RD54 Stopped Spinning

Robert Jarratt robert.jarratt at ntlworld.com
Mon Apr 6 01:05:30 CDT 2015

> -----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:
> > 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).



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