setting bias for power FETs?

[bdeko]

Hi everybody, I'm new to this forum.
I have an Electrovoice 81PMX power mixer that was blowing fuses. The previous owner had replaced all the power FETs and did a pretty crappy job. I put in new ones (IRF640 and IRF9640) and now it's OK, but there is a note on the power amp board saying I need to adjust the bias when replacing FETs. What exactly am I looking for when I adjust it?
I'm not used to high power stuff and my FET theory is shaky.
Can anyonelet me know how to adjust this properly?

Thanks!

[unclejed613]

first, attach a dc voltmeter across the emitter resistors and bring up the amp on a variac. if the voltage across the emitter resistors goes over 100mV before you get to full line voltage, shut it down!!!!!

if you are able to get to full line voltage, adjust the bias pot until you get between 10 mV and 20 mV across the emitter resistors. the bias pot is usually near a transistor or diode "stack" (usually a red blob shaped thing in a clip that screws to the heat sink). you will also want to adjust the offset by attaching a voltmeter across the speaker terminals and adjusting the offset pot (usually near the amp board input cable) until you have 10mV or less across the speaker terminals.

[XEAGLEKEEPER]

Unclejed Dont you mean source resistors instead of emitter.
Dave

[bdeko]

Thanks guys, but as I was adjusting the bias, I would see the voltage swing up to 15mV or so, and then right back to 0. I thought I needed to increase a bit more, but then true to Murphy's law, all four PNP FET's blew (these are double the price of the NPNs).
Before I try this again, I'd like to figure out why this happened. I measured across the source resistors (they are 0.33ohm, 2W). Could using a magnetic screwdriver affect it? I can't think of anything else. Help?

[AndrewT]

Hi,
are the output devices set up as source followers
or
as CFPs?

I think the source follower requires more bias to reduce crossover distortion.

I wonder if the amp is actually a quasi complementary set-up to eliminate the need for expensive 9640?

How many pairs are fitted to each channel of the output stage?

Any chance of posting a schematic?

[bdeko]

I don't have a schematic. There is an initial BJT NPN/PNP pair, then four pairs of 640/9640. I'm not used to the FET amps so much. Also, I was measuring the bias with no signal applied. Do I need a signal? I have a sine wave source.

[AndrewT]

Hi,
output bias and output offset are both measured and re-set, if necessary, with the input shorted to ground and with the output open circuit (=no load).

4pair of 120W devices should be able to give 150W to 200W of output power if designed to be reliable.

15mV on Vre is tiny if measured correctly.
There appears to be another fault.

You need a search out a schematic.

[ifrythings]

Originally posted by bdeko
Thanks guys, but as I was adjusting the bias, I would see the voltage swing up to 15mV or so, and then right back to 0. I thought I needed to increase a bit more, but then true to Murphy's law, all four PNP FET's blew (these are double the price of the NPNs).
Before I try this again, I'd like to figure out why this happened. Help?

And this is why I use a light bulb (60w's or so) in series with the mains when I first fire up or change settings. It's cheap current limiting and saves you many expensive parts. Also check or replace the bias pot, if it has a dead spot (aka open area) this could kill the amp.

AndrewT

I wonder if the amp is actually a quasi complementary set-up to eliminate the need for expensive 9640?

That could be very possible.

Thats easy to check, if (-) rail goes to center leg of fet thens it's complementary, but if it goes to the source then it's quasi complementary (all N-channel Fets) (Gate will be a few 3-4 volts higher the the (-) rail)

[bdeko]

It definitely is complementary.
What do you mean about the light bulb in series with the mains? Where exactly do you wire the bulb? I'm curious 'cause I really don't want another $40 going up in smoke.
The bias pot is OK; checked it with a Fluke multimeter and turned it slow - no dead spots.

[ifrythings]

I have a light bulb with wires and holder straped onto a chunk of wood and on the wires are aligater clips.

As for were to put it, I usually pull out the fuse and put clip the leads onto the fuse holder, then the bulb is in series and stuff can't get hurt.

DO NOT use a high watage like, or it's usless, I use a 60w bulb.

you have to use a Incandescent light bulb too, the mini fourecents don't work.

Have you checked the bias transistor? for shorts? for leaky junctions? pull it out of circuit for testing.