H7826 PSU, No Signal On Transformer Outputs

Rob Jarratt robert.jarratt at ntlworld.com
Mon Nov 6 14:14:18 CST 2017

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Brent
> Hilpert via cctalk
> Sent: 06 November 2017 19:44
> To: General Discussion: On-Topic and Off-Topic Posts
<cctalk at classiccmp.org>
> Subject: Re: H7826 PSU, No Signal On Transformer Outputs
> On 2017-Nov-05, at 9:14 AM, Rob Jarratt via cctalk wrote:
> > I have moved on with the work on the H7826 PSU in the TURBOchannel
> Extender.
> > However I have found that there appears to be no signal at all on the
> > output windings of the transformer. I have posted all the details of
> > what I have been doing here
> > http://robs-old-computers.com/2017/11/05/repair-progress-on-the-h7826-
> > power- supply/, and I would welcome any suggestions as to what might
> > cause this?
> "I then checked the primary side of the transformer and I could see a
> You don't describe the nature of the signal, but regardless, seeing a
> signal at some point on the primary is not a guarantee that current is
> through the primary, current being necessary for induction and getting
> anything out of the secondaries.

I saw alternating signals on a number of the inputs to the transformer, but
I didn't note which pins at the time :-( Surely all you need is an
alternating voltage to produce an output on the secondary side?

> It depends on the primary side driver arrangement, (going by the H7861
> schematic DEC used some funky arrangements, is the 7826 schematic
> available?), For example you might see a voltage signal from the high-side
> driver, but the low-side driver could be faulty.=

I have not looked for a schematic, but I severely doubt there is one, in
this case the markings on the board don't even have component labels! I
could try to trace it out, but it is quite difficult with this board.



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