Aleph J illustrated build guide

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#4,201
Extreme_Boky said:
Don't know... but it is a quick check.
Click to expand...

Will do, but The examples of that problem I've seen usually result in big problems, here it plays and measures correctly. Just one cold side.

I used the Deluxe 4 chassis, predrilled and checked before mosfets originally tightened . But, was hot before "fixing" maybe a keratherms got damaged?

Sounds like I will have to wait for Zenmod's return to figure this one out.

Russellc
 
#4,203
Russellc - the voltages across the source resistors that you posted looked "off" to me. You posted numbers like 44.x while I was expecting to see numbers like 0.44x.

Check your mosfets for shorts after fiddling with the mounting bolt. Use your DMM to check for continuity between each of the three pins and some bare spot on your sink. No pin should show continuity with the sink - if it does, you have a short and need to remove and remount that mosfet. Depending on how the Aleph protection circuit functions, a short at the mosfet may not cause a fuse to open.

I've had a few problems with shorting the mosfet to the sink over the years. Usually, I'd not de-burred the mounting hole sufficiently after drilling/tapping and some lip would pierce the silpad and short the mosfet. One time it was a nearly microscopic aluminum shaving that got stuck in the silpad that caused a short. Now I try to be extra careful with drilling and use ceramic insulators and goop instead - they also perform just as well as Keratherm and cost less.
 
#4,204
Eric said:
Russellc - the voltages across the source resistors that you posted looked "off" to me. You posted numbers like 44.x while I was expecting to see numbers like 0.44x.

Check your mosfets for shorts after fiddling with the mounting bolt. Use your DMM to check for continuity between each of the three pins and some bare spot on your sink. No pin should show continuity with the sink - if it does, you have a short and need to remove and remount that mosfet. Depending on how the Aleph protection circuit functions, a short at the mosfet may not cause a fuse to open.

I've had a few problems with shorting the mosfet to the sink over the years. Usually, I'd not de-burred the mounting hole sufficiently after drilling/tapping and some lip would pierce the silpad and short the mosfet. One time it was a nearly microscopic aluminum shaving that got stuck in the silpad that caused a short. Now I try to be extra careful with drilling and use ceramic insulators and goop instead - they also perform just as well as Keratherm and cost less.
Click to expand...

You get the prize. Yup, just when it appeared I was total wacko, it's even worse.....

One meter, on the hot side reading correctly, .41v. Other side with different meter, 40 mv! No wonder it was cold!
One meter auto range, one not.
All is now right with world of AlephJ! Geeze!

I can hardly believe it played with such low bias.

Russellc
 
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#4,211
How could ANYONE...? or set their DMM to read current instead of voltage across a resistor... oh forget it 😀

I never mind making laughable errors as long as I learn... and I don't have to desolder. I hate desoldering...

:cheers:

Edited to add - a fun but educational thread could be... Errors we've made... :rofl:
 
#4,212
ItsAllInMyHead said:
How could ANYONE...? or set their DMM to read current instead of voltage across a resistor... oh forget it 😀

I never mind making laughable errors as long as I learn... and I don't have to desolder. I hate desoldering...

:cheers:

Edited to add - a fun but educational thread could be... Errors we've made... :rofl:
Click to expand...

One of my personal favorites Is hooking up the meter, only to discover the leads I hooked up go to a different meter than the one I'm looking at!

Russellc
 
#4,214
Russellc said:
One of my personal favorites Is hooking up the meter, only to discover the leads I hooked up go to a different meter than the one I'm looking at!

Russellc
Click to expand...

Yup ... did that more than once. I often use three or four meters at once, so have to make a real effort to always reference the correct meter to the action ...

So I usually set them up in a pattern to make it easier to remember.

I also occasionally fiddle at the wrong pot, wondering why nothing happens... only to find that I de-adjusted another parameter this way 🙄
 
#4,216
Hi guys, I've received a bunch of IRFP240PBF for my Aleph J build but have a question regarding matching them. The only power supply I have to hand is a 19v laptop supply so what value/wattage resistor would I need to match Vgs for an Aleph J? Also, should the FETS be mounted on a heatsink for matching?
I tried 20 Ohm 3W as mentioned in another thread but the resistor got very warm!
 
#4,217
matchingfets.jpg

Here's the basic diagram that I found. As I only have basic electronics knowledge this is the only one that made sense to me!
 
#4,218
I used three resistors in parallel to achieve 24R with a 30w rating and my resistors STILL got hot while testing each mosfet for only 30s. Ideally, you want to test at the same voltage and current levels that they will see in the completed circuit, but I found that mosfets that match at some power dissipation level also tend to match well at other power dissipation levels.

Thus, the final measure I made for my in my big Aleph-X amps was 15v at 0.5A current. In the finished amp, they run at 21v and 1.5A and they match nearly exactly with one another.

In my Aleph-X page (linked below) search on "Matching Mosfets" and you'll find lots of details.
 
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