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woutzor 29th March 2009 07:08 PM

Rockford Fosgate P800.2 [Repair]
Hi guy's.

I'm trying to repair P800.2 amplifier that I got from a friend. It doesnít power up so I opened it up. Some pictures of the inside with visible burn marks.

Fried power supply FET:

Burnt source resistors:

Fried drive circuit transistor?? Q2 called p2G = PMBTA56:

Burnt and broken trace:

It seems that the left side of the amp is blown for most of the part.
The right side is good (No leaks in output Fetís between Gate and Drain / Source resistors measure 0.1Ohmís).

After removing the left side and measuring most of the components this is what I found:

As you can see I desoldered most of the defective components and fixed the broken trace.

I've made a list of components that need to ordered so far:But I can't find this component or any replacement part. Q2 is a little SMD PNP transistor with the code p2G (PMBTA56)

Does anyone know a good alternative part?

If I need to measure some other parts let me know ;)

Perry Babin 29th March 2009 07:26 PM

Philips uses a P in the prefix. Motorola uses an M. The MMBTA56 is essentially the same part.

Check the gate resistors for the power supply FETs.

Also check all of the surface mount resistors that are connected to the gate AND to the source resistors for the outputs. When the source resistors open, the SMD resistors often open.

The diode at the end of the burned trace is likely shorted.

You should read through the following threads if you haven't done so:

woutzor 29th March 2009 10:00 PM

Thank's Perry for reviewing my problem.

Great stuff, I'll just replace the SOT-23 PMBTA56 with a TO-92 MPSA56 :D

I've measured the Gate resistors from the Power supply FET's.
  • R8 [680] = 68,5Ω
  • R10 [680] = 68,5Ω
  • R58 [680] = 69Ω
  • R59 [680] = 68,9Ω

The readings of the Gate resistors of the output FET's.
  • R248 [510] = 51,8Ω
  • R249 [510] = 51,7Ω
  • R250 [510] = 51,5Ω
  • R251 [510] = 51,6Ω
  • R252 [510] = 55,2Ω <- ?
  • R275 [510] = 52,5Ω <- ?
  • R276 [510] = 51,5Ω
  • R277 [510] = 51,9Ω
  • R278 [510] = 51,7Ω
  • R279 [510] = 51,6Ω
The Source resistors (0,1Ω ) of the output FET's that I haven't unsoldered are good.
They all (R274, R244, R242 and R240) measure 0,4Ω. 0,4Ω - 0,3Ω (internal resistance DMM) = 0,1Ω
The Source resistors I desoldered measured between 180Ω and 3KΩ.

Correct!! The diode was shorted indeed shorted, gave me a perfect .000 in both directions! Its a 1N4003 diode which is rated to 200V peak.. I only have a bunch of 1N4001 diodes so I'll have to order those ;).

Yes, I've read those threads for most of the part. To be honest, I came across one of those threads when I was looking for repair information about this amplifier.

Thanks for the help so far Perry, I've learned allot from the Basic Car Audio Amplifier Repair guide!

If there is anything I have to measure before I order everything, let me know. Thanks again :cheers:

Perry Babin 29th March 2009 10:07 PM

I'd replace the two that have the question mark.

You need to replace all of the 0.1 ohm resistors.

If you haven't done so, double-check the resistors directly connected to the source resistors. It's rare to have open source resistors and not have at least two of the SMD resistors (connected to the source resistors) fail.

Use the MMBTA56 instead of the TO-92 case transistor. I've done it before but it's difficult to do it without stressing the pads. Replacing the SMD transistor is easier than replacing through-hole transistors.

woutzor 30th March 2009 04:23 PM

I have placed a bunch of 51Ω SMD resistors to my order.

Desoldered the other 0,1Ω Source resistors of the Left output.
How many Watt's do these 0,1Ω resistors have to be? I can choose between 2 or 5 Watt.

After desoldering the 0,1Ω Source resistors, I measured the resistors connect to the Source trace/pad:
  • R237 [101] = 101,1Ω
  • R239 [101] = 101,6Ω
  • R241 [101] = 101,4Ω
  • R243 [101] = 100,8Ω <- ?
  • R245 [101] = 102,5Ω <- ?
  • R265 [101] = 101,0Ω
  • R267 [101] = 101,5Ω
  • R269 [101] = 101,0Ω
  • R271 [101] = 101,1Ω
  • R273 [101] = 101,3Ω

I guess these seem alright. The ones that are a bit off are R243 and R245. Should I replace them?

Well the problem about the MMBTA56 is that I can's find a shop who sells it :(. only sells the TO-92 variant so I have to settle with the MPSA56.

To be on the safe side, I'm also going to replace transistor Q1, Q3 and Q4 in this picture:

Do you know a good alternative transistor for the MPSA56 and MPSA06? If so, I'll have to look for them if they are available in both TO-92 and SOT-23 case.

Otherwise I have to go with this solution:
  • Replace Q2 and Q3 with MPSA56's
  • Replace Q1 and Q4 with MPSA06's (these two are originally 1GM = MMBTA06)

Thanks again for your help :cheers:

Perry Babin 30th March 2009 05:59 PM

Mouser has a worldwide site.

The original resistors are 2w.

The 100 ohm resistors are within tolerance.

Look for these two resistors and check them. One should be marked 390. The other should be marked 470.

woutzor 30th March 2009 09:26 PM


Mouser has a worldwide site.
Thanks for the webshop ;)


The original resistors are 2w.

The 100 ohm resistors are within tolerance.
Great! :D


Look for these two resistors and check them. One should be marked 390. The other should be marked 470.
Are you referring to R390 and R470? I can't find any of those :( (maybe I need a new pair of glasses, lol)
If you are, please give me a hint on where to look.
I've got a schematic of this amplifier that I recieved from RF tech a couple of days ago. If you need it or any pictures of the amplifier, let me know :cheers:

Perry Babin 30th March 2009 09:39 PM

R238 > 470
R264 > 390

woutzor 31st March 2009 03:35 PM


Originally posted by Perry Babin
R238 > 470
R264 > 390

R238 [470] = 45,1Ω (measured in circuit)
R264 [390] = 41,9Ω (measured in circuit)

Tonight I'll unsolder them and check the real resistance.

woutzor 31st March 2009 09:17 PM


Originally posted by woutzor

R238 [470] = 45,1Ω (measured in circuit)
R264 [390] = 41,9Ω (measured in circuit)

Tonight I'll unsolder them and check the real resistance.

Here's a picture of the two resistors (Red arrow) R238.jpg

I've unsoldered the two resistors, this is what I measured:

R238 [470] = 47,2Ω
R264 [390] = 41,8Ω

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