Hey there!
I purchased a Carvin FET 450 for use with my guitar a couple months ago, and while it initially sounded fine with occasional cutouts, one day it stopped producing sound altogether. Now, I didn't return it because I knew I would have to do some work on this thing and I got a rather good deal on it. I replaced the large bulk caps on it (two 10,000 uf 80v caps) and two of the second largest caps (2,200 uf 25v) and decided to power it on to check my work. I made sure all the quick connects and ribbon cables were inserted exactly as I had found them (obviously before I powered it on). All was fine for a minute - it sounded pretty much as I expected, maybe with a little more hum than was ideal. However, after that period of time, I heard a horrible spitting sound coming from the amplifier and saw smoke starting to leak out of the chassis. I immediately powered it down and opened it back up to observe what had happened. As per the title, some pretty burnt and exploded resistors were there to greet me. Per the markings on the board, R68, 62, 72 and 74 were either beginning to cook or had completely blown up. I excavated all of these to observe the damage, which looked pretty nasty as you can see on the attached images. Some scorching of the board, destroyed traces and what looks like a hole blown through the board, and although I think that was a via that before the resistor above it exploded it and made it larger.
Before I cleaned and removed the blown resistors
Damage on the underside of the board
After I cleaned the board
After looking through the schematic, it's pretty clear the damage occurred only on channel 2, which makes a bit of sense considering that I was only driving one channel of the amp. What doesn't make sense is the fact that it was channel 1 is was driving! Now, before I go ahead and repair the damage, I obviously have to figure out what went wrong. I have a few theories, but this is honestly where my experience with and understanding of electronics ends, which is why I'm coming here for help. My main idea is that someone messed around with the trimmers for the channel bias and the current limiter to match the output of the aging filter caps, (That's P1, 51 and 101) although I can't see how that could draw enough current to blow up what looks like 600 mW carbon film resistors.
If anyone has any ideas as to what went wrong or what I did wrong, it would be very much appreciated! One thing I think I might have done wrong was I think I plugged into and out of channel 2 of the amp, but I don't see how I would have gotten any sound out of it considering I only had the channel one volume turned up and channel two was all the way down.
Sorry for the length of this post, I wanted to be as detailed as I possibly could. If anyone wants any extra information, I'd be happy to provide it.
Thanks in advance!
I purchased a Carvin FET 450 for use with my guitar a couple months ago, and while it initially sounded fine with occasional cutouts, one day it stopped producing sound altogether. Now, I didn't return it because I knew I would have to do some work on this thing and I got a rather good deal on it. I replaced the large bulk caps on it (two 10,000 uf 80v caps) and two of the second largest caps (2,200 uf 25v) and decided to power it on to check my work. I made sure all the quick connects and ribbon cables were inserted exactly as I had found them (obviously before I powered it on). All was fine for a minute - it sounded pretty much as I expected, maybe with a little more hum than was ideal. However, after that period of time, I heard a horrible spitting sound coming from the amplifier and saw smoke starting to leak out of the chassis. I immediately powered it down and opened it back up to observe what had happened. As per the title, some pretty burnt and exploded resistors were there to greet me. Per the markings on the board, R68, 62, 72 and 74 were either beginning to cook or had completely blown up. I excavated all of these to observe the damage, which looked pretty nasty as you can see on the attached images. Some scorching of the board, destroyed traces and what looks like a hole blown through the board, and although I think that was a via that before the resistor above it exploded it and made it larger.
Before I cleaned and removed the blown resistors
Damage on the underside of the board
After I cleaned the board
After looking through the schematic, it's pretty clear the damage occurred only on channel 2, which makes a bit of sense considering that I was only driving one channel of the amp. What doesn't make sense is the fact that it was channel 1 is was driving! Now, before I go ahead and repair the damage, I obviously have to figure out what went wrong. I have a few theories, but this is honestly where my experience with and understanding of electronics ends, which is why I'm coming here for help. My main idea is that someone messed around with the trimmers for the channel bias and the current limiter to match the output of the aging filter caps, (That's P1, 51 and 101) although I can't see how that could draw enough current to blow up what looks like 600 mW carbon film resistors.
If anyone has any ideas as to what went wrong or what I did wrong, it would be very much appreciated! One thing I think I might have done wrong was I think I plugged into and out of channel 2 of the amp, but I don't see how I would have gotten any sound out of it considering I only had the channel one volume turned up and channel two was all the way down.
Sorry for the length of this post, I wanted to be as detailed as I possibly could. If anyone wants any extra information, I'd be happy to provide it.
Thanks in advance!
pretty common.
people recap boards and end up blowing them up.
guess would be the 15 volt supply was hooked up wrong and
nailed the p channels full on.
or when output devices were re attached
to the heatsink one of them was shorted to the heatsink.
usually advised to not reuse mica isolation pads.
and check all pins with a meter to make sure there is
no short to the heatsink before power up
then on power up use inline bulb limiter.
On power up the bulb would have stayed lighted
telling you there was a problem.
and you could have just turned off power before damage
people recap boards and end up blowing them up.
guess would be the 15 volt supply was hooked up wrong and
nailed the p channels full on.
or when output devices were re attached
to the heatsink one of them was shorted to the heatsink.
usually advised to not reuse mica isolation pads.
and check all pins with a meter to make sure there is
no short to the heatsink before power up
then on power up use inline bulb limiter.
On power up the bulb would have stayed lighted
telling you there was a problem.
and you could have just turned off power before damage
Yeah I'll definitely live and learn from this - light bulb limiter always from now on. Thought I could get away with not having one because it was a 'simple' repair on the face of things - the hubris of a relative beginner I guess. Not gonna give up on this thing though, I am absolutely gonna give repairing those traces a shot. This time with a light bulb limiter and after making sure everything is absolutely going where it should be with a meter, paying close attention to the heat sink.
Thanks for your input 👍
Thanks for your input 👍
I've worked on several FET 900s and found that they will oscillate themselves to death if the wires and ribbon cable are not positioned perfectly. Not sure how the FET 450 is built but I would imagine they are similar. You will need an o'scope to see the problem.
Craig
Craig
I'm 53. I use series lamps from my 16's when a teacher called Todoruck at ENET N°1 teached us how to use it.