Huf75337p3 in place of irfz44n?

[spooney]

Trying to get a very cheap pyramid amp up and running for a family member and I have some HUF75337P3's on hand that i was wondering if they might work to replace the blown irfz44ns in the power supply.Guy has no money(hence the pyramid amp) so I'm trying to do this as cheaply(but still reliable as a pyramid amp can be) as possible. Any info would be greatly appreciated.

[Perry Babin]

They should work without any mods. If the new FETs start to heat up at idle and the amp isn't drawing excessive current (drawing less than ~2 amps), the drive circuit is intact and the original gate resistors are higher than 68 ohms, replace them with 68 ohm (or lower) resistors.

[spooney]

is there anything else in particular i should look for in this amp knowing that it was killed by shorting the positive and negative power input cables via a 16 gauge wire? The reverse protection diode was toast as well as the ps fets. its a 1n5403 i believe

[Perry Babin]

Shorting B+ to ground at the terminals won't cause the RP diode to fail. That will only fail if the B+ and ground connections are reversed.

If the RP diode opened and all of the FETs opened (allowing reverse voltage to be applied to the ICs in the power supply), those ICs may have failed. You should power up the amp with the FETs out of the circuit to see if the drive voltage on the gate pads for the FETs is as it should be (~5-6v DC when measured with a multimeter, ~50% duty cycle square wave that swings to ~10v, when viewed on a scope).

[spooney]

i have some 30.1 ohm resistors on hand. are those too low to replace the 99 ohm resistors currently in the amp with

[Perry Babin]

You may not need to replace the 100 ohm resistors (unless they're defective or out of tolerance). If you do need to replace them, the 30 ohm resistors should be OK.

[spooney]

got a chance to get back into this amp tonight. I checked the gate pads and they had almost 5 volts on them(4.9) at least on the side I checked before installing the new fets(I neglected to check the other side which I now realize may have been a mistake). So anyways new fets installed the amp appears to power up fine through a 5 amps fuse.It has a little jump in current draw at turn on but then settles right down and idles below one amp even after sitting for approximately 10 minutes.

Not sure what made me check it again at this point but I rechecked the gate pads only to find that one side has 5 volts on the gate pads and the other side has 11 volts on the gate pads. Amp is still powered on fine at this point and the rail voltage is consistent from the left to the right channel but I still have two fets with 5 volts on the gates and two with 11 volts on the gates.All gate resistors are original to the amp and are still within tolerance.

Not sure if this has any adverse affect but I used a small 1n4003 diode in place of the original 1n5400 reverse protection diode as I have yet to obtain a replacement.Would this cause my issue?My gut tells me no but I figured I would ask.is it dangerous to use this diode even temporarily?

[Perry Babin]

It sounds like you may have an open ground trace.

What's the voltage on all 3 legs of the FETs with 11v on the gates?

[spooney]

well I checked the pins of the fets and got the following voltages

pin 1 11 volts
pin 2 12.6 volts
pin 3 6.6 volts

Before I even measured the voltages though it was plain to see where the bad ground trace was.I can't believe I missed it.Couldn't have been much more obvious as it was near where I had already done some soldering. A couple strips of desoldering braid later and the voltages are now all consistent for all ps fets so i guess that just leaves me with the rp diode to replace. Will it cause any trouble to do some testing with the temporary 1n4003 in place?I'm hoping the local radioshack has a compatible diode in stock because i'd really hate to make an order with mouser when I really only need one diode.My amp tinkering has been cut way down ever since my son was born so i'm just scraping by on old parts for this one

[Perry Babin]

RS has some suitable diodes.

The 1N4003 is OK for now.