odd punch 200.2 issue

[spooney]

I have a punch 200.2 (older model) here that i just got done replacing a channel and some power supply fets in. The bad channel had the irf540's and irf9540's replaced as well as the .1 ohm source resistors. Power supply just had the original ps fets(slips my mind what they were now) replaced with some huf75337p3's that I had on hand. All other ps components appeared to be good. After replacing all of what i thought were the defective components i put the board back into the heatsink and set the bias. After setting the bias I ran some sine waves through it and verified that I had output at the speaker terminals by using my meter to check the voltage(late at night when I finished up). Today I got the chance to run the amp to some speakers and at very low to low volume everything seems fine with the amp. Now when I try to run it at low to moderate volume or higher for some reason it makes my test head unit cut out/reset itself like power is getting unhooked and then hooked right back up . At the same time it is also making a clicking/popping sound through the subwoofer i was using for testing. I tried some different source material and no matter what i played i get similiar results.At this point I was thinking it was my test head unit so i hooked it up to another amp and then the problem went away. So something about this punch 200.2 is doing some weird stuff to my deck and itself. Its like it is somehow feeding back through the rcas into the head unit and causing it to shutdown. At no point does the amp ever shut off...just the head unit. My guess is there is a problem with the amps audio inputs but I'm not sure where to start looking

[Perry Babin]

Are you testing it in the car or are you using a 12v power supply on the bench?

If it's a bench power supply, does it have a variable voltage?

Were the power supply FETs blown?

Were the replacement IRF540s the same exact part number (IRF540N >> IRF540N or IRF540 >> IRF540N)?

[spooney]

replacements for IRF540's were actually IRF540N's. Replacement IRF9540's are still 9540's. testing the amp on a bench power supply. I tried upping the voltage to 15 volts but it was no help.my supply is only 22 or 25 amps continuous but problem starts way before that amperage. power supply fets were blown but gate resistors and gate drive signal appeared to be good.

[Perry Babin]

Reduce the power supply voltage to ~10v to force the power supply to go to full duty cycle. If the current draw doesn't increase when you decrease the supply voltage, the amp's supply is likely OK.

I've had 3 Rockford amps that wouldn't work properly with the IRF540N. One oscillated, one drew excessive current when driven to ~1/2 power (sounds like the same thing you're experiencing), the last one wouldn't power up (went into protect as soon as remote voltage was applied). In all 3 instances, I replaced the IRF540Ns with IRF540As (Fairchild Semiconductor) and they all worked properly with no other changes.

http://www.mouser.com/ProductDetail/...EvOvQtGw%3d%3d

As a side note, the rest of those IRF540Ns worked properly in other (non-Rockford) amps.

[spooney]

Hmmm thats strange but hopefully a swap of those parts does the trick. I thought the IRF540N's would work because I've used them as a replacement for 540's in the past. Rockford must not like them for whatever reason.I'll give those a shot and see what happens . Thanks again

[spooney]

as long as I have to re order parts is there any problem with putting irf540's back in the amplifier? I just noticed that the IRF540PBF's appear to be a little cheaper but if there is an advantage of using the IRF540A i will just get those.Also curious to know if IRF9540PBF's are a problem or are they just the lead free version of a IRF9540? I purchased the IRF9540PBF's and they are currently in the amp. Just would like to know if I should swap those while I'm back inside the amp.

[Perry Babin]

I can't remember any instances where the lead free versions caused problems in class AB amps.

The IRF540 (PBF or not) should work but I know that the IRF540A worked in the amps I dealt with.

[spooney]

Just wanted to drop a quick update. I was able to source some irf540's locally(540a's weren't available) and using those in place of the irf540n's did indeed solve my issue. What is it about the N version that causes these issues?

[Perry Babin]

Maybe someone else can tell you why they don't work. I don't have a clue. I know that by the time I figured it out, I had about as much hair left as Homer Simpson.

This is another example of why I recommend exact replacements unless the substitute parts have been tested thoroughly.

[spooney]

well in any case you have helped me put yet another amp back into service(one of my favorites at that) . Thanks for always being there to help the little guys like me out.