Rockford Punch Amp repair

[Dr Zeus]

I didnt list the model number because I honestly dot know exactly what this amp is. I worked on a Punch 75 before this, and this amp basically has twice of the same type of outputs; possibly making this amp a Punch 150 (75wRMS x 2 I guess). The front cover says Punch 401s; 1200Watts. There are no details on the back cover, and the crossover is switch/knob style; where some of the earlier Rockfords used cards.

Needless to say, someone probably thought this amp was actually 1200w and ended up blowing one of the channels out. Someone before me clipped off the emitter resistors. I went ahead and removed the output transistors out of the circuit due to them being shorted and still soldered in. The serviving channel still works and does not need service. The power supply of this amp is also still in decent shape.

Digikey has the IRF540 and IRF9540 transistors, but they do not have the 0.10 ohm 3w resistors. Mouser sells them for $1 a pop and they are 1% tollerance. I have some 0.22 ohm 3w resistors in my possession - can I use 0.22 instead of 0.10 for this amp? Whats the limitation of these 0.22 ohm resistors as emmiters? I have a lot of them...

The largest caps in the amp (4700uF, 50v) are puffed out on top like I've not seem before. I'll need to replace these... Is there a good part number for caps like these?

[Dr Zeus]

Well, maybe this cap will work... In stock

Digi-Key - 338-1627-ND (Manufacturer - SLPX472M050C3P3)

[Perry Babin]

The model number is a 401s. I think Rockford used the 3x power rating on thier amps as a joke (or possibly to compete with other amps with wildly inflated power ratings).

Those caps are probably OK. If you push down on the dome and it pops back up, the caps are almost always OK. If you're concerned, cut the shrink-wrap so that you can remove the plastic circular cover. If the cap has vented, it will need to be replaced. If the vents have not opened, the caps are likely OK.

In an amp with FET outputs instead of BJT outputs, the large, low value resistors are 'source' resistors, not 'emitter' resistors.

The amp has current sensing to shut it down when the load is unsafe. With the 0.22 ohm resistors, the current sense circuit will shut down the amp with a much lighter load and may even shut down with a load that's safe for the amp to drive.

If speakers were connected to the amp with the source resistors out of tolerance or with them out of the circuit, the protection circuit components for the current sensing have probably been damaged. They are likely to be the SMD resistors directly connected to the source resistors.

[Dr Zeus]

On the caps, both are popped up like a can of soup gone bad. The dome's are solid and not abe to be pushed down - like theres a solid mass inside. It feels different than a similar amp I worked with. I'm thinking about just ordering a few of those caps for the collection anyhow.

Speakers were likely connected - as well as the gain controls maxed out before I got this amp. I know for sure one of the SMD resistors is bad as it is not reading right - 3.9ohm (Says 390 on the back) resistor came off the pcb while getting tested.

So far parts list:

Two 4700uF 50v caps
Two IRF540 outputs
Two IRF9540 outputs
Four 0.10 ohm 3w resistors
One 3.9ohm SMD or equivalent

After posting this thread I found out that this model 401s amp is rated for 100w X 2 RMS. Not bad but its funny about the LED faceplate saying its a 1200w amp.

[Perry Babin]

An SMD resistor with the mark '390' is a 39 ohm resistor.

Did you remove the plastic cover from the cap to confirm that it was swollen?

[Dr Zeus]

I think the CAPs are fine.

I had ordered some parts to start fixing this amp; however under repair I noticed one of the output driver transistors is shorted from a previous experience. Its a small surface mount, with 2A on its back.

I just wanted to confirm before ordering; is 2A a MMBT3906L transistor?

http://www.marsport.org.uk/smd/mainframe.htm

[Perry Babin]

What's the circuit board designation for that transistor?

The datasheets (download) tell you what the markings will be).

[Dr Zeus]

You mean Q116? I lifted the center leg and this transistor is measuring 100 ohms from left leg in photo to center leg. This transistor is very close to the output transistor which caused the most board damage; in fact the center leg is connected to the source resistor; where the 39 ohm also fried from the 3rd leg of the output transistor. I've started replacing components and am stuck on this Q116.

[Perry Babin]

This is a protection circuit component. It appears to be defective.


There is a transistor marked 2A that has it's collector and base shorted. I didn't know if you were possibly concerned about that one.

[Dr Zeus]

I think Im concerned. Shouldnt I be? Its a MMBT3906L right?

I have not applied power to this amp since starting the replacement and repair; just notice this 2A transistor w/ a problem.