gwiley
Experienced Member
The simplest circuit in the H740 supply (PDP-8/M), the +15V output, is giving me trouble. I'm seeing ~20Hz oscillations when the +15V output is loaded above 0.5A. Q1 in both of my supplies is a 2N3791, a PNP power transistor (not a Darlington). However, the schematic identifies Q1 as part number MJ2500, which is a Darlington. I'm curious what Q1 part number others have in their H740 supplies.
I've tested all of the transistors in the +15V circuit and swapped Q1, the pass transistor, with the same part from another supply. I noticed the circuit that drives the base of Q1 has an asymmetrical impedance. Q3 pulls it high but a 10K (R38) to ground pulls it low. I tried reducing the value of R38 to 3K, and the oscillation now happens at a higher load of around 0.8A. Reducing R38 by a lot isn't a good solution though because 39 volts squared / R38_small can dissipate a lot of power. I'm thinking that swapping Q1 with maybe the correct MJ2500 is the right thing. Seems like the MJ2500 Darlington is the right choice here. With a 10K pulldown it needs a pretty high beta to drive an amp on the +15V output.
I checked Roland's post: https://forum.vcfed.org/index.php?threads/what-did-i-do-to-my-pdp-8-today.77577/page-33#post-1247620 and I see an MJ2500 in his 11/10.
Does anyone else have a 2N3791 in their supply? I wonder if there was an ECO to change this transistor and I'm just not finding it.
I've tested all of the transistors in the +15V circuit and swapped Q1, the pass transistor, with the same part from another supply. I noticed the circuit that drives the base of Q1 has an asymmetrical impedance. Q3 pulls it high but a 10K (R38) to ground pulls it low. I tried reducing the value of R38 to 3K, and the oscillation now happens at a higher load of around 0.8A. Reducing R38 by a lot isn't a good solution though because 39 volts squared / R38_small can dissipate a lot of power. I'm thinking that swapping Q1 with maybe the correct MJ2500 is the right thing. Seems like the MJ2500 Darlington is the right choice here. With a 10K pulldown it needs a pretty high beta to drive an amp on the +15V output.
I checked Roland's post: https://forum.vcfed.org/index.php?threads/what-did-i-do-to-my-pdp-8-today.77577/page-33#post-1247620 and I see an MJ2500 in his 11/10.
Does anyone else have a 2N3791 in their supply? I wonder if there was an ECO to change this transistor and I'm just not finding it.