Rockford fosgate P425.4 DC offset smoked R53

mikep28

Member
2015-02-20 5:28 am
CORRECTION: this is a 450.4

Got this amp to fix. LF channel outputs were no good. Both of them shorted badly. Replaced both of them and the associated PZTA06 and PZTA56 's. Powered amp up and channel plays but distorted. Other channels play great. Was doing a little probing around now no audio / protect light not on and the PZTA06 and PZTA56 for that channel get smoking hot fast. Noticed LF + has nice toasty -30VDC on it. Powered amp off and noticed R53 was also crispy. Anyone know what value R53 is and anybody have a schematic for this amp? I have no way of knowing if R53 has anything to do with that circuit etc.
 
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mikep28

Member
2015-02-20 5:28 am
I did the base is 2 on the MPS and 1 on the MMBT. I was going try to flip it and see if I could get it working. I'll just wait for the proper part

R53 is 49.9 ohms its in the power supply page 1. I have the diagram for a 4004. Same as far as that goes.. Installed a 50 ohm in that spot. Amp produces clean audio on the remaining 3 channels. Wait on the MMBTA6521 to arrive. Thanks Perry
 

mikep28

Member
2015-02-20 5:28 am
Hope pic shows up. The 3 conductor connector on the right I assume 100 series parts.
 

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mikep28

Member
2015-02-20 5:28 am
Ok back to this beast:
Installed the proper bias transistor. Still no bias possible. No output on LF.

So far : R150, R121 both bad. Q103, Q110 shorted. I believe D104 or D105 was open. Replaced both. No bias still. No voltage on any leg of the junction connector to the 6521 on that channel. Don't see anything else bad in that the channel. The PZTA56 and A06 both good. I'm going to pull the board from the chassis and see if I have a burnt trace or something. Any idea? I have the bill of materials and the schematic for his amp if you want it Perry.
 
I'm assuming that you've already confirmed that all of the resistors around the bias components are within tolerance.

Desolder D102, 103, 104 and 105, one at a time (you can leave them desoldered as you desolder the group). Check for biasing after you desolder each one.

Be careful, these are part of the protection circuit and removing them could allow excessive current draw or other problems.
 

mikep28

Member
2015-02-20 5:28 am
Yes I've checked most the resistors. Everyone looked ok. I think I found the problem. The 6521 on the MEHSA showed 4 ohms to ground. Cut that lead of the connector and sure enough It's shorting on the pad. Removed the connector from the board. I tried a MPS6521 by crossing legs but I still get distorted output on that channel. It'd been a long day for me so I came in. According to the diagram it looks like I flip legs 1 and 2. On the board it's BEC and my part is E B C.