crate bfx100 blows fuses - possible dc in output

[Metalmaster06]

I sent enzo this email, below is his reply. He requested that I start a thread, and here we are.


So, i've seen you popping up on almost every amp forum i've searched. Figured i'd drop you a line and see if you could point me in the right direction.
>
> I have a crate bfx100 that started blowing fuses on startup.
> w/ a fresh fuse it will output throught the main speaker a hum at full volume (60hz maybe? sounded similar to hum from a singlecoil) until it burns up the fuse. I removed the smaller board w/ the power transistors on it and tested them thoroughly and both checked out.
>
> No obvious scorch marks on the board and no bad solder joints. I went through them visually and with my meter checking continuity just to make sure for cold joints.
>
> This is one of my two amp projects i'm working on to familiarize myself w/ amplifier innards. The other is a fender red knob twin i have a thread for on these forums. PDF64 has given me some pointers on that one.
>
> I'm guessing it may be a transformer issue but i have not checked a transformer before. I have no problem reading technical literature and if you prefer to point me to some resources on the subject, by all means. I have experience with printed circuit boards, soldering, and using a multimeter.
>
> Thanks for any info you can offer,
> Nick

Enzo's reply:

Everyone leaps right to their transformer whenever there is a problem. Once in a while a transformer does fail, but it is quite rare. Disconnect the transformer secondaries from anything and apply mains power to the primary. if the transformer alone still blows fuse, it may be bad. I suspect not. Loud hum and blowing fuses generally means DC on the output, usually from shorted output transistors. Disconnect the speaker and see if the fuses still blow.

And please, start a thread for the amp in the forum. That way others can benefit from the efforts, and you will receive the insights of the whole group instead of just me.

[Metalmaster06]

I have since disconnected the speaker load and it still blew the fuse.
I need to acquire more fuses, but from some reading i've done it looks like I need to check the rectifier diodes.

Any pointers would be greatly appreciated.

[Metalmaster06]

just got back w/ more fuses.

no blown fuse w/ transformer secondaries disconnected. i assume that means the power tranny is still good.

pulling the main board to test the bridge rectifier.

EDIT: pulled bridge rectifier out to test out of circuit. Tested good.

if it is dc voltage in the output, then somewhere a capacitor is leaky and letting it through into places it shouldn't be, correct? i'm gonna trace the + and - 40V and see if something is crossed.

[olddawg]

If it has output transistors, check them for shorts. If it uses an output IC float the rail supplies. Just as Enzo says, you most likely have a failure in an output device resulting from a bad load situation. Hum and blowing fuses is a classic symptom of shorted outputs in an SS amp. The more fuses you put in it and the more times you turn it on, the more you will continue to damage the amp taking out driver transistors, etc. CHECK THE OUTPUT DEVICES FOR SHORTS!

[Metalmaster06]

i have pulled the output transistors (Q5 and Q6) and tested for shorts and opens. both spec out fine out of circuit.

i've only used 3 fuses. one when i first had the problem, one w/ speaker load disconnected, one in right now unblown, but not powered up since transformer test.

so you're suggesting starting at the front end of the output section and checking everything as i go?

[olddawg]

i have pulled the output transistors (Q5 and Q6) and tested for shorts and opens. both spec out fine out of circuit.
i've only used 3 fuses. one when i first had the problem, one w/ speaker load disconnected, one in right now unblown, but not powered up since transformer test.
so you're suggesting starting at the front end of the output section and checking everything as i go?

I'm saying that unless you have a bench variac with an ampmeter or in the worst case a lightbulb limiter, you can cause a lot of damage by repeatedly blowing fuses on power up. If you still have a short with the finals removed you could have a shorted rectifer, a failed cap, regulator, or a driver, etc. I would start by static checking every active device, checking the resistance of the rails to ground, etc with the amp unplugged. Can you post a schematic?

[Metalmaster06]

gotcha. heres the schematic. I do appreciate your expertise in this field. At this point i have more time than money, and i'm willing to learn a few new tricks to keep my gear in shape.

[Phostenix]

With the amp off, check the resistance to ground from both sides of R64 & R65 (4 different measurements). That should help narrow down which power rail is shorting.

[olddawg]

Check the resistance of pin 3 and 9 (+/-40v rails) on J25 for a short. If one is shorted, float it to isolate on which side of the connector the short is. This is an FET power amp BTW. I would really suspect the power FETS and those zeners.

[Metalmaster06]

here goes

From both sides of R64 to ground
rectifier side .5 ohm
after (side closer to d24) 270 ohm

from both sides of R65
rectifier side 3.36kohm
after(side closer to d25) 3.44kohm

From j25 pin 3 to ground 3.35k ohm
From j25 pin 9 to ground 0 ohm but continuity.
even when j25 is disconnected and measured from main board
no continuity to ground on either pin on the output board

from these measurements my +40v rail is grounding out somewhere on the main board, not on the smaller output board after J25
If i'm thinking logically, that should make me question components in the power supply section and also the components on the schematic in map coordinates E1-F2

until i hear from you pros whethere i'm thinking through this properly, i'm gonna start checking components from pin 9 on J25 back to ground to see what might be shorting out.

Thanks again for pointing me in a direction that should yield some results

[Metalmaster06]

ok, this thing is driving me nuts.

tested as many caps as i could see that relate to the +40v rail and nothing showing up short or open.
resistors are spec.

found and repaired one lifted trace that looked like stress damage from board flexing. maybe thermal expansion.

no change to the resistance to ground measurements. Don't want to power it up because i haven't changed anything.

any suggestions?

[olddawg]

ok, this thing is driving me nuts.

tested as many caps as i could see that relate to the +40v rail and nothing showing up short or open.
resistors are spec.

found and repaired one lifted trace that looked like stress damage from board flexing. maybe thermal expansion.

no change to the resistance to ground measurements. Don't want to power it up because i haven't changed anything.

any suggestions?

Have you checked zener diode D24? Have you foalted the bridge rectifier? You just need to float the components on the +40v rail. If you have floated the connecto,r and the short is befor ethe output stage, just lift components one leg at a time. Chances are it will be a diode or a cap or an active device where the 40v line goes to somewhere else unless you have a solder splash or something. But you're right, if the +40v rail reads to ground, don't power it up until you find the short.

[Metalmaster06]

i pulled the bridge rectifier out and it tested just fine. no solder splashes. it was opened once before to fix a loose jack but done very carefully. the zener a d24 tested ok out of circuit as well. checked all the caps in the power supply section. none open or shorted. i don't have a bench power supply to test them properly however.

in circuit, when testing d24 to ground i was getting some weird readings, maybe because caps were slowly charging from the test voltage. figured this would lead me to a shorted or open component, but backtracing it didn't reveal anything strange. out of circuit the voltage reading through it was a little higher than expected. I expected .64v but got something in the neighborhood of .74v, but maybe that's just me testing the zener w/o a power supply across it.

I'm gonna get some sleep and hit it again tomorrow, fresh eyes and all might show me something i missed

[olddawg]

i pulled the bridge rectifier out and it tested just fine. no solder splashes. it was opened once before to fix a loose jack but done very carefully. the zener a d24 tested ok out of circuit as well. checked all the caps in the power supply section. none open or shorted. i don't have a bench power supply to test them properly however.

in circuit, when testing d24 to ground i was getting some weird readings, maybe because caps were slowly charging from the test voltage. figured this would lead me to a shorted or open component, but backtracing it didn't reveal anything strange. out of circuit the voltage reading through it was a little higher than expected. I expected .64v but got something in the neighborhood of .74v, but maybe that's just me testing the zener w/o a power supply across it.

I'm gonna get some sleep and hit it again tomorrow, fresh eyes and all might show me something i missed

D24 is a 16v zener. It regulates the +16v supply which is derived from the +40v rail. I'm not sure what you mean by .74v with the amp unplugged? You may have to open a trace to separate the power supply section. If the short goes away then it has to be somewhere between the power supply and P24. Right?

[booj]

I just worked on one of those. The output fets were screwed down so tight that the insulators were compromised. If the outputs looked good, look at the insulators.