RD53 Restoration - test-drb@ccmp.vtda.org

16 May 2010

Yes "blipping" the coil with a 9V battery moves the heads. When the drive
was powered on I also measured the voltage by attaching a probe to each end
of the coil cable, in that case I measured about 1.3V only. So I am guessing
the coil is OK. Next step is to swap the boards back again.

Regards

Rob

...
  -----Original Message-----
 From: cctalk-bounces at classiccmp.org [mailto:cctalk-
 bounces at classiccmp.org] On Behalf Of Rob Jarratt
 Sent: 16 May 2010 20:13
 To: 'General Discussion: On-Topic and Off-Topic Posts'
 Subject: RE: RD53 Restoration

 Thanks, never thought of the obvious, will dig out a 9V battery and try
 that.

 Thanks again.

 Regards

 Rob

  -----Original Message-----
 From: cctalk-bounces at classiccmp.org [mailto:cctalk-
 bounces at classiccmp.org] On Behalf Of Tony Duell
 Sent: 16 May 2010 20:03
 To: cctalk at classiccmp.org
 Subject: Re: RD53 Restoration

 Many thanks for the reply. I am not an electronics expert  unfortunately, but

 Alas sometimes I forget that not everyone had a mis-sepnt  shildhood...

 > will look at your suggestions. One thing I did (perhaps wrongly)  was
  measure
 > the voltage with one probe connected to the casing, not to the  other
  pin on

 This problaby is not the right thing to do. The point is that you  need
  to
 be able to pass a crurrent through the positioner coil in either
 direction (one way will exert a force to move the heads towards the
 spindle, the other way will exert a force to move the heads away form
 the
 spindle). Now if you had one side of the coil grounded (same voltage  as
  the chassis) yoy would need power supplies of
both polarities wrt
 ground
 to be able to do that.

 What is more commonly done is a circuit based on what is called a  'full
  H
 driver. Basiecaly it means both sides of the load (the coil in this
 case)
 can be driven. If one side is pulled towards ground tand the otehr  side
  to, wayu, +12V, the heads moce one way. But if
the first side is  taken
  to
 +12V and hte second side to ground they move the other way.

 If oyu measure the voltage wrt ground on one end of the coil you are,
 effectively, only testing half the circuit. There could still be a
 fault.

 > the cable. I will try across the two pins to see what that is like.  I
  don't
 > have a DC supply I can use to "blip" the coil, unless I can press 
the
  PSU

 Sure you do. I bet you'd get some movement using a small battery. P

 > from my MicroVAX II into service for this? Is there a danger I  might
  pass
  too much current and burn the coil? 
 There is, which is why you 'blip' it. Actually, a battery is probably
 the
 safest, a primary (non-recharageable) 9V battery is not going to  supply
 > enough current to do much damage.
 >
 > -tony 