Fender Prosonic..Gain Bleed Through and Weak Treble Response?

[Jecarroll7]

This is my first Prosonic repair, appreciate any help.

First of all, the amp sounds great. Good tone, no buzz etc. Tubes checked/replaced and voltages are clean. The only evident mod I see is someone put an impedance selector in, to select 4-8-16 ohms. Their work looks clean and functional. I believe its also had the service tech channel switching mod done (per elsewhere in this forum) as I do not have any noticable noise when switching from normal to drive on the front panel.

However, on the normal channel:

When the volume is turned up, but no master volume, you get sound if Gain 2 happens to be turned up, (if you turn all three tone knobs off, the sound goes away) If Gain 2 is off the master volume works as it should and eliminates all sound when turned off.

Also there is a weak Treble response, sound volume doesn't increase with treble increase but it does "hollow out" the bass and mid tones.

On the drive channel:

With the volume turned up, but no master volume you get sound if gain 1 or gain 2 are turned up.

Treble response is the same as above.

I don't have another Prosonic to compare agains't. Is this a quirky design flaw or do I have (separate/related) circuit issues?

Hopefully you can follow the above, any thoughts?

[Chuck H]

I had an early Prosonic. It had no such issues. I would suspect that some mod or service upgrade has caused the bleed problem. If you look at the original schem you'll see that the amp not only decouples the output of the OD circuit, but grounds the output. This would have been done ONLY if there was bleed after simply decoupling. I have built the Prosonic circuit into other amps and found that both are needed to eliminate channel bleed. If any of the mods (that, If I understand what I've read) reduce switching noise eliminate one of these two functions then bleed may occur. Since the amp you have has been modded I can't really advise much beyond this.

[Jecarroll7]

Chuck, very helpful thanks. I was comparing the rev E schematic for the combo against the schematic for the head, which I understand came from the factory with the impedance selector. Not into it too deep yet but did see R18 on the head calls for 120K, and on the combo calls for 100K. Like you I'm guessing the mods have something to do with it and need to dig a bit deeper. Nothing like trying to figure out what someone else did.

Any thoughts on the treble response? Should the volume increase, or stay relatively flat and hollow out the bass and mid? I think I know the answer but in the interest of being thorough....

[Enzo]

Is this treble response thing a separate issue, like when actually using a channel? Or do you mean the treble is an issue when experienceing this channel 2 crosstalk symptom?

[Jecarroll7]

Hi Enzo;

The weak treble response is constant. What I don't know (since this is my first prosonic,) is a weak treble normal for a prosonic? Bass and mids sound about right to me when comparing to other fender amps, meaning each get's louder when turned up. If I turn all three down, and bring just the treble up it just sounds weak to me. If I bring up the bass and mid, then bring up the treble it doesn't get louder, it just hollows out (mixes with?) the bass and treble. Difficult to explain I guess but I'm wondering if its related to this gain bleed issue I'm chasing.

I'm actually working on it now so maybe I can post additional later today.

[Jecarroll7]

Update - Looking like a few of the double pots may be mis-wired. A few wires certainly not following the rev E schematic, and I've noticed that someone (factory?) didn't uniformly follow the A-B assignment of each double pot. Most are wired as A side to the nut, at least one is wired as B side to the nut.

I'll get it sorted further after dinner.

[Jecarroll7]

Fixed - Listening to a nice sounding amp. Rewiring the Gain 2 (p3) and Master Volume (p7) pots to match the schematic took care of it. Have no clue what the last guy was trying to do.

Thanks for everyone's help.

[Chuck H]

Great. This would have been near impossible to diagnose remotely but I did figure it was mod related. I owned an early Prosonic. Aside from some very minor switching noise and some really crappy sounding speakers (always used it with an external cabinet) it was an excellent amp that didn't lack treble or suffer channel bleed. IMHO Zinky hit a home run with this amp. I think two other notorious Fender Pro shop amps were based on similar circuits as well. Glad it worked out.

[Jecarroll7]

No doubt you put me on the right track Chuck, really appreciate the help.