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Fender Princeton 65' treble response and Distortion

Started by knutolai, September 19, 2012, 05:54:27 PM

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knutolai

Hi all! My first post!
I have a Princeton 65' guitar amp which I have always treated with care. The other day however when I turned it on the signal (playing through the clean channel) was distorted and lacking of any treble response. The distortion sounds a bit like the effect from a dist. pedal however when the notes decays they all end in a fart-like crackling sound.
I have no idea how I came to harm the amp. Does any of these symptoms sounds familiar to anyone?/Any suggestions? I'm planning to change the fuse and unscrew the speaker to check if it is torn in any way. I have found the schematic, but cant make a lot of sense out of it.

any response is highly appreciated!  <3)

Enzo

Who says you "harmed" it?   That light bulb in the bedroom ran for 15 years, then one day, POOF, it blew out.   Is that anyone's fault?  No, it just happened.  Like that light bulb, paqrts in an amp can fail, so can connections between perfectly good parts.

First thing to do is connect a different speaker tot he amp to see if the problem is the speaker.

knutolai

Hm yes Ill try that and get back to you. Thanks for the input ;)

knutolai

Ok so I've checked the speaker and the PCB and both look alright apart from the thermistor which is covered in a white substance. This might just be isolation but I thought it might be worth a mention. Any suggestions on what to do next?

DrGonz78

#4
A thermistor covered in a white substance??? I want to see this... Take pics for me and all to see.

Or it's just the glue they put in there to hold in place just in case? This sounds strange.
"A person who never made a mistake never tried anything new." -Albert Einstein

Roly

Five will get you ten it's mounted against a heatsink (or should be) and the white substance is thermal compound for thermal coupling.
If you say theory and practice don't agree you haven't applied enough theory.

knutolai

10 points to you Roly!!
The amp was put together in 1999. Would breaking down in 2012 be considered long durability? I really don't know as this is the first amp I have ever owned apart from a Roland MicroCube :P

DrGonz78

#7
The thermistors are there at the entrance of power stage and they are resistors that change their resistance in response to the amp heating up. This is different than the output transistors tip142 & tip147 that are mounted to a heat sink w/ the thermal compound coupling the connection between the two. Now those transistors I can imagine a white substance, but have only seen glue holding thermistors tightly to the PCB board.

So I have worked on a few of these 1999 boards and that would indicate to me that it is made in Mexico rather than China. My best recommendation is to fully solder up that board and make sure every connection is connected to the board nice. As you take off the board you will need to make sure that you document where each connection belongs. Take a sharpie and write J9 or J11 on the plastic part. When you get to putting it all back together you will need thermal grease on the heat sink and make sure the heat sink bar is on the right way. Take a sharpie and write an arrow pointing up to ensure this is correctly put back together. This amp probably just needing some solder, but then after you do all this play the amp and see if there is any noticeable changes. At that point this only can be good for the amp if you are handy w/ a solder iron...

If the amp is still acting up then we can start looking into troubleshooting.

"A person who never made a mistake never tried anything new." -Albert Einstein

knutolai

Great!
Ill do this and upload pictures as soon as I can find the time. Thanks a lot for your input! I'm hopeful for this project!

Roly

13 years is not bad, but it's not great either.  It depends a lot on what sort of life its had, just in a bedroom or gigged hard, lived by the seaside in salt air and had drinks poured in it, or kept in an air conditioned studio... 
If you say theory and practice don't agree you haven't applied enough theory.

knutolai

#10
Ok so here is a picture of the thermistor labled "RT1" (in the center, covered with white stuff). Its not mounted against the heat sink, but placed right next to it. That was just my bad English running loose ;)

I dont know how to crop it for this topic, but Ill supply a link to it instead: http://i.imgur.com/GYK4g.jpg

I have removed the board from the housing and looked over the soldering. The soldering looks very good, though I will go over it with my soldering iron to make sure. Also I will check for contact between the various connected nodes.

DrGonz78

#11
Okay I see... Yeah I am learning as well as we all go here too!! lol  :duh But I was imagining other type of thermistors than this one. Larger one's located near the fuse and transformer input stages on the board. Maybe I am even thinking of a different Fender FM65, princeton212 or something like the RocPro??? Props to Roly for his greater experience than mine.  :dbtu: Sorry I even had the wrong idea going on the output transistors for this amp...

Yeah if all the soldering looks nice and shiny that is good to know about the condition of the amp. If you flow solder on at least the components like wire wound resistors and filter caps that might be just enough. I like to practice soldering a whole board at a time as I am new at this and it helps to practice.

This might not fix anything yet, but now you will know the condition of the board as a whole. Also, this will get you really familiar with the entire amp. Be careful and take some care putting it back together. Remember to clean off the heat sink and apply new thermal grease when you put the board back together.
"A person who never made a mistake never tried anything new." -Albert Einstein

knutolai

Update:
I have, at long last, gotten around to re-solder the whole board. Next up is checking all the cables for damage. Is changing the thermal putty-thingy for the transistors mounted on the heat sink important?

DrGonz78

The thermal compound is old and used by now for over a decade. I would take some isopropyl alcohol and some Qtips to really get that stuff out of the way. It is important that this compound fill in the cracks that may exist between the heat sink and the chassis. Fresh grease is way better and more reliable. The last thing we want here is for the output transistors to overheat.

Keep up the good work and let us know if the soldering makes any difference on the amp. Good luck.
"A person who never made a mistake never tried anything new." -Albert Einstein

phatt

Hi knutolai,
           I just had a quick looky at schematic and assuming we are both looking at same then

Check voltages at TP3 and TP4  about +/- 40ish VDC either side of common. (GNDA on drawing)

If they are OK then check TP5 and TP6 you want to read close to +/- 15VDC.
My guess is these are marked on the PCB so should not be hard to find em all :tu:

Also check those efx loop sockets they are prone to fail one day. :-X
Test by inserting a short cord into loop which bypasses the internal switch which is the bit that fails and can cause all sorts of problems.  :grr

There is also a mute switching setup on the input socket and that maybe faulty causing issues.
hopefully better minds here will have more info.
Phil.