disgnosing a shorted transformer?

marko · 21st September 2010, 05:05 PM

i have a rockford 250.2 (silver/gold), just replaced all the power supply transistors as they had obviously blown

, also swapped a couple or outputs out that were bad now the outputs all seam ok but the power supply still has a short somewhere..

i think (hope not) that the transformer is shorted but not sure how to test it?

Mark.

pjp · 21st September 2010, 05:13 PM

Disconnect the secondary, power it up, see if it gets hot ?

marko · 21st September 2010, 05:59 PM

on closer inspection the transformer looks like it got more than a little hot, looks like the amp is a right off unless i can wind a new one but this is new teritory for me!

Perry Babin · 21st September 2010, 06:07 PM

Connect only remote and ground to the amp (no B+). What is the DC voltage on the gate legs of each of the power supply FETs?

marko · 21st September 2010, 06:38 PM

Quote:

Originally Posted by Perry Babin

Connect only remote and ground to the amp (no B+). What is the DC voltage on the gate legs of each of the power supply FETs?

8.23v on one bank of 3 and 0.8 on the other

Perry Babin · 21st September 2010, 06:44 PM

It appears that you have some defective driver transistors and possibly some defective gate resistors.

Did you confirm that all gate resistors were within tolerance?

What is the DC voltage on pins 9 and 10 of the TL494?

What is the DC voltage on the bases and emitters of all of the power supply driver transistors (Q1, Q2, Q4 and Q5?)?

marko · 21st September 2010, 07:06 PM

Quote:

Originally Posted by Perry Babin

It appears that you have some defective driver transistors and possibly some defective gate resistors.

Did you confirm that all gate resistors were within tolerance?

What is the DC voltage on pins 9 and 10 of the TL494?

What is the DC voltage on the bases and emitters of all of the power supply driver transistors (Q1, Q2, Q4 and Q5?)?

2 of the driver transistors look shot (very hard to tell but the have a small bump in the middle!)

2 of the gate resistors are shot, one is reading 88 ohms and the other is blown apart, there again these sm's are so small i miss them

i don't want to put +/- 12v on as the amp starts to draw 30amp rather quickly..

Perry Babin · 21st September 2010, 07:53 PM

Replace the parts that you know are defective and recheck the gate voltage with no B+ applied. All should be approximately the same (which is typically ~5v).

marko · 22nd September 2010, 05:16 PM

Q1,2,4 and 5 appear to have p1a 59 on them, hard to tell as they are so small, is this correct? can't seam to find much info on them..

resistors should be easy to find although in 500's!!

Perry Babin · 22nd September 2010, 05:34 PM

They are generally marked 1A (MMBT3904) and 2A (MMBT3906).

21st September 2010, 05:05 PM	#1
marko diyAudio Member Join Date: May 2004 Location: donnyuk	disgnosing a shorted transformer? i have a rockford 250.2 (silver/gold), just replaced all the power supply transistors as they had obviously blown , also swapped a couple or outputs out that were bad now the outputs all seam ok but the power supply still has a short somewhere.. i think (hope not) that the transformer is shorted but not sure how to test it? Mark.

21st September 2010, 06:07 PM	#4
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Connect only remote and ground to the amp (no B+). What is the DC voltage on the gate legs of each of the power supply FETs? __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

21st September 2010, 06:44 PM	#6
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	It appears that you have some defective driver transistors and possibly some defective gate resistors. Did you confirm that all gate resistors were within tolerance? What is the DC voltage on pins 9 and 10 of the TL494? What is the DC voltage on the bases and emitters of all of the power supply driver transistors (Q1, Q2, Q4 and Q5?)? __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

21st September 2010, 07:53 PM	#8
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Replace the parts that you know are defective and recheck the gate voltage with no B+ applied. All should be approximately the same (which is typically ~5v). __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

22nd September 2010, 05:34 PM	#10
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	They are generally marked 1A (MMBT3904) and 2A (MMBT3906). __________________ 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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21st September 2010, 05:13 PM	#2
pjp diyAudio Member Join Date: Nov 2006 Location: under the rainclouds	Disconnect the secondary, power it up, see if it gets hot ?

21st September 2010, 05:59 PM	#3
marko diyAudio Member Join Date: May 2004 Location: donnyuk	on closer inspection the transformer looks like it got more than a little hot, looks like the amp is a right off unless i can wind a new one but this is new teritory for me!

22nd September 2010, 05:16 PM	#9
marko diyAudio Member Join Date: May 2004 Location: donnyuk	Q1,2,4 and 5 appear to have p1a 59 on them, hard to tell as they are so small, is this correct? can't seam to find much info on them.. resistors should be easy to find although in 500's!!

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