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DistantStar001

Apple //c Power Problem

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A while ago, my trusty //c decided to be a little less than trusty when it burned out both the fuses on the motherboard and in the power supply.  I've since replaced the fuses and managed to trace part of the fault to the DUV26 Transistor at Q2 on the voltage regulator board, so I replaced that too.  Now it no longer burns its fuses (good news!)  Unfortunately, I'm still not getting a 5v output from the regulator.  

 

I've removed and tested all of the capacitors.  The all tested good and showed no signs of bulging or leaking, so I put them back and continued probing.  That's when I came across something odd.  The HA17339 IC is grounded on pin 11.  

 

I looked up the data-sheet and pinout, and from what I can see pin 12 should be the only ground.  11 does have the expected continuity (with resistance)  with + on 11 to - on 13 (but not the reverse), as well as from pin 10, with + on 11 and - on 10 (again not in reverse), which seems to correspond to the data-sheet.  However, the sheet makes no mention of 11 to ground.  

Is this normal?  Or should I be replacing this IC?

 

Also are there any other points of failure that I should be looking for?  As always any assistance is appreciated.  🙂 

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Posted (edited)
10 hours ago, trash_44fr said:

Maybe this will help you.

Unfortunately, I already have this.  While, it was very helpful in identifying some components, my //c is a little newer.  So the schematics don't exactly match.  From what I can tell, the IC in question isn't even present in the regulator documented.  

 

Thanks though.  Even if it didn't help with the problem at hand, this is a terrific manual and very informative!  Honestly, every //c owner should give it a look over.  🙂 

Edited by DistantStar001

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So the good news is that I was able to determine that the HA17339 has failed and I was able to install a replacement.  However, it didn't solved anything.  The lines that connect pins 11 and 12 are still shorted elsewhere on the board as well, and I still don't know if this is normal.  All I do know is that they're not supposed to be on the chip.  

It also occurred to me that I forgot to mention which regulator I'm working on.  It's an Astec AA 7343A.  Hopefully that will help. 

I've been looking everywhere for a schematic for this.  But so far, I haven't had any luck.  If anyone has this (or even just a layout of the traces on the PCB) it would be extremely helpful.

 

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Posted (edited)

I replaced the regulator!  So now my //c is back and running!!! :-D  

 

For those who might be curious, the short between pins 11 and 12 is normal as it is present on the working regulator as well.  I still haven't given up on the original.  But now I have a working example as a guide to diagnose whatever the defective component is.  

 

So far, I've replaced the DUV26 Transistor at Q2, the HA17339 IC, all of the electrolytic compactors, and the regulator abled IC2 (not sure why it was abled that way).  I've also checked all of the diodes in and out of circuit.  Basically, I've been poking blind.  Hopefully the working one will point me in the right direction.  I'll post again if I figure it out. 

 

An additional question:  I noticed that the text has a noticeable rainbow-bleed on the screen.  Admittedly I'm using an old SAHRP CRT TV from the 90's, but its a bit more severe than it should be.  I remember a similar issue just before the old regulator died.  It took several years. And at first, I thought it was just the natural result of having been in storage for more than a decade.  But it got really bad the day it died.  Then the video started to cut out, and it just shut off and wouldn't power on again.  So I'm a little concerned about this.  Any suggestions on how to keep my beloved //c healthy would be appreciated.

Edited by DistantStar001
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Posted (edited)

Very valuable information, thanks for sharing!. I also have an Apple //c+ with a flaky PSU and I'll need to put some time into it, noy sure if they're the same PSU tho...

 

Old capacitors or a noisy PSU can cause a bad picture.

Edited by jltursan
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11 hours ago, jltursan said:

Old capacitors or a noisy PSU can cause a bad picture.

Thanks!!!  Looks like a noisy PSU.  Kind of a shame, though.  It's the original.  Maybe replacing the filter cap will help matters.  In the interim, I spliced a din connector onto an old laptop charger.  It seems to have cleaned the image up considerably.  Although I will probably want to recap the internal regulator when I get a chance.

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On 4/16/2021 at 2:50 AM, jltursan said:

Very valuable information, thanks for sharing!. I also have an Apple //c+ with a flaky PSU and I'll need to put some time into it, noy sure if they're the same PSU tho...

 

Old capacitors or a noisy PSU can cause a bad picture.

The //c+ has a different psu for sure. They dropped the external brick with the linear transformer and rectification diodes and ran the mains directly into the //c+.

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17 hours ago, nick3092 said:

The //c+ has a different psu for sure. They dropped the external brick with the linear transformer and rectification diodes and ran the mains directly into the //c+.

True!, well, I haven't been able to find any troubleshooting guide for the model and its PSU; so I'm afraid that I need to start opening it up and reclacing caps.

 

Thanks!!!  Looks like a noisy PSU.  Kind of a shame, though.  It's the original.  Maybe replacing the filter cap will help matters.  In the interim, I spliced a din connector onto an old laptop charger.  It seems to have cleaned the image up considerably.  Although I will probably want to recap the internal regulator when I get a chance.

 

Nice, good to know!

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