5 Apr
2020
5 Apr
'20
11:12 p.m.
-----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:
haven't
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 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).