Crate GX1600 blowing fuses (and general SS help)

I have a Crate G1600XL that is blowing fuses within about a second of powering up. I get some hiss from speakers and that's it. I am using the recommended 5A 250V fuses. I am assuming this denotes a short somewhere, I kinda ruled out the transformer since it worked for a second (should I?), the filter caps and rectifier diodes "test" ok, so I am thinking of pulling all the transistors* off the output section (?) and testing them. I am on the right track here or is there something I haven't considered? (out of my admitted ignorance, I took Elec101 in school that's it).


* TIP31C x2 , MJ15001(NTE60) x2, MJ15002(NTE60) x2

ps any flowcharts for trouble shooting ss amps or is there just too many variations (I have some pretty decent generic ones for crts)
First, let me suggest staying away from NTE. VERY important is that both those transistors do not cross to NTE60, the MJ15002 crosses to NTE61. One is NPN and the other PNP. If no other reason than this, I can buy the MJ parts from Mouser or somewhere like that for under $4 each. I found the NTE60 at Newark for over $10 each. Why waste money when the real part is a lot cheaper than the replacements?

MJ15002 is getting rare, so I'd probably move up to MJ15003 and MJ15004. I just checked, and Mouser has them in stock.

yes, THE most likely cause of the fuses blowing is shorted output transistors. You will need to check. You don't need to remove them first. Measure for shorts while they are mounted. There is nothig a circuit can do to make shorted transistors appear not shorted. But if they appear shortede, THEN take them out and recheck. That is the only way to really know, because some other circuit element could make them appear shorted when they are not.

Transformer failures are extremely rare, but you cannot rule one out just because the amp tries to power up.

Try this:

There are three pairs of long 10 watt resistors on the power amp board. One pair ir right close to the two large main filter caps. There is a diode between the two, and another diodealmost between them. Those diodes are in parallel with the output tansistors. SO if either of those diodes checks as shorte3d, you have a shorted output transistor. The diodes themselves are not likely to be shorted, though anything is possible.

You want to test the transformer? Unplug the two red wires from it over near the fuse. Now power up. If the fuse holds, the transformer is OK, but if the fuse pops, the tranny is shot.

Now, STOP blowing fuses. Look up "light bulb limiter", then make one and use it. VERY simple tool, but VERY useful.
I put my schematic away, so I have no specifics, but just to be sure, look on the primary side for any caps across the primary or other parts that COULD be taking the fuse. In fact, if I recall this has a universal transformer, so it is possible someone has one of the primary wires on the wrong post causing this issue. Otherwise, it is looking like the tranny.
this amp did work and I am the first one to open it up (helping guitar player in my band) so I'd rule out the primary being on wrong posts (and looking at them they seem to be correct) ... there are no components besides the fuse and switch between AC in and where the pcb outputs to the primaries. So yeah ... we have some line voltage issues in our room but I dunno if they are severe enough to kill a transformer.

side note I see a lot of cheap mj1500x transistors, marked made in mexico, being shipping from china, I assume these are counterfeit, does it matter?
It certainly matter if the parts are real or not. Stick with a known supplier like Mouser. SOme guys buy internet/ebay parts all the time, and it scares me to death. And we all the time hear the stoies about fake parts arriving.

Isn't there a disc cap on the power amp board, right in the middle of all those mains push-ons next to the fuse? It is right beside one of the large filter caps. That cap is right directly across your mains. As a last chance, just make sure it is not shorted.
Isn't there a disc cap on the power amp board, right in the middle of all those mains push-ons next to the fuse? It is right beside one of the large filter caps. That cap is right directly across your mains. As a last chance, just make sure it is not shorted.

it is not shorted.

I was doing some other tests with another couple guys and tested the transformer and it appears to be ok, I hooked everything up again including secondaries and used a 3A slo fuse and a resistor down near the output end of the board caught on fire immediately. I tested the 6 transistors for shorts, not that. I don't know the value of that resistor because it burned now though, it might be red|brown(?)||black(?)|gold (21 ohms 5%) it reads 27 ohms on my meter is there anything like this in the area to the left of the right most pair of 10 watt resistors? there are 3 resistors there the other 2 are 2200 and 1000.

also the fuse/circuitbreaker doesn't pop when I disconnect the secondaries and plug it in now. ugh.
I have the paper schematics, and they are 11x17, way larger than my scanner, and I donlt want to post it in sections. SO contact Loud Technologies - the owner of Crate - and ask for the schematics for this. Make sure to get the model number correct, and include your serial number. They will ask for you not to post it on the internet. But at least you will have it so we can all refer to part numbers.

The G1600XL is the same schematic as the G160XL, that might help them find it. In fact, the preamp schematic set itself is labelled "G-XXX". Power amp drawing is 16-048-01 160 watt power amp.

Your resistor didn't short and before it burnt was probably not defective. resistors are passive, and only burn up when something else causes excess current to flow through them.

I am not sure which resistor you mean. At the end of the board opposite the fuse is a pair of power resistors. near them I do see four resistors, three are more or less to the side of them, and one off the end. Of the three, I see a 1000 ohm and a 22k (not 2200) and the one closest to the corner of the neat sink is 22 ohms, it is R21. It is in series from your main V- rail serving V- to the left half of the power amp schematic. If that burnt, I have to think you have more bad small transistors. One is the single round transistor probably has its own small heat sink fins not far from your burnt resistor. Also on your heat sink are the four large transistors, and in the center two mid-size ones, and over towards teh fire is one more smaller one. That smaller one is the bnias transistor. See if it has failed and check the bias adjust pot trimmer nearby.