| IIe motherboard short [message #375074] |
Sun, 28 October 2018 16:34  |
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Originally posted by: cb meeks
I'm trying to find out why one of my IIe motherboards does not work.
I checked the resistance from pin 1 (GND) and pin 3 (+5V) & between pin 1 (GND) and pin 4 (+12V) of the power connector (on the board) and I get very high resistance of 600-800 Ohms.
I have a working IIe and when I do the same thing, I get very low resistance (50-80 Ohms).
So I think there may be a short somewhere.
I'm going to look over the entire board for anything obvious...what else can I do to help find the short?
Thanks for any suggestions.
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| Re: IIe motherboard short [message #375075 is a reply to message #375074] |
Sun, 28 October 2018 16:47  |
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Originally posted by: James Davis
On Sunday, October 28, 2018 at 1:34:55 PM UTC-7, cb meeks wrote:
> I'm trying to find out why one of my IIe motherboards does not work.
>
> I checked the resistance from pin 1 (GND) and pin 3 (+5V) & between pin 1 (GND) and pin 4 (+12V) of the power connector (on the board) and I get very high resistance of 600-800 Ohms.
>
> I have a working IIe and when I do the same thing, I get very low resistance (50-80 Ohms).
>
> So I think there may be a short somewhere.
>
> I'm going to look over the entire board for anything obvious...what else can I do to help find the short?
>
> Thanks for any suggestions.
Would look for the opposite. An open circuit, sort of. A short would cause lower resistance = 0-ohms.
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| Re: IIe motherboard short [message #375077 is a reply to message #375075] |
Sun, 28 October 2018 17:23 |
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Originally posted by: cb meeks
On Sunday, October 28, 2018 at 4:47:50 PM UTC-4, James Davis wrote:
> On Sunday, October 28, 2018 at 1:34:55 PM UTC-7, cb meeks wrote:
>> I'm trying to find out why one of my IIe motherboards does not work.
>>
>> I checked the resistance from pin 1 (GND) and pin 3 (+5V) & between pin 1 (GND) and pin 4 (+12V) of the power connector (on the board) and I get very high resistance of 600-800 Ohms.
>>
>> I have a working IIe and when I do the same thing, I get very low resistance (50-80 Ohms).
>>
>> So I think there may be a short somewhere.
>>
>> I'm going to look over the entire board for anything obvious...what else can I do to help find the short?
>>
>> Thanks for any suggestions.
>
> Would look for the opposite. An open circuit, sort of. A short would cause lower resistance = 0-ohms.
So, why is it when I put the continuity tester on my GOOD motherboard (between PIN 1 and PIN 3) I get an audible tone? When the bad board, I get 650Ohms?
I would have thought on the good board there would be no audible tone because GND and VCC shouldn't allow current when unplugged?
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| Re: IIe motherboard short [message #375078 is a reply to message #375077] |
Sun, 28 October 2018 17:43 |
Michael J. Mahon
Messages: 1772
Registered: October 2012
Karma: 0
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Senior Member |
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cb meeks <cbmeeks@gmail.com> wrote:
> On Sunday, October 28, 2018 at 4:47:50 PM UTC-4, James Davis wrote:
>> On Sunday, October 28, 2018 at 1:34:55 PM UTC-7, cb meeks wrote:
>>> I'm trying to find out why one of my IIe motherboards does not work.
>>>
>>> I checked the resistance from pin 1 (GND) and pin 3 (+5V) & between pin
>>> 1 (GND) and pin 4 (+12V) of the power connector (on the board) and I
>>> get very high resistance of 600-800 Ohms.
>>>
>>> I have a working IIe and when I do the same thing, I get very low
>>> resistance (50-80 Ohms).
>>>
>>> So I think there may be a short somewhere.
>>>
>>> I'm going to look over the entire board for anything obvious...what
>>> else can I do to help find the short?
>>>
>>> Thanks for any suggestions.
>>
>> Would look for the opposite. An open circuit, sort of. A short would
>> cause lower resistance = 0-ohms.
>
>
> So, why is it when I put the continuity tester on my GOOD motherboard
> (between PIN 1 and PIN 3) I get an audible tone? When the bad board, I get 650Ohms?
>
> I would have thought on the good board there would be no audible tone
> because GND and VCC shouldn't allow current when unplugged?
>
>
A continuity test from +5 to ground is not very informative unless it
measures a very low resistance (<1 Ohm, that is, a short) or a very high
resistance (>100kOhms, a virtual open).
Try powering the board with an Apple power supply (which is protected
against shorts and opens) and explore its behavior: monitor output and
speaker output. This is informative, telling you whether the processor is
executing the reset code (“beep”), the video generator is running, etc.
If nothing happens at all, then it’s time to start taking voltage
measurements at critical locations and verifying that the clock oscillating
is working.
Needless to say, troubleshooting an Apple (or most other things) is a
many-spendored thing, and experience and accurate information are critical.
Good luck!
BTW, as noted in the earlier response, a short can only cause a resistance
measurement to drop, never increase. Conversely, an open can only cause a
resistance measurement to increase.
--
-michael - NadaNet 3.1 and AppleCrate II: http://michaeljmahon.com
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| Re: IIe motherboard short [message #375084 is a reply to message #375078] |
Mon, 29 October 2018 03:56 |
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Originally posted by: James Davis
On Sunday, October 28, 2018 at 2:43:22 PM UTC-7, Michael J. Mahon wrote:
> BTW, as noted in the earlier response, a short can only cause a resistance
> measurement to drop, never increase. Conversely, an open can only cause a
> resistance measurement to increase.
>
> --
> -michael - NadaNet 3.1 and AppleCrate II: http://michaeljmahon.com
Thanks for the confirmation, Michael.
James
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| Re: IIe motherboard short [message #375262 is a reply to message #375083] |
Thu, 01 November 2018 11:26 |
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Originally posted by: cb meeks
On Monday, October 29, 2018 at 3:48:36 AM UTC-4, James Davis wrote:
> On Sunday, October 28, 2018 at 2:23:47 PM UTC-7, cb meeks wrote:
>> So, why is it when I put the continuity tester on my GOOD motherboard (between PIN 1 and PIN 3) I get an audible tone? When the bad board, I get 650Ohms?
>>
>> I would have thought on the good board there would be no audible tone because GND and VCC shouldn't allow current when unplugged?
>
> Sounds like the connector or the solder joints attaching it to the motherboard may be faulty. But it could be something to do with different versions of motherboards, too.
>
> Have you got a copy of SAMS Publishing's "Apple II Plus/IIe Troubleshooting & Repair Guide" by Brenner? It might be of great interest to you for your A2 repair projects: < ftp://public.asimov.net/pub/apple_II/documentation/hardware/ misc/sams/Sams%20Apple%20II%20Troubleshooting%20&%20Repa ir%20Guide..pdf>
I'll take a look at that book. Thanks!
So, as an update....I checked all of the electrolytic caps and all of them except two showed infinite resistance. Which is what I would expect. Two of them showed 600+ ohms. So, I clipped those two. The GND to +12V short seems to be gone now. However, the GND to +5V is still there.
So I thought...what the heck. I clipped all electrolytic caps. (I figured I could recap the board anyway). The short is still there.
I'm really stumped on this one. I will read through that book and see if I can find some insight.
Thanks for any suggestions.
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| Re: IIe motherboard short [message #375268 is a reply to message #375262] |
Thu, 01 November 2018 13:25 |
Michael J. Mahon
Messages: 1772
Registered: October 2012
Karma: 0
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Senior Member |
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cb meeks <cbmeeks@gmail.com> wrote:
> On Monday, October 29, 2018 at 3:48:36 AM UTC-4, James Davis wrote:
>> On Sunday, October 28, 2018 at 2:23:47 PM UTC-7, cb meeks wrote:
>>> So, why is it when I put the continuity tester on my GOOD motherboard
>>> (between PIN 1 and PIN 3) I get an audible tone? When the bad board, I get 650Ohms?
>>>
>>> I would have thought on the good board there would be no audible tone
>>> because GND and VCC shouldn't allow current when unplugged?
>>
>> Sounds like the connector or the solder joints attaching it to the
>> motherboard may be faulty. But it could be something to do with
>> different versions of motherboards, too.
>>
>> Have you got a copy of SAMS Publishing's "Apple II Plus/IIe
>> Troubleshooting & Repair Guide" by Brenner? It might be of great
>> interest to you for your A2 repair projects:
>> < ftp://public.asimov.net/pub/apple_II/documentation/hardware/ misc/sams/Sams%20Apple%20II%20Troubleshooting%20&%20Repa ir%20Guide.pdf>
>
>
> I'll take a look at that book. Thanks!
>
> So, as an update....I checked all of the electrolytic caps and all of
> them except two showed infinite resistance. Which is what I would
> expect. Two of them showed 600+ ohms. So, I clipped those two. The GND
> to +12V short seems to be gone now. However, the GND to +5V is still there.
>
> So I thought...what the heck. I clipped all electrolytic caps. (I
> figured I could recap the board anyway). The short is still there.
>
> I'm really stumped on this one. I will read through that book and see if
> I can find some insight.
>
> Thanks for any suggestions.
>
I’m not sure what you mean by a “short”.
It’s normal for there to be current flow from +5 to ground—after all,
that’s what makes the computer work. And the polarity of any resistance
measurement is also critical to any device containing active devices or
diodes.
As I said before, resistance measurements from a power rail to ground are
not particularly informative or meaningful.
Try the board with a short-protected power supply, like the Apple supply.
If it just “clicks”, then you probably do have an excessive load problem,
but if it doesn’t, the video and audio output of the board will provide
useful information.
Don’t worry—there is very little risk in connecting a good Apple power
supply to the board, and its the only way to assess the behavior of the
board.
BTW, re-capping is practically never needed for Apple main boards. If a
capacitor does fail, it’s almost always “open”, not shorted. Main board
electrolytics live in a relatively cool environment, and seldom “dry out”.
--
-michael - NadaNet 3.1 and AppleCrate II: http://michaeljmahon.com
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| Re: IIe motherboard short [message #375272 is a reply to message #375268] |
Thu, 01 November 2018 14:38 |
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Originally posted by: frank_o_rama
A good start would be plugging in the power supply and checking the output voltages with it disconnected from the board. If all 4 voltages are in spec, then try it connected. A bad electrolytic could take out another component like a power transistor that could show up as a short. But, replacing all your old electrolytics now won't tell you that. The supply would still not work if that was the problem.
An ESR meter can measure leakage without the capacitor under test being disconnected, unlike a continuity test. Helpful for power supply problems, which tend towards out-of-spec caps.
f
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| Re: IIe motherboard short [message #375284 is a reply to message #375074] |
Thu, 01 November 2018 16:36 |
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Originally posted by: cb meeks
On Sunday, October 28, 2018 at 4:34:55 PM UTC-4, cb meeks wrote:
> I'm trying to find out why one of my IIe motherboards does not work.
>
> I checked the resistance from pin 1 (GND) and pin 3 (+5V) & between pin 1 (GND) and pin 4 (+12V) of the power connector (on the board) and I get very high resistance of 600-800 Ohms.
>
> I have a working IIe and when I do the same thing, I get very low resistance (50-80 Ohms).
>
> So I think there may be a short somewhere.
>
> I'm going to look over the entire board for anything obvious...what else can I do to help find the short?
>
> Thanks for any suggestions.
Couple of notes...
I don't have an ESR meter so no real way of testing how good a cap is.
I don't normally recap my vintage computers for the sake of recapping. Especially my Apple gear.
I have plugged the board into a known, working A2 power supply. Nothing comes up on the screen. There are two LED's on the board. The LED closest to the PSU lights up. The LED on the other side of the board (near the cassette jacks, etc.) doesn't light up at all.
I will have to take some more measurements to see what the voltages are around the board. I jumped the gun too soon by clipping the caps. But I can get those replaced quickly.
I didn't try connecting a speaker previously. But I can try that too.
Thanks
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| Re: IIe motherboard short [message #375289 is a reply to message #375284] |
Thu, 01 November 2018 17:02 |
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Originally posted by: frank_o_rama
That second LED near the cassette jacks lights up for a beep, if no speaker is connected.
Try adjusting the video signal level pot near the cassette ports. Sounds like you might have multiple problems (no beep=bad ROM, RAM, processor; no video=something wrong in the video circuit.
f
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| Re: IIe motherboard short [message #375313 is a reply to message #375289] |
Fri, 02 November 2018 06:55 |
retrogear
Messages: 245
Registered: November 2012
Karma: 0
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Senior Member |
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One good starting point would be check 65c02 pin 37 for approx 2V. If 0V or 5V then then you have a clock problem. The master clock is divided down and goes to the uP at 37.
Larry G
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| Re: IIe motherboard short [message #375314 is a reply to message #375289] |
Fri, 02 November 2018 07:05 |
retrogear
Messages: 245
Registered: November 2012
Karma: 0
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Senior Member |
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Since you have a known working power supply and if the voltages stay correct, check the 65c02 pin 37 for approximately 2V. That is divided down from the master clock. If it's 0V or 5V then clocks are dead and we can start back tracking to the master.
Larry G
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