RF 325.2 blows fuses

brandes.cm · 29th August 2011, 12:55 AM

Circa 2006 325.2
I bought this amp not working, like everyone I buy. This one had the supply fets blown and by the looks of it one of the outputs was "repaired" as well. I replaced the supply fets and gate resistors. I don't see anything wrong with the outputs, transistors and resistors measure fine.

If I switch power on for a split second I see +-rail voltage along with +-15V supplies. If I keep it on longer it will blow the fuse I have in-line with B+.

Wondering if I should pull the outputs to verify the supply is operational; is there an easier way? I did apply just remote power and seen the outputs from the 494.

cory

nigelwright7557 · 29th August 2011, 01:09 AM

What i do with this type of problem is remove output transistors and feed VAS output back into LTP to close the loop. I can then check for dc offset and bias without blowing output mosfets.
If there is still a fault with outputs removed I start pulling all the transistors and diodes and check them out of circuit. While the transistors are out I check all resistors and check capacitors for shorts.

Perry Babin · 29th August 2011, 01:12 AM

Set the bias pots fully counter clockwise.

brandes.cm · 29th August 2011, 01:25 AM

Bias pots are set CCW, should have mentioned that.

Perry Babin · 29th August 2011, 01:34 AM

Power it up through a current limiter and measure the DC voltage across all of the 0.1 ohm source resistors. Do all read 0.000v DC?

If not, which ones read greater than 0.000v DC?

brandes.cm · 29th August 2011, 01:42 AM

The channel that was previously "repaired" is reading ~28mVDC across each one. The other channel is reading 0.

Perry Babin · 29th August 2011, 01:49 AM

Which 0.1 ohm resistors, specifically, have voltage across them?

brandes.cm · 29th August 2011, 01:55 AM

It would be R280 and R281, but the problem is J200 was cut from the board. It looked like a solder connection, but it had none.

I need to look more closely at the previous repairs, most of the time that is the issue.

Does measuring across those resistors show signs of leakage?

Perry Babin · 29th August 2011, 02:07 AM

Measuring the voltage tells you where the current is flowing to narrow down the channel with the problem or to determine whether the excess current is due to a problem in the output stage or elsewhere.

29th August 2011, 12:55 AM	#1
brandes.cm diyAudio Member Join Date: Sep 2009 Location: Wisconsin	RF 325.2 blows fuses Circa 2006 325.2 I bought this amp not working, like everyone I buy. This one had the supply fets blown and by the looks of it one of the outputs was "repaired" as well. I replaced the supply fets and gate resistors. I don't see anything wrong with the outputs, transistors and resistors measure fine. If I switch power on for a split second I see +-rail voltage along with +-15V supplies. If I keep it on longer it will blow the fuse I have in-line with B+. Wondering if I should pull the outputs to verify the supply is operational; is there an easier way? I did apply just remote power and seen the outputs from the 494. cory

29th August 2011, 01:09 AM	#2
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	What i do with this type of problem is remove output transistors and feed VAS output back into LTP to close the loop. I can then check for dc offset and bias without blowing output mosfets. If there is still a fault with outputs removed I start pulling all the transistors and diodes and check them out of circuit. While the transistors are out I check all resistors and check capacitors for shorts. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.

29th August 2011, 01:12 AM	#3
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Set the bias pots fully counter clockwise. __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

29th August 2011, 01:34 AM	#5
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Power it up through a current limiter and measure the DC voltage across all of the 0.1 ohm source resistors. Do all read 0.000v DC? If not, which ones read greater than 0.000v DC? __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

29th August 2011, 01:49 AM	#7
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Which 0.1 ohm resistors, specifically, have voltage across them? __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links


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29th August 2011, 01:25 AM	#4
brandes.cm diyAudio Member Join Date: Sep 2009 Location: Wisconsin	Bias pots are set CCW, should have mentioned that.

29th August 2011, 01:42 AM	#6
brandes.cm diyAudio Member Join Date: Sep 2009 Location: Wisconsin	The channel that was previously "repaired" is reading ~28mVDC across each one. The other channel is reading 0.

29th August 2011, 01:55 AM	#8
brandes.cm diyAudio Member Join Date: Sep 2009 Location: Wisconsin	It would be R280 and R281, but the problem is J200 was cut from the board. It looked like a solder connection, but it had none. I need to look more closely at the previous repairs, most of the time that is the issue. Does measuring across those resistors show signs of leakage?

29th August 2011, 02:07 AM	#9
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Measuring the voltage tells you where the current is flowing to narrow down the channel with the problem or to determine whether the excess current is due to a problem in the output stage or elsewhere. __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

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