Memphis MCD16-4KW rail voltage???

[Deadly Sones]

I'm working on a couple memphis 4K's. They both arrived with several (almost all) the transistors removed. Both input and output.

I found a bad A1023 near the driver board so I replaced both of them for that side. I checked the C1027's and they were good. I powered the amp up with 15 amp fuse and it turned on no protection. The transformers were making a slight pulsing noise and the gates were pulsing 2.?vdc and down.

I installed all the power transistors and powered it back up. The transformers are now quiet, it powers up fine on a 15 amp fuse.

With the output resistors removed I checked the output gate pads. One side I get -115.5vdc and the other side I only get .257v....

Here is a pic of the output section. What should I be looking at next? I should have equally + and - voltage between the rails should I not?

Thanks in advance.... I think she is almost reborn...

[Perry Babin]

That sounds normal for the gates. One output source it tied to the negative rail. The gate will be about 5v above that. The other source is swinging from rail to rail and the gate is about 5v above the average source voltage. For the output that has its source swinging from rail to rail, the drain will be on the positive rail.

Do you have a scope?

[Deadly Sones]

Unfortunately no scope yet Perry.... its still on the table if there is something I can check.

[Deadly Sones]

Tested the drain of the other rail... I get +117.9vdc on one and -115.5vdc on the other... NICE!!!

Is it safe to install the output transistors?

[Perry Babin]

If you measure the DC voltage from gate to source on each bank of outputs and only one bank reads more than about 1v, it's likely safe. If you get something significantly different, post the results.

[Deadly Sones]

All locations read 0.000vdc from gate to source.

[Perry Babin]

I'd expect half of them to be on. If they're all off, it should be safe to install the outputs. Clamp them to the sink before applying power.

[Deadly Sones]

Awesome!! Thanks Perry. Unfortunately I only have a few IRFP360's (I need to order 12 new ones per amp)... can I install a couple good old ones just to test/ensure working correctly? where should they be located? Does it matter?

[Perry Babin]

I don't know if it will tell you definitively that the drive circuit is working properly but if it works with a few good FETs, it's likely to work when you install all of them. Check the used FETs to confirm that there is absolutely no leakage from the gate to either of the other legs (meter on ohms) before installing them.

[Deadly Sones]

K, will do... thank you sir... I have like 9 of them around here so, I could install 8 of the 12... I'll double check that they are good before installing.