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

[berskyboy]

Excellent; to see how it behaves compared to the other one, try these measurements again using the diode test mode:

Diodes   Fwd, Rev            Description       Measurement
{BA, BB}   0.13, inf   CR20a, CR20b      BA: 0, OL BB: 0,OL
{DA, DB}   0.30, 0.37   CR25, CR23        DA: OL,OL DB:
.
I stopped because should I put Q1 and Q2 back into the PSU?

And for each of Q1 and Q2 now out of circuit, also measure

Q1
Red to Black         
Lead 1 to Lead 2.       0       
Lead 2 to Lead 1        0.1

Q2
Red to Black         
Lead 1 to Lead 2        0.8
Lead 2 to Lead 1        0.6

[sigma7]

Excellent; to see how it behaves compared to the other one, try these measurements again using the diode test mode:

I stopped because should I put Q1 and Q2 back into the PSU?

No, make these measurements with Q1 and Q2 removed, but that is a good question.

Diodes  Fwd, Rev            Description      Measurement
{BA, BB}  0.13, inf  CR20a, CR20b      BA: 0, OL BB: 0,OL
{DA, DB}  0.30, 0.37  CR25, CR23        DA: OL,OL DB:

And for each of Q1 and Q2 now out of circuit, also measure
Q1
Red to Black         
Lead 1 to Lead 2.      0       
Lead 2 to Lead 1        0.1

Q2
Red to Black         
Lead 1 to Lead 2        0.8
Lead 2 to Lead 1        0.6

Are those measurements using a "diode test" function? From the readings I'd guess it was a resistance mode.

So I can peruse the manual, what is the brand and model of DMM you are using now?

[berskyboy]

Are those measurements using a "diode test" function? From the readings I'd guess it was a resistance mode.

So I can peruse the manual, what is the brand and model of DMM you are using now?

Here's a photo of it. ASTrOAI CM2KOR
https://www.dropbox.com/scl/fi/2giwzufwhe0l051stcxix/ASTrOAI.HEIC?rlkey=b7got1wh776cwtdchmoxarybb&st=i8ld7461&dl=0

that was in diode mode.

[sigma7]

Here's a photo of it. ASTrOAI CM2KOR
https://www.dropbox.com/scl/fi/2giwzufwhe0l051stcxix/ASTrOAI.HEIC?rlkey=b7got1wh776cwtdchmoxarybb&st=i8ld7461&dl=0

that was in diode mode.

Test CR18 on the component side (one end still disconnected from the board) in both directions using the diode test mode.

Note the manual says on page 17 that to enter Diode Test mode, you have to press the "Func" button after rotating the switch to the diode test/continuity position. When in that mode, the diode symbol will be shown in the display.

[berskyboy]

Test CR18 on the component side (one end still disconnected from the board) in both directions using the diode test mode.

CR18 in diode mode
OL, OL

CR18 in resistance ohm mode
33.4, 0.45

[sigma7]

Test CR18 on the component side (one end still disconnected from the board) in both directions using the diode test mode.

CR18 in diode mode
OL, OL

Did you see the edit I made that says "Note the manual says on page 17 that to enter Diode Test mode, you have to press the "Func" button after rotating the switch to the diode test/continuity position. When in that mode, the diode symbol will be shown in the display."?

If not, try the test again in diode test mode.

[berskyboy]

thank you for that

CR18 in diode mode
0.462V, OL

[sigma7]

thank you for that

My pleasure.

Test Q1 and Q2 again, out of circuit, use the diode test and check all 6 permutations:

Red to Black
Lead 1 to Lead 2
Lead 2 to Lead 1
Lead 1 to case
case to Lead 1
Lead 2 to case
case to Lead 2

[berskyboy]

Diodes	Fwd, Rev	Description
{BA, BB}	0.13, inf	CR20a, CR20b
{DA, DB}	0.30, 0.37	CR25, CR23
.
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

And for each of Q1 and Q2 now out of circuit, also measure

Red to Black
Lead 1 to Lead 2
Lead 2 to Lead 1

Now my new measurements:


Diodes  Fwd, Rev            Description      My Measurement
{BA, BB}  0.13, inf  CR20a, CR20b      BA: 0.126, climbing BB: 0.126,climbinng
{DA, DB}  0.30, 0.37  CR25, CR23        DA: 0.26, 0.277 DB: 0.267, 0.260

Transistors    Red-Blk    Description. My Measurement:
{1-2, 4-5}    1.25    Q4, Q3 c-b.  0.775, 0.767
{2-1, 5-4}    0.62    Q4, Q3 b-c.  0.572, 0.566
{1-3, 4-6}    0.70    Q4, Q3 c-e.  0.661, 0.681
{3-1, 6-4}    1.89    Q4, Q3 e-c.  1.583, 1.585
{2-3. 5-6}    0.64    Q4, Q3 b-e.  0.612, 0.618
{3-2, 6-5}    inf    Q4, Q3 e-b.  1.712, 1.725

Q1
Red to Black       
Lead 1 to Lead 2.    0v       
Lead 2 to Lead 1      OL

Q2
Red to Black       
Lead 1 to Lead 2      0v
Lead 2 to Lead 1      0v

[berskyboy]

Test Q1 and Q2 again, out of circuit, use the diode test and check all 6 permutations:

Red to Black
Lead 1 to Lead 2
Lead 2 to Lead 1
Lead 1 to case
case to Lead 1
Lead 2 to case
case to Lead 2

Q1
Red to Black
Lead 1 to Lead 2 0v
Lead 2 to Lead 1 OL
Lead 1 to case 0v
case to Lead 1 0v
Lead 2 to case 0v
case to Lead 2 0v

Q2
Red to Black
Lead 1 to Lead 2 0v
Lead 2 to Lead 1. 0v
Lead 1 to case 0v
case to Lead 1 0v
Lead 2 to case 0v
case to Lead 2 0v

[sigma7]

Test Q1 and Q2 again, out of circuit, use the diode test:

Q1
Red to Black
Lead 1 to Lead 2 0v
Lead 2 to Lead 1 OL

Check this again, if it is a short circuit, it should be bidirectional.

With Q1 and Q2 and CR18 still out of circuit, do these again using the diode test (we are looking for additional problems):

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

[sigma7]

Oh maybe you just did that... wait while I review

[sigma7]

Oh maybe you just did that... wait while I review

Most looks good.

Test Q1 and Q2 again, out of circuit, use the diode test:

Q1
Red to Black
Lead 1 to Lead 2 0v
Lead 2 to Lead 1 OL

Check this again, if it is a short circuit, it should be bidirectional.

With Q1 and Q2 and CR18 still out of circuit, do these again using the diode test (we are looking for additional problems):

Diodes	Fwd, Rev	Description
{AA, AB}	0.46, inf	CR18, CR19
{CA, CB, CC, CD}	0.5, inf	CR1, CR2, CR3, CR4
{EA, EB}	0.38, inf	CR26, CR24
{FA, FB}	0.41, inf	CR22, CR21
.
Check the following again with the daughterboard removed:
Transistors
Red-Blk		Description
{3-2, 6-5}	inf	Q4, Q3 e-b

[berskyboy]

Check this again, if it is a short circuit, it should be bidirectional.

With Q1 and Q2 and CR18 still out of circuit, do these again using the diode test (we are looking for additional problems):

Morning,

Here's a few more measurements:

{AA, AB}   0.46, inf   CR18, CR19   AA:0.469, OL AB: 0.476, OL
{BA, BB}   0.13, inf   CR18, CR19   BA: 0.132, climbing BB: 0.132,climbinng
{CA, CB, CC, CD}   0.5, inf   CR1, CR2, CR3, CR4   CA: 0.473,OL CB:0.480, OL CC:0.476, OL DC:0.468, OL
{DA, DB}   0.30, 0.37   CR25, CR23   DA: 0.26, 0.277 DB: 0.263, 0.275
{EA, EB}   0.38, inf   CR26, CR24   EA: 0.392 , climbing, EB: 0.392, OL
{FA, FB}   0.41, inf   CR22, CR21   FA: 0.392, OL FB: 0.391, OL
.         
Transistors         
Red-Blk      Description   
{1-2, 4-5}   1.25   Q4, Q3 c-b   0.775, 0.767
{2-1, 5-4}   0.62   Q4, Q3 b-c   0.572, 0.566
{1-3, 4-6}   0.70   Q4, Q3 c-e   0.661, 0.681
{3-1, 6-4}   1.89   Q4, Q3 e-c   1.583, 1.585
{2-3. 5-6}   0.64   Q4, Q3 b-e   0.612, 0.618
{3-2, 6-5}   inf   Q4, Q3 e-b   1.712, 1.725 // 0.731 , OL
         
{7-8, 10-11}   0.00   Q2, Q1 e-b (in parallel with pulse transformer)   Ov, 0v
{8-7, 11-10}   0.00   Q2, Q1 b-e (in parallel with pulse transformer)   0v, 0v

[sigma7]

Here's a few more measurements

From those, I don't see any obvious issues on the main board with Q1 and Q2 removed from the circuit.

(For future reference, I'm assuming the lower Q4, Q3 c-b measurement is due to a difference in DMM characteristics, and that Q4, Q3 e-b is with the daughterboard installed.)

Temporarily overloading the output is a condition that the PSU design anticipates and normally protects from and survives without damage.

So the likely cause of the failure of Q1 and Q2 is the solder bridge(s). Since a solder bridge is an unusual cause of failure, we can't be confident that Q1 and Q2 are the only components that need to be replaced.

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.