Fender 85 painful squeal :(

[westozwind]

Hi all,
I'm really impressed with the content of this site and wondered if you could give an electronics gumby a hand to fix my amp.
My beloved Fender 85 (Born April 22 1991) has developed a (very) painful high pitched squeal when turned on. I have had this amp for 2 years, and the previous owner had in in Far North Queensland, where it was subjected to a warm, high humidity climate.
I have gone through all the troubleshooting basics:-
  Nothing plugged in (Squeal)
  Input 1 with cord, no guitar (Squeal)
  Input 2 with cord, no guitar (Squeal)
  Input 1 with cord, guitar (Squeal)
  Input 2 with cord, guitar (Squeal)
  Input 1 and 2 cords, no guitars (Squeal)
  Input 1 and 2 cords, guitars (Squeal)

If you click the link below you can hear the painful noise

http://www.perthwindsurfing.com/fender85/fender85-noise.wav

From my basic knowledge I would assume that a capacitor or transistor is on the blink and causing the squeal to occur.

Here are few pictures:-


As you can see the input jacks have been well used and show signs of corrosion from the humidity.


Rear information

So after the initial troubleshooting, it was out with the screwdrivers to see what was going on.

Below are pics from the basic examination of the circuit board that I performed

The only discolouration to the top of the circuit board I can see


The worst solder I can find (Under the area of discolouration, surprise!)


Circuit board details


As the unit is quite old, I was planing to take a holistic approach to repairing the unit.
Which components are likely to be the culprits in this case (1, 2, 3 or all)?

Are there others that would cause the issue?

I am happy to de-solder and replace the pairs of capacitors/transistors, but need help identifying the ones to pick on, and in what order.

Because of the age I will replace the input jacks as well.

Here are a few more pics of the circuit board




The schematic has already ben posted on this site, but click the link below to see it again.

http://www.perthwindsurfing.com/fender85/Fender-Studio-85.jpg

I'm sure with your expert advice I can crack this one and thank you all in advance for your assistance.
Regards

Andrew
 

[J M Fahey]

Hi Westoz. Those discoloured areas might even be called "normal", because that resistor works hot, and it´s unlikely it´s the cause of your problem.
From what I hear, much worse than the "squeal" itself, you have a terrible case of "hum". You sound as if your guitar cable were made out of unshielded parallel mains cable (a.k.a. zipcord). Replace your jacks with original ones and probably all will be fine.
Good luck.

[westozwind]

Thanks for the reply JM
I am currently sourcing the appropriate sockets and will let you know how I get on.
They don't call Western Australia (WA) Wait Awhile for nothing

[westozwind]

woo hoo.
After a 3 week wait, I have the sockets
Will change them over tonite and let you know the results

[westozwind]

OK, Jacks replaced.
The bad news is that the noise when I switched on the amp is horrendous

Click the link below to listen.

---!!!WARNING!!!---

Make sure you're PC speakers are not turned up loud

http://www.perthwindsurfing.com/fender85/20090528_amp_noise.wav

---!!!WARNING!!!---

So, is it a capacitor?

[J M Fahey]

Hi Westoz.
Now you are much worse than before, so I must conclude that either you did not use the same type of jacks, or some tracks or pads were lost/broken when replacing them.
I assume the "horrible noise" dissapears turning down the volume.
To check that your noise is indeed coming from an input grounding problem, locate the trace that joins R1, R2, and C1 and ground it, that means: strip both ends of a short piece of wire, (say, 6 or 8 inches), solder one end to any pad in that track (you have 3 of them), look around for a ground track (usually thicker than others, besides they show continuity with the chassis) and solder there the other end, the noise should dissapear.
Post your results. A clean picture of that part of the PCB would help. Thanks.