rock fosgate 250 a2 help.

[bitpoop]

on my 250 a2 I would like to know how to check the fet's
I have a ohm meter, the amp has several different fet chips.

 

[Perry Babin]

Read both the Basic Amplifier Repair page and the Basic Transistor Testing pages (link in sig line below).

 

[bitpoop]

thanks for the link. looks like every fet chip in the amp is shorted.
the left side matches the right side for shorts,- like u 1620 leg 1 and 3 short on both sides.
I can get most of the replacement fet chips on ebay, but I cannot find U1620.
is there a replacement chip for u1620?
for 5NO5HD, I was told to use irf 3205 - is that correct?

 

[Perry Babin]

Don't buy semiconductors from ebay. There are too many counterfeit parts being sold there.

Buy from digi-key, mouser, arrow or future electronics.

The rectifiers appear shorted because the outer legs of the rectifiers are connected across the transformer windings.

The IRF3205s are generally good substitutes for the power supply FETs. If the gate resistors are greater than 51 ohms, replace them with 47 ohm resistors.

 

[bitpoop]

thanks perry, if u need car help send me a message.
most people never change the power steering fluid on there cars,
u can tell by smelling the fluid it will smell burnt when bad.
the same with rear axle drive cars, people never change the gear oil. (every 15k miles)

 

[bitpoop]

the gate resistors r150,r152,r250,r252
all are 12 ohm's

 

[Perry Babin]

I should have been more clear. I was referring to the power supply gate resistors. They are R19 and R20. They are likely 68 ohms.

 

[bitpoop]

ok the gate resistors r19 = 68.1 and r20= 67.7.
can i test the rectifiers on the circuit board or do they need to be removed first?

 

[Perry Babin]

The rectifiers can be checked in the circuit. If you don't read anything near 0 ohms between the center leg and the outer legs, they are likely OK.

 

[bitpoop]

rectifiers I count 6 - 3 on each side.
they all read n/a on ohm's with any leg combination.
I have not done the test with a diode tester yet.
what I don't under stand is, when i test for ohm's the meter jumps,
for a split second then goes to n/a?

 

[Perry Babin]

There's only one rectifier on each side (CR1 and CR2).

 

[bitpoop]

for the gate resistor, 48 ohm's.
is this ok? thin film.
irf 9540 are on backorder.
Mouser Part #: 279-RN73C2A48R7BTDF
Manufacturer Part #:
RN73C2A48R7BTDF
Manufacturer:
TE Connectivity
Description: Thin Film Resistors RN 0805 48R7 0.1% 10PPM 1K RL

 

[Perry Babin]

Digikey has the 9540s. They also have the 47 ohm resistors:
P47ACT-ND

Did you check all of the 12 ohm gate resistors also?

 

[bitpoop]

r142,r150,r152 & r242,r250,r252 all read 12.3 ohm's
gate at voltage regulator Lm337 and Lm337sp = r25,r26,r27 I get no readings R28=117 ohm's.
I was testing random resistors and some of them I get no reading.
not sure if i am making contact with the metal.
sometimes the meter flashes a reading for a split sec. then goes n/a?

 

[Perry Babin]

Is your meter auto-ranging?

 

[bitpoop]

I am using a cheap harbor freight cen-tech 7 function digital multimeter.
it has 5 ohm settings from 200 to 2000k. its set at 200 ohm's
I do have an old fluke meter from the 80's in the garage I could use, the cen tech is a lot easier to use do to size.

 

[Perry Babin]

Some resistor values will be much greater than 200 ohms. If your meter isn't auto-ranging, you have to change the setting to read the value of the resistors.

Reading the resistors in the board is not going to give accurate readings for higher value resistors. If a resistor looks like it has overheated, it needs to be checked, otherwise it's likely OK.

 

[bitpoop]

ok I pulled out my fluke 73 meter and this changes everything.
I now get ohm readings from all the chips.
on the 5NO5HD chip I get 4 ohm's on legs 1 and 3 is that to low?

 

[Perry Babin]

That's too low.

MTP75N05HD

 

[bitpoop]

new meter = R26= 219 OHM'S, R28=240 OHM'S
R25= 1089 OHM'S, R27= 120.5 OHM'S
it looks like r25 with 1089 ohm's is bad.
I don't understand how a peace of nickel alloy fails.