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mike49504 30th September 2006 08:02 PM

rockford fosgate 500s
hi just wondering if i should check anything else before powering it up I replaced all the outputs IRF540 and 9540 they were all blown Ireplaced the power transistors IRF3205 all blown I replaced the 317 and 337 and i replaced the u1620 and 1620 r diodes they were blown also i replaced the a06 and 56 transistors they checked ok but still repaced them wondering if i should check anything else before i power it up to see if it works

Perry Babin 1st October 2006 05:26 AM

What happened to that amp??? I can't remember ever having one with blown rectifiers.

Check the gate resistors for ALL of the FETs.

Check the large source resistors near the output transistors. They must be within tolerance for the amp to be reliable.

Check the power supply driver transistors near the TL494 IC. The PNPs (likely marked 2A) often fail when the power supply FETs fail.

Power it up through a current limiter or a 10 amp fuse. Have the transistors clamped tightly in the heatsink for the best protection (in case you've missed something).

mike49504 1st October 2006 07:36 AM

fosgate 500s
the drivers transistors marked 2a have 1 lead on one side 2 lead's on the other how do i check thoose not sure if i did it right i set my meter to 20k ohm put the neg probe of my multimeter on the one lead and on the side with 2 leads i put the positive on one lead and got 1.00 and nothing on the other lead went to the next one did the same but i got 1.00 on one lead and got 0.70 did i do this right or not if not how do u check them if i did does this mean they are good or bad?

Perry Babin 1st October 2006 01:49 PM

Does your meter have a diode-check function?

Do you have an oscilloscope?

Download the datasheet for the 2N3906/MMBT3906. It will show you the base/collector/emitter pin configuration.

mike49504 20th October 2006 11:35 AM

hi i replaced the transistors amp fires up worked for about 1min and wnet dead no output but the amp comes on stays on and 2 transistors in the power supply heat up almost instantly the 3205's are heating up only on the right side what might that be from

Perry Babin 20th October 2006 12:01 PM

Open gate resistors or blown PNP driver transistors (typically labeled 2A) can cause those symptoms. Check the resistors and drivers for both banks of FETs. It's also possible that the NPN driver has failed (marked 1A) but that doesn't happen often. If you have them, replace both PNP drivers. It only takes a couple of minutes to change them.

This is one time that a scope would be helpful.

If you didn't know it, you can connect only the ground and remote to the amp and the power supply IC will run. When you have good FETs in it (or no FETs in it), you can check the gate drive with no chance of damaging the FETs. Since you only have a voltmeter, you can measure the DC voltage on the gates of the FETs. The voltage on the gate leg of the PS FET should read ~1/2 of the remote voltage when you don't have the B+ supply connected to the amp. Touch the black meter lead to the ground terminal of the amplifier or to the third leg of the FET when measuring the gate voltage.

As far as I know, this only works for Rockford amps.

mike49504 20th October 2006 12:19 PM

hi i checked the drivers and it has blown drivers and had one blown resistor thanks for your help i greatly appericate it also wondering where i might be able to learn how to use a scope thinking about buying one shortly

Perry Babin 20th October 2006 12:34 PM

Tektronix has at least two good documents. The older one is in print. I scanned it but it's too large to email (36megs). The other is very good but it's too technical/complex for someone trying to learn the basics.

I have a basic tutorial on the scopes page of my site. There are a couple of flash demos that will show you the basics.

If you're going to buy from ebay, email me the auction numbers and I'll try to offer some suggestions.

This is the more complex document from Tek. Read it after you've read through my page.

mike49504 20th October 2006 01:40 PM

just wondering i checked the drivers again do i set my meter to ohm's or the diode setting because when i set it to diode setting they measure the same 2a's same reading 1a's same readings if i set it to ohms the transistor measure way differnt like from collector to emitter on 1 of them it reads .853 and the other 1 reads 36.75 on the 2a's and then going from the base to emitter 1 reads 6.86 and the other is 36.94 just wondering if that means they are shorted and going fromemitter to collector 1 reads .853 the other reads 36.75

Perry Babin 20th October 2006 01:48 PM

It's difficult to check them in the circuit because the bases and emitters of the 1A and 2A transistors are connected together.

Apply enough solder to the base and emitter terminals to allow your iron to heat both terminals. With the iron, GENTLY lift the two rear terminals off of the board until the transistor is at a 30-45 angle. Use some desoldering braid to remove the solder from the gap between the transistor terminals and the board. With the two terminals disconnected, you can accurately check the transistors.

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