Fender Prosonic Issues

[23dmorris]

I have a little bit of a dilemma going on and was wondering if anyone could help me. I popped open my 96 Custom Shop Fender Prosonic to take a look at the innards and noticed some charring on what looks like the output tranny board? Particularly where those two diodes are in the middle. I know it's a long shot because of what little info I have, but the tubes, caps and everything are all original to the amp. Time for a recap maybe? OT gone bad? Bad diode? The tone is great, but it buzzes real bad when turned up. I can hear a faint "spark" noise coming from that charred area in the center. Anyone have an idea? Thanks in advance.

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[ehdwuld]

Re: Fender Prosonic Issues

if the tubes and caps are 25 years old then thats always a possibility
does it sound ok
you never mentioned what issues you were having

 

[Joey Voltage]

Re: Fender Prosonic Issues

I don't see much charring in your picture, I see a lot of flux, if it concerns you, you can clean that up easily with some rubbing alcohol or naphtha. That said, a little bit of browning especially around an area on the board where there is substantial heat dissipation is normal. When you see sparks, smoke, and the smell of ozone pouring out is when you know you have a bigger problem. A component or area that is obviously black, is when you should start to be concerned.

The area you pointed out is for the the switching supply and the bias supply FWIW.

 

[23dmorris]

Re: Fender Prosonic Issues

I looked at the schematic and it calls for the bias resistor to be at 15watts 220 ohms. Currently there are two stacked resistors wired in parallel. One is a 1k ohms and the other is 470 ohms. My meter reads 320 ohms which would be correct since they're in parallel. Do you think that could be my problem? Wrong impedance values?


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[hamerfan]

Re: Fender Prosonic Issues

I don't see here problems either. 21 years isn't bad time to see a tech. The sparks and the smell says at least one component is not up to the job. See a tech now.

 

[Joey Voltage]

Re: Fender Prosonic Issues

I looked at the schematic and it calls for the bias resistor to be at 15watts 220 ohms. Currently there are two stacked resistors wired in parallel. One is a 1k ohms and the other is 470 ohms. My meter reads 320 ohms which would be correct since they're in parallel. Do you think that could be my problem? Wrong impedance values?


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Sorry to leave you hanging. The schematic shows it as a 220r 1w (r103) which is above fine, I don't see anything rated for 15W which would be silly for a bias supply. you would never really need a high wattage resistor for the bias supply, since no current is drawn from the grids of the power tubes, and the only current that is tugging on the supply is from the resistive loads themselves (resistors).

The parallel resistors were most likely used to get the bias voltage in range for the set of tubes that were fitted, so nothing strange there. Also the parallel configuration of resistors increases their power dissipation rating.

If your amp is working fine as is and sounds good I wouldn't sweat it. If you think you do have big problems ahead I would take it to a reputable technician. The supply caps are probably fine. the first stage in them going bad is typically very noticeable loss of both treble and bass frequencies. If they are really done, you will notice that the amp will lose volume during big transients like hitting a chord, and then come back to full volume during the decay.

 

[GoldenVulture]

Re: Fender Prosonic Issues

I have a little bit of a dilemma going on and was wondering if anyone could help me. I popped open my 96 Custom Shop Fender Prosonic to take a look at the innards and noticed some charring on what looks like the output tranny board? Particularly where those two diodes are in the middle. I know it's a long shot because of what little info I have, but the tubes, caps and everything are all original to the amp. Time for a recap maybe? OT gone bad? Bad diode? The tone is great, but it buzzes real bad when turned up. I can hear a faint "spark" noise coming from that charred area in the center. Anyone have an idea? Thanks in advance.

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That's not all original. It's obviously been worked on. It's also had a bit of a hack job done on it.
It certainly needs a clean up.
Here's what an untouched board looks like.[ with the exception of the resistor in the bottom right corner ]

 

[Joey Voltage]

Re: Fender Prosonic Issues

That's not all original. It's obviously been worked on. It's also had a bit of a hack job done on it.
It certainly needs a clean up.
Here's what an untouched board looks like.[ with the exception of the resistor in the bottom right corner ]
View attachment 83064

I wouldn't go as far as to say it's a hack job. Trust me there are far, far worse. Indeed the amp has been worked on, but only so far as to adjust the bias voltage for both the fixed and cathode bias modes, most likely a new set of valves didn't bias up with the factory values. Yes, maybe this could have been done a little neater, but I wouldn't consider that a hacksaw approach.