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No it's not shorted. Because I'm testing the PSU standalone, without the drive connected. But the answer might be correct anyway.
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It's the marked resistor (R2) that gets hot. In the schematics it's specified to be "Fixed res. 25W 100Ohm". Since it's sitting in parallel with the bulk cap there will be 38V over it.
P=U^2/R gives P=38^2/100 -> P=14.4W
I'll guess that that will generate som heat at least.
If you are checking the power supply removed from the load R1 and R2 will be equal temperature. Do you have your transformer strapped for 240 Volts?
R1 gets less warmer. It's on the 18V rail, power dissipation is just less than 4 Watts. The transformer is strapped for 117V, and I do have a 117V power outlet at my work bench. Makes life much easier when restoring old DEC machines.
I got the drive in working condition, running nicely with a TD8E board in my PDP-8A. But I have used an external power supply until now. Now I have modified the power supply a bit, I put an external 50W resistor on a heat sink outside the drive (non destructive modification, it's possible to rebuild the drive in original condition). But when I measure the 38V rail I get about 43V! The 18V rail is just right!
Is there anyone with a TU56 drive that can measure those two rails so I have something to compare with?
I'm also looking for better documentation on the H725 power supply, in the TU56 maintenance manual there isn't much. But under the electrical specifications the supply voltages are 115/220V AC and +5V or +10V and -5V. On other places in the document it says 110V and I have also seen 117V.
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