Hello,
I'm hoping to receive some advice about repairing my PDP 11/34a's 5V power supply. I've made some progress, but I'm still having issues.
When I received the machine, the power supply's fuse would blow immediately. I traced this to a shorted D12 and Q2.
I replaced D12 with a ues2605r, the parts list calls for a ues602r. From what I can tell, the replacement has a higher PIV at 200v instead of 100v, and a slightly higher forward voltage at 1.25 vs 930mv of the original (assuming UES602R == UES2602R). Would this be an issue?
I replace Q1 and Q2 with NOS ASI 2N5302 transistors. I also replaced output capacitors C11 and C16. I, replaced input cap C1 with the cap from a working supply.
This solved the fuse blowing, and the power supply worked with no load, and with a 1ohm load on it.
However, with a .33 ohm load, the voltage would start slowly dropping. I turn it off before it gets below 3.9V.
This seems to be related to heat, as the starting voltage will only start near 5V after the supply has been turned off for a while. Knowing this, I started looking for components that were hot. I found Q5, which wasn't hot, but was getting warm. To test, I sprayed Q5 with a can of compressed air upside down to cool it down, and immediately the voltage went back up. So, I ordered a NOS D44C12 to replace it.
This did not solve my problem, unless the new part is also bad. It seems to behave the same way as before. When I spray it, the voltage comes back up.
Any advice? I'm not familiar with power supplies, so I don't know where to go from here. I have an oscilloscope, so I can check things if anyone has suggestions. I also have a working H7441 to compare to.
Also, here is a link to the schematic on page 9 in FMPS MP00271. H7441 Field maintenance
Thanks
I'm hoping to receive some advice about repairing my PDP 11/34a's 5V power supply. I've made some progress, but I'm still having issues.
When I received the machine, the power supply's fuse would blow immediately. I traced this to a shorted D12 and Q2.
I replaced D12 with a ues2605r, the parts list calls for a ues602r. From what I can tell, the replacement has a higher PIV at 200v instead of 100v, and a slightly higher forward voltage at 1.25 vs 930mv of the original (assuming UES602R == UES2602R). Would this be an issue?
I replace Q1 and Q2 with NOS ASI 2N5302 transistors. I also replaced output capacitors C11 and C16. I, replaced input cap C1 with the cap from a working supply.
This solved the fuse blowing, and the power supply worked with no load, and with a 1ohm load on it.
However, with a .33 ohm load, the voltage would start slowly dropping. I turn it off before it gets below 3.9V.
This seems to be related to heat, as the starting voltage will only start near 5V after the supply has been turned off for a while. Knowing this, I started looking for components that were hot. I found Q5, which wasn't hot, but was getting warm. To test, I sprayed Q5 with a can of compressed air upside down to cool it down, and immediately the voltage went back up. So, I ordered a NOS D44C12 to replace it.
This did not solve my problem, unless the new part is also bad. It seems to behave the same way as before. When I spray it, the voltage comes back up.
Any advice? I'm not familiar with power supplies, so I don't know where to go from here. I have an oscilloscope, so I can check things if anyone has suggestions. I also have a working H7441 to compare to.
Also, here is a link to the schematic on page 9 in FMPS MP00271. H7441 Field maintenance
Thanks