Help! Amp Problem!

[spj6106]

my (4yr old) fender princeton 65 is making crackly noises. it sounds almost like the speaker is bad and needs to be replaced, but i've done that and it still makes the sound. an electrician said it might be a component in the amp itself.
i wanted to know if anyone here has had this problem and also a solution. any help would be appreciated
thanks.

also, it's not a tube amp.

 

[John Phillips]

The most likely cause is one of a small number of hot-running components (two Zener diodes, and some high-power resistors - but it's usually the diodes) having got so hot they have unsoldered themselves from the circuit board and are now making a poor contact.

This is a VERY common fault with lower-priced PCB-built amps, not just Fender (although they do seem to be quite prone to it) - the makers trim every cent on build cost to the extent that they won't use slightly higher-rated diodes or even mount them properly on standoffs from the board. It doesn't normally show up for a few years, so they avoid having to fix it under the warranty mostly.

If it's not that it could be a filter cap that's come loose and cracked its board joint as the hot-melt glue they use to secure them has become brittle with age and heat. After that the range of faults becomes more obscure and much less common...

In both cases you need to get the board out (not difficult, but a bit time-consuming, you need to undo basically everything in the amp except the power transformer), clean up the solder joints with a solder-sucker, bend the ends of the component leads down tightly onto the board - this will help keep things from vibrating loose even if they get hot - and resolder the connections, preferably using a little more solder than Fender did.

I would say "if I had a dollar for every one of these I've had to do..." but actually I have about $40 for every one of these I've had to do .

 

[TheAmpNerd]

Originally posted by John Phillips
The most likely cause is one of a small number of hot-running components (two Zener diodes, and some high-power resistors - but it's usually the diodes) having got so hot they have unsoldered themselves from the circuit board and are now making a poor contact.

This is a VERY common fault with lower-priced PCB-built amps, not just Fender (although they do seem to be quite prone to it) - the makers trim every cent on build cost to the extent that they won't use slightly higher-rated diodes or even mount them properly on standoffs from the board. It doesn't normally show up for a few years, so they avoid having to fix it under the warranty mostly.

If it's not that it could be a filter cap that's come loose and cracked its board joint as the hot-melt glue they use to secure them has become brittle with age and heat. After that the range of faults becomes more obscure and much less common...

In both cases you need to get the board out (not difficult, but a bit time-consuming, you need to undo basically everything in the amp except the power transformer), clean up the solder joints with a solder-sucker, bend the ends of the component leads down tightly onto the board - this will help keep things from vibrating loose even if they get hot - and resolder the connections, preferably using a little more solder than Fender did.

I would say "if I had a dollar for every one of these I've had to do..." but actually I have about $40 for every one of these I've had to do .

Ahhh my friend
John...

I understand completely.

$40 for every fricking amp.

I know, and damn if you wanted them to builc them
right in the first place. But once you know the secret.

and the cash flows.

Damn, if they just built it right to begin with!!!!!

 

[John Phillips]

I'm not complaining too much since I get paid to fix them, but it does become boring after a while .

BTW, a word of warning if you're trying to fault-find on one of these amps - don't run it with the board out, even for a short time under no-signal conditions. The power transistors are so close to the minimum requirement as well that they'll cook if they're not bolted to the heatsink, even at idle. One of these I did early on caught me out like that...

 

[TheAmpNerd]

Originally posted by John Phillips
I'm not complaining too much since I get paid to fix them, but it does become boring after a while .

BTW, a word of warning if you're trying to fault-find on one of these amps - don't run it with the board out, even for a short time under no-signal conditions. The power transistors are so close to the minimum requirement as well that they'll cook if they're not bolted to the heatsink, even at idle. One of these I did early on caught me out like that...

Oh the damn.


Going through all the BS

And

Then you find that rare gem that
you get to work on too. : )

That makes it all fun to work on.