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Old 22nd January 2007, 06:22 AM   #1
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Default RF Power 500bd problem

I got nice Rockford Fosgate Power 500bd amp with pretty unusual problem. It powers up fine (if +/- around 35 V on rails sounds right for this amp), idles perfectly fine without excessive draw and DC in output, protection LED is off. When feeding a signal to it, it plays, but very quietly. But when I increase signal amplitude or a gain, it acts as relay, sending a full DC to output. And what's disturbing, that I can't really find anything wrong with it I appreciate any help. Thank you!
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Old 22nd January 2007, 08:25 AM   #2
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Probably some cooked transistors. I've got an 800a2 that blew a couple (and some mosfets as well).

You can easily see the mosfets which tried to catch fire in this one, but what you should probably be looking for are 3-legged (two on one side, 1 on the other) transistors which are cracked. There is at least one in this amp the last time I looked at it. I need to get it fixed at some point ...

http://www.wcroller.com/los/linkpics/800a2innards.jpg

edit - The transistor which needs replacing (at least one) is just to the right of the second line of black capacitors (going from top to bottom) next to the 12-legged IC and small blue capacitor.

Kind of hard to see with the crappy camera I used, but i'll get better pics some day and figure out all of what I need to repair this thing as well..
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Old 22nd January 2007, 11:33 AM   #3
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valterdaw:

35 volts would be low rail voltage for this amp. You can't measure rail voltage on the transistors. You have to measure it across the red and black twisted pair. 35 volts on the tab of the transistor would be about 70 volts of rail.

If the outputs and the drive circuit for the outputs are Ok, you should read ~1/2 of the actual rail voltage on the tabs of the output transistors. The 6215s should have negative voltage and the 3415s should have positive voltage (black meter lead on the negative speaker terminal).

You said that there was no excessive DC across the speaker terminals. Precisely how much DC is there?

Can you post a list of DC voltages on the IC U17?

Does the DC voltage on the spealer leads go away when the signal is reduced or does it remain until the amp is switched off?



NVMDSTEvil:

MOSFETs are transistors.

You need to replace all 8 mosfets in the supply.

The small transistors near the TL494 are the drivers for the power supply FETs. Q3 and Q4 are the most likely to fail.

The gate resistors also need to be checked.
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Old 22nd January 2007, 01:30 PM   #4
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A fresh readings:

U17 (LM6171):

pin1: less then -0.01 mV
pin2: 2 mV
pin3: 0.4 mV
pin4: -9.74 V
pin 5: less then -0.01 mV
pin6: -24.3 mV
pin7: +9.82 V
pin8: less then 0.01 mV


And the re-measured rail voltages on twisted pair even better: -32.2/+33.8.

DC voltage goes away with reduction of signal without switching amp off.

Thanks a lot!!!
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Old 22nd January 2007, 01:49 PM   #5
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How much DC is there on the output (no signal)?

The voltages on the IC look about right. I'd expect the see the output (pin 6) to be a bit higher but it's probably within the margin of error for that IC.

You can't measure the rail voltage on this amp like other amps. The rails are floating. You have to measure the rail voltage by placing the black lead on the black wire and the red lead on the red wire.

After you re-measure the rail voltage, check the DC voltage on the tabs of the 3415s and 6215s (black meter lead on the ground terminal of the amp). Are they all at 1/2 of the rail voltage?
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Old 22nd January 2007, 01:57 PM   #6
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Rail voltage is 65.9V, on outputs (they are strangely 540 & 9540) it's close to 1/2 of it.
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Old 22nd January 2007, 09:09 PM   #7
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If you have +1/2 rail on all of the 3415s and -1/2 rail on all of the 6215s, I think the drive circuit and the outputs are OK.

It could be a problem with the signal getting to U17 or U17 could be defective. Does the DC remain on the outputs long enough to measure the voltage on U17? You don't need to have a speaker on it. When you measure the voltage, you need to make sure that there is DC on the outputs when taking the readings. I need to see how U17 is reacting to the offset.

Also measure the DC on the output. It's important to have the black lead on the negative speaker terminal and the red lead on the positive speaker terminal. The polarity (+/-) is critical here.

Did you ever buy a scope? It would be nice to know if the signal getting to U17 is clean.
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Old 23rd January 2007, 02:03 AM   #8
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Thanks for the info Perry.

Wouldnt happen to know what mosfets make suitable replacements would you? I took the part number down and looked on digikey and found nothing a few years ago. Checked some "mom&pop" type electronics repair stores in case they had a couple kicking around but no luck there either.. only reference i've actually found is in an old Sony 30x2 amp that I picked up from a friend because it was toast.. parts parts parts
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Old 23rd January 2007, 02:22 AM   #9
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If the gate resistors are 68 ohms and none are open or out of tolerance, IRFZ48s or HUF75339P3s will make good replacements. If you have to replace any of the gate resistors, I'd suggest replacing all 8 gate resistors with 47 ohm components and using IRF3205 FETs.

The 2G transistors are an MMBTA56. If/when you order parts, you should also order the MMBTA06 (1G marking) in case they have failed.

If possible, it would be nice if a moderator would move this to its own thread.
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Old 23rd January 2007, 03:23 AM   #10
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Yes, would be appreciated

I'll proabably pick up all the parts and do everything. Good experience.
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