tech question on crate bx-100

[the-rodman]

my brother plugged in another amp into the input,it blew the main fuse,so i checked it out,and found one of the irfp9140l,seemed to ohm bad,i took it out and it stopped blowing the fuse,so i figured that had to be the problem..so i got the replacement ,a nte 2905,cause they dont make the 9140 anymore,put the new 2905 very carefully (static sensetive)and it still blows??? any ideas out there? thanks,rod

[Enzo]

IRFP9140.

Mouser has 268 of them in stock for $1.63 each. Stock # 844-IRFP9140PBF

Allied has 215 of them from IR at $1.10 each Allied Stk#: 273-1658
Allied has also 132 of them from Vishay blowing out at 84 cents. Allied Stk#: 503-0463

Digikey has no stock.

Not so sure the NTE2905 is so good a sub for the 9140. The NTE is a 12A 200v 150w part, while the 9140 is a 21A 100v 180w part. Also the NTE has a higher on resistance.


But I don;t think that spec difference is blowing your fuses.

You found at least one bad part, but other power transistors could also have failed, and when transistors fail, they often burn out resistors associated with them. So check for that.

And while you are in there, check the rectifiers in the power supply.

[the-rodman]

thanks enzo...crate bx-100

the thing that confuses me is,the fuse blows,i take out the 9140,it dont blow anymore,i put the new 2905,and it keeps blowing,the question is if there was other bad componets,shouldnt it keep blowing even if i leave the 9140 out? does that make any sense??? thanks rod p.s the little board that has those 9140's ,dont have many parts on it ,i will check them out.....also.......i can check the bridge rectifier,which i know has tenencies to blow in alot of different applications,which sucks to check because,i would have to take the main board off to do that,but there again if it were bad,the fuse usually blows,...thanks for your help! rod

[Enzo]

The 9140 completes a circuit that allows excess current to flow. When it is gone, whatever else is wrong, no longer has a path to send current through.

[J M Fahey]

the question is if there was other bad componets,shouldnt it keep blowing even if i leave the 9140 out? does that make any sense???

Yes, it does. As Enzo stated, it is in series with other elements.
In particular, it is in series with Q6, IRFP140 which is a *strong* suspect.
Replace both and, as suggested, by the original part, not some equivalent.
Remember to replace micas, and check insulation from chassis/heat sink using the diode scale on your meter, both ways.
Even if they are both right, new and shiny, an excess bias would turn them into a practical short between +40 and -40.
Use a series lamp limiter *always* and test with no speaker or any load connected.
Other strong suspects are Q3 and Q4. Any of them bad would overbias the output transistors.
The same would happen with an open Q2 (MPSA06).
Play it safe and replace all five. Be very careful when pulling them, the one item you can't buy over the counter is the PCB.
Please pull both Q5 and Q6 and measure voltage across R47 and R51, that's the Mos real bias voltage, double check by measuring across the source and gate pins , at the board, with them out od circuit.
You should have around 3,5V, and that value should vary with the bias pot setting.
Only after you are satisfied with that, you can re-insert Q5 and Q6.
I almost forgot: you must have less than 100mV DC on TP8, the output terminal.
Good luck.
PS: it's not very probable, but even IC3 , NE5532 *might* be bad, but leave that for now.

[the-rodman]

crate bx 100

hey guys a super duper thanks for the help,it worked!! it was the 2 big rectifers, q5 and q6............enzo even sent me to a place that had them cheap!!!