2 Nov
2015
2 Nov
'15
12:35 p.m.
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Mattis Lind
Sent: 02 November 2015 13:20
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Testing H7864 (MicroVAX II) PSU With No Load
2015-11-01 17:35 GMT+01:00 Robert Jarratt <robert.jarratt at ntlworld.com>:
Here is a link to what I have now http://1drv.ms/1KQkTBp
There are Eagle sch files, and PNG files for those who do not have Eagle.
I took a quick look at the schematics and I had hard times following the
schematics, but I guess I asked for it.
I did warn you! :-) :-), but thanks for looking and trying to make sense of them.
>
Maybe you can publish your schematic somwhere. It is much easier
> to come
up
> with ideas to pin point problems if we all view the same schematic.
> I
think it
> would be possible to identify the problem without comparing the
> two PSUs
at
> least in this case where there is some kind of fundamental problem.
I do intend to publish the schematics, but right now I am pretty
sure they
are:
a) full of mistakes
b) not drawn logically
c) there are a couple of areas I couldn't trace without major
surgery on the board.
Well. I guess that anything is better than nothing.
Anyway. I think that the signal is produced somewhere behind the transformer
T301 and then passes through a number of driver stages (Q405 and Q408) until
it gets to the Q301. You could check the signal on T301 and onwards. The
designation for the connector of the small boards are all
X1 while there X1, X2, X3 mentioned on one schematic so I am a little bit
confused. Then there are four small boards which presumably have Xn
connectors but I can only find three in your schematics. But this is maybe a
result of the fact the schematics are incomplete or me misunderstanding
something.
Yes, I wasn't very disciplined with the labelling. Basically on the big schematic,
called Secondary I have drawn the connectors for each of the riser boards. The text near
the connector identifies which riser board it is the connector for and the connector
schematic only has one connector, but it will always be called X1, I didn't make it
correspond to the Secondary schematic. I will fix this. Note also that the small riser on
the primary side is included in the Primary Control schematic, I didn't do a separate
schematic, which was a mistake I think.
The circuitry C403 and R407 doesn't make sense to
me. It has to be
a drawing mistake.
Thanks, I will check this. I have found lots of mistakes, I am sure this is another one.
Maybe it is a good idea to to focus on one area and to make that part of the
schematic complete. Like the part surrounding the main switch transistor. Then
try to follow the signal back through T301 and trace that part out.
Yes, I agree, the Primary Control is the one where the incorrect operation seems to be
happening, Q301 does not appear to be switching. I think this schematic is as complete as
I can make it, although it could still have mistakes....
Some time ago I got a scope on Q301, and I could see the b-e never went high enough to
switch it on. There were some very short and very low pulses every 40us, but that was all.
When I compared this to the working one, I could see a signal, still every 40us, but
definitely switching the transistor on during the cycle.
If the PWM signal is produced on the secondary side then there has to be some
small PSU providing power to this circuitry at startup so that the PSU can
bootstrap. But I cannot see where this is.
Good Luck!
/Mattis
>
> Regards
>
> Rob
>
>