Mesa Pulse 600 Pwr Xstr Replacement procedure

[x-pro]

10V zener + 0.7..0.8V reverse biased zener = 10.7...10.8V

[nevetslab]

The replacement zeners have arrived in our shipping dept but regrettably I now await recovery from my injuries before I can install the two pair of zeners that were never installed on the MosFET PCB. Have removed the solder from the PCB solder pads and have it ready to complete.

Miss being in the shop/lab!

[nevetslab]

2nd day back in the shop, now able to walk again and picking up the pieces of the four months of recovery since being run down.

My 9.1V zener diodes arrived days after my injury, and I just got them installed into their PCB mounting holes. Odd that there weren't any zeners in the holes for D5 thru D8. Sucked out the solder, bent the leads to fit and installed them.

I also see the Trim-1 Bias pot is at the outside end of the heat sink assembly, and is NOT that trim pot on the small daughter board that has the ribbon cable that jumps across the heat sink assembly to mate with the rear panel PCB.

No instructions on setting the bias on this output stage. So, using the output stage schematic and three voltmeters, one across the output of the amp, one across the zener diode string between the upper and lower gate busses, and the third across the source resistors (one at a time). When I began, not yet having tweaked the bias pot, bringing up the variac watching voltage across those source resistors, I was getting a lot of voltage drop, meaning the MosFET's were being turned on hard. So, turned the variac down low and began turning the bias pot counter-clockwise and watched the voltage drop across those 0.22 ohm Source resistors until I only had about 0.15V across any of the twelve Source Resistors. And, the power consumption likewise dropped way down, so that original setting for the other MosFET's that failed must have been conducting a lot more than these replacement parts I installed.

Looking across the two gate buses now, I'm seeing 0.55VDC, and idle current from the wall is only 0.33 Amps. I haven't yet connected signal to the amp to see if it passes output yet. Looking at my clock, I'm going to have to wait until tomorrow or Monday to resume this procedure so I don't miss my afternoon bus ride home.

Any thoughts on what might be a typical idle currrent thru the MosFET's that are now installed? The new Output MosFET's are Exicon ECX10N20 and ECX10P20, replacing the BUZ901P's and BUZ906P Bilateral MosFET's.

I'll know more when I get back to the shop to resume this procedure.

Onward

[g1]

Glad to hear you are back on your feet and in the shop!

[The Dude]

Yes! Great to hear you are healing up and back at it nevets!

[nevetslab]

I ended up setting the bias for an average of 10mV across the 0.22 ohm Source Resistors of the output stage, which is 45.5mA per MosFET. All were reading within the range of 9.9mV to 10.3mV. Idle current/wattage was 0.95A/85W @ 120VAC. The gate voltage from upper to lower busses measured 1.05VDC. Clipping into 8 ohms was a bit over 40V. At 40V out, I was measuring 69mVDC across the 0.22 ohm Source resistors, 314mA per FET. There was a DC offset of +250mV on the output for some reason.

Amp behaved just fine, and sounded solid with my Fender P-Bass plugged in. I did have to change the input tube, as I was getting garbled noise as the note faded out,

I suppose I could have set the bias a bit lower, but seemed fine. Main issue was turning the bias level down quickly as it was way too high for these MosFET's compared to the BUZ901P/906P MosFET.s that were installed before.