VAXmate PSU

[Rob Jarratt]

> -----Original Message-----
> From: Brent Hilpert <bhilpert at shaw.ca>
> Sent: 06 April 2020 00:12
> To: rob at jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
> <cctalk at classiccmp.org>
> Subject: Re: VAXmate PSU
> 
> On 2020-Apr-05, at 2:53 PM, Rob Jarratt wrote:
> > I have obtained a scope trace as you suggest. R32 is still lifted so
> > the
> > UC3842 is powered by the bench PSU, but I am using the full 240VAC (no
> > variac). The channels are:
> > 1.	Ch1. 555 timer.
> > 2.	Ch2. D19 Anode
> > 3.	Ch3. D19 Gate.
> > 4.	Ch4. Q1 Source.
> > The picture is here:
> > https://rjarratt.files.wordpress.com/2020/04/h7270-control-pulse-width
> > -modul
> > ator.png
> >
> >
> > You are correct, I have mixed up the values with R30 and R31, the
> > correct value is 15K. I have updated the schematic, and rearranged it
> > to look diagrammatically more like a sample diagram in the TI
> > datasheet for the UC3842. The updated schematic is here:
> > https://rjarratt.files.wordpress.com/2020/04/h7270-control-pulse-width
> > -modul
> > ator.png
> 
> 
> The links for pic and schematic don't seem to be correct.
> I picked up a modified 15K+15K schematic from the blog web page but
haven't
> found the scope pic.
> 

Sorry, that looks like a cut and paste error, here is the link to the scope
picture
https://rjarratt.files.wordpress.com/2020/04/h7270-primary-scr-trigger.png

I used a 100ms timebase for the capture and then "zoomed in" a bit 


> Keep in mind at a low scope timebase resolution (long timebase) (as per
your
> previous scope pics), a brief spike on R13 may not show up in the scope
display
> (too brief for the discrete per pixel display interval, but it could have
charged
> C18 enough where it will linger longer and show up on the display. DSOs
can
> present some vagaries like this. If you haven't done so, examine the
traces at a
> higher timebase resolution (it could have been recorded in the sampling,
just
> not being seen).