6 Apr
2015
6 Apr
'15
8:41 a.m.
On 04/06/2015 01:05 AM, Robert Jarratt wrote:
There will have to be some electrical insulation as the
drains of the low-side transistors can't be commoned.
Make sure you select a transistor with equivalent threshold
voltage (the gate voltage where it starts to turn on).
Jon
-----Original
Message-----
From: cctech [mailto:cctech-bounces at classiccmp.org] On Behalf Of Jon Elson
Sent: 06 April 2015 01:48
To: General at classiccmp.org; Discussion at classiccmp.org:On-Topic Posts
Subject: Re: RD54 Stopped Spinning
On 04/05/2015 04:40 PM, Robert Jarratt wrote:
pop-
I tested each FET using a component tester (Peak
DCA55) and it
appeared to show that they were all good. I believe they are FETs
because the XT-1140 schematic someone posted shows FETs and the
component tester also reported them as FETs. I do wonder if one of
them is only partially working, I seem to remember that the one you
point out as having smooth ramp was indeed the one with the wobble. If
I have further trouble I will follow Tony's suggestion and swap them
around to see if the wobble moves, but detaching them from the
heat-sink looks to be difficult. Regards Rob
I'd just get some FETs that are
compatible (probably MUCH better parts
available today) and replace any suspect ones with new units. Are these riveted to the heat sink? I vaguely remember a
drive built like that.
Jon
I might do what you suggest. They are IRF521 N-Channel and IRF9521 P-Channel
parts, I'll have to find equivalents. Not knowing too much about the
significance of the main parameters, I am not sure how critical the
equivalence needs to be. For example it appears that the IRF521 has
Vdss=60V, Rds(on)=0.3ohm, Id=5A-8A (temperature dependent), while a
similarly numbered modern part I found (IRF540Z) has Vdss=100V,
Rds(on)=0.0265ohm, Id=36A .
The heat sink is not riveted on, but appears to be glued on. I have not
really tried to remove it yet; to test the parts I removed all 6 of them
(the heatsink is a single one across all 6 parts).