crate GX-65 blows fuses

[movek1]

Hi, i am a pro luthier, but very, very amature, amplifier tech.
I bought this amp broken, for the purpose of learning.
So when i got the amp it had bad output transisitors, and a blown fuse.
So i put new output transistors (tip142 and tip147) and the correct 2a fuse, instead of the 4a that was in there. Now when i turn it on it blows the fuse.
So i got this far, now what

[movek1]

so far i have been going by Enzo recomendations,

When an amp blows fuses, the list starts about like this:
shorted output devices
shorted main rectifiers
shorted low voltage rectifiers.
main power rail shorted to ground
shorted filter cap
mis-wiring

How do i check bridge rectifier, and do i need to check it off the board

also i got the shematic from crate and its horribly blurry, if anyone has a better copy let me know
thank you

[Gtr_tech]

You need to check the drivers and possibly the bias section.

[movek1]

very sorry, im learning. what are the drivers

[Gtr_tech]

Pull the op amp (5532) at IC3. Check the output pair for shorts. Pwr up....fuse hold? Replace the op amp. Best to leave the spkr unhooked for testing so you can check for DC at the output.

You can check for shorts in the pwr supply/output section right at the main filter caps C37, C40 (4700uF). Check each cap for short to gnd, and check across the terminals that see voltage (+ term for the pos side, - term for the neg side). If you have both output devices shorted it will show at the caps...the active terminals will read shorted to each other.

[movek1]

first off, thank you so much Gtr Tech for your help.
Ok im trying to understand, bear with me,you said pull the out/put amp (5532)out and check for short, is that the black chip in the pic, and which legs would i check, (i checked all legs and no shorts) and what is 5532

second you said check the output pair for shorts, is that what is in the pic below, i just installed them and there are no shorts with those

third i checked the caps but want to make sure because you can see in the pic below the 2 main caps are C40 and J21. C37 is a little ceramic cap off to the side.

anyway i checked for short across + and - each cap and no short (i know im doing this wrong) but i checked + to ground and - to ground and both are shorted, again thank you for your help

[Gtr_tech]

Ok...they used a different op amp at IC3...no big deal. Just about any dual op amp should work fine with a bias touchup. You might not be able to read a short with a meter. Alot of the time you need the op amp in the circuit operating for the problem to show itself.

Next.....your pix shows that there is no insulating pad under one of the output transistors. This is a no-no. The collector is connected directly to the metal surface on the back of that transistor....and your rail voltage is there. When there's no pad/mica there, you're grounding the rail voltage via the heatsink. IE, fuse will blow. Get that taken care of now before going any further. It could be the only problem.....

[teemuk]

And always, always use somekind of method to limit current when you power up the amp for the first time to test if it's fixed. If there are still faults like possible short circuits in the amp, then at least the amp isn't pushing full current to such circuits to toast things up.

A crude current limiter will save you from smoke, from many broken parts and frustration.

There are few threads about a "light bulb current limiter", check them out. Build one. It will pay itself back in no time.

[J M Fahey]

Hi.
For once, I see a big short there, you installed the left transistor straight into the heatsink, without mica insulator.
Get a couple micas and some thermal grease, and insulate both transistors, since I don't trust reused micas much.
Or you might use SilPads, which do not need grease.
After remounting and tightening screws , check that the central leg on each transistor is not connected to heatsink. The left one is grounded now for sure.
Also build a light bulb lamp series limiter (google it) to let you measure something instead of the second the fuse allows before blowing.
They seem to have used a TL072 instead of the NE5534. Hey! 1 dollar in 1000000 amps means ONE MILLION DOLLARS!!! Don't call them cheap !!!!
Or if you want to see it under other light, it pays for 2 hours 30 minutes salary for an amp stuffer in Guang Dong Province, China (official minimum salary 39 US cents an hour)
Good luck.

[movek1]

thank you everyone for your help.
OK, i know, no insulating pad, big rookie mistake. what is the purpose of the insulating pad and why the thermal or silver grease.
About the curent limiter, im getting a variac, im not sure exactly which variac i need, theres so many different ones on ebay, but i have seperate thread about this
So i put an insulating pad in there,(horray no shorts on either o/p transitor) put a new fuse, powered it up.
OK it powers up, holds power, had it on for about 20 minutes, you can switch A/B channel, But plug signal to it and nothing, no signal.
The DC at the output where i would hook the speaker up is 14.4mv
WHAT NOW

[J M Fahey]

Now it's (relatively) easy, compared to fixing a blown power amp.
Continuing with classical Enzo recommendations, inject sound (MP3, CD, keyboard, computer line out, even a guitar) into the Power Amp in /Line in/Effects return jack , which usually has rusty/grimey contacts, previously squirting a little Deoxit or similar into it and working the plug in/out a few times.
Also check the same way the earphone jack which is wired as to disconnect your speaker when you plug phones there.
I suppose you have checked that the big 5W wire resistors on the power board are not open, have you?.
Good luck.

[movek1]

OK, the jacks were the first thing i checked, usually the first thing i do is clean all the jacks and switches with deoxit.
On the front of the amp, from left to right, there is an input, insert, and external speaker jack. I plugged the headphones into external speaker, and could here nothing with my guitar, but very faint with +4dbu signal.
As far as the 5 watt .47ohm resistors, when you say open, do you mean a break in the line (no signal or resistance from one end of wire to the other end), because they show me about .5ohm on my ddm.
Also, arent they expected to get hot or at least warm up because the .470 dont but the 270ohm do.
and why do most of the things on the service schematic differ from whats actually on the board

[Enzo]

What do you mean "most things differ?" Are your referring to the parts that are not installed, though there is a place for them? As far as I can tell your latest photos are of places for parts not installed?

I think you will find just about everything on the SCHEMATIC will be found on the board. The schematic describes the circuit of your amp model. The board is used for other amp models as well as yours. SO some of those places are where parts for other models would go.

Imagine your car was built with no radio. There would still be an ampty spot where a radio would go in your dash board.

Open means no conplete circuit. A burnt out light bulb or a blown fuse would be examples of things that went "open."

[52 Bill]

The posted schematic doesn't seem to match your specific amp, but they should be close enough to work from. Do you have the correct schematic?

First off, have you checked the speaker? The Extension speaker jack is wired in series with the main speaker, so if you plug headphones into that jack it will probably not work.

I did notice that the 0.47 ohm emitter resistors have been replaced before, and the schematic calls for 0.33 ohms. By the way these will not get hot while the amp is at idle, unlike the 270 ohm ones in the power supply which are supposed to get hot.

And in the last set of photos that you posted, R63 is burned. Has it gone open?

[Enzo]

Oh, I see what you mean. Your GX65 is a GX65a I believe. See if these drawings fit better.

Try these: