Power Supply: Fixed

By -
Testing continued on the power supply. I borrowed a few 4Ω 255W resistors from the Power Systems lab so I could try and load test the supply.
     With four of these resistors wired in parallel we get 1Ω. Because of Ohms law this means we will get a 5A current from the 5V output. This would give about 25W of power, (P=V*I). 900W is more than enough to satisfy this.
The result of having the load was a drop in the output voltage. The 5V line dropped from its normal 5.07V to around 4.96V. I hooked the oscilloscope back up to measure the +8V and +5V lines as they power up.
Compared to before this was a much better graph. There is a much lower spike in the +5V output, it jumps to 5.8V and then quickly settles down to 4.8V to 4.9V (according to my multimeter). The power supply appears to be working just fine now. No more random tripping and no more burning resistors.

Next on the agenda: Powering up the machine and running a few basic tests. Posting a list of installed options.


Edit: I forgot to mention when I posted this that one other thing I did was reform the capacitor C5, which is the filter cap for the +8V line. I think that definitely helped to smooth out the +8V voltage spikes seen in the previous graphs. I have attached the schematic below for those interested. C5 is located in the top middle. You might need to right click and hit "view image" to zoom in.


Comments

  1. UnknownJune 7, 2017 at 6:08 AM

    The Living Computer Museum in Seattle has a lot of running DEC gear and may be of help in your restoration. They have extensive restoration experience. http://www.livingcomputers.org/

    ReplyDelete

Post a Comment

Popular posts from this blog

Power Supply: The Mystery Continues

By -
Image
I continued testing the power supply. It would work most of the time it was powered on and give the correct voltages, but then sometimes when it was powered it the +5V circuit breaker would trip. I managed to capture this on the oscilloscope The peak voltage was 5.16V and then the breaker trips and it goes down to around 1V.  Time to keep investigating the G824 module to see if there is something wrong with it. (Michael from RICM has been very helpful with troubleshooting so far).  I borrowed a nice mixed signal scope from the EE shop. It blows my HP 54200A out of the water. The +5V regulator is supplied by the +8V line. So Michael suggested looking at both of the lines as they power on to see what they look like. Not Tripped Tripped Above are the results of this. I measured the peak voltages for both lines when they tripped and didn't trip. The +5V line averaged around 6.6V when it didn't trip and averaged around 12V when it did trip. The
Read more

Serial and Other Diagnostics

By -
Image
Warren came up from South Dakota to help us get the serial ports working and do some more diagnostics. Sunday night he made a cable to connect the serial card to the serial port on our computer tower. We are using Warrens modified copy of MTTTY (Multi Threaded TTY) as our TTY emulator. It is an old Windows program so we are running it in Wine and it has been working pretty well so far.  After On Monday... We toggled in two simple programs. One was a send/receive program and the other was a send. The send would take the data input on the right switches and display the character or number associated with that number in MTTTY on the PC. The code for that is as follows: 0200 / 7604  LAS 0201 / 6046  TLS 0202 / 6041  TSF 0203 / 5202  JMP 0204 / 5200  JMP  The LAS instruction loads the AC with the switch register. TLS Sends the AC to the serial port. TSF will skip the next line if the flag is set. JMP will jump to 202 and then the next JMP will jump to 200. This program w
Read more

Setting up our new Raspberry Pi0-based Flip Chip Tester (FCT)

By -
Image
  Today's task: get our Flip Chip Tester (FCT) up and running. For those who don't know, Flip Chips (in DEC parlance) are small circuit boards with a handle opposite an edge connector, about the size of a 3x5 card, that have a small number of discrete components on it. Each Flip Chip provides a specific set of components to a machine. The one pictured here is an M617, which provides six four-input NAND gates (two per IC). The Flip Chip would be inserted into a slot, and a wire-wrap backplane would connect it to power, ground, and upstream and downstream components. One of my favorite things to explain to students is how the PDP-12's CPU can quite literally be repaired. When FCs go bad, it's usually because some IC on the FC has failed. Of course, you can just swap out the whole FC (if you have a spare). However, they can also be easily repaired if you know what to replace and you have equivalent IC packages. Enter the Flip Chip Tester. Before he died, Warren Stearns pro
Read more