Ashdown Five-Fifteen 100w combo keeps blowing fuses. Help me?

[docSanchez]

So as the title says my Ashdown Five-Fifteen 15" 100w combo amp blows it's fuse immediately after being turned on. The amp is solid state, so I can't do my usual and blame it on the tubes

I should mention here that by fuse I mean all 3 fuses; the main fuse and the 2 internal PCB-mounted fuses.

Unfortunately I realize this symptom could mean almost literally anything at this point, so I need some help narrowing down and getting to the bottom of this. I would like at some point to be able to use this amp again instead of just having it as a fuzzy nightstand

Everyone's help is greatly appreciated. This forum has helped me fix an amp before, the know-how is definitely floating around here, I just need some help.

Thanks!

[R.G.]

A lot depends on what you can do safely.

Do you already know how to work inside an amp with the AC power connected to it and turned on safely? If you don't your options are limited.

[Euthymia]

First, to debug this, you'll need a lightbulb limiter.

The first things to check on instant fuse blow on a SS are the power transistors, then the power supply rectifier(s). Those can often be tested in-circuit using the diode test function of a DMM.

If you get a dead short between any two terminals of a power transistor or rectifier diode or bridge, pull it and test it again.

[Enzo]

What RG said, safety is important.

By far the most likely reason for blowing fuses is shorted power transistors. The next thing on the list would be rectifiers, and they are way down the list from the transistors.

[docSanchez]

The amp has 3 power transistors, two TIP142 and one TIP31c. To be honest, I'm not sure what readings indicate what when testing a transistor so I'll just put them up here.

Putting the red cable of my DMM onto the middle terminal, I measured the DMM's black cable to the left terminal and got:
TIP142 (the first one) 1848
TIP31c 1744
TIP142 (the second one) 704

Going the other direction, with the DMM's red cable still on the center but with the DMM's black going to the right terminal of the transistor, I read
TIP142 (first) 011 (no decimal, just what the display read)
TIP31c 1605
TIP142 (second) 004

So... I don't know if that is how you are supposed to measure these things but that's what I did. If this is wrong, correct me and I'll redo it.
Meanwhile... onto the rectifier

[docSanchez]

So for the rectifier I measured each of the adjacent pairs of terminals, reading from 540 - 570. Again, I don't know what that means but for none of these readings have I gotten a short or broken connection.

P.S. thank you all for the safety warnings. I am cautious by nature, so I keep pretty safe working on things like this and know when it is time to quit when things get over my head.

[Enzo]

I haven't dug out the schematic yet, but could one of those TIP142s be a TIP147 instead?

When we look at the three legs of those transistors screwed to the heat sink, we could call the legs 1,2,and 3. But what they really are is base, collector, and emitter, or B-C-E. Left to right.

So it looks like from the center to the right, or C to E, you have a very low resistance. A collector to emitter short, we'd say. That generally means shorted. Those transistors must be removed from the board. We then test each again to verify the part itself has the problem instead of something else inside the amp fooling us. And then either the good checked part is installed back or a new part is installed.

If one is a 142 and one is a 147, it is VERY important which one goes where. Also, you will probably find a mica square under the transistor and a plastic washer around the screw. These insulate the part from the metal heatsink, and are VERY important as well.

[docSanchez]

You're right, the one on the right (the last one I listed) is a TIP147.
Thank you for your help!

One last question though...
for future reference, what should the collector/emitter read resistance-wise?

[R.G.]

If you're sure you can do this without electrocuting yourself,
(1) put together a lightbulb limiter. It will pay for itself in fuses you don't have to buy and hours you don't have to spend.
(2) divide and conquer; open the connections between filter caps and rest of the amp. If the fuse does not blow, it's in the amp. If it does blow, it's in the power supply and no amount of squinting power transistors will make it work.
(3) If 2 shows it's in the amp, check the transistors for shorted, including shorted to chassis/heatsink. Enzo's comments are on target, as usual. Shorted transistors will show nearly zero ohms if they're shorted. Testing has to be done with it disconnected from the rest of the circuit to mean much.
(4) there are other things. These are just the first places to look.

[docSanchez]

Yeah, I'll put together a lightbulb limiter (sounds like something I'll be using fairly frequently) and go through the steps you outlined. I'll do that sometime in the next week and see how that goes... fingers crossed...

[docSanchez]

Well, I pulled the TIP142 and the TIP147 and you're right; they're shorted across the collector-emitter.

Now, I have a new TIP142 and TIP147 in my hand, but I am reluctant to stick them in the amp without knowing why they blew in the first place. I don't want them frying on me right when I stick them in there again. So could anybody spell out for me what probably happened? The amp was on loan to a friend when it blew.

[Anodyne Audio]

I was the service manger for Ashdown in Canada for three years. The 5-15 commonly blew output devices. There is a mod as well, depending on age. If it has 0.22ohm emitter resistors it requires the mod, which includes replacing the emitter resistors with 0.33ohm as well as bias circuit changes. sorry, can't recall exactly what needed to be done. Ashdown are reasonable folks, give them a call, they should help.

[g1]

"So could anybody spell out for me what probably happened? The amp was on loan to a friend when it blew." 'Nuff said, this is surely one of the most common causes of amp fialure.
Perhaps pushing the amp hard or trying to hook up extra speakers. Or maybe just coincidence. Check the speaker to make sure it still works.
See post above regarding mod, the emitter resistors are larger and should be near the output transistors. Make sure they are still ok and see if it needs the mod or not.

[docSanchez]

@Anodyne Thanks for the tip! Unfortunately, I believe I have the 0.33 ohm resistor version. I have emailed Ashdown and am waiting to see what they will have to say.
@g-one the speaker is still good, and the output transistors are definitely fried for sure; pulled them out and found the collector-emitter had shorted. So hopefully this mod will be the key (and with any luck Ashdown will provide me with the info I need to do said mod!)

Fingers crossed!

[docSanchez]

I did get a hold of an Ashdown representative, who was fantastically helpful. I am pretty sure I fixed the problem (with his (and your!) invaluable help, of course!) and I'll post what I did after a couple of days, just to make sure the amp keeps truckin' and in case anyone else goes to scour the internet to fix this same problem.