LisaList2

Quote from: sigma7 on September 22, 2026, 02:36:13 PMI'm assuming the "-" prefix in these readings doesn't indicate negative voltage... if it does then let me know.

that denoted a measurement.  Sorry I wanted to separate  it.

But what about this value?:

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

Quote from: sigma7 on September 22, 2026, 02:36:13 PMSo 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?

This is what I found.

BUX48A
TRANS, BIPOLAR, NPN, 450V, 15A, TO-3

https://canada.newark.com/multicomp-pro/bux48a/trans-bipolar-npn-450v-15a-to/dp/23AJ5969

from AI or google:

Yes, the BUX48A can replace the 2N6308 in most high-voltage power supply, inverter, and switching applications. It matches the package type, exceeds all critical maximum voltage and current limits, and provides equivalent or better switching speeds. Solitron Devices, Inc. +3

https://solitrondevices.com/datasheets/2n6306_8-ds.pdf

Quote from: berskyboy on September 22, 2026, 02:54:44 PMBut what about this value?:

C. ..

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

The fact that E - H show the SG3524 attempting to run indicates the soft-power circuit is working, so I think it doesn't matter if either you weren't able to probe it or my predictions were incorrect.

Quote from: berskyboy on September 22, 2026, 03:02:26 PMThis is what I found.

from AI or google:

Yes, the BUX48A can replace the 2N6308 in most high-voltage power supply

Neither AI nor google have any direct knowledge, they've only collected bits of information from elsewhere, and sometimes combine them incorrectly, so they can't be considered an authoritative or reliable source. eg. AI was previously telling you to replace a part that isn't there.

So I'm hoping there is a better source, if not, we will compare specifications in an attempt to confirm the suggestion is reasonable.

I'm found and bought from this site the 2N6308

https://www.tedss.com/2028023156

Quote from: berskyboy on September 22, 2026, 09:38:40 PMfound and bought from this site the 2N6308

Good choice!

It looks like there are very few TO-3 transistor packages still being made. The plastic case alternative will not be trivial to retrofit in the DataPower PSU.

Quote from: berskyboy on September 22, 2026, 09:38:40 PMfound and bought

While you're waiting for those to arrive, anticipate if you will need to procure some additional items to install them.

If you can remove the mica insulators from the old transistors without breaking them, you can re-use them.

In addition, thermally conductive grease/compound is needed with mica insulators. In a pinch, if the old parts have plenty and it is clean, you can re-use the grease, but cleaning it off and using new grease can help avoid issues. The fancy heatsink compound used for computer CPUs/GPUs is the same type of material (but more expensive) and may be easier to get locally, or you may already have some.

Watch some videos to see how much grease to use... the final layers should be very thin (eg. almost translucent), too much will make thermal transfer worse. You may need slightly more for broad contact if the heatsink surface isn't very flat. Keep grease away from where the leads enter the transistor case.

There is also the option of replacing the grease and mica with a TO-3 shape silicone insulator pad. I've not read confirmations that this works ok in this PSU, but I expect so. The silicone pad option does not need grease; clean off all the old grease if switching to a silicone pad.

If any of the TO-3 mounting screws were difficult to remove, the solder joint between the threaded mounting post and the circuit board may have broken. This is best resoldered without the screw and transistor in place. On the bottom of the main board, carefully check for cracks in the solder around the outside of the threaded mounting posts and resolder if the joint is suspect. Avoid putting more solder in the hole (and remove solder in the hole if possible).

Important: When re-installing the TO-3 transistors, you must complete the physical mounting before soldering the leads. ie. clean and grease the mating surfaces, assemble, and fully tighten the phillips head screws before soldering anything. Soldering too soon introduces mechanical stresses that can damage the transistor and circuit board.

The DataPower PSU was originally wave soldered after the TO-3 transistors were installed. As a result, the transistor mounting screw holes have solder in them. Check carefully that the transistors are rigidly clamped down against the heatsink; if not, the solder in the screw holes may be preventing full engagement of the screws. If you have appropriate equipment, you may be able to remove the solder from the screw holes, or you can use a 6-32 tap or something similar to clear solder from inside the threaded hole. You do not need to re-solder the mounting screws once re-installed.

Once the screws are tightened, check that the insulator has not been compromised by measuring the resistance between the heatsink and the case of the transistor. It should have a very high resistance. The black coating on the heatsink is non-conductive, so you'll need to pierce it, scrape a bit off, or find a spot that isn't black. Measure the resistance from one spot to another on the heatsink to confirm you've established good contact.

Double check that all other components (such as CR18) have been re-installed to return the PSU to the fully assembled state.

Quote from: sigma7 on Yesterday at 03:25:19 PMWhile you're waiting for those to arrive, anticipate if you will need to procure some additional items to install them.

If you can remove the mica insulators from the old transistors without breaking them, you can re-use them.

I think I can use them and wipe them off

Quote from: sigma7 on Yesterday at 03:25:19 PMIn addition, thermally conductive grease/compound is needed with mica insulators.

I have ordered this.

https://canada.newark.com/wakefield-thermal/120-sa/thermal-grease-compound-packet/dp/00Z1244?CMP=e-email-sys-shipping-GLB-Product

Quote from: sigma7 on Yesterday at 03:25:19 PMWatch some videos to see how much grease to use... the final layers should be very thin (eg. almost translucent), too much will make thermal transfer worse. You may need slightly more for broad contact if the heatsink surface isn't very flat. Keep grease away from where the leads enter the transistor case.

would this be a good video for that?

https://www.youtube.com/watch?v=ufTi-lH4nw4

Quote from: berskyboy on Yesterday at 07:18:51 PM

Quote from: sigma7 on Yesterday at 03:25:19 PMWatch some videos to see how much grease to use... the final layers should be very thin (eg. almost translucent), too much will make thermal transfer worse. You may need slightly more for broad contact if the heatsink surface isn't very flat. Keep grease away from where the leads enter the transistor case.

would this be a good video for that?

https://www.youtube.com/watch?v=ufTi-lH4nw4

No.

Edit to add some details:

To be fair, if it is done that way, it might be fine, and probably is in many cases, especially if you are doing service for a fee and the time consumed to do the job is a concern. But I think the video is probably not considered a demonstration of ideal technique by many, and we can hope the Lisa PSU might be useful for another 40+ years, so taking the time to do a good job is warranted.

It is a good point that if the old material is dried out, re-using it would be a last resort.

My criticisms of the approach shown:

1. Not keeping the transistor leads clean: Apparently the silicone oil can migrate past the glass insulator around a lead and contaminate the transistor inside the case. Supposedly this takes a long time, so probably only an issue for ultimate longevity. I don't know what consequences there are to contaminating the inside, perhaps not relevant here to a switching/power transistor. Regardless, aside from saving the time it takes to be careful, there isn't any advantage to putting grease on the lead.

2. Using a power tool to attach the hardware: the time saved over dozens of repairs is completely wiped out when you have to fix a stripped/damaged tapped hole in a heatsink or threaded standoff. There are specific circumstances where a power tool might be used, but it should not be implied as standard procedure.

3. Not starting with clean surfaces: the best thermal contact is when the surfaces are as close together as possible. This can easily be compromised if the surfaces are not clean.

4. Using more grease than necessary: As above, the best thermal contact is when the surfaces are as close together as possible. The grease is there only to fill the gaps (since air is a good insulator), so there should be as little as possible. This is not a case of "the bigger the gob, the better the job" as AvE sometimes says tongue-in-cheek when tearing down equipment. (If you're not familiar with AvE, you might find him amusing, although some of the vernacular may be a bit opaque if you're not Canadian.)

Not specific to the video is checking the quality of the mating surfaces. Quality-control cutbacks are sometimes revealed as non-flat surfaces, raised dings, as well as burrs around holes. These prevent close contact and result in poor heat transfer (which results in failed transistors as they overheat). At minimum, check for and remove burrs around holes in heatsinks.

I may add more thoughts/opinions later.

Quote from: sigma7 on Yesterday at 07:36:30 PMTo be fair, if it is done that way, it might be fine, and probably is in many cases, especially if you are doing service for a fee and the time consumed to do the job is a concern. But I think the video is probably not considered a demonstration of ideal technique by many, and we can hope the Lisa PSU might be useful for another 40+ years, so taking the time to do a good job is warranted.

Good point.  40+ years, yes a definite goal to aspire to.  I'm in your debt for all the help you have shown me, and even these tips for success.  (ie Goal of 40+ years), which is "inline" with how they designed and made the Apple Lisa (from my exposure it looks like they didn't cut corners and did the best job to last 40+ years). 

I will do more research on the topic and find other videos.

Quote from: berskyboy on Yesterday at 11:49:43 PMdid the best job to last 40+ years

except for the I/O board battery though 

(Could they have known?)

Quote from: stepleton on Today at 03:28:34 AM

Quote from: berskyboy on Yesterday at 11:49:43 PMdid the best job to last 40+ years

except for the I/O board battery though 

(Could they have known?)

I think someone expected as much - witness the 16 year range of the RTC.