I won a local auction for an Apple Lisa!!!

[berskyboy]

I'll work on some measurement instructions to look for further failures on the daughterboard. In the meantime:

Use the image below to diligently check for any remaining solder bridges. If any remain, then Q1 and Q2 may be destroyed again after replacement, and further parts may be damaged too. Shiny solder joints hide subtle details, so after fully checking it once, it can be helpful to rotate the board and check again from a different angle.

If you find any bridges, take a picture/make a note of where they are as that information can help locate and/or eliminate additional components to suspect.

Here's a photo of the board and maybe some solder bridges?  I thought the dot close together would be the same join?

https://www.dropbox.com/scl/fi/hw98nypaj0xsqwhhfgoer/Soldure.Bridges.heic?rlkey=sa3nlcvognw0gys8tra246itb&st=dzc64371&dl=0

[sigma7]

Remove the daughterboard and check it while disconnected from the main board:

With the black lead on pin 5 of the daughterboard's connector, put the red lead on each of the other connector pins and measure with diode test mode, and again with resistance mode.

Black to Pin 5, Red to Pin x

Pin x	DiodeTest	Resistance	Signal
1	inf	6k	+12 sense
2	inf	10k	line sense
3	1.1	>5k	+18V
4	inf	50k	on/off
-	-	-	ground
6	inf	2.6k	+5 sense
7	inf	2.5k	current sense
8	inf	>100k	/shutdown
9	inf	inf	/power fail
10	inf	inf	pull
11	inf	inf	push
12	1.1	>1k	logic +5
13	inf	inf	/delay reset

[sigma7]

Here's a photo of the board and maybe some solder bridges?  I thought the dot close together would be the same join?

The red arrows don't point to solder bridges as far as I can tell.

Referring to the black and white diagram, there should not be any solder that bridges across a white gap from one black section to a different black section. It can be very difficult to see a bridge in a photo, so you should look at the narrow white sections and carefully compare to your board.

[berskyboy]

With the black lead on pin 5 of the daughterboard's connector, put the red lead on each of the other connector pins and measure with diode test mode, and again with resistance mode.

Black to Pin 5, Red to Pin x
Pin x   DiodeTest Resistance  Signal. Measurements
1   inf   6k   +12 sense.     1.628
2   inf   10k   line sense.    1.842
3   1.1   >5k   +18V           0.949
4   inf   50k   on/off.        OL
-   -   -   ground
6   inf   2.6k   +5 sense.      1.196
7   inf   2.5k   current sense. 1.163
8   inf   >100k   /shutdown.     OL
9   inf   inf   /power fail.   OL
10   inf   inf   pull.          OL
11   inf   inf   push.          OL
12   1.1   >1k   logic +5.      0.871
13   inf   inf   /delay reset.  OL

[TorZidan]

sigma7, are you a human?

I am growing more-and-more suspicious that you may be not:
You've been guiding berskyboy relentlessly to fix his power supply.
You've been posting voltage measurements of a working power supply, photos and custom-made schematics. I have't seen such level of dedication in humans...

Kudos if you are!

[ried]

Piling on to say the same. What a heroic effort!

[berskyboy]

Yes I know for sure I'm 100% grateful!! Plan B would be this clone PSU,

https://68kmla.org/bb/threads/lisa-1-8a-psu-clone-by-warmech.47154/page-2

but I am still hopping to get the old gal running.  LOL.  Lisa would be a girl right?

[sigma7]

The short circuit of Q1 and Q2 would result in a current having most of the available energy from the main capacitors (plus more from the power utility until the fuse blew).

This current goes through windings of pulse transformers T4 and T5, so we need to check that there is still continuity through those windings.

Refer to the drawing, and measure the resistance between each of these pairs (Q1 and Q2 still removed from the circuit):

T4:5 - T4:6
T5:5 - T5:6

In each case, the resistance should be close to 0.0 ohms.

[sigma7]

Normally, with Q1 and Q2 out of circuit (either completely, or with the 4 phillips head mounting screws removed such that their cases are disconnected), the main transformer will not receive any switching current.

In this state, the only output voltage will be +5Standby.

With the high power circuitry inactive, we can (hopefully) check some of the control circuitry without having a fault in the high power circuitry blowing the fuse.

Double check that Q1 and Q2 are disconnected (4 screws removed) or completely removed from the circuit board.

Ensure a good fuse is installed.

Connect the test jig load circuit to the card edge, and turn its small switch "off".

Connect the line power and turn on the power strip; there should not be any smoke, flashes, odours, blown fuse or other dramatic evidence of something happening. If something does, stop here, disconnect the power and report what you observed.

If Q1 is still mounted on the board, measure the DC voltage from its case to ground. If the voltage exceeds 1 Volt, immediately remove power and stop here.

The small switch should still be in the OFF position, and the power strip will be on. Check the DC and AC voltages from pin 3 of the daughterboard connector to ground. This should be about 16 to 18V DC, and should have an AC voltage of less than 1V; if the DC and AC voltage are in that range, continue with the next step. If not, stop here, remove power, and report what you observed. If the DC voltage is about half or double, investigate whether the 115/230V jumpers are not installed correctly or damaged or not making proper contact. If the DC voltage was zero, check the fuse.

Remember to set the meter back to DC volts after measuring the AC volts.

Check the DC voltage across R33 (from one end to the other, not to ground) & across R2. (These are two large resistors beside the large black capacitor C8.) They should be about 150-170 V each (positive or negative, depending on which way you probe them): if so, continue with the next step. If not, then the primary rectifier, line filter, and associated components are suspect - stop here, remove power, and report what you observed.

Check the DC voltage at pin 12 of the daughterboard connector to ground. This should be about 5V: if so, continue with the next step. If not then Z2 on the daughterboard is suspect - stop here, remove power, and report what you observed.

Make sure the interlock switch has been engaged. Set the small switch (on the test jig connected to the card edge) to the ON position. There should not be any smoke, flashes, odours, blown fuse or other dramatic evidence of something happening. If something does, stop here, disconnect the power and report what you observed.

Collect measurements from the following steps A - H and report what you observe. As before, stop, disconnect power and report observations if anything happens that isn't expected.

These measurements are much more easily done if the black lead of the DMM can be secured to ground, freeing up a hand to operate the switch. eg. clip the black lead to the grounded side of a load resistor. If nothing else is available, inserting the black probe into the grounded mounting hole near the card edge connector and leaving it leaning against the edge may work. Be careful to not dislodge the probe while power is on; you might want to tape it in place. Turn off the power strip if you need to fiddle with this connection.

On/Off in the following refers to the small switch on the test jig attached to the card edge. The power strip can remain on (the behaviour of some of the following measurements is different if the power strip is turned off/on instead of the test jig's switch).

A. Turn off, locate pin 13 of the daughterboard connector (at the CR15 end), then while watching the voltage at pin 13, turn on: the voltage should rise from a low voltage about 0.5V to about 4.8V over a few seconds.

B. Turn off, then while watching the voltage at pin 8 of the daughterboard connector, turn on: the voltage should rise briefly, from near 0 to near 5V, then return to 0. This takes less than a second but should be visible. If it is stuck at 0, double check the interlock switch has been bypassed.

C. Turn off. Carefully (be careful the probe does not slip and cause a short circuit) hold the red probe to the lower end of R30 on the daughterboard (R30 is at the left edge/corner). The voltage should be near 4V.  Then while watching the voltage, turn on: the voltage should drop immediately to near 0. If the voltage remains high (around 4 VDC) then the soft-power circuit is suspect.

D. Turn off. Carefully (be careful the probe does not slip and cause a short circuit) hold the red probe to the + (plus) side of C14 on the daughterboard (not far from R30). Then while watching the voltage, turn on: the voltage should rise over a few seconds from near 0 to near 4.8V. If stuck (around 0 VDC), then the brownout circuit is suspect.

E. Turn off, then while watching the voltage at pin 10 of the daughterboard connector, turn on: the voltage should drop from about 2.4V to about 1.4V for a noticeable but brief period (less than a second). If you don't see the brief drop, turn off, switch the DMM to AC Volts and when you turn on again, you should observe the AC voltage briefly rise from 0V to perhaps 1V (the value will depend on the DMM), then return to 0 before you turn off again. If no activity is observed, then Z2 on the daughterboard is suspect. Set the DMM back to DC Volts.

F. Turn off, then while watching the voltage at pin 11 of the daughterboard connector, turn on: the voltage should drop from about 2.4V to about 1.4V for a noticeable but brief period (less than a second). If you don't see the brief drop, turn off, switch the DMM to AC Volts and when you turn on again, you should observe the AC voltage briefly rise from 0V to perhaps 1V (the value will depend on the DMM), then return to 0 before you turn off again. If no activity is observed, then Z2 on the daughterboard is suspect. Set the DMM back to DC Volts.

G. Turn off, locate R7 on the main board. R7 is on ceramic standoffs beside a silvery heatsink plate. One end should measure around 16-18V. While watching the voltage at the other end, turn on, the voltage should rise briefly, from about 1.6V to 5-15V (varies depending on the speed of your DMM), then return to 1.6. If no activity is observed, then Q4 or T5 is suspect.

H. Turn off, locate R5 on the main board. R5 is on ceramic standoffs near the center beside R4. One end should measure around 16-18V. While watching the voltage at the other end, turn on, the voltage should rise briefly, from about 1.6V to 5-15V (varies depending on the speed of your DMM), then return to 1.6. If no activity is observed, then Q3 or T4 is suspect.

[sigma7]

are you a human?

Ah, the eternal question.

Thanks for your kind words; I'm hoping that describing this example of the process will be useful to others someday too, but 'are you not entertained' is sufficient reward.

A computer that can still be repaired at the component level by moderately technical users is, I think, a fortunate anachronism.

[berskyboy]

Refer to the drawing, and measure the resistance between each of these pairs (Q1 and Q2 still removed from the circuit):

T4:5 - T4:6
T5:5 - T5:6

In each case, the resistance should be close to 0.0 ohms.

T4:5 - T4:6. in ohms mode 0 ohms
T5:5 - T5:6. in ohms move 0 ohms

[berskyboy]

Normally, with Q1 and Q2 out of circuit (either completely, or with the 4 phillips head mounting screws removed such that their cases are disconnected), the main transformer will not receive any switching current.

In this state, the only output voltage will be +5Standby.

With the high power circuitry inactive, we can (hopefully) check some of the control circuitry without having a fault in the high power circuitry blowing the fuse.

Double check that Q1 and Q2 are disconnected (4 screws removed) or completely removed from the circuit board.

Ensure a good fuse is installed.

Connect the test jig load circuit to the card edge, and turn its small switch "off".

Do I connect CR18 again?  I would presume so.  I also took out the heatsink and screws for T3, and T4.  And should the daughter board be placed back in before testing?

[sigma7]

T4:5 - T4:6. in ohms mode 0 ohms
T5:5 - T5:6. in ohms mode 0 ohms

Ok, good.

Do I connect CR18 again?  I would presume so.  I also took out the heatsink and screws for T3, and T4.  And should the daughter board be placed back in before testing?

You can reconnect CR18 or leave it disconnected at your convenience.

The daughterboard should be in place.

T3 and T4 don't have a heatsink or screws. If you mean Q1 & Q2, at a minimum, the screws should be removed, if the heatsinks are then loose, removing them too is wise.

[berskyboy]

Normally, with Q1 and Q2 out of circuit (either completely, or with the 4 phillips head mounting screws removed such that their cases are disconnected), the main transformer will not receive any switching current.

In this state, the only output voltage will be +5Standby.

With the high power circuitry inactive, we can (hopefully) check some of the control circuitry without having a fault in the high power circuitry blowing the fuse.

Double check that Q1 and Q2 are disconnected (4 screws removed) or completely removed from the circuit board.

Ensure a good fuse is installed.

Connect the test jig load circuit to the card edge, and turn its small switch "off".

The small switch should still be in the OFF position, and the power strip will be on. Check the DC and AC voltages from pin 3 of the daughterboard connector to ground. This should be about 16 to 18V DC, and should have an AC voltage of less than 1V; if the DC and AC voltage are in that range, continue with the next step. If not, stop here, remove power, and report what you observed. If the DC voltage is about half or double, investigate whether the 115/230V jumpers are not installed correctly or damaged or not making proper contact. If the DC voltage was zero, check the fuse.

-DC: 17 V
-AC:  0.111 V

-Fuse OK

Remember to set the meter back to DC volts after measuring the AC volts.

Check the DC voltage across R33 (from one end to the other, not to ground)

-161V

 & across R2.

-161V

(These are two large resistors beside the large black capacitor C8.) They should be about 150-170 V each (positive or negative, depending on which way you probe them): if so, continue with the next step. If not, then the primary rectifier, line filter, and associated components are suspect - stop here, remove power, and report what you observed.

Check the DC voltage at pin 12 of the daughterboard connector to ground. This should be about 5V:

-4.97V

if so, continue with the next step. If not then Z2 on the daughterboard is suspect - stop here, remove power, and report what you observed.

Make sure the interlock switch has been engaged. Set the small switch (on the test jig connected to the card edge) to the ON position. There should not be any smoke, flashes, odours, blown fuse or other dramatic evidence of something happening. If something does, stop here, disconnect the power and report what you observed.

-ok

Collect measurements from the following steps A - H and report what you observe. As before, stop, disconnect power and report observations if anything happens that isn't expected.


These measurements are much more easily done if the black lead of the DMM can be secured to ground, freeing up a hand to operate the switch. eg. clip the black lead to the grounded side of a load resistor. If nothing else is available, inserting the black probe into the grounded mounting hole near the card edge connector and leaving it leaning against the edge may work. Be careful to not dislodge the probe while power is on; you might want to tape it in place. Turn off the power strip if you need to fiddle with this connection.

On/Off in the following refers to the small switch on the test jig attached to the card edge. The power strip can remain on (the behaviour of some of the following measurements is different if the power strip is turned off/on instead of the test jig's switch).

A. Turn off, locate pin 13 of the daughterboard connector (at the CR15 end), then while watching the voltage at pin 13, turn on: the voltage should rise from a low voltage about 0.5V to about 4.8V over a few seconds.

-0.388V to 4.64V (then stays at 4.6V)

B. Turn off, then while watching the voltage at pin 8 of the daughterboard connector, turn on: the voltage should rise briefly, from near 0 to near 5V, then return to 0. This takes less than a second but should be visible. If it is stuck at 0, double check the interlock switch has been bypassed.

-0.01v to 4.6V then down to 0.025v

C. Turn off. Carefully (be careful the probe does not slip and cause a short circuit) hold the red probe to the lower end of R30 on the daughterboard (R30 is at the left edge/corner). The voltage should be near 4V.  Then while watching the voltage, turn on: the voltage should drop immediately to near 0. If the voltage remains high (around 4 VDC) then the soft-power circuit is suspect.

-no 0V, but I might not be putt the probe correctly

D. Turn off. Carefully (be careful the probe does not slip and cause a short circuit) hold the red probe to the + (plus) side of C14 on the daughterboard (not far from R30). Then while watching the voltage, turn on: the voltage should rise over a few seconds from near 0 to near 4.8V. If stuck (around 0 VDC), then the brownout circuit is suspect.

-4.63V

E. Turn off, then while watching the voltage at pin 10 of the daughterboard connector, turn on: the voltage should drop from about 2.4V to about 1.4V for a noticeable but brief period (less than a second). If you don't see the brief drop, turn off, switch the DMM to AC Volts and when you turn on again, you should observe the AC voltage briefly rise from 0V to perhaps 1V (the value will depend on the DMM), then return to 0 before you turn off again. If no activity is observed, then Z2 on the daughterboard is suspect. Set the DMM back to DC Volts.

-goes 2.25v to 1.43v then back to 2.25v

F. Turn off, then while watching the voltage at pin 11 of the daughterboard connector, turn on: the voltage should drop from about 2.4V to about 1.4V for a noticeable but brief period (less than a second). If you don't see the brief drop, turn off, switch the DMM to AC Volts and when you turn on again, you should observe the AC voltage briefly rise from 0V to perhaps 1V (the value will depend on the DMM), then return to 0 before you turn off again. If no activity is observed, then Z2 on the daughterboard is suspect. Set the DMM back to DC Volts.

-2.25v then 1.84v then 2.25v

G. Turn off, locate R7 on the main board. R7 is on ceramic standoffs beside a silvery heatsink plate. One end should measure around 16-18V. While watching the voltage at the other end, turn on, the voltage should rise briefly, from about 1.6V to 5-15V (varies depending on the speed of your DMM), then return to 1.6. If no activity is observed, then Q4 or T5 is suspect.

-1.621v to 11.3V then 1.63V (resistor to ground)


H. Turn off, locate R5 on the main board. R5 is on ceramic standoffs near the center beside R4. One end should measure around 16-18V. While watching the voltage at the other end, turn on, the voltage should rise briefly, from about 1.6V to 5-15V (varies depending on the speed of your DMM), then return to 1.6. If no activity is observed, then Q3 or T4 is suspect.


-1.631v to 9.72V then 1.63V (resistor to ground)

[sigma7]

-1.631v to 9.72V then 1.63V (resistor to ground)

I'm assuming the "-" prefix in these readings doesn't indicate negative voltage... if it does then let me know.

The measurements E-H suggest the SG3524 on the daughterboard is attempting to drive the power circuit, so it is probably good. The other measurements suggest the ancillary circuitry is also working.

We haven't thoroughly checked the secondary side of the PSU, but since you have replaced the output capacitors, and we confirmed the rectifier diodes look like diodes, there is a good chance it is ok.

I suspect we found and corrected the original fault (a broken solder joint at the trimmer potentiometer R11), and the fault that caused Q1 & Q2 to fail, solder bridge(s).

So I suspect that replacing Q1 & Q2 (and ensuring there are no solder bridges remaining) is likely to be sufficient to repair the fault that blows the fuse.

The original 2N6308 transistors are no longer made, so finding a substitute is the usual option, although one could look for old-stock originals on eBay etc.

A few posts back, you referenced a Newark/Element14 cart with BUX48A transistors in it. Did you find references that indicate this is a suitable substitute?

Does anyone else have a recommendation for a currently available 2N6308 substitute that works in this PSU?