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Yet another backlit Portable power problem---where to probe?

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Hi all, excited to be a new member.


My friend and I have been debugging a recapped 5126 lately and are looking for some advice. Once again this machine was recapped some time ago  :)


The machine is in OK shape overall with a well-behaved 40MB hard drive. Unfortunately, leakage from the three caps under J13 (left-hand kb/trackpad port) took a toll on the circuit board. We've recently bridged two pins of J13 that were attached to broken traces over to their cousins on the other side of the board. Also, I just now finished wiring the interrupt switch directly to the Misc GLU to work around a rotten via just above RP201.


The board has a flaky SWIM. There is a replacement on standby. However, we suspect the power regulation may also be off, and this may have caused the SWIM to break in the first place. So we'd like to fix the power first before we exchange that chip.


The problem is similar to the behaviour discussed in this thread from 2015. Plugged into the wall and with the battery installed, the system runs well enough, although some screen flicker occurs when the drive spins up. Unplugged, the battery meter shows the power plunging very quickly. It takes about 15 seconds or so before the computer gives up the ghost and shuts down altogether.


The thread just linked suggests to me that bad continuity in the region south of J13 can cause problems like these, and given all of the continuity problems seen so far in that area, I wouldn't be surprised if more continuity issues were at fault, so it seems useful to check for that first. The thread suggested measuring continuity between C26 and pins 14/15 of the op-amp U1M, but those seem to be fine. What else might be worth probing?


For what it's worth, we're using the original charger and a replacement 6v sealed lead-acid battery of the usual kind.


Thanks for any help!


Edited by stepleton
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I will keep checking.


To narrow my search: the op-amp area has ten transistors, three capacitors, two ICs, two resistor packs, and a diode. Are any of these things not related to the power circuit that I'm debugging?


Checking out traces underneath J13 is difficult. Before I go get my desoldering gear ready, are there any traces that go under J13 that I should be sure to check?


Many thanks for any hints,


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They all are related. The majority of the transistors are for switching on and off various circuits. the 2N7002 transistors. 


Then the Op-Amp is all related to 5.2V regulation, battery level monitor, and the low-battery lockout. As well as the charging/not charging indicator. 


Also the two 1uF capacitors are debounce caps for both the NMI and Reset buttons. make sure those are connected as well as the pullups on the underside of the PCB. 

Edited by techknight
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OK, I think I am making some progress---I've discovered two more bad vias in the vicinity of C26. Can anyone answer about the following:


1. Does the negative lead of C26 ultimately connect to the end of R204 that's closest to the bottom edge of the board?


2. Does the right "leg" of Q207 (the lead furthest from the op-amp---the source of that FET) ultimately connect to the end of R151 closest to the nearby kb/mouse connector header?


3. What does pin 12 of RP201 connect to?


Thanks again for any help!

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Hah, I can answer question 1 for myself! The answer is NO. C26 does not connect to R204.


Bad probing on my part---I picked the wrong via on the way to Albuquerque. I found out the hard way after actually wiring the two together. The machine still worked, but something inside made an ominous hissing noise. After booting, the flashing Apple menu low-battery indication was blinking, and a dialog box popped up warning that the computer would shut down in a few seconds. I'm pretty sure the low-battery lockout was engaged, too.


This was an embarrassing mistake---I'll be more careful in the future  :O


Now, with the C26-R204 bodge removed, but the Q207 (source) to R151 connection still in place, the machine is now slightly better than how it was before, with mostly minor changes, and one big one. Minor changes:


1. When plugged in with a fully-charged battery, the machine now agrees that the battery has a full charge. Previously, the battery desk accessory would show the battery about 2/3 full. Now the meter nearly tops out (always a few pixels shy of full).


2. Screen flicker observed during hard-drive spin-up doesn't seem quite so bad as it has been, although maybe this is wishful thinking.


The big change: the portable's gotten more confident about its battery. Starting plugged in, with a full battery indication, the disk spinning, and the backlight at minimum intensity:


1. After unplugging, there are a few seconds of the battery remaining full before charge appears to diminish rapidly. After a minute it is mostly gone, the indicator hovering just above the horizontal "low" marker.


2. Shockingly, the Portable remains on, which is new! It will continue running for at least another minute with the power wavering just above the marker. There is considerable backlight flicker, but the machine does not shut down, which is new. (Previously it would shut down after unplugging in under 30 seconds.)


3. Plugging back in after two unplugged minutes, the flickering stops and the power rebounds, showing full charge after about 5m30s. It had nearly made it that far after 5 minutes, but then the HD spun down as usual, allowing the battery to charge the rest of the way.


I may next try to see just how long it will run.


Other observations:


After drive spin-up, the meter jumps down to about 4/5 full but then gradually restores itself over the next half minute or so. (Perhaps this is normal---it does take power to start those platters spinning.


So, good progress! But not perfect yet...


Thanks for reading---always happy for hints!


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And the answer to the question "how long will it run on the battery" is:


At around 1 minute: the battery meter has already dropped to a few pixels above the "reserve power" line.

At around 36 minutes: the dialog box announcing "reserve power" (and "dimming the screen") appears.

At around 44 minutes: I notice the battery meter dip beneath the "reserve power" line for the first time

At around 76 minutes: the dialog box announcing that we're down to 25% of reserve power appears.


So that's where I put the machine to sleep and plug it back in.


That's a big functionality improvement, but still very mysterious!



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Also, do you know for a fact the battery is good? 


Are you using true cells? or are you using one of those cheapo black UPS 6V batteries which I have had nothing but problems with? 


The heavy flickering means the voltage is getting low, almost to the threshold point which is roughly 5.8VDC from the battery. 


6.4V is fully charged. 5.8V is dead. According to the design. So if you rapidly fall to 5.8 and below, your battery is junk. 


Monitor it with a good DMM. Also monitor the 5.2V rail simultaneously with another DMM to keep an eye on the the system. This will iron out a few variables as there are too many to try and factor out at the current moment as things stand. 

Edited by techknight
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Could well be the battery, which is in fact one of the conventional 6V sealed lead-acid batteries. It is a new one, so I would hope it would be dependable for a little while, even if they do become flaky before long.


I will see if I can get access to a good bench power supply so I can observe the behaviour of the battery monitor at different voltages.


I'll plan on measuring the +5v at the floppy port.


@Macdrone no cap replacement on the display yet, but since the flickering is so well correlated with low battery readings from the battery monitor, I'm betting it's still related to power management.


What is the recommendation for replacement battery cells as of April 2017? Older threads mention items that are no longer in production...


More as it comes, and thanks as usual...

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Like I mentioned before, you need to take measurements with a DMM at the battery to make a determination. Easier than dragging out a power supply. 


However, you can use a bench top power supply for voltage regulation, That would rule out the machine I guess. 

Edited by techknight
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Hello again, back now with some observations now from some bench power supply experiments!
A collaborator and I attached the supply to the battery compartment power contacts and left the wall power adapter unplugged. Here's what we observed:
1. Battery charge sensing
First of all, the battery meter DA seems to show reasonably correct indications for different voltages from the bench supply. At 6.4v the battery meter is usually full except for a single pixel, and at 5.8v the battery meter is completely empty. A completely full (all black) meter never appeared at any voltage we were brave enough to try. Intermediate voltages within the 5.8v-6.4v bounds seemed to produce sensible and proportional battery meter indications.
We noticed that the battery charge indication varied depending on power demand. Depending on factors like whether the drive was spinning, the intensity of the backlight, and whether the computer had been idle long enough to reduce the clock speed, the battery DA could vary by up to five or six pixels. This variation seemed most pronounced to me in the middle of the voltage range.
Photos of these experiments are available if they would be helpful.
2. Current draw
With the hard drive spinning and the backlight on at its highest intensity, the bench power supply indicated that the Portable drew in the range of 1.2 to 1.3 amps, with the median draw at 1.23A.
With the hard drive not spinning and the backlight at the lowest intensity (but still on), the Portable drew between 0.5 and 0.55 amps, median 0.5A.
As one might expect, current draw spiked when the hard drive spun up. This transient draw was considerable, with the power supply reporting around 2.4A and once as high as 2.6A. We're not sure if this is normal for the Portable or if the drive is consuming too much power in its old age.


3. Voltage regulation
We monitored the Portable's 5v and 12v rails with the help of an oscilloscope, probing at the +5 and +12 lines of the floppy drive port. During monitoring, we varied the supply voltage, varied the backlight intensity, allowed the hard drive to spin down and back up, and did random things on the computer itself (starting programs, etc.) to try and encourage hiccups on the power supply lines.
The 5v line never varied from its value (5.1v IIRC) no matter what we did. We set the oscilloscope to trigger on fairly small deviations from this baseline value, and never managed to trip a threshold in either direction.
The 12v line (which we observed holding 11.8v steadily) sagged momentarily only one time during our experimentation, when that 2.6A draw reported above occurred.  The power dip went as low as 9.5v. We observed no other deviations, even during multiple other drive spin-up operations.
4. Other notes
The Portable's wall adapter was measured as supplying 7.5v under no load.
When plugged into the Portable, the voltage across the battery terminals (with the microswitch depressed and no battery installed) was measured at 6.86v.
My collaborator had their own 6v sealed lead-acid battery (the "wrong kind" we discussed earlier---the Cyclon replacements are still being shipped). Installing this battery in the Portable and then unplugging the wall power, we observed the same rapid voltage drop we observed with my own battery. Restoring the wall power showed the same almost-as-rapid voltage rise.
During our session, I observed once after rebooting the plugged-in Portable that the battery meter DA showed the battery as "half full" but not charging (no lightning icon). Only after unplugging the Portable and plugging it back in again did the charging icon appear.
I think that's about all we have to relate at this point. Please feel free to suggest further experiments. I will have fresh Cyclon batteries very soon, but I'm starting to wonder whether the charging circuit in this Portable might also have issues.
Thanks for reading!
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Everything is normal, within normal limits. Your power and control circuitry as well as the window comparator is fairly healthy. I dont see any issues thus far. 


The fact that the battery meter never makes it to 100% concerns me a bit, but that could be the transistor in the ADC circuit as well as an out-of-tolerance 10K resistor thats on the backside of the board next to the PMU IC. Or the PMU IC itself. 


check your battery level when the power supply is set to 6V. it should read roughly 1 quarter, or 25% charge. maybe a tiny bit more, or less. but it should NOT be grossly close to 100 or grossly close to 0, if so then more attention needs paid to out of tolerance resistances in the charge monitoring path. (or a disconnected resistance) 

Edited by techknight
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Thanks for having a look!


I think we're in pretty good shape with regards to the power indications you describe. I don't have ready access to the bench supply we used for yesterday's experiments, but I do have the photos I mentioned. We don't have an exact 6.0V reading in any of those, but if you interpolate between the readings we do have, it seems to be in the right ballpark.


(Our bench supply had a very sensitive voltage control knob, so it was difficult to obtain any specific setting---just a gentle tap on the knob would shift the output voltage by .05v.)


With the drive spinning and the backlight at maximum intensity (at least until the Portable throttled it down due to low power):



With the drive not spinning and the backlight at minimum intensity (but still on):



If these look healthy to you---not counting the never-quite-full power indication---it might well be that it's just the battery that's unhealthy. Fortunately, my Cyclon replacement has arrived today. I'm keen to try it out (hooking it up via alligator clips at first), but before I do, I'd be grateful for advice on how to approach charging for best results from the battery. Is it better to leave the computer plugged in as much as possible? Should I only charge after the battery has had a chance to discharge most of the way? And so on.


There is an enormous amount of folk advice out there about the care and maintenance of lead-acid batteries, and plenty of discussion of gremlins like sulfation and memory effect, but perhaps the Portable community has more specific and informed guidance.


(From the photos: the signature on the display was obtained when the signer gave a talk at my university fourteen years ago. He said that Portables were cool machines  ;D



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Everything is factory normal there. 


Also, make sure your battery wiring harness is good, clean, unpinched, or has no corrosion that can increase the resistances in the wire, creating a voltage drop between the battery and the logic board itself. 


If your clipping your bench supply directly on the battery leads, it kinda rules that out. 


But it might be worth doing a good inspection of that. 

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  • 2 weeks later...

So, after taking a little break to finish up that NeoWidEx thing over in the 1,2,3,Lisa forum, I'm back to bug hunting in my Portable.


I don't trust my charger circuit now. When I leave the thing plugged in to my wall adapter, but with no battery installed, the voltage at the battery terminals varies slowly between 6.6 and 6.8v. Maybe it makes sense for it to wander randomly with no load, but that still seems fishy to me.


Question: C116, positive terminal. Is it supposed to connect to pin 20 of U15H (PMGR)?


I think a trace there might be broken. If it is, would it have anything to do with battery charging?


There are a couple of vias between C116 and U15H, including one very close to C116's negative terminal---that's close enough for me to worry about rot! (And the via connects to one of the internal PCB layers, I think, so it's impossible to trace it by eyeball.) So, are the other continuity checks I should do if I'm worried about this trace?


Thanks for the advice about the wiring harness. I would have told you that it looks fine, but unfortunately just now the positive lead has snapped off of the 9v battery terminal. Easily repaired, but annoying!


More as the search continues...

Edited by stepleton
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if I recall, C116 is the contrast network filter for the LCD display. 


as far as the battery terminals varying with no battery, thats not surprising considering the power supply circuit isn't designed to run without one! 

Edited by techknight
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Huh. Well, fair enough. Thanks for the quick reply.


The C116 issue gets deep into "ain't broke, don't fix" territory from the user's perspective, since contrast has always worked well enough for me. Considering that the negative terminal is tied to ground, maybe it's doing some kind of coupling or smoothing that we can get away without doing under most circumstances...


(NB: obviously I'm not an EE.)


(Also, just to check: does "contrast" mean the same thing as "backlight intensity" in this case?)

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Via rot around C11 brings the latest additions to the sad census: traces connecting U13C (buffer) pin 2 to U14J (VIA) pin 10, and U13C pin 12 to U14J pin 14 are also damaged. I'm curious what those buffers and that VIA do...


The positive leads of C10 and C11 both seem to be more or less on top of vias as well, and I'm not sure whether those are sound.

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