Fender pro 185 help needed

[jasonguitar]

I'm having trouble trying to repair a Fender Pro 185 for a friend. The filter caps had dried up and the 2 resistors R175 & R176 the 2-270 ohm 7watt had been charred. I replaced the caps and put in 2-270ohm 10 watters and the amp sounds terrific but the resistors start smoking after 10 mins. Any suggestions? Thanks.

[J M Fahey]

Hi Jason.
R175/176 means nothing to me without the schematic.
Post it or a working link.
Anyway, those look like the voltage dropping resistors for the preamp, please post the voltage (referred to ground) on both ends of each resistor.
They should not be touching the board (they will cook it) but leave at least 5mm (or 1/4") free space.
A picture would help.

[phatt]

Hi ,
   If we are talking same or similar circuit then This little snip might be helpful?
FWIW I was trying to understand a *Pro 1000* Fender a while back. (Similar circuit)
            (& Edit whoops,,,make that *Performer 1000*) 
The heat stress on these R's and the two following Zeners was just insane.
The Zeners where a first degree burn on the old temp/bias probe digit sensor,,, my Finger.  
Those big R's are only 1mm off the board and the Zeners make contact to the PCB. The wisdom of the design is beyond me. :duh
I notice that some of these circuits use very similar ideas but obviously some don't have problems yet others are just a fire hazard.

From memory I paralelled 2x 470 R 10Watt to replace each 270 R.
(you might have to trial and error a couple of values)

Mounted them off the board on a metal bracket to chassis.
This dropped the heat from both the R's and the Zeners now are only just warm.
The Amp will now at least last a lot longer than it would have.

The Amp BTW was for a friend Who I jam with every couple of weeks so my backyard fix is obviously working. cross my burnt finger and hope it stays that way.
Cheers Phil.

[Enzo]

He's on other boards with this.  His PT wires were mixed putting 70vAC across one side of the bridge instead of 35v.  SO instead of 50VDC he had 90VDC dropping through the resistors to the zeners.

[J M Fahey]

Oh well !!
Thanks Enzo.
You've saved us all a lot of time.

[Enzo]

That explains the heating resistors, but he is still sorting out the damage the overvoltage did in the poper amp.

[phatt]

Hi Enzo,
           Me not having the benifit of experience like some of you chaps,, I don't see enough gear to make a well considered comment but having repaired a few SS fenders from this era, (as I stated) some seem to have no issue while others are just a few degrees away from self destruction.
IMHO,, it may have been more inteligent (and cost effective) to impliment a second winding for the preamp circuitry.

And yes you do have to have enough talent to know how to connect the secondary otherwise you are just playing with fire.

It's honerable to try and help a friend but plain silly to attempt things you don't fully understand.
I turn away quite a few kids with exotic Amps cause I know darn well that if I make a booboo I have to cough up the repair bill.

The above Performer 1000 was fixed on the understanding that if it went poof my mate was prepaired to loose the amp,, as he did not like it that much anyway.
Cheers Phil.