• Please review our updated Terms and Rules here

Restoring a DEC PDP-11/05

wh3016

Experienced Member
Joined
Jul 20, 2022
Messages
50
Location
Massachusetts
Yes. It could be a reason for it not working. You have pullups on the M930. That is why they are high in the backplane. You somehow need to figure out where those signals is cut.

DEC used purple for DC LO and yellow for AC LO. Check the continuity from the PSU for those wires.

It can be that the backplane somehow is modified. It is hard to tell.
I have continuity for both AC and DC LO the connector on the PSU all the way to the backplane.

Looking back, I only had one of the M930’s plugged in to the backplane. Perhaps I’ll take another look and see if having both unibus slots makes a difference.
 

MattisLind

Veteran Member
Joined
Sep 30, 2013
Messages
1,019
Location
Stockholm, Sweden
A second M930 should not be necessary. A short unibus works well with just one M930.

I can ot understand how there can be continuity while at the same time you meassure different levels at different locations of the same signal.
 

AK6DN

Veteran Member
Joined
Aug 23, 2010
Messages
1,075
Location
Silicon Valley USA
A second M930 should not be necessary. A short unibus works well with just one M930.

I can ot understand how there can be continuity while at the same time you meassure different levels at different locations of the same signal.

Could be a connection that is poor and has a high(er) resistance. 'Continuity' at very low current may show ok, but may cause a voltage drop across the junction.
 

MattisLind

Veteran Member
Joined
Sep 30, 2013
Messages
1,019
Location
Stockholm, Sweden
Yes. That could an explanation. The PU on the M930 is around 500 ohms as far as I remember. So getting a measurment of 3.5 and 4.11V indicate a resistance along the way in kOhm range. Definetely worth investiagting further.
 

wh3016

Experienced Member
Joined
Jul 20, 2022
Messages
50
Location
Massachusetts
Yes. That could an explanation. The PU on the M930 is around 500 ohms as far as I remember. So getting a measurment of 3.5 and 4.11V indicate a resistance along the way in kOhm range. Definetely worth investiagting further.
Could be a connection that is poor and has a high(er) resistance. 'Continuity' at very low current may show ok, but may cause a voltage drop across the junction.
Honestly given my experience with vintage computers, I wouldn’t have considered oxidation and corrosion on things to be an issue. I suppose when you take into account that there was at one point a mouse living inside, (I found a whole peanut in the power supply) the idea of corrosion seems more plausible. I had vacuumed and wiped down the inside and card slots with denatured alcohol, though I never checked the slots that were populated.

Usually when I clean out things of this nature, I apply the alcohol to a cotton swab, but with a bunch of little tabs inside the card slot, I find they just tend to get ripped up. I’ll pick up a can of contact cleaner and report back.

I’m thinking of taking another look at the AC DC LO’s. It seems odd that I can get a measurement at one place and not the other. Any advice on this?
 
Top