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

[Steve Conner]

Yeah, believe it or not! Just because something is solid-state doesn't automatically mean it'll sound bad.

[liquidice]

all right, 3 days later, amp is back home. 53 bucks, gave him 60 cuz he was nice.

New transistors and resistors, everything else checked out 'ok'.

Interestingly enough, amp sounds a little louder when using normal settings, and the tone seems 'richer' than before too. Placebo effect, or could new transistors give this thing a little more 'ommmph'????

[km6xz]

Congratulations on your new-again amp and finding an ethical tech.

[g1]

Good deal, even 10 years ago. If I recall correctly there are more than one of each type power transistor in that model. It is possible that one or two had failed in a way that did not blow the fuse (open rather than short) at some earlier time. Or one or two emitter resistors. If that were the case the remaining pair would work harder and the amp may have seemed down on power. Also this would have exaggerated the overheating condition with the lack of heatsink. So it is possible it may sound better now than before.
Otherwise, transistors are basically good or bad. They don't wear out or get weak like tubes.

[AllAmpedUp]

I love this forum...
Anyway I just bought the same amp (the 112 plus for $100) it is the 1996 Anniversary model....which means it has a silly dime sized sticker.
There was a popping noise (very loud speaker pop/snap) whenever it was turned off and I also new about the Reverse Analog (Type-C) pot....I bought it anyway.

So, I replaced the pot with a Type-A (via a spare-parts Champion 110) and I am planning to add a 100w L-PAD (power-soaker) as well.

For the popping, I experimented by adding an on/off switch on the speaker (+) wire but after 5 min of playing on it, the TIP147 is fryed - aka blowing fuses.

But to ensure its not just the TIP147 (and, yes, I will replace both TIPs) I will also remove a few other items just to double check them (CR14 diode, C41 cap and the R66 resistor)

My question...
Does anyone think that the popping noise could have been a "weak/arcing" transistor (that finally shorted) or could it have been something more?

[52 Bill]

For the popping, I experimented by adding an on/off switch on the speaker (+) wire but after 5 min of playing on it, the TIP147 is fryed - aka blowing fuses.

But to ensure its not just the TIP147 (and, yes, I will replace both TIPs) I will also remove a few other items just to double check them (CR14 diode, C41 cap and the R66 resistor)

How do you know that the problem is a bad TIP147?

[JoeM]

A lot of these Fenders (and others too) pop on turn off. My Fender 85 (SS) which I've had for nearly 20 years does and still works fine, so it's not the popping that caused the failure.

[AllAmpedUp]

How do you know that the problem is a bad TIP147?

I tested it with a MM..it is shorted between the C&E yet the TIP142 is ok.
All the other parts are reading "normal" or close too it since they are all still attached and I know that the only way to correctly test is to remove the part

I just realized that the failure was due to a improperly seated Heat-sink as I remember thinking ....."hmm, that screwed back on harder than I remember"....
When I re-traced my steps, I realized it was spun 180 degrees and the lip of the PCB was between the the plate/sink...and I didn't use any thermal compound.
It wasn't an accident that I posted on this thread...lol

[AllAmpedUp]

A lot of these Fenders (and others too) pop on turn off. My Fender 85 (SS) which I've had for nearly 20 years does and still works fine, so it's not the popping that caused the failure.

Thanks Joe, Yeah....I have read that!
Even though my Champion 110 also pops (a little) , the 112+ pop is uncomfortably LOUD...and is going to be a deterrent when re-selling it.
I am just curious if the popping is ever related to a tip failing?

In this case, I am 95% sure I screwed up the heat-sink ....thus the delay in the TIP failure....it seem very logical, now...at least to me.

After I replace the TIPs and reinstall the PCB with a properly aligned heat-sink (...and thermal compound...), I will resume my testing of the Speaker on/off switch.
Assuming the popping noise wasn't related to the TIPs, I wonder where that surge/ground causing the "pop" noise actually goes??

[Enzo]

If an amp just sits there pop-pop-popping, then SOMETHING is breaking down. But if you refer to the pop at turn off, no, that is not failing output transistors. IF they were failing, they wouldn't wait until you turned the amp off.

[AllAmpedUp]

If an amp just sits there pop-pop-popping, then SOMETHING is breaking down. But if you refer to the pop at turn off, no, that is not failing output transistors. IF they were failing, they wouldn't wait until you turned the amp off.

I believe the TIP failed because of sloppy heat-sink install by me....
The popping is un-related.
I have confirmed in another thread that the on/off relay is cheap, should be replaced and may even need an resistor across it (arc snubber) if it persists with the popping after new switch is installed.

I am loving the help here, it as a great community...glad I found it and joined.
My local supplier only sells the NTE TIPs...they are cheap (or so I read here), so I will have to go to another vendor.

[DrGonz78]

The funniest and saddest thing I have seen yet... I got this Fender M-80 to work on and clean up. So I take the sucker apart and realize the last person to work (or totally #$#$ up) on this amp had put the heat sink on the bottom side of the chassis!!! I am looking at the PCB just getting pulled down flexing by all the pressure of the screws. Just a blank space between the board and the inside of the amp. I just stopped at that point lit up a cigarette and cracked open a beer, started laughing out loud by myself.

So I give props to Fender on that one for making an amp that can withstand a moronic setup like that! Anyway, I put it all back together and saved it's life! However, isn't it strange how someone could have set that amp up like that w/ no care in the world and the damn thing just keeps on working? Then other people take way more care and make a way smaller mistake than this one and that ends up shorting their output transistors or other components, it just seems ironic.

[AllAmpedUp]

Very true, I shouldn't pound on the entire design and designers as it is not any more sensitive to hacks like me noodling around with it than any other amp. I admit, freely, that it has a clean channel that sounds very very good.

Thanks for the insight, Doc.

[Steve Conner]

I guess the odds of survival depend on how hard you drive it. It's like running your car with no water in the cooling system, it would probably make it to the shops, but you wouldn't get far in the Pikes Peak hill climb.

If you drive an air-cooled VW Beetle, that might be a bad analogy.

[DrGonz78]

Ok I am gonna be completely honest about this damn M-80!!! Truth is this thing had bugs and eggs of the bugs that had lived in it!!! It was covered in some kinda oil like residue!!! It gave me the Hee-Bee-Gee-Bees!!! Worst fricking thing I have ever touched in my entire life nearly! I mean the smell of this thing could make you sick and then soldering on it made me sick!! I felt like throwing the chassis in the garbage, but something made me stomach this challenge. Not even sure why I thought this amp could ever work efficiently again.

But the damn thing still worked!!! So I cleaned and cleaned and then I Cleaned it over and over again, till I felt sick. Then I took a day off or two. Came back and tested it out for a couple of hours and it would not die. I mean this thing should be in a damn museum as a tribute to Fender!!! The person I got it from said it would shut down after about 15 minutes. Never once since I put my love into it has it stopped working. The moral of this story is that I will probably end up keep this damn destined to be a lemon cockroach motel oil slick of an amp forever. As a tribute to Fender!!! So that is the rest of the story.