power supply resistors popping everytime i change them

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need help please, everytime i change the power supply transistors, they snap , crackle , pop as soon as i turn the amp on
this is really tripping me out, i know im missing something but cant figure it out...2 amps

RF 60 DSM....i turned the VR (2 of them)on the board to see what would happen, as soon as i turned it the slightest bit, the transistor blew (2...x...irfz40)...and they keep on blowing now.

225hcca dig ref....this one is really messing with my head...they use smp60n06-18 transistors..10 of them....i tried using rfp70n06 as well as ssp60n06.....cant find smp60n06-18s.....both the rfp's and the ssp's....blow immediately....when amp is turned on...5 per time....ouch

what am i missing here, do the gate resistors have to be changed to a different value ?

the dsm uses 68 ohm gate resistors
the 225 hcca uses 10 ohm gate resistors

help me out fellas this is getting expensive...

 

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oops i meant transistors are popping...not resistors

 

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need help please, everytime i change the power supply transistors, they snap , crackle , pop as soon as i turn the amp on
this is really tripping me out, i know im missing something but cant figure it out...2 amps

RF 60 DSM....i turned the VR (2 of them)on the board to see what would happen, as soon as i turned it the slightest bit, the transistor blew (2...x...irfz40)...and they keep on blowing now.

225hcca dig ref....this one is really messing with my head...they use smp60n06-18 transistors..10 of them....i tried using rfp70n06 as well as ssp60n06.....cant find smp60n06-18s.....both the rfp's and the ssp's....blow immediately....when amp is turned on...5 per time....ouch

what am i missing here, do the gate resistors have to be changed to a different value ?

the dsm uses 68 ohm gate resistors
the 225 hcca uses 10 ohm gate resistors

help me out fellas this is getting expensive...

Have you checked the outputs to ensure that they are not shorted? You may want to remove the secondaries, of the xformer, to isolated the PS from the audio stages. Also, if you can get your hands on a current-limited power-supply, that will help out a lot.
It seems like something outside of the power supply is causing this. Also, the amp is not loaded when you apply power, correct?

 

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this how the problem started, one day i was testing the amp and forgot to secure the transistors against the heatsink....was playing it pretty loud, and the next thing i knew some of the transistors in the power supply were on fire....oops


225hcca:

ive changed all of the output transistors from 2n6488/2n6491 to
BD911/BD912.......all of them

as well as the rectifying diodes both +/- rails

the amp is not loaded to any speakers, i have a battery and a charger in my rooom....

ive changed the PS transistors 3 times....and the same thing happens everytime....one time i didnt even connect the remote and the transistors fried....first it was the transistors on the right(5).....then left(5).....and now right (5)

i dont have a current limiting ps....if i use a fuse or a ps from a computer would that work ok?

 

[TO-3]

this how the problem started, one day i was testing the amp and forgot to secure the transistors against the heatsink....was playing it pretty loud, and the next thing i knew some of the transistors in the power supply were on fire....oops


225hcca:

ive changed all of the output transistors from 2n6488/2n6491 to
BD911/BD912.......all of them

as well as the rectifying diodes both +/- rails

the amp is not loaded to any speakers, i have a battery and a charger in my rooom....

ive changed the PS transistors 3 times....and the same thing happens everytime....one time i didnt even connect the remote and the transistors fried....first it was the transistors on the right(5).....then left(5).....and now right (5)

i dont have a current limiting ps....if i use a fuse or a ps from a computer would that work ok?

Hmmmmmm. I don't know about the 911,912 parts, so I don't know how comparable they are to the 6488,6491s. Possibly a bias problem? On the other hand, if the remote is not connected and the PS mosfets are blowing, it is quite possible the trasformer is shorted. I'd remove that before you continue with power testing. As you know, the fets are grounded through the transformer.
A computer PS should be ok to use, as it should have internal short-protection circuitry, but make sure it puts out enough amperage for a idle amp.

 

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the bd911/912 are ST microdevice products...these are the cross reference for the 2n6488/91's...but thats all i really know about them...i changed them on my very last attempt 3/3 to fix the power supply...not from the beginning

the burnout with the disconnected remote happened once then never did it again...trips me out

yeah i agree i think i need to check out the transformer, the top looks ok, but i can barely see some burn/char/whiteash marks around the underside...i have to pull it out completely to look underneath it..any good way to verify a shorted transformer?

how would it get shorted in the first place?

 

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yeah i agree i think i need to check out the transformer, the top looks ok, but i can barely see some burn/char/whiteash marks around the underside...i have to pull it out completely to look underneath it..any good way to verify a shorted transformer?

how would it get shorted in the first place?

Too much heat, trying to source too much power, vibration causing the protective coating to wear off. By your description of white ash, etc., that may be the culprit. In that case, you either need to scrap the amp, or try to figure out the wiring ratios and rewind the core. The best way to remove the xformer is by turning the power up to max, on the iron, then putting more solder on the secondary leads until it heats up the rest of the solder and lets you pull out the lead. Becareful when doing this, as sometimes the manufacturers bend the leads under the board and. Make sure you have some heavy duty needle-nose pliers handy to help you to remove the bastard. Power up the amp after you remove the secondary leads. If it still shorts, the primary is bad, if not, quite possibly the secondary is bad.

 

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ok, got the transformer out....really hard to tell by looking though if its shorted...theres melted enamel in a dripping position on the underside....if its shorted what should it be reading on a ohmeter?

cause im getting 3 and 3.6 ohms...doesnt look to hard to rewind if need be....primary has 6 turns, secondary has 11-12 turns, always wanted to try modifying it anyway

which brings me to another question, how high can i designate the new rail voltage ? im sure you know that the hcca series has to be loaded down to make its power....what if i increase the rail voltage ? to say 40 ish volts???

what does core size have to do with everything ?

thanx

 

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ok, got the transformer out....really hard to tell by looking though if its shorted...theres melted enamel in a dripping position on the underside....if its shorted what should it be reading on a ohmeter?

cause im getting 3 and 3.6 ohms...doesnt look to hard to rewind if need be....primary has 6 turns, secondary has 11-12 turns, always wanted to try modifying it anyway

which brings me to another question, how high can i designate the new rail voltage ? im sure you know that the hcca series has to be loaded down to make its power....what if i increase the rail voltage ? to say 40 ish volts???

what does core size have to do with everything ?

thanx

Sounds like it is fine. If it was shorted, you would see a value that looked like a short >1ohm. I'm not exactly sure what you mean about designating a new rail. Do you want one or two? and for what?

What is the unloaded output voltage now? I haven't wound transformers in about 7 years, and I only followed the instructions for it. Winding xformers was never my thing. I am sure you can add 1 or 2 more windings without saturating the core of the transformer too much. If your output wave starts looking less than square, you may wish to back off a winding or so. Maybe somebody else in another section can give you a proper tutorial on windings/cores.

 

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update....when i measured the transformer, after i pulled it out i got 1 ohm on the primary and secondary...so maybe it is shorted out afteralll????? 1.1 ohm reading

i got that other ohm value when it was still in the board...i guess if i can find the same gauge wire and measure the resistance through 1 meter i should be able to tell for sure if its shorted huh?

when i said designating a new rail voltage, i meant if it was possible to increase the rail voltage, so i could get higher power at a higher impedence, instead of having to drop it low.

btw, thanx for your help...you know of a good way to tell if the transformer is shorted ?

 

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btw, thanx for your help...you know of a good way to tell if the transformer is shorted ?

Have you measured between the primary and secondaries to see if they are shorted??