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Old 7th February 2009, 12:20 AM   #1
fongman is offline fongman  United States
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Join Date: Jan 2009
Default Rockford fosgate 800a2

Thank you perry for all the help. while I'm waiting for the parts i will try to temp a harder amp. Its a fosgate 800a2 that i bought and did not work. The first time i put power to the amp it turn on for about one min. And dead. I open it up and see that c20 r15 c19 is missing. I then check power without the remote and 14v. With the remote to power 10v. I have a drain some where. I check the Fet power and the seem fine. I check the outputs 5 IRF9640 and 5 IRF640 found that are usual reading.
Resistance from leg 1-2
Q211: 62 ohm
Q212: 164 ohm
Q213: 164 ohm
Q214: 164 ohm
Q215: 164 ohm
Q216: 79 ohm
Q217: 79 ohm
Q218: 79 ohm
Q219: 2.4 ohm
Q220: 9.1 ohm

Resistance from leg 2-3
Q211: .4 ohm
Q212: .6 ohm
Q213: .6 ohm
Q214: .6 ohm
Q215: .6 ohm
Q216: 60 ohm
Q217: 60 ohm
Q218: 60 ohm
Q219: .6 ohm
Q220: 1.3 ohm

After I measure those i found two transistor looks like its been change. found a Lm317T WKO049849 malaysia next the 10th IRF640. Across the other side same place a LM317T W99019735 china ST. I don't think these are stock parts.

Resistance from leg 1-2
Q211: 4.5 ohm
Q212: 3100 ohm


Resistance from leg 2-3
Q211: 5.5
Q212: 120 ohm

I wil measure the gates resistor and post later.
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Old 7th February 2009, 12:26 AM   #2
fongman is offline fongman  United States
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gates resistor are

Q211: .4
Q212: .4
Q213: .4
Q214: .4
Q215: .4
Q216: .4
Q217: .4
Q218: .4
Q219: 60 ohm
Q220: 59.8 ohm

If i'm correct the gates are the one under the outs and i think there are 10
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Old 7th February 2009, 03:01 AM   #3
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Location: Louisiana
Q211, 219 and 220 appear defective. The emitter resistors for Q219 and 220 appear to be open also. Cut the legs free on the defective ones and then you can recheck the outputs to determine if they're defective. They all have to be changed but having them out of the circuit will allow further testing.

After removing the shorted outputs from the circuit, then you can reliably check the gate resistors. They generally fail open.

I'm not sure why you're reading something less than 10 ohms. U3 and U4 are supposed to be the LM317 and LM337. You should read ~the same as the resistor connected between the legs (nothing less than ~120 ohms.

Mouser:
511-LM317T
512-LM337T

With these amps, you can typically expect at least 20-30 parts to fail or to have to be replaced.

Have you ever replaced parts on the MEHSA insulators?
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Old 7th February 2009, 06:36 AM   #4
fongman is offline fongman  United States
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I have not, but i will try. I will buy one of the Mini Butane Iron Gun
and try my best. I think i will replace 5 of the IRF9640 and 5 of the IRF640. I had a friend open up his amp to compare and he had 337t and 317t on the two that are replace with china and may parts. Are this the original? Before i oreder anything i will cut the legs and retest them 1st.
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Old 7th February 2009, 06:51 AM   #5
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Location: Louisiana
You are a long way from ordering parts. There are many things that need to be checked.

When you order the outputs, order 10 of each so that you're more likely to get 5 of each with a matched date code.

The radio shack butane torch/iron works well.
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Old 7th February 2009, 07:29 AM   #6
fongman is offline fongman  United States
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I was going to cut the legs but can not find a good size cutter, so i will have to pick one up? some how i think that the IRF9640 and IRF640 are bad. Do you think i can cut the leg and try to power up and see if the led lights up?
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Old 7th February 2009, 03:00 PM   #7
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Location: Louisiana
You can try it. On many amps, you can cut out the bad components and they will power up. Generally, these do significantly more damage and will not produce audio from the blown channel.

The following graphic shows a typical repair and the parts that fail (or have to be replaced) when the outputs fail.

http://www.bcae1.com/temp/punch800a2repaired.swf
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Old 7th February 2009, 06:21 PM   #8
fongman is offline fongman  United States
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wow thats alot of parts but clean soldering. I hope mind don't need that many parts.
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Old 9th February 2009, 06:02 AM   #9
fongman is offline fongman  United States
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okay, I cut the legs of

q311:
q312:
q314:
q315

q316:
q317:

Left the legs on

q318
q319
q320

and now power up. measure

r340: 83 ohm
r342: 118.6 ohm
r344: .4
r346: .4
r362: .4
r366: .4
r368: .4
r370: .4
r372: .4
r374: .4

Found 2 resistor are bad. So are 5 IRF9640 and 3 IRF640. What other parts do you think i should check?
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Old 9th February 2009, 06:16 AM   #10
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You need to replace all of the IRF640s and 9640s in that channel. You also need to replace R340, 342, 344, 346 and 347.

You also need to check R337, R338 and R339. Pull the resistors above before checking these resistors.
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