My Answers are inline.?
?
On Wed, Aug 28, 2024 at 07:37 PM, Bill Berzinskas wrote:
I had looked at ripple on my scope a bit, but given the supply is unloaded I'm not sure it matters.? ?I'm seeing a few supply rails in the ~1v p-p range, of those supplies I had tried to jumper a temporary cap across.?
?
====
Unloaded is fine, it is the easier case for the linear regulator. However, 1Vpp at output is too high. Checking ripple across ?C1, C3, C4, C6 is a good idea. However, it is also possible caps are fine and ripple at the output is because regulator is not functioning and following the ripple of the input. ?You can use a DVM in AC mode to measure ripple across them, schematic has expected ripple in loaded condition, should be even less unloaded. Schematic shows ripple of the cap on one side, in regulation other side of the cap is fixed so you can take it as ripple across.?
?
I saw no real changes here.? ?Not sure I understand the "probably obvious" part of C4 / Ground lead, should I just clip to chassis then?? ? ? I can re-measure as needed just want to make sure I'm doing it as expected.? ?
?
===
Both sides of C4 is live, one side is at -100V other side is at +31.8V. Ground lead of the scope probe and ground of this circuit are shorted through ground (earth) prong of the power plugs. Connecting ground lead of scope to any side of C4 is same as shorting that side to ground through scope probe. A DVM across C4 is good enough, oscilloscope at output is useful with ground connected to ground for catching high frequency oscillations.?
?
I do believe the resistors around the trimpot are good.? I just measured the pot from center to one leg and do notice some odd behavior.??
On the ends, I get 0 and 1.6k but as I turn from 1.6k downward it actually peaks up to 1.9k, then comes back down.? ? No voltage change?
is seen until its past the "hump" there, at which point I can adjust from ~67-70v.? ? Getting the gang of 3 pots out looks mildly painful but certainly possible.??
?
===
If ripple at rectifier side is ruled out trimpot mid point is the most interesting node. It is a simple voltage divider, it shouldn't be able to produce -7.7V with -80V input.
?
You can pull out F3, apply -1V to -100V terminal with the unit off. You should see ~ -155mV at CW terminal and -215mV at the other side of trimpot. These voltages are low enough that they won't turn on Q8/CR14 junctions. This checks the division ratio.
?
If this is good. There are only three components touching that node, Q8, CR14, and R33. Emitter of Q8 is below -7.7 so CR14 or B-E junction can't be the one pulling that node to -7.7. Since you replaced Q8, I don't think it is C-B junction. C7 is other one but if you replaced it too. Interesting puzzle.?
?
Ozan
?
