Crate g130cxl blowing fuses

[cminor9]

A friend brought by an old crate head, wanted to see if I could fix it. It is blowing fuses. He said he plugs it in, it hums for a few seconds, and then pops the fuse. He recently had the amp in the shop recently to replace 2 switches on the front panel (those small push switches, one is a channel switch and the other is for the chorus). It started popping fuses after he had the work done. Obviously it passed the tech's QA, though, so it would seem that it worked OK at some point after the servicing was done.

Since it was blowing fuses, I figured I'd start at the power supply first. I unhooked the power transformer and verified that it is outputting voltage. The filter caps are rated for 50v, and I got 60vac on the PT secondaries. I figured that sounded about right, since after rectification that should be ~ 43vdc. I checked the rectifier diodes, and they are all right too.

I told him to get some more fuses (his last one was blown, so there was only so much diagnosis I could do) and to order some new filter caps (they aren't too expensive on this amp). The amp is about 15 years old, and I figured filter caps might be at fault because I had nothing else really to go on.

So, now what? I don't know much about SS amps. So how should I troubleshoot this? It's not like I can pull tubes. Did I take the right approach in starting from the power supply, given the circumstances?

[J M Fahey]

Well, 99% the output transistors are blown, check them for shorts, also the drivers driving them, plus check that ballast resistors have not opened.
YouŽll need a series lamp limiter, blowing fuses only allow about one millisecond to measure anything, not enough.
Search this Forum, IŽm sure the schematic (or that of a close brother) was posted here.

[nosaj]

Your right, because at 15years the electrolytics could be at issue and if the power supply ain't right nothing is. Save your self some fuses and make a dim bulb tester. And just because another tech QA'ed never assume the work is right, just work your way through the schematic and see,verify. I made a mistake today that cost me too much time grabbing what was supposed to be a 220k resistor turned out to be a 220 ohm. Always verify I'm eating those words now.

Crate will give you schematics for free, they just email you an NDA which you return and get the schematics.

Jason

A friend brought by an old crate head, wanted to see if I could fix it. It is blowing fuses. He said he plugs it in, it hums for a few seconds, and then pops the fuse. He recently had the amp in the shop recently to replace 2 switches on the front panel (those small push switches, one is a channel switch and the other is for the chorus). It started popping fuses after he had the work done. Obviously it passed the tech's QA, though, so it would seem that it worked OK at some point after the servicing was done.

Since it was blowing fuses, I figured I'd start at the power supply first. I unhooked the power transformer and verified that it is outputting voltage. The filter caps are rated for 50v, and I got 60vac on the PT secondaries. I figured that sounded about right, since after rectification that should be ~ 43vdc. I checked the rectifier diodes, and they are all right too.

I told him to get some more fuses (his last one was blown, so there was only so much diagnosis I could do) and to order some new filter caps (they aren't too expensive on this amp). The amp is about 15 years old, and I figured filter caps might be at fault because I had nothing else really to go on.

So, now what? I don't know much about SS amps. So how should I troubleshoot this? It's not like I can pull tubes. Did I take the right approach in starting from the power supply, given the circumstances?

[52 Bill]

Solid state or tube doesn't really make that much difference if you look at the amp in basic sections. Both have preamps and power amps and power supplies.

If a tube amp is blowing fuses the first place we usually look at is the power tubes and then the power supply. In a solid state amp we do the same thing. It may not be as easy as pulling out a tube, but metering an output transistor or power supply diode looking for a short is not all that hard either.

As JM stated, build yourself a light bulb limiter and save some fuses. Also disconnect the speakers, as it will be easier and safer to work on the amp with them out of the circuit.

Your description of hum then blown fuse sounds to me like there could be voltage on the speaker output. This would point to shorted output devices. Check all of the output transistors for shorts. Do this first with them in circuit and then if you find any odd or low readings, you may have to remove one or all to test out of circuit.

Edit: I just saw Jason's post regarding the caps, and yes they could be a problem, but I suggest that you wait to replace them until you know that they are involved in the problem with the amp. Fix first then update or modify.

[cminor9]

wow, thanks a lot guys!

So first, it sounds like I took the right approach in starting on the power supply end of things. yay! I'm learning!

I did search the forum, I found something close to this schematic but not it exactly. I guess if I need to I can contact Crate. It seems like there are a few things I can do before I have to go there, like checking the output section a bit closer.

I'll do the limiter too. Good idea. Also, maybe I'll take those switches out of the circuit to eliminate them. They are just channel switching after all. They are sort of bugging me in that when a circuit (or anything else) stops working the first thing you should ask is "what was the last thing that was changed?" That thought had crossed my mind, but I figured they were simple switches. Good reminder, nosaj.

As for testing the transistors, I am a dummy when it comes to this. Can I do this with a decent multimeter? Is there somewhere I can read up on how to test transistors? Wouldn't this vary across types of transistor?

EDIT: found this: http://www.kpsec.freeuk.com/components/tran.htm That tells how to test transistors. I'll give that a go. I guess I need to find out which type they are, which means I might need to get that schematic after all.

[J M Fahey]

YouŽll find much better explanations but just as a minor starter
1) yes, any multimeter which has a "diode test" function.
Usually coincides with the 200 ohm or 2 Kohm scale, and has a small diode sign by it.
2) get the transistor datasheets, read the pinout there (although all metallic TO3 have the same , *most* plastic power transistors too, but be aware, although uncommon, there may be variations there)
Basically you want to find around 0.6 to 0.7 V across a BE or BC junction in one way, a much higher value the opposite way, and always a high value CE both ways ... although *some* transistors as popular TIP142/147 or MosFets have a "backwards" internal diode, which may mistake you into thinking you have a short.
3) you will always find a short in-circuit, nothing in parallel will hide it, but to be certain, pull the suspect part and re measure outside.
Multiple output transistors in parallel are common in mid-high power amps such as yours, and just one short will look like all are shorted, measuring outside clears the issue.
4) look around, many repairs similar to yours have been discussed here.
5) last but not least ... good luck.

[Jazz P Bass]

May I kindly advise that you leave the switches alone until you verify what the problem is.

[cminor9]

thanks again for all of the great advice. He is bringing the amp back over maybe next week. We'll see what we can do. I'll post the results.

[Enzo]

Those TIP142/147 transistors have three legs. Even a plain old ohm meter will tell you if they are shorted. The legs on the transistors left to right are: base, collector, emitter. Usually collector to emitter shorts. Just go down the row and see if any legs are shorted together.

[cminor9]

Well, finally got to look at this again. I checked the transistors out, and the TIP-32s near the power section all tested good.

So then I moved onto the big cylindrical ones. They were MJ15001 and MJ15002. One of each of these showed continuity across each combination of C, B, and E in both directions. Uh-oh. It was a pain, because I had to remove a jumper later in the circuit to isolate each one. So I would assume the ones that showed continuity in all directions across all pins would be bad, correct?

Any tips on replacing these? It looks like it'll be a royal pain. They are bolted to a heatsink and have two legs soldered to the board.

[Enzo]

Suck the solder off of two pins and pull the bolts. I see that as more fun than a lot of things I have to do.


Yes, shorted is shorted, which is bad.


Sorry, TIP142/147, I was thinking of a different Crate model. MJ15001/2 should be available in the parts market. If not, then MJ15003/4 would work.

You call them cylindrical, fair enough. I describe them as looking like a cowboy hat with a couple legs on the bottom. In the industry that shape or "package" is called a TO3. For Transistor Outline #3. The smaller tab types like the TIP32 are called TO220. Note the TO designation is the shape of the part, and says nothing about it electrically.