On 2025Sep 5,, at 11:35 PM, Rob Jarratt via cctalk <[email protected]>
wrote:
>
> Thanks Scott. I removed a couple of other caps on the 12V output, tested them
> and then put them back as they seemed OK. I have tested the ESR (in circuit)
> on all the other electrolytics and they all seemed fine. I tested a few
> resistors but not particularly rigorously though.
>
> The resistive load I applied was calibrated to the specs of the PSU, above
> the minimum load and below the maximum. I didn’t load the -12V and +15V
> outputs though, only +5 and +12. The -12V output has no minimum current spec.
>
> I can always consider the meanwell supplies but so far I have been able to
> repair my PSUs and I hope to do the same here.
>
> I am going to try installing it back in the machine today and see how it
> fares.
Here is yet another (4th) version of the schematic, redrawn for greater
functional clarity:
http://madrona.ca/e/misc/OlivettiM24PS.pdf
<http://madrona.ca/e/misc/OlivettiM24PS.pdf>
This version follows the component IDs of the marked-up version of the main
Olivetti schematic, though I’m not clear
where these IDs came and they don’t seem to have much order (perhaps they came
from grid locations on the PCB?).
These IDs are inconsistent with the main Olivetti schematic and Olivetti
ThOfOp, which seem to use a component type
designator rather than component instance ID. There are some number of errors
or inconsistencies in the Olivetti docs.
- - -
Some ThOfOp further to the Olivetti manual:
This is a 'self-oscillating’ design in that the switching-oscillation is built
into the primary drive circuit - there is no independent oscillator driving the
primary driver.
The primary drive circuit is basically a dual-supply push-pull amplifier
driving the main transformer primary, though the drive output has a capacitor
C141) and a winding of the control reactor transformer (T365) in series in the
circuit.
The feedback loop to make it oscillate goes through the base-drive transformer
(T366) with its two secondaries feeding the bases of the two drive transistors.
The main feedback path is presumably via T748.4,5. I would guess winding
T365.5,6 is a 2nd-level regulation control altering the drive level. The T366
base-drive secondaries would be polarity-oriented so they drive the bases in
cross-over, that is one transistor ON driving the primary feeds back (with
delay) to eventually drive the other transistor ON. So to speak, the
oscillator is a high-power magnetically-coupled positive/negative-pulsing
astable flip-flop. You see that in the +/- pulses on the input to the output
rectifiers (per your scope pics). The Olivetti ThOfOp says the output
rectifiers are half-wave, but they’re two half-wave rectifiers operating on
opposite polarities from a center tap .. which is a full-wave rectifier.
Not sure, but I surmise that C141 may be a/the primary determinant of the
oscillation frequency, while the 4 caps around the driver bases (C131,etc.)
determine the intra-cycle pulse periods and delays. Or C141 may be there to
keep any DC drive imbalance out of the primary (..?).
Regulation-control operation in summary:
- Q83,85 form a differential pair for the regulation error sense.
- Q83.B is the reference input, supplied by U49.
- Q85.B is the sense input, supplied by V+5s from the +5V output via
divider R80,R87,RV89.
The trail of regulation-control influence, by example:
- A + increase of V+5s results in increased drive current thru
Q85.BE, sending Q85.C more +.
- This drives Q86 to greater conduction, thus Q86.C goes lower
to GND.
- This lowers Q44.B, reducing its conduction and so reducing
current through the control reactor input winding (T365.7,9)
- The reduced current in the reactor input winding reduces
T365’s core saturation.
This enables the primary driver pulse currents flowing
through the primary-side control winding (T365.3,4) to have more
magnetic influence in the T365 core, which is to say those
driver currents now see a higher impedance to their current flow.
- The current through the main transformer primary winding
(T748.1,2) is thus reduced, counteracting the increased V+5,
thus completing the control loop.
- - -
As the oscillation is built into the primary drive circuit, presumably the
control system has a lower limit that inhibits shutting the primary drive
completely off because if it did, the primary oscillation would stop. This
lower limit may be designed into the maximum impedance of the T365
primary-control winding, but it may also involve the (relatively high) minimum
load specs per the manual.
You didn’t specify the currents for your dummy loads. The min spec for the
V+15 is 1A, i.e. 15W. That isn’t trivial so you may want to ensure the total
dummy load is accounting for that: total power of dummy load(s) > total power
of min output specs.
Hypotheses, in the absence of other measurements: the control circuitry is
faulty and holding the primary drive level to a minimum, enough to maintain
oscillation but not enough to produce significant output. The primary drive is
minimised by zero current in the control reactor input winding (T365.7,9) and
increases with increasing current in that winding.
Observe: Measure the V across the 16*100 resistor bank when the PS is
functioning, perhaps at different load levels, and and when it is in fault.
There are all sorts of failures in the control circuitry that could send the
supply into minimum drive (basically anything that shuts off current in the
T365 input winding).
The Olivetti ThOfOp suggests fault-detection circuitry (involving Q59,75) can
latch-up the supply into minimum drive.
I haven’t looked into that circuitry in much depth, but the latch-up mechanism
seems to be:
- at lesser/minimum drive the T365 core goes out of saturation,
allowing the now-varying magnetic field from remaining
primary drive pulse current to induce V in T365.1,2.
- this is rectified and filtered and supplies drive to Q86 via Q75 (if
Q75 conditions are appropriate).
- this holds Q44 off to shut off T365 input current, so keeping the
primary drive at minimum.
