8 Nov
2015
8 Nov
'15
2:08 p.m.
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Robert
Jarratt
Sent: 02 November 2015 23:18
To: 'General Discussion: On-Topic and Off-Topic Posts'
Subject: RE: Testing H7864 (MicroVAX II) PSU With No Load
I have been doing some further checking of the schematic for the primary control section,
because that is where the failure was, and I have found some significant errors. I have
been over it as carefully as I can and hopefully it is now much more accurate. I have also
made the connections to the primary side of the transformer correct, although as I remark
on the schematic, I don't know how the transformer is wired in relation to the
secondary outputs. Regarding the apparent short between the base and the emitter of Q301
(as measured, not as drawn - I have corrected the drawing), seems to be just the low value
of R410, the resistance I measure b-e is 39R, which is exactly the marking on this
resistor. So I think this would explain that. One other thing worth noting is that there
is evidence of heat around D408, but it tests OK (in circuit).
The updated Primary Control schematic is still here: http://1drv.ms/1KQkTBp
Regards
Rob
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of
Mattis Lind
Sent: 02 November 2015 21:44
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Testing H7864 (MicroVAX II) PSU With No Load
2015-11-02 22:05 GMT+01:00 Robert Jarratt <robert.jarratt at ntlworld.com>:
Thanks for spotting the drawing mistake. I have noted an apparent short
before, and I have some notes I made at the time, but that was so long ago
(almost a year!), and the notes don't seem to conclude whether I found the
short or not. I think I may have put it down to some of the transformers at the
time, but I can't be sure. I will investigate this further as this is quite
promising,
but it looks like I will need to desolder some components...
Regards
Rob
I have just uploaded all the corrections. So the connector names on
the risers should now correspond. I have also corrected the area on
the primary control module around C403 and R407.
Regards
Rob
Checked the new one. There are still something weird. The base of Q301
it is short circuited to the emitter (which if that is true would of
cause your problem). But I think this is a drawing mistake. The base
signals goes to the
D405 diode which is connected to emitter Q405 and emitter Q406 which
is then connected to emitter Q301... Please check this again. Maybe
there are a short circuit component that makes you trace it wrong (if
you do it by ohm-meter)
/Mattis