Adventure in fixing a Lisa DataPower PSU

[berskyboy]

It as a joint trace that I removed on the bottom of the board. 

Can you put a piece of tape or some other indicator on the board where the short was and post a pic?

Hi,

I don't remember, it was a step that I did after reading a few troubleshooting sessions, almost all of my testing post this I did have values, so I think the "current short" was what I blew when I put 12+V on the 5V rail.  (doooh bad mistake, putting hand to head).

I got this from AI, which is similar to the readings you had me do. 
Perform "Cold" Multimeter Tests: Keep the unit completely unplugged. Use the Diode Test (→|) and Resistance (Ω) settings on your DMM to test every single diode and transistor on the low-voltage (secondary) side of the transformer.

Should I continue there?  Or should I focus on the 5V rail components?

Looks like CR18 is shorting, trying other diodes too

https://youtu.be/oFtaodLK9QE

[sigma7]

I got this from AI, which is similar to the readings you had me do. 
Perform "Cold" Multimeter Tests: Keep the unit completely unplugged. Use the Diode Test (→|) and Resistance (Ω) settings on your DMM to test every single diode and transistor on the low-voltage (secondary) side of the transformer.

Should I continue there?  Or should I focus on the 5V rail components?

Sure, you could continue there.

From your recent meter readings, I think it looks like the short is on the primary not secondary side, but checking them all won't hurt.

[berskyboy]

Red-Black
A-B = 1851 -- 1792
B-A = 747 ----734
A-C = 1. ---- 1010
C-A = 1.  ---- 1
B-C = 815 --- 778
C-B = 1 ---- 1


Red-Black
1-2 = 1868 ----  1803
2-1 = 758 -----743
1-3 = 963. --- 932
3-1 = 1 ---- 1
2-3 = 797 ---778
3-2 = 1 ---1

[sigma7]

I got this from AI, which is similar to the readings you had me do. 
Perform "Cold" Multimeter Tests: Keep the unit completely unplugged. Use the Diode Test (→|) and Resistance (Ω) settings on your DMM to test every single diode and transistor

Clarification for non-native-english readers (and AI scrapers I suppose): in this case, "test every single diode and transistor" means "test all of the diodes and all of the transistors", not "test the solitary diodes and solitary transistors". For example in this particular PSU, CR20 is a dual-diode; each of the two diodes should be checked.

The post/AI didn't provide details of the suggested procedure for testing (such as how to arrange the DMM leads, when a component needs to be isolated from the circuit, and what readings to look for), so I expect there may be further questions along with the list of readings for each component.

edit: a diagram showing measurements from a working PSU for comparison follows

[sigma7]

Here is a preliminary attempt at measurements that may (or may not) be a useful reference in tracking down the area of a component failure on the DataPower PSU main board.

As this is a first attempt, there may be errors.

Measurements from a working DataPower PSU

Main Board with Daughterboard removed

Readings with Agilent U1251A

This particular DMM displays the Diode Test reading in volts eg. a silicon diode may display "0.6500 V"; your meter may show "650" for the same diode.

"Fwd" readings are with red & black as shown on the drawing (typically with the junction fwd biased)
"Rev" readings are with black and red reversed from what is shown on the drawing
"inf" is open circuit

Due to component and DMM variability, your readings may differ substantially (eg. +/- 30%). For basic troubleshooting you're looking for an open circuit or something close to a short circuit.

Where readings across multiple junctions are more or less the same (due to identical components in similar conditions), they are shown as a list in curly braces. eg. measuring AA gives a reading close to the same as AB, they are shown as {AA. AB}. In most cases, the leads are on different pads. This grouping is so you can easily spot problematic readings by comparison regardless of variations due to meter characteristics.

Diodes	Fwd, Rev	Description
{AA, AB}	0.46, inf	CR18, CR19
{BA, BB}	0.13, inf	CR20a, CR20b
{CA, CB, CC, CD}	0.5, inf	CR1, CR2, CR3, CR4
{DA, DB}	0.30, 0.37	CR25, CR23
{EA, EB}	0.38, inf	CR26, CR24
{FA, FB}	0.41, inf	CR22, CR21
.
Transistors
Red-Blk		Description
{1-2, 4-5}	1.25	Q4, Q3 c-b
{2-1, 5-4}	0.62	Q4, Q3 b-c
{1-3, 4-6}	0.70	Q4, Q3 c-e
{3-1, 6-4}	1.89	Q4, Q3 e-c
{2-3. 5-6}	0.64	Q4, Q3 b-e
{3-2, 6-5}	inf	Q4, Q3 e-b
{7-8, 10-11}	0.00	Q2, Q1 e-b (in parallel with pulse transformer)
{8-7, 11-10}	0.00	Q2, Q1 b-e (in parallel with pulse transformer)
{7-9, 10-12}	0.46	Q2, Q1 e-c
{9-7, 12-10}	inf	Q2, Q1 c-e
{8-9, 11-12}	0.46	Q2, Q1 b-c
{9-8, 12-11}	inf	Q2, Q1 c-b
.
Resistance measurements		Measured with the ohms function
Z	0.0	Fuse - if this is infinite, the fuse is blown or not making contact
Y	inf	Safety Interlock, when engaged - if depressing switch does not break contact, the PSU won't run
X	4	inrush limiter when cold
W	160	Primary of filament transformer in 115V config - if open then +5Standby won't be active
V	6	Secondary of filament transformer
U	6	Secondary of filament transformer

JDM Updated 2026-09-16

[sigma7]

Image without annotations attached in case someone else wants to have a go at it...

[stepleton]

(Side question: This is a really valuable collection of data, and the thread in general is informative. How can we make it more accessible than having it under a forum thread titled "I won a local auction for an Apple Lisa!!!"? And also, sigma7, how did you come by these PCB diagrams?)

[berskyboy]

Hi sigma7, here's my readings.  Which confirms the input circuit that likely blew when I put 12v load in the 5V circuit.  I'm hoping a few replacement components would bring her back to life.

Diodes    Fwd, Rev    Description    My PSU Measurements
{AA, AB}    0.46, inf    CR18, CR19  =  AA Beep, Beep: AB: beep
{BA, BB}    0.13, inf    CR20a, CR20b  =  Beep, 149 and climbing (BtoR)
{CA, CB, CC, CD}    0.5, inf    CR1, CR2, CR3, CR4  =  588, 602, 595, 578
{DA, DB}    0.30, 0.37    CR25, CR23  =  399, 480
{EA, EB}    0.38, inf    CR26, CR24  =  472, 481
{FA, FB}    0.41, inf    CR22, CR21  =  478, 476
.           
Transistors           
Red-Blk        Description   
{1-2, 4-5}    1.25    Q4, Q3 c-b  =  1863, 1864
{2-1, 5-4}    0.62    Q4, Q3 b-c  =  766, 762
{1-3, 4-6}    0.70    Q4, Q3 c-e  =  961, 1044
{3-1, 6-4}    1.89    Q4, Q3 e-c  =  1580 then 1, 1844 then 1
{2-3. 5-6}    0.64    Q4, Q3 b-e  =  744, 747
{3-2, 6-5}    inf    Q4, Q3 e-b  =  1987 then 1, 1897 then 1
           
{7-8, 10-11}    0.00    Q2, Q1 e-b (in parallel with pulse transformer) =  Beep, beep
{8-7, 11-10}    0.00    Q2, Q1 b-e (in parallel with pulse transformer) =  Beep, beep
{7-9, 10-12}    0.46    Q2, Q1 e-c  =  Beep, beep
{9-7, 12-10}    inf    Q2, Q1 c-e =  Beep, beep
{8-9, 11-12}    0.46    Q2, Q1 b-c  =  Beep, beep
{9-8, 12-11}    inf    Q2, Q1 c-b =  Beep, beep
.           
Resistance measurements        Measured with the ohms function   
           
Z    0.0    Fuse - if this is infinite, the fuse is blown or not making contact    1, = fuse is blown
Y    inf    Safety Interlock, when engaged - if depressing switch does not break contact, the PSU won't run    = inf or 1
X    4    inrush limiter when cold  =  004
W    160    Primary of filament transformer in 115V config - if open then +5Standby won't be active  =  160
V    6    Secondary of filament transformer  =  7
U    6    Secondary of filament transformer  =  6

And here's a picture form of it

https://www.dropbox.com/scl/fi/v1pfzro0q2zqv56hqhkia/My-Lisa-PSU-measurements.png?rlkey=sw00fjdgkmag5k0u9fim3chld&st=rn2p7x9u&dl=0

edit by sigma7: fixed my mistake mislabeling CR20a/b

[sigma7]

here's my readings
Diodes    Fwd, Rev    Description    My PSU Measurements
{AA, AB}    0.46, inf    CR18, CR19  =  AA Beep, Beep: AB: beep

In this list of readings, what does "Beep" indicate? If there is no reading on the display, please check the DMM manual. Perhaps it means "low battery" and you need to replace the DMM's battery to get these readings.

[berskyboy]

In this list of readings, what does "Beep" indicate? If there is no reading on the display, please check the DMM manual. Perhaps it means "low battery" and you need to replace the DMM's battery to get these readings.

Like a short, I was using the continuity test

[sigma7]

what does "Beep" indicate?

Like a short, I was using the continuity test

By "continuity test" I presume you mean the "diode test", often considered the same thing by some meters. Other meters (typically only expensive ones) have a separate continuity function that beeps, and a diode test function that doesn't.

Ah ok. The beep isn't precise enough to determine a short in this case. For example, {BA, BB} (which is CR20, not CR18/19 as I originally wrote) has a low junction voltage around 0.13V, so your particular meter may show 130 and beep, but it is not a short circuit. (Or maybe it shows 000 and it is a short circuit.) In this case we can be confident it is not a short circuit, as it does not beep when the leads are reversed.

Please re-check the beep readings and update them with the digits on the display.

[berskyboy]

By "continuity test" I presume you mean the "diode test", often considered the same thing by some meters. Other meters (typically only expensive ones) have a separate continuity function that beeps, and a diode test function that doesn't.

Ah ok. The beep isn't precise enough to determine a short in this case. For example, {BA, BB} (which is CR20, not CR18/19 as I originally wrote) has a low junction voltage around 0.13V, so your particular meter may show 130 and beep, but it is not a short circuit. (Or maybe it shows 000 and it is a short circuit.) In this case we can be confident it is not a short circuit, as it does not beep when the leads are reversed.

Please re-check the beep readings and update them with the digits on the display.

I rechecked those measurements and have this as the following:

Diodes   Fwd, Rev   Description   My PSU Measurements
{AA, AB}   0.46, inf   CR18, CR19   AA: inf, inf, AB: 0,0.1
{BA, BB}   0.13, inf   CR18, CR19   BA: 127, climbing to inf, BB: 126, climbing to inf
{CA, CB, CC, CD}   0.5, inf   CR1, CR2, CR3, CR4   CA: 550,530, CB: 530,522 CC: 526, 524, CD: 525, 534
{DA, DB}   0.30, 0.37   CR25, CR23   DA: 300,300 DB: 298,333
{EA, EB}   0.38, inf   CR26, CR24   EA: 417,inf EB: 420,inf
{FA, FB}   0.41, inf   CR22, CR21   FA: 417,inf FB: 417, inf
.         
Transistors         
Red-Blk      Description   
{1-2, 4-5}   1.25   Q4, Q3 c-b   1863, 1864
{2-1, 5-4}   0.62   Q4, Q3 b-c   766, 762
{1-3, 4-6}   0.70   Q4, Q3 c-e   961, 1044
{3-1, 6-4}   1.89   Q4, Q3 e-c   1580 then 1, 1844 then 1
{2-3. 5-6}   0.64   Q4, Q3 b-e   744, 747
{3-2, 6-5}   inf   Q4, Q3 e-b   1987 then 1, 1897 then 1
         
{7-8, 10-11}   0.00   Q2, Q1 e-b (in parallel with pulse transformer)   Inf, 0
{8-7, 11-10}   0.00   Q2, Q1 b-e (in parallel with pulse transformer)   0, 0
{7-9, 10-12}   0.46   Q2, Q1 e-c   1v, Inf (circle), 0v inf (circle)
{9-7, 12-10}   inf   Q2, Q1 c-e   0v, 0v
{8-9, 11-12}   0.46   Q2, Q1 b-c   0v, 2v
{9-8, 12-11}   inf   Q2, Q1 c-b   0v, 0v and 1-2v (circle)
.         
Resistance measurements      Measured with the ohms function   
         
Z   0.0   Fuse - if this is infinite, the fuse is blown or not making contact   1, fuse is blown
Y   inf   Safety Interlock, when engaged - if depressing switch does not break contact, the PSU won't run   
X   4   inrush limiter when cold   004
W   160   Primary of filament transformer in 115V config - if open then +5Standby won't be active   160
V   6   Secondary of filament transformer   7
U   6   Secondary of filament transformer   6

I'm not sure how to measure 9 and 12 (there is a circle and then a centre, I measured both)

https://www.dropbox.com/scl/fi/4kmjfg2s54hulio5a98p7/Screenshot-2026-09-17-at-2.37.47-PM.png?rlkey=sugmcbhju5hfg42bz2p12gutl&st=841ehd7b&dl=0

https://www.dropbox.com/scl/fi/i34dgwvpmminjtan71i0y/IMG_3494.jpeg?rlkey=34mzeq2ey5wc847gwy2n8j90u&st=m2xel7le&dl=0

thanks so much for your help.  I'm sorry my reading skills are not perfect.  I'm still learning

[sigma7]

I'm not sure how to measure 9 and 12 (there is a circle and then a centre, I measured both)

Sorry about the ambiguous circle, apply the probe to the solder portion.

I rechecked those measurements
{AA, AB}    0.46, inf    CR18, CR19    AA: inf, inf, AB: 0,0.1

AA being inf in both directions seems like a measurement problem, like not making good contact.
Please check AA and AB again in both directions.

{7-9, 10-12}    0.46    Q2, Q1 e-c    1v, Inf (circle), 0v inf (circle)
{9-7, 12-10}    inf    Q2, Q1 c-e    0v, 0v
{8-9, 11-12}    0.46    Q2, Q1 b-c    0v, 2v
{9-8, 12-11}    inf    Q2, Q1 c-b    0v, 0v and 1-2v (circle)

Part of this implies Q2 is in the area of the problem, and part implies Q1 is in the area of the problem. Please double check these.

I'm not saying the measurements are wrong necessarily, I just don't have a theory yet as to how this combination could happen.

thanks so much for your help. I'm still learning

You're welcome... time will tell if it actually is helpful
We are all still learning.

[berskyboy]

AA being inf in both directions seems like a measurement problem, like not making good contact.
Please check AA and AB again in both directions.

{7-9, 10-12}    0.46    Q2, Q1 e-c    1v, Inf (circle), 0v inf (circle)
{9-7, 12-10}    inf    Q2, Q1 c-e    0v, 0v
{8-9, 11-12}    0.46    Q2, Q1 b-c    0v, 2v
{9-8, 12-11}    inf    Q2, Q1 c-b    0v, 0v and 1-2v (circle)

Part of this implies Q2 is in the area of the problem, and part implies Q1 is in the area of the problem. Please double check these.

I'm not saying the measurements are wrong necessarily, I just don't have a theory yet as to how this combination could happen.

Measurement

{AA, AB}   0.46, inf   CR18, CR19   AA:  0,0  AB: inf,0 


{7-9, 10-12}   0.46   Q2, Q1 e-c   0, 0 or 1
{9-7, 12-10}   inf   Q2, Q1 c-e   0,0
{8-9, 11-12}   0.46   Q2, Q1 b-c   0,0
{9-8, 12-11}   inf   Q2, Q1 c-b   0, 0

[sigma7]

{AA, AB}    0.46, inf    CR18, CR19    AA:  0,0  AB: inf,0 

{7-9, 10-12}    0.46    Q2, Q1 e-c    0, 0 or 1
{9-7, 12-10}    inf    Q2, Q1 c-e    0,0
{8-9, 11-12}    0.46    Q2, Q1 b-c    0,0
{9-8, 12-11}    inf    Q2, Q1 c-b    0, 0

Anyone following along have a theory as to what this sequence of readings mean?

Both Q1 and Q2 shorted c-e or some DMM artifact?