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Live from VCF West 2017 8M Power Up

jhalbrecht

Experienced Member
Joined
Mar 25, 2015
Messages
93
Location
Mesa, Az
I'm exhibiting some S-100 equipment at VCF West 2017.

Earlier today a couple nice guys taught me a bit about my Ebay pdp8m purchase that I brought along hoping for just such a meeting. Jack and ?Vince?. We got bold, pulled the cards and checked the power supply during the show Saturday morning.

Re assembled after borrowing a hex wrench from the VCM to remove the knob.

Hey it powered up, works a bit but when I set addresses to zero and pressed 'dep' the RUN light came on and stayed on, can't 'halt'. Jack said something about a 5v sense line that may be sensitive to ripple?? Any suggestions?



PDP 8M First Test VCF West 2017.jpg
 

vrs42

Veteran Member
Joined
Nov 24, 2009
Messages
655
Location
Beaverton, Oregon
Hey it powered up, works a bit but when I set addresses to zero and pressed 'dep' the RUN light came on and stayed on, can't 'halt'. Jack said something about a 5v sense line that may be sensitive to ripple?? Any suggestions?

Well, I'm back from VCF-W and slowly catching up on email and such.

I think Jack's point about checking the supplies for ripple and making sure DC_OK is stable
are spot on. Without those in the "known" category, we can't be sure what we are seeing.

Assuming the supply is solid, I've been thinking about what else might be wrong. The
drawing at
http://bitsavers.org/pdf/dec/pdp8/pdp8e/PDP-8M_Engineering_Drawings_Nov73.pdf
isn't too bad, and on D-CS-5409668-0-1 sheet 1 of 2 (page 49), you can see that
the HALT and SS switches should assert STOP_L. Either they aren't doing that due
to a problem with E9, E13, or (more likely) E10, or the CPU isn't halting when told to.

There might also be some odd interaction with SW, so try HALT and SS with SW up
and also with it down.

I'll also see if I can follow the CPU prints and see what could be wrong there.

Apparently I have not yet drawn the schematic in Eagle. Can you do scans of your
FP board, or can someone point me to some?

Vince
 

jhalbrecht

Experienced Member
Joined
Mar 25, 2015
Messages
93
Location
Mesa, Az
Hey it powered up, works a bit but when I set addresses to zero and pressed 'dep' the RUN light came on and stayed on, can't 'halt'. Jack said something about a 5v sense line that may be sensitive to ripple?? Any suggestions?


View attachment 40178

I'm back from VCF now. So many projects, so little time! Wish I had found this box of extras for the PDP 8m before I left. Extender board, top connectors, book of engineering drawings, and huge blueprints and a couple spare M8330 (two different rev levels - note crystals) "M8330 KK8E Timing board, replaces M833" according to the DEC Omnibus Modules list by Doug Jones.

So next task is to find the DC_OK. Is that a Boolean signal? Or a different 5v supply to check for ripple?

I've got three pictures here. Boads and placement after reassembly at the show. M8330 board rev details and the extra goodies I wish I had brought to the show.

PDP 8M Board Configuration After VCFW 2017.jpg

PDP 8M M8330 rev detail.jpg

PDP 8M SN PR03 08004 Accessories documentation.jpg
 

jackrubin

Veteran Member
Joined
Mar 12, 2004
Messages
846
Location
Chicago, IL and Buchanan, MI
You'll find all the voltages on male Faston connectors at the back of the backplane. At the show we measured +5 and +/- 15 but we couldn't look at ripple. The center gray wire (marked V2) is DC-OK. Make yourself a couple of 6" extender wires with 1/4" female connectors and you can plug them right onto the power inputs to the backplane so you can get a good scope connection without worrying about the possibility of a probe slipping (don't ask me how i know, yadayada).

Jack
 

jhalbrecht

Experienced Member
Joined
Mar 25, 2015
Messages
93
Location
Mesa, Az
Made the cables. I always see a lot of debate on how to measure ripple. Here is an image from my old owon scope. AC coupling. Looks like about 200mv ripple across ground and V2. Frequency is a bit unstable at 4.1khz instability on the 2 and 3rd digits.

20140309_384631.jpg
 

vrs42

Veteran Member
Joined
Nov 24, 2009
Messages
655
Location
Beaverton, Oregon
Here's something that should help you make use of the extender card:
http://bitsavers.trailing-edge.com/pdf/dec/pdp8/pdp8e/Omnibus_legenda.pdf

For instance "STOP L D2S" means that STOP_L can be observed at the
"S" position of side 2 (solder side) of the "D" edge connector (left, as
viewed with the component side toward you, right if you are looking at
the solder side). (That means you needn't worry about how to connect
to pin 10 of IC E10.)

Pins (and edge connectors) are labeled ABCDEFHJKLMNPRSTUV from
right-left on the front, or left-right on the back.

With any luck, the female connectors of your extender card should have
lead bends that you can hook a probe to.

Vince
 
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