Starting up a PDP 8/e

[silcreval]

Its that time of year, and my latest project is to try and restore a pdp 8/e to its former glory.

Hardware is all clean, but the first job in hand is to get he PSU working. This was powered up 4-5 years ago without issue, but I want to be careful.

I've removed all cards from system, so looking for recommendations to start things up. I dont have a load test card, which would be ideal at this point. If anyone has any projects they can point me to I'd be interested in making a few of those. Will post some pictures once its on the bench.

 

[djg]

8/e is linear power supply so doesn't need minimum load for testing. I have a selection of power resistors I've gotten from hamfests that I hook up as needed for load testing. When I can I also like to measure the voltage drop across the connections in the power wiring since resistance can increase with age.

 

[thunter0512]

If you like you could reform the electrolytic capacitors in the power supply by isolating each from the rest of the power supply circuit and then using a current limited power supply slowly ramp up the voltage starting at 1V in steps of 1V and wait until the current drops to zero (or very close say 1 mA) before each next step.
Keep increasing the voltage until you reach the rated voltage of the capacitor.
I usually limit the current to just 10 mA, but 50 mA is fine too. I wouldn't go higher.
If you notice prolonged (> 1 minute) current at the set limit then the capacitor is faulty and should be replaced.

 

[silcreval]

If you like you could reform the electrolytic capacitors in the power supply by isolating each from the rest of the power supply circuit and then using a current limited power supply slowly ramp up the voltage starting at 1V in steps of 1V and wait until the current drops to zero (or very close say 1 mA) before each next step.
Keep increasing the voltage until you reach the rated voltage of the capacitor.
I usually limit the current to just 10 mA, but 50 mA is fine too. I wouldn't go higher.
If you notice prolonged (> 1 minute) current at the set limit then the capacitor is faulty and should be replaced.

Thanks for that tip - I may give that a try.

 

[bobaboba]

The 8/e psu has an under-voltage supply trip IIRC so after any offline capacitor reforming ramping up the overall supplies needs it to be disabled. After several 8/e psu refurbs I’ve yet to find a large electrolytic which needed replacement - good quality components eh? In fact the psus overall seem pretty robust and long lasted.

 

[ftcnet]

The 8/e psu has an under-voltage supply trip IIRC so after any offline capacitor reforming ramping up the overall supplies needs it to be disabled.

That's a new one to me. The power supply does have over-voltage protection on the +5V in form of a SCR. Perhaps you're thinking of the the delayed assertion of PWR OK H ? That signal stays L until the DC voltages stabilize at their nominal values. The PWR OK H logic is relevant once you have some modules installed, but not when you're bringing up the power supply.

 

[thunter0512]

The 8/e psu has an under-voltage supply trip IIRC so after any offline capacitor reforming ramping up the overall supplies needs it to be disabled. After several 8/e psu refurbs I’ve yet to find a large electrolytic which needed replacement - good quality components eh? In fact the psus overall seem pretty robust and long lasted.

Perhaps you misunderstood what I wrote. The 8/e power supply's electrolytic caps are reformed individually and in isolation from everything else by lifting one of the screw connectors on the top of the capacitor to be reformed. Nothing else sees the slow ramp-up of the voltage on the now isolated cap.