In the end, I went with the mid-90's Motorola parts because the price was right and the seller got me matching date codes (so I could try and get two as closely matched as possible).
In the end, I went with the mid-90's Motorola parts because the price was right and the seller got me matching date codes (so I could try and get two as closely matched as possible).
I just buy a few extras and grade them myself. I recently did this with 2N5886G transistors (80V 25A 200W 4MHz TO-3) and out of four, two matched to within a couple millivolts at the expected operating current. The original design did not even use emitter ballasting on the parallel transistors so I added that as well when I repaired it.
I just buy a few extras and grade them myself. I recently did this with 2N5886G transistors (80V 25A 200W 4MHz TO-3) and out of four, two matched to within a couple millivolts at the expected operating current. The original design did not even use emitter ballasting on the parallel transistors so I added that as well when I repaired it.
That's exactly what I did as well. I needed two, so bought four (with matching date codes to up my odds) and then used my cheap Chinese transistor tester to get the Hfe and base voltages, followed by a run through my homemade curve tracer to verify the current characteristics.
I ended up with two that were about a 1% match.
Those will read as shorted as they have jumpers across them at the back panel. Those protect the sense inputs.
Those will read as shorted as they have jumpers across them at the back panel. Those protect the sense inputs.
I’ve got those removed for testing, as I thought about that. It may be a bad cap after some more testing. But that wouldn’t cause the problems I’m seeing I don’t think. I wonder if it could be the Op-Amp? Hmmm, looks like the OP-05CP is equivalent to the uA741. Might grab one and see.
Those will read as shorted as they have jumpers across them at the back panel. Those protect the sense inputs.
I’ve got those removed for testing, as I thought about that. It may be a bad cap after some more testing. But that wouldn’t cause the problems I’m seeing I don’t think. I wonder if it could be the Op-Amp? Hmmm, looks like the OP-05CP is equivalent to the uA741. Might grab one and see.
Did you check Q3?
Those will read as shorted as they have jumpers across them at the back panel. Those protect the sense inputs.
I’ve got those removed for testing, as I thought about that. It may be a bad cap after some more testing. But that wouldn’t cause the problems I’m seeing I don’t think. I wonder if it could be the Op-Amp? Hmmm, looks like the OP-05CP is equivalent to the uA741. Might grab one and see.
Did you check Q3?
It's on my short list. It's not shorted, but it could still be bad. I'll have to take it out to test it, but I need to fix my cheap Chinese component tester first, as it shit the bed the other day. (I may have blown the ATmega after hooking it to a 9000uF cap that wasn't discharged by I clipped it to a blue-yellow wire instead of a yellow-blue wire in something I was testing...)
Weird thing is, the supply semi-works at the 1V setting (I get 1.5V out), but once you pass 2V or so you basically lose regulation and get 28V. On the 10-20V setting you get 32V.
I guess Q3 could be saturating very early or something? I just don't get what could have killed it. It worked fine the other day! It hasn't even been plugged in!
And Sean, you're right, I double checked and I had the RV/RC jumpers removed (when buzzing out some wires) and thought they were the sense jumpers when I looked last night. (The supply is upside down right now, easy 5AM mistake to make I suppose.)
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(In the numbers I gave above, I forgot to add 0V is 0.4V. Last time the unit was working, it was 0.000xxxV, so that indicates a problem outside of the switches, right?)