Simple rig to match mosfets?

[phydauex]

I was using the light bulb limiter I built, because I was so paranoid about blowing up anymore output devices. When I first started working on it, the one channel was completely gone and after replacing all the parts went to fire it up and check voltages, the other channel melted down in front of me. So I was a little nervous.

Not yet as I don't have a dummy load capable of any where near full output for this amp. I'm trying to think if I know someone who has one I can go hook it up to.

The drop in current was on both channels, not just the one I just put in new outputs. They were both at @25mv with the board out, then after being back together and running for a bit I checked again and both sides were like @ 12mv. No other voltages changed, the bias on the 47ohms was still at +/-3.6v and the drain 80v. I woke up this morning thinking about it and wonder if the +/- 15v supply is acting up. I have to go back and check it out. All of the ballasts were replaced along with most of the 47 ohm and 1.5k gate resistors, also both limiter transistors (Q108,109, 208,209) and their diodes & zeners.

That business about the grounds would explain alot, although the only thing disconnected was the short jumper from the breaker to board. I swapped the long wire that goes to the switch and connected it to the board so I could have the board up and still turn it on. All three of the ac leads coming in are all on the board. Unless the +/- 15v supply that lives on the little board riding on the heatsink gets it's ground from the fan/heat sink bolt on the bottom of the chassis.

[woodyc]

How about a schematic for that little rig? Or inside pix?

There's almost nothing in the box...

[Sowhat]

I'm not sure if this is simple or not, that depends on what you have laying around in your shop. Here's the rig I use to match BJT's and MOSFETS:



These days matching is kind of an academic thing or just a double check in my opinion. Parts manufactured today are so much tighter in tolerance that usually as long as you get parts from the same manufacturing batch or tube, you're usually good to go. Many years ago I really had to match transistors and MOSFET's because the ones I would buy were all over the place. Today they are usually tighter than the published specifications. Still, I think it's good to be real certain that things are matched, especially with parallel output devices. The test's are simple and quick and they can save you a potential meltdown with some higher powered systems with quartets, sextets and even octets of complimentary power transistors or MOSFETS.

[J M Fahey]

Try this very simple one: