Fender Princeton 65' treble response and Distortion

[knutolai]

Hi Phatt
Thanks for the tips!
I found the points you listed on the PCB however I have never done this type of troubleshooting before so I have a few basic/stupid questions:
-What do you mean by "either side of common"? I can't find a GNDA label on the PCB (I might just not have looked hard enough   xP ) Im guessing GNDA is a ground and that I should measure the voltage between the TPx points and ground?

-When I do these measures should the amp be powered or shut off?

again thanks everyone for all the great input!

[DrGonz78]

"Either side of common" is common ground. Read on and reference the schematic as you read too. Look at the P4 connection on the schematic and observe what is on each side.

To find the GNDA reference it is found all over the place on the schematic. In this case look at the power supply part of the schematic where it shows TP3 TP4 etc... On the PCB board you will find a center tap part of the transformer that is connected by way of the P4 faston. That is essentially ground or GNDA, and also you can pull out the DMM to test continuity to ground. Do the test for continuity to ground with the amp off to become familiar with all these point that reference back to ground. The chassis is a reference to ground as you will soon discover. I would get some alligator clips to clip the black probe of the DVM to chassis ground. Look on the schematic and find common grounds to double check it is connected securely. Look for the AC safety ground wire that is bolted to the chassis... That is ground reference point. With amp off clip one probe on P4 and touch the other probe to safety ground clip on the chassis. Do this with the continuity setting of the DMM and you will see .002 or something very close to zero ohms. Most DMM's have a beeping noise that lets you know there is continuity too. So, what are you noticing?

You will need to have the amp on to test for voltage. So secure that black probe to chassis ground and then use red probe to touch the test points to read voltage. Read as much as you can about safety tips before you proceed testing a live circuit, especially if this is your first time.

[phatt]

Well DocGonz beat me to it and thanks chum great advice. :tu:
Anyway I'm not about to rewrite my whole post again so some stuff will be repeated here. :cheesy:

Ok I'll try my best to explain it.

First comes safety!!!
All watches rings and sacred crosses or any kind of metal pendants around the neck are OFF. :grr

Ware shoes and maybe even put a thick mat under your feet.
Ideally work on a wooden table/bench.

Now find the MAINS wiring and understand exactly where those wires run.
Most modern stuff seems to cover really dangerous terminals but don't bank on it.

Yes, Common is ground or GNDA


First maybe learn to understand the basic principal of operation.
Amplification is achieved by floating the AC voltage on a DC voltage potential.
The noise you hear is the AC wiggle made larger while ridding on the DC . (not to be confused with Mains ACV at the wall socket)

To do this you first have to convert/transform the AC mains voltage down to the needed voltage, Secondary winding of power transformer does this.
Then the secondary AC is Rectified to DC and then filtered. Rectification is via the 4 diode bridge.

In this case you have both a *Positive rail and a Negative rail* and of course Common is zero volts and often ground in the audio circuit.

The common rail will be where those 2 big can capacitors are joined on the PCB.
Nearly always the biggest things on the board so it's hard to miss them.
The other 2 cap terminals will be one positive and one negative. You are looking
for C65 and C66 both 2,200uF.
Than is the main supply for the circuit. So you have +40/0/-40 VDC supply rails.


The other lower voltages for opamps are derived by a drop resistor from the above rails (R118 and R119 330 R wirewound ceramic) and the voltage is held constant/regulated via a Zener diode (D46, D47 16 volt Zeners) across the rail to common. These are often prone to failure.
also if C69, C70 have failed that can give problems.

So now (if you have grasped the idea) Turn the power on and With DMM set to the highest setting, usually 1,000VDC range,, black lead on common rail and red lead on the two test points you should read one at 40 VDC positive and the other obviously will read the same but with a - sign in front.

(I say highest setting because you never know for sure what you are about to read.
Once you see the 40 volts then switch down to the 200VDC range for better accuracy.)

Now again for the lower voltage test points, It does not have to be exact so within a volt or so.
If one side reads 39 and the other reads 41 it's ok.
Same for low rails. one side might be 15 the other 16,, close enough. :tu:

If these voltages are way off then we know there is a problem with DC rails and you need to fix those first.

Thing to look for;
Burn marks under those zener diodes and also those 5 Watt drop resistors, R118 and R119.
Bulging or leaking can caps,, not to be confused with the brown glue that might look like leaking goo. It's usually hard and brown while electrolyte is clear wet and sticky muck.

Having said all that I'd still say start at the easy things first,, like the input socket.
I believe it has two inputs so try both and wiggle the plug gently side to side to see if that mute switch is stuffed,,Same for efx loops.
Also if it has 2 speaker sockets,, then try the other one.

Meantime google some tecko words like rectification, diode operation, transformer concept,, It is amazing how much stuff you can find once you catch onto a few teckky words the Technicians use.
If you don't understand anything then ask first ,,, Oh yeah and my favorite advice for those trying to get there head around electrical stuff;
""Don't touch anything you can't outrun"" 8|
Phil.