Rockford Power 500.2 Amp Troubleshooting power supply. - diyAudio
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Old 31st August 2008, 03:02 PM   #1
kds9591 is offline kds9591  United States
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Unhappy Rockford Power 500.2 Amp Troubleshooting power supply.

I'm working on a Rockford power 500.2 car amp troubleshooting the power supply. Originally all the output MOSFETs and power supply MOSFETs were shorted and replaced and some of the .1 ohm power resistors were open and replaced. This amp uses IRF3205 MOSFETS for the power supply and IRF9540 and IRF540 for the MOSFET outputs.

When it powers up it draws over 30 amps but yet the driver transistors Q1, Q2, Q4, Q5 were checked and are fine. On a hunch I replaced the TL494 I.C. and no change. Because of the high current draw I can't troubleshoot with the voltage on or I'll blow the MOSFET's I replaced. It draws almost a amp of currrent with a 5 ohm power resister in series with the 12VDC power input.

I'm leaning towards thinking the problem is a power xformer with some shorted turns but wanted some tips or advice on anything else I should check before condeming this amp.

Any help would be greatly appreciated.
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Old 1st September 2008, 12:52 AM   #2
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If you replaced the outputs, set the bias pots fully CCW. If it still draws excessive current, measure the DC voltage across the 0.1 ohm source resistors. If you read 0.000v across ALL of the 0.1 ohm resistors, apply only remote voltage and ground and confirm that you have a clean square wave on the gates of the power supply FETs.

If that's OK, pull the center legs of CR1 and CR2 and power the amp up normally. Does it continue to draw excessive current?
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Old 1st September 2008, 05:43 AM   #3
kds9591 is offline kds9591  United States
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Thanks Perry for your reply.

The bias pots were turned counterclockwise when I replaced the MOSFETS. I applied just the remote voltage and ground and the waveform on the gates of MOSFETS Q10, Q6 looks typical and clean. The waveform is missing on the gates of Q11, Q7.

I cut the center legs of CR1 and CR2 and when powered up normally the amp now draws about 3 amps, Q10 & Q7 start to get hot, Q11, Q7 stay cold because of no signal.

I didn't measure the voltage across the .1 ohm resistors with the amp powered up normally because I did not want blow the fuse in my meter and blow the MOSFETS I just replaced. I accurately checked all of them before and replaced the ones that measured open or changed value.

Anything else I can check, maybe more problems in the output section?
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Old 2nd September 2008, 10:10 PM   #4
shagone is offline shagone  United States
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did you check the drive stage for the outputs for any shorted components? you never mentioned that so i figured i would ask.

Good luck!
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Old 2nd September 2008, 10:27 PM   #5
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You need to determine the reason you have no signal on the gates of Q7 and 11. If you don't have a signal on either side of the gate resistor, the driver transistors are probably blown. Check the signal on pin 9 of the 494. If you have a square wave there, Q2 and/or Q4 may be defective.
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Old 3rd September 2008, 03:42 AM   #6
kds9591 is offline kds9591  United States
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Thanks again for your reply Perry.

I doublechecked for the drive signal at pins 9 & 10 of the TL494 IC and there was 11 volt drive signal from pin 10 but only a .5 volt drive signal from pin 9 with some slight distortion. I checked the driver transistors out of circuit before and they were fine but I replaced them anyway.

The signal on pin 9 did not change so I lifted R2 the 1Kohm resistor which is tied to the base of Q4 to ground and the signal came up & now matched pin 10. R2 measured 1K ohms. I rechecked Q11 & 7 and resistors R20 and R46. The only thing left in the circuit that could be dragging the drive signal down from pin 9 was the transformer itself. C24,C22,C20,C26 are all in parallel so if any were resistive it would affect both drive signals. CR1 and CR2 checked ok and the center pins are still cut.

I put a 50Khz 10 volt sine wave signal across the primary windings of 1 & 2 and checked the signal with a scope across the secondary windings 6 & 7 and the signal was a 15 volt sign wave.

I did the same with the primary windings of 3 & 4 and secondary windings of 5 & 6 and the waveform on the secondary side was less than 1/2 volt and somewhat distorted.

At this point with the above results I gotta believe there is some shorted windings either on the primary side or secondary side for one side of the transformer.

Any last thoughts or ideas before I condem this amp?
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Old 3rd September 2008, 04:09 AM   #7
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If the 1k ohm resistor drags the output of pin 9 down and the DC voltage on pins 8 and 11 is the same (when 9 is being dragged down), the 494 is defective.

The output of the 494 is not connected to the windings so shorted windings can't drag the output down (particularly when the B+ is not connected).

Replace the 494 and solder the 1k ohm resistor back in place. If the drive signal on the gates is OK for all power supply FETs (no B+ connected), the amp should power up normally. If it does, solder the legs of the rectifiers back into the board.

I don't know what the output impedance of the sine wave generator is but it's probably relatively high. The impedance of the secondary is going to be much higher due to the higher number of windings. If there were a shorted winding, it's likely that the generator would have been dragged down when on the secondary windings also.
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