General Category > Lisa Troubleshooting and Repair

Lisa 2 PSU Woes

<< < (2/4) > >>

stepleton:
For what it's worth, I don't think we know if the Zener has failed --- I think sigma7@ was saying that we just haven't tested whether it's still Zenering. For example, we haven't tried to reverse bias it with a voltage and see if it conducts if the voltage exceeds a threshold. The easiest way to do that would be to take it out of circuit, which you could do by lifting a leg as mentioned. Bear in mind that you wouldn't just want to apply a voltage directly to both sides of the diode: you want a resistor to limit the current in case the zener does start to conduct.

The fact that in-circuit it's +18V on either side could mean that it's working fine, but that the SCR next door isn't responding to its gate.

sigma7:

--- Quote from: warmech on June 14, 2022, 03:58:33 am ---it's +18v on either side of CR9. Is that to say that the zener aspect of CR9 has failed?

I checked the number on the diode and it's a Motorola part: 1M12ZS2 - I assume this is just a generic 12V zener diode?

--- End quote ---

Since the voltage is +18 on both sides, that means the voltage across the diode itself is ~0V, which means it is either a dead short or there is only leakage current through it. Since you measured it as looking like a diode, then it isn't shorted, so no current.

Since there is +18 at the cathode, there is a circuit to the +12 rail.

Since it is +18 at the anode, it seems there is no circuit to ground on that side. That could be due to R13 being open. However, +18 at the anode should trigger the SCR, applying the crowbar. Since there is no crowbar effect, that's a second clue (or problem), so both could be due to no circuit from CR9 to both (R13 and CR10). Another possibility is that CR9 is connected to both R13 and CR10, but there is no connection to ground for R13 and CR10.

I suppose another failure mode could be that CR10 is shorted from gate to anode, cathode open, in which case R13 would burn up. So check for a short from the gate of CR10 to +12 rail if R13 is open.

The leads on CR9 look corroded, so check the diode lead is truly connected to the circuit board. ie. not in a hole with solder around it that doesn't actually make contact. This isn't normally a problem with boards that used to work, but if you've removed and re-installed the corroded component, the new solder joint might not be good.

Once you get the crowbar working, you may want to disable it to figure out why the regulation isn't working, but check the capacitors etc. will tolerate the high output voltages.

Come to think of it, since the regulation isn't working, the PSU may continue to drive all the current through CR10 rather than going click click click, causing CR10 to fail. So make very brief tests and check the temperature of CR10 (don't burn your fingers!) I think the fuse may save you from this condition, but not certain.


I think 1M12ZS2 is probably a 12V zener. Closest datasheet I could find easily is in 1988 Motorola Rectifiers and Zener Diodes Databook (Thanks Al!) https://archive.org/details/bitsavers_motoroladaRectifiersandZenerDiodesData_33936696/page/n393/mode/1up

FWIW, 1N5292 datasheet at https://archive.org/details/bitsavers_motoroladaRectifiersandZenerDiodesData_33936696/page/n437/mode/1up

edit: found 1M12Z5 at
https://archive.org/details/1965MotorolaSemiconductorDataManual/page/n58/mode/1up
Where
1 = 1Watt
 M = Motorola
   12Z = 12V Zener
        5 = 5%

No indication about S2, but does mention "series matched sets" with better tolerances were available, so perhaps that's 2%.

warmech:
Excellent news - the crowbar is now working! It was this line right here:


--- Quote from: sigma7 ---there is no connection to ground for R13 and CR10
--- End quote ---

that finally clicked in my head. Sure enough, testing for continuity showed zero connection between that whole trace along the edge of the PCB and ground. The issue ended up being a bad trace coming off a capacitor which has since been fixed and the PSU is now (un)happily chirping away as expected due to an overvoltage issue. Now, the secondary ground connection coming off of R29 was serviced by that trace so it's connected correctly again, but there is no change in voltage when attempting to dial it in either way. It was too hot to stay in my shop any longer, so I'll test more after work is over and the A/C has some time to cool things off out there.

Not out of the woods, but certainly seeing daylight through the canopy now. Thank y'all both so much - we've got to be in the homestretch now.

Edit: I'd like to state for the record that I feel like a complete idiot for missing this, lol.

sigma7:

--- Quote from: warmech on June 14, 2022, 04:30:31 pm ---... the PSU is now (un)happily chirping away as expected due to an overvoltage issue.

--- End quote ---
Great progress!


--- Quote ---Now, the secondary ground connection coming off of R29 was serviced by that trace so it's connected correctly again, but there is no change in voltage when attempting to dial it in either way.
--- End quote ---
... confused by that... R29 (the voltage adjust trimpot) doesn't connect to ground, does it? Resistor on one end, transistors on the other end and the wiper, no?

warmech:
My bad - that was to say that the secondary ground coming off of C33 (that is tied to R29 by way of R27 - and the +5v sense circuit) was connected to that. Fingers typed faster than my thoughts could keep up with.

Navigation

[0] Message Index

[#] Next page

[*] Previous page