Fender SS amp - heatsink failed - now blowing fuses - I've got details.....

[liquidice]

ok, I'll try and spell this out as much as I can to make this easy for any responders, and 'thank you' ahead of time:

Picked up a Fender Princeton 112 Plus amp for pretty cheap and have been messing with it since I bought it. I recently changed out the volume pot as it was installed linear, and I wanted audio.

I've only played the amp for short periods of time since doing the before mentioned change. Today I played abit longer, and suddenly there was a 'pop', and then the audio signal faded away I cracked the amp open to see what was cooking (something smelled hot). Turns out, I installed the heatsink upside down, and it wasn't even touching the metal bar that the transistors were attached to. More research online had me on my way to Radio Shack for thermal compound and some new fuses.

I completely took out the circuit board to inspect for any burn marks. Didn't find any. I then layed on a layer of thermal compound, installed correctly, replaced fuse (bought slow-burn). With some speaker wire to extend my reach so I wouldn't have to put the amp back into the case, I powered it on; loud hum, fuse fried up within 2-3 seconds.

SO, I'm already thinking I caused some 'heat' damage from not having that heatsink installed correctly the first time. Transistors first to look at?

So here is more info before you ask: I don't have a multimeter, the transistors are "tip142 and tip147" which seem cheap online. Only info I have on caps is 2200uf 50v. I can't solder worth crap, but I'll try my butt off

So what should I do? Should I order transistors and caps?...or just transistors?...or take to local repair guy?

[Jazz P Bass]

My take on it is the Tip transistors failed due to lack of a heatsink.
There may be other damage.
The heatsink exists for the transistors.
If your soldering, desoldering skills are not up to it, take it to a tech.

[liquidice]

Well, maybe I'd at least attempt new transistors. Since I know what I need, anyone have a good-cheap place to order from?

[52 Bill]

I agree with JPB, one or more of your output transistors has shorted due to the lack of heatsinking. This is a common problem with that series of amps.

Not having a meter will make it difficult for you to repair the amp. You could just replace all of the outputs and maybe you could get lucky and that will fix the problem. But if there are any other shorted parts, you risk damaging the new parts when you power up the amp again.

If you don't have one build yourself a light bulb limiter. And get yourself a meter.

[g1]

Are the output transistors fully plastic or metal back types? If metal make sure all mica spacers and insulating bushings/washers are intact. The collectors must be fully isolated from the heatsink.
As it was working for a while before it blew, most likely it overheated as mentioned above. If you never removed the output transistors from the metal bar the insulators should be ok. There are probably power resistors connected to the output transistors which may have opened and will also need to be replaced. Make sure the Tip142 & Tip147 you order are the same type (all plastic or metal tab). Don't use slow blow fuses unless they are specified.

[liquidice]

crap, this is getting deeper than I would like. I wish I could do the work myself, but not fully confident. If I could get these parts cheap enough with shipping, I might attempt swapping them out. My fear is I'm going to take it to an electronics repair guy, and this cheap amp will end up costing quite abit more.

Not sure if any of you guys are repairmen, but based on what you know so far, can you ballpark what you would charge?

[R.G.]

As a side comment, get a multimeter. Without one, you're essentially working blind. You can find them at Harbor Freight and possibly other places for under $10 for a minimal-function one. A multimeter would, for instance in this case, let you tell if a transistor is shorted, a resistor is open, and if transistors are insulated from the heatsink, all possible issues with your amp. It's easy to destroy more components than a multimeter would cost while working on a power amp.

[liquidice]

how about this one from Lowes?

If I get one, I'm going to need some suggestions on what exactly to use it on.... I hate requesting to be "spoon fed" information, so I'll do my best to do searches online first.

Thanks everyone so far for speaking up...

[km6xz]

I like the suggestion to take it to a tech, because there might be more defects along with the transistors and firing it up without testing and verifying could make is much worse and destroy the new transistors.

The minimum equipment needed is a multimeter, sinewave signal generator, a light bulb limiter or better, a metered Variac and for more in depth trouble shooting, an oscilloscope. Anything less and it is just guess work. The tech will have these, know how to use them and get it repair in an hour or less. Techs charge anywhere from a flat rate by equipment type or by the hour, which for this would be a better deal to pay for 30-60 minutes bench time at $40-80 an hour.

[Steve Conner]

Agreed, not really worth spending money on a multimeter if you don't know what to do with it.

[liquidice]

alright well, waiting for a callback from an amp tech who operates out of his basement. Hopefully I can score a deal off him. I don't want to turn a 150 dollar amp into a 250+ dollar amp. It was a pretty powerful SS amp, so kinda sad to see it sit there, wounded...

[J M Fahey]

Well, sorry for what I say, but *now* itīs a $10 amp.
Now, if you have it repaired .... things change.

[liquidice]

Yeah yeah, I know. Well, lesson learned I guess. We'll find out how expensive of a lesson in a couple days. Took amp to a dude who's kinda a legend around here. Not sure how long it will take him to get to the amp, but I'll fill you all in once I get it back.

[R.G.]

Agreed, not really worth spending money on a multimeter if you don't know what to do with it.

That's true; however, he's already a little bit pregnant since he's disassembling and reassembling things and (for the moment) getting it wrong. I was actually thinking that the use of the MM for continuity testing would be a great place for him to start.

On the other hand, if he can be convinced to take it to a tech, he'll get it back working faster and more reliably than using it for his learning 'victim'.

[liquidice]

funny, but if this amp didn't sound so good, it probably would be my experiment amp.

I quite honestly never thought a SS amp would sound so good. Given, the overdrive channel sux, but most of the Fenders do. But the clean channel, my goodness, it sounds so good! I'm gonna get the amp back, plus I'm checking out a boutique OD pedal, and this puppy will sing!

I'll still reserve this amp as a "practice" amp, and save up for something else (SuperReverb, Twin Reverb, or Hot Rod Deville) for playing out (don't play out at all).

Considering I only paid 65 for the Princeton 112 Plus, even if the tech charges me 100 bucks, I'll be ok with that. That is his hourly rate anyways