Hi Chris,
Thanks for the info. Unfortunately I have already ordered the
IRFP460 replacement FET's from an electronics supplier -- they have a
minimum $ order so I'll wind up with a LOT of transistors and fuses
(in case anyone else runs into the same problem, contact me for a
good deal on these items...).
Homier is sending a replacement board but it may take up to 2 weeks
so I'll probably try to repair the present board in the interim.
I found a schematic for the Grizzly 7x12 in the 7x10 group photos
section in a folder titled ESC. This confirmed what I had determined
from looking at the ckt board: the output FET's are in parallel but
they do not use source resistors to ensure equal current sharing. My
theory on why my unit failed is that this particular board has 2SK790
FET's from different manufacturers; I suspect that the device
characteristics were sufficiently different that they didn't share
the current equally causing the device which took the bulk of the
current to eventually fail. Others who experience a similar failure
might check to see whether both devices have the same manufacturer's
logo as a check on my supposition.
Also, I had gotten into the habit of putting the fwd/rev switch into
the center=off position when fiddling with the chuck, this to ensure
that the chuck wouldn't move in the unlikely event of a failure
occurring while my hands were in harm's way. I can't decide whether
this may have contributed to the failure. Based on examining the
schematic I will in the future use the big red button instead of the
fwd/rev switch to ensure the chuck can't move -- don't know whether
this will help but it certainly won't hurt.
Further, I intend to add some holes in the case below the heat sink
and to add a spacer to ensure the clear plastic shield is held away
from the heat sink so air can circulate past the sink. These holes
will also help any metal chips which wander into the electronics
enclosure find their way back out. Again, this may not help but it
won't hurt.
Having had my favorite toy disabled by an electronics failure I'm
trying to avoid a repetition.
John
--- In 7x12minilathe@..., Chris Wood <chrisw@s...> wrote:
John:
It sounds like the MOSFETs have blown. We have replacements that
are higher
capacity.
ProductID=1221
Regards,
Chris Wood
LittleMachineShop.com <www.littlemachineshop.com>
The premier source of parts and accessories for mini lathes and
mini mills.
221 E. Walnut St. #110, Pasadena, CA 91101
(800)981-9663 * Fax (626)584-5844
-----Original Message-----
From: John [mailto:moran03@e...]
Sent: Sunday, March 23, 2003 8:32 AM
To: 7x12minilathe@...
Subject: [7x12minilathe] 7x12 Blows Fuses ?
My 7x12 which is now 2.5 months old popped its fuse shortly after
startup Friday, just as I was beginning a parting cut at low
speed.
I installed the replacement fuse and , with the speed control in
the
OFF position and the tool backed away, the lathe started as if it
was
at full speed and the fuse popped again in less than a second.
This is a relatively new unit so I called Homier and they promised
to
call me back on Monday. I'm just wondering if this is a common
problem; my understanding is the Homiers use an SCR based
controller
which is less prone to failure than the MOSFET types. My ckt board
has a sticker with FC250J/110V and the large power devices are
identified as K790 -- the board uses surface mount devices except
for power handling which are thru-hole. Anybody have more info on
the Homier controller?
Up until this happened I was very happy with the controller since
it
allowed speed to be reduced to 12rpm, slower than most references
in
the 7x10 group specify. I did notice that when the speed control
was
in the OFF position that rotating the knob slightly(as alowed while
OFF) made the chuck would move slightly and groan a bit. Since it
seems to be live even while OFF I always turn the wall outlet off
after use.
John
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