? Well, the reverb circuit is incredibly noisy, and the farther I turn up the reverb, the noisier and brighter it gets. I've traced it out and it's right to the schematic.

Any suggestions?

[martin manning]

Look carefully at the reverb grounds?

[Stevem]

That reverb circuit is the same as in a very many Fender black face and silver face tube amps, there’s nothing tricky in there.

What do you mean by what you describe as noisy, are you talking about 60 hz hum as you open up the reverb level control?

If so it may simply be because you have the side of the pan labeled output near the power transformer.

Another common issue with these reverb circuits is that the Aluminum of the rca end on the cables and the jacks on the amp tarnish up over the years badly enough so that the whole circuit looses it’s ground connection, which of course then means there’s no shield being formed around the return cable on back to the 12AX7 .
Take note of this below!

When Fender started to make the Bandmaster and Showman reverb head they had to make the cabinet near 4” taller to keep the reverb pan far enough away from the power transformer to have a low enough 60 hz noise floor with the reverb turned up to not folks complain about it.

A easy way to start the process of elimination and see where this issue is coming from is to place a short across the end of the rca jack that goes into the output side of the pan.
This is easily done with a clip lead or a needle nose plyer.

This is the end of the cable that when the reverb is turned up will produce a buzz much like what’s heard by touching the end of a guitar cable when it’s not hooked up to a guitar, but the volume is up.

Conversely with a adapter you can pull the signal off of the send cable to the pan and feed that into another amp to take a listen if you do not have a scope.

[Matthews Guitars]

It's not picking up hum, it's picking up white noise and plenty of it. And as you turn up the reverb control, the overall tone of the amp gets brighter and brighter.

This amp has been rebuilt to a large extent and it wasn't me who rebuilt it. I got it as a leftover and abandoned project from the estate of a friend of mine, who was the local amp guru,
and I had to put some time and effort into completing it so it would run at all. Fortunately that wasn't too difficult. However, the overall wiring layout has changed drastically from the
factory layout. My friend had decided to improve it. And then he abandoned it. It had sat there untouched for at least fifteen years when I got it.

Once I had asked about it, and he said he didn't think I was ready for it. That's actually laughable because I often stepped in to fix what he couldn't figure out. That's just because I use
test equipment, I trace signals with a signal generator and o-scope. He used a cheap VOM and guesswork. Don't ask me why.

The original reverb cables are long gone. I use fully shielded cables and test with the reverb tank a few feet from the amp, so stray fields should not be a problem.

When I dive back into it I'll start with adding some shielded cables. The schematic of this reverb circuit does match that of a typical Fender amp. I traced it out to be sure.

Aside from the reverb issue the amp works perfectly.

[Stevem]

All the above you posted aside, please apply a short across the input to the 12AX7 recovery amp input and see if your issue still takes place.

[Matthews Guitars]

I realize it's kind of a long shot but I'd like to find some clear photos of other examples of the 400PS showing the wiring. I want to compare and check a few things. Be sure I'm not missing anything. I found a loose ground wire buried deep in the control wiring bundle and need to try to track down where it goes. A few other things, too, that may be best solved by referring to another unmolested example.

[Matthews Guitars]

Upon close inspection I see that the reverb circuit's components are mostly of the wrong values and the circuit itself doesn't really make sense.
For one thing, the reverb control has two shielded cables attached to it, presumably "in" and "out" to the potentiometer, and it doesn't resemble the 400PS reverb schematic at all.

So I'll just strip down the reverb circuit and build it according to the print.

There are aspects of the current circuit that resemble Fender reverb circuits, but for other models, such as the Deluxe Reverb's reverb schematic. The values of resistors that are off for the 400PS schematic seem to match the DR reverb schematic pretty well.

It's just a mess and I'm going to rebuild it from the schematic. It's often easier to start over from the beginning.