Mods for Fender Ultimate Chorus Amp

[SyncTronX]

Hi,

I'm new here but was wondering what mods can be done to help this amp?

Specifically it seems the drive channel is horrible, nasaly, thing, weak,
buzzy, no defined distortion.

Any Ideas?

Also seems at stage volumes, even small stage, the amp just
falls apart. the nice clean tones and chorus just start sounding
like chit.

Thanks.

[SyncTronX]

I found the DV offset a little much about about .5 VDC on the
16 V rails.

TP 5 @ 16.68
TP 6 @-17.10

The 40VDC rails were close to on the money:
TP3 @ 39.15
TP4 @-39.12

What is the simplest and best way to balance
this 16V rails out?

[Enzo]

Change the element that regulates it - either a zener or a three terminal regulator. I forget which in this amp.

[SyncTronX]

It has 1n53538 16V 5 w zeener on each side.

Filtered by caps and resisters 240ohm & 270ohm, and caps to the rails.
the resistors are off by an ohm or two, but not much. Will have to find
the right zeeners.

Here the schematic:

http://www.fender.com/support/amp_sc..._Schematic.pdf

[Enzo]

Thanks, I was just too lazy to find my own schematic. I would have actually had to change CDs in my drive.

I think if you look closer, those diodes are 1N5353B And those are common 5 watt zeners. Mouser or wherever.


But while that is HOW to balance your rails, the 1N5353B is a 5% device. perfectly good ones will "zene" at 15.2 to 16.8v. Your readings are not far off. And in fact, you should take readings directly across the zener instead of to ground. temperature also has a slight effect on zener voltage.

I very seriously doubt that tiny difference in rail voltage will have any effect on tone.

Have you tried connecting the amp to different speakers? And are you sure the existing speakers are wired in phase?

[SyncTronX]

Thanks, I was just too lazy to find my own schematic. I would have actually had to change CDs in my drive.

I think if you look closer, those diodes are 1N5353B And those are common 5 watt zeners. Mouser or wherever.


But while that is HOW to balance your rails, the 1N5353B is a 5% device. perfectly good ones will "zene" at 15.2 to 16.8v. Your readings are not far off. And in fact, you should take readings directly across the zener instead of to ground. temperature also has a slight effect on zener voltage.

I very seriously doubt that tiny difference in rail voltage will have any effect on tone.

Have you tried connecting the amp to different speakers? And are you sure the existing speakers are wired in phase?

Enzo,

Yeah I kinda figured, so I popped the schematic back up here.

I got some new zeeners at the local Fry's electronics, the wonderful
NTE devices they carry, in hope of zene-ing to 16Vs on the rails. I kept
thinking not that far off an maybe some slightly leaky caps....

Plugging into a cab is good. The distortion channel is the pits,
thin, weak, chitty; call it a barf-a-matic.

I just keep thinking the rails are operating the op amps just enough
over to start screwing up the drive channels op amps just engought
to hate it. That, along with trying to think how to reduce, reduce,
reduce clipping using different diodes in CR7 - CR10 (schematic block D7) in the clipping circuit, and CR11, CR12 (block D5) is a clipping circuit.

Thinking of upgrading caps at C6, C11, C15, C17, perhaps a few other
coupling caps and then changing out some key resistors in the
final transistor circuits with some higher rated and carbon films.

All the big cement resistors are okay and all tightly matched, within .01 ohm.

Wondering if it will make any difference.

Sync

[Enzo]

The amps are not precision circuits, so don;t expect them to be. Look up an op amp data sheet. 17v is not going to hurt it, it is happy as a clam at least up to +/-18v in most cases.

Look at D7-10, they are really just steering diodes. The LED LD1 is the real clipping component. Different color LEDs have different voltage drops, you can experiment with a battery and a resistor on your bench. You can also experiment with 2 or 3 iN4148 in series, or adding a diode in series with the LED, or whatever. That will affect the clipping, but not the EQ and such.

D11,12 - you sure those are not there for noise reduction? I am not sure, but I bet so. You could lift one end of each to disable them and see what results. That doesn't look to me like where someone would stick clipping diodes. See how the clean channel has the same circuit with D1,D2?

I think diodes like D4, 5, 13, 14, 18, 19, 22, 23 are more for limiting than any clipping.

And since when did Fender ever have a clue how to make a decent sounding overdrive channel anyway?

[SyncTronX]

The amps are not precision circuits, so don;t expect them to be. Look up an op amp data sheet. 17v is not going to hurt it, it is happy as a clam at least up to +/-18v in most cases.

Could be, I don't know.

Look at D7-10, they are really just steering diodes. The LED LD1 is the real clipping component. Different color LEDs have different voltage drops, you can experiment with a battery and a resistor on your bench. You can also experiment with 2 or 3 iN4148 in series, or adding a diode in series with the LED, or whatever. That will affect the clipping, but not the EQ and such.

I did, D7 - D10. Seems they help contol the diode.
It also seems to clean things up a bit also.

D11,12 - you sure those are not there for noise reduction? I am not sure, but I bet so. You could lift one end of each to disable them and see what results. That doesn't look to me like where someone would stick clipping diodes. See how the clean channel has the same circuit with D1,D2?

I noticed that also. But as long as I was in there anyway, I figured I change them too, CR11, CR12.
Perhaps it did nothing. But is did something, maybe taming the circuit just a little bit.

I think diodes like D4, 5, 13, 14, 18, 19, 22, 23 are more for limiting than any clipping.

Hmmm, Yeah That is kind of why I stayed away from them. Windering of
D28, 29, 40, 41

And since when did Fender ever have a clue how to make a decent sounding overdrive channel anyway?[/QUOTE]
I finished it up.

What a huge difference everything made.

Now at least the voltages throughout are within .05V ov each other
with all the opamps.

I replaced some of the key caps in favor of film caps.
Some of the film caps were upgraded.
Some were replaced with oil caps.

All of the IC .47 electrolytics were changed to film.

Two new powersupplies were built, the HV rails received
HEXFREDS and caps for noise reduction.

The Low voltages rails received new Zeeners
and hi temp filters and 150 % more capacitance.

The amp really sounds outstanding for what it is.
Even with the crappy fender speakers.

[rockin roy]

signal quality

I brought one of these home to fix an input jack )Thanks for posting the schematic). I was going to replace the two monoceramic caps on the input with some nice polystyrene, Then I notice the preamp dumps the signal into a 22 Mfd electrolytic. They might as well have use paper sacks instead of speakers. Please tell me I'm reading this wrong.