Aguilar DB751 failure

[nevetslab]

One of our Aguilar DB 751 Bass amps arrived from our rental inventory with the note 'Won't Turn On'. Pulled the cover....and oh, yeah....I'd agree with that statement.





The T16A Mains fuse blew, of course. The outputs are Exicon 20N20 & 20P20 Lateral MosFET's, in TO-3 cans, socketed. I know I can turn regular Power MosFET's on and off with my Fluke 8060A DMM in Semiconductor Test mode, which has over 8VDC behind it's 1mA current source. I can't get these to turn on or off, whether plugged into the PCB or removed.

I was likewise somewhat unnerved by seeing the outside wrappings of their Toroidal Power Xfmr burned away from the heat of the nearest radial Power supply cap mounted on the PCB. While it wasn't physically touching the xfmr, there was enough heat to melt the wrappings. After removing the power amp module, and checking the bridge rectifier (which is a common failure in these amps), which checked out ok, I replaced the mains fuse and plugged it in, ran up the variac watching for any arcing on the xfmr as well as sudden current surge to be shown on the power analyzer. All came up nominal, so other than the burn area on the power xfmr, looks like the chassis survived.

Aguilar owns the rental gear of theirs in our rental depot, so I'll contact them with my findings, and see if they just want to replace the amp module (having trace failure on Q106 & supporting parts), or put it all back together and return the entire amp.

Is there some trick to doing basic go-no go testing on Lateral MosFET's?

Exicon ECF20N20 Lateral N-Ch Mosfet.pdf

Exicon ECF20P20 Lateral P-Ch Mosfet.pdf

[Jon Snell]

They normally fail short circuit. To test these, you need to charge the gate.
Hold the MosFet by the case or the tab but don't touch the metal parts of the test probes with any of the other MosFet's terminals until needed.
First, touch the meter positive lead onto the MosFet's 'Gate'. Now move the positive probe to the 'Drain'. You should get a 'low' reading. Connect the gate to the source and drain to source should return to open again.

[J M Fahey]

I know I can turn regular Power MosFET's on and off with my Fluke 8060A DMM in Semiconductor Test mode, which has over 8VDC behind it's 1mA current source. I can't get these to turn on or off, whether plugged into the PCB or removed.

Try this.

Originally intended to match Vertical MosFets (IRFP250, etc.) but will work fine with Laterals. (Excn , BUZ*** , etc.)
All these are enhancement types, only Verticals will turn-on with around 3.5V Vgs; Laterals typically with less than 1Vgs

IF there is somebody alive inside that case, it WILL turn-on, you have 30V and up to 15mA available, and IF device is shorted, R1 limits that.

Just in case, make R1 1W

[nevetslab]

Aguilar's Tech Support replied to my email & photos, describing the damage found, and they're sending me a shipping box/inserts and shpg label, wanting to have the amp back as is, so I won't get a chance to try the two methods Juan and Jon provided. Unless I get thru the new batch of repairs before receiving the shpg box for the Aguilar DB751. Thanks, mates!

[g1]

They normally fail short circuit. To test these, you need to charge the gate.
Hold the MosFet by the case or the tab but don't touch the metal parts of the test probes with any of the other MosFet's terminals until needed.
First, touch the meter positive lead onto the MosFet's 'Gate'. Now move the positive probe to the 'Drain'. You should get a 'low' reading. Connect the gate to the source and drain to source should return to open again.

Negative probe stays on source, yes?

[nevetslab]

Negative probe stays on source, yes?

I was wondering about that myself

[Jon Snell]

Negative probe stays on source, yes?

That depends how your DVM is wired but one way yes. You will know straight away.

[J M Fahey]

Ok, follow Aguilarīs instructions on that one, after all they literally *own* it , but just for kicks/practice do the *very easy* test (meter has just *one* component ) on any random Mosfet (any kind) you have lying around, in your junkbox, suspect parts box, even fresh out of the package ones, just to get the hang of it.

Post results.