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Old 12 August 2021, 11:32   #1
rjnorthrow
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A500+ Serial and Parallel faults

In an otherwise working Amiga 500+, I have problems with the parallel and serial ports.

I was unable to use Amiga Explorer successfully so decided to build the test dongles as described here.

The results:





The power rails are lighting up the LEDs correctly on the dongles.

I removed the battery from the A500+ years ago and there was little to no corrosion around it. I can't see anything now.

I swapped out the CPU, Gary, Paula and Denise from spare A500 chips I had bought with no effect. I swapped around the CIA chips similarly with no effect (I have no spares).

I tested the dongles on an A1200 and they both work fine on that, giving green across the board.

I am using v1.18 of Amiga Test Kit. All other tests work, the only other slight problem is that the right audio channel is weaker than the left.

Any help to diagnose this would be greatly appreciated!
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Old 12 August 2021, 18:18   #2
solarmon
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Check the data traces going to the CIA chips.

Did you do a CIA test in ATK?
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Old 12 August 2021, 18:33   #3
rjnorthrow
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Thanks for replying. I will check the traces.

Yes, I did the CIA ATK tests, the first one shows that all passed and I'm not sure what I'm looking at in the second test
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Old 12 August 2021, 18:33   #4
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Also check U38 and U39, and traces/components ofrom if from the serial and parallel ports. These chips can also fail and may need to be replaced.
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Old 12 August 2021, 19:17   #5
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Can you show a picture of to the CIA test.
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Old 12 August 2021, 19:49   #6
rjnorthrow
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I'll get a picture of the CIA test.

I have buzzed out some of the traces, eg from 8 and 12 on the parallel port to U7->16 and U8->40&3 respectively for example and that seems fine. I'm using the diagram here:

https://archive.org/details/A500_Plu...mode/2up?q=CIA
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Old 12 August 2021, 19:55   #7
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Result of the CIA test:

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Old 12 August 2021, 20:21   #8
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Is there a way to test the U38 and U39 line drivers with a simple multimeter?
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Old 12 August 2021, 20:40   #9
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The CIA test results look OK, so it is not an issue with th data lines going to them.

I would suspect U38 and/or U39.

It would probably be best to use an oscilloscope or logic probe to see if there any activity on the those chip pins. But is not uncommon for them to need replacing.
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Old 12 August 2021, 20:47   #10
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OK, thanks for your help. I suspected that I would need a logic probe or oscilloscope to test those chips.
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Old 12 August 2021, 20:56   #11
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Just to rule it out, did you inspect and tested the serial and parallel port pins themselves? I.e. they are not oxidated, corroded or damaged?
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Old 12 August 2021, 21:05   #12
rjnorthrow
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Yes, I was buzzing out the serial signals from the port pins so I can rule that out. I inspected the parallel port sockets and those also look fine, I can see shiny copper inside. My multimeter probes do not fit in the socket holes but I think I can rule that out too.
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Old 12 August 2021, 22:01   #13
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Oh dear, I realised that I'd left the standard Denise in the machine from when I was testing and, after removing it and fitting the Super Denise, I broke the socket :-(

The machine turns on, there is no display and the green power light flashes. I think I'll have to send it off for repair.
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Old 12 August 2021, 22:32   #14
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Oh, sorry to hear that!

But how did you break the socket itself? Make sure you haven't put Denise back in the wrong way - I know I've done that before.

Since it is an A500+ then I assume it also has battery corrosion, that needs cleaning up and repairs work anyways?
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Old 12 August 2021, 22:38   #15
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Actually, in my haste I might just have done that - put Denise in the wrong way around! I will check that tomorrow as I have put it all away. I just assumed I broke the socket with my clumsy chip removing/replacing skills. It definitely isn't a good idea to attempt these things when one feels tired!

Luckily, I removed the battery before any damage was done, the board is perfectly clean. I replaced the battery with a coin cell and diode and the clock circuit is working perfectly so I know that is OK. I guess it may need a re-cap although they all look fine too.
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Old 12 August 2021, 22:51   #16
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I was kidding myself if I thought I could wait until tomorrow to test it! I've just got it all out again and you are absolutely correct! I put Denise in the wrong way round. Phew. I have the display again. I think I have mangled the socket a bit but it's obviously working.
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Old 13 August 2021, 01:40   #17
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I have ordered replacement parts for U38 and U39.
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Old 13 August 2021, 01:50   #18
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Look at Amiga PCB explorer (https://www.amigapcb.org/) for the Amiga 500; the single Amiga 500 PCB revision available is for a revision 8A board which is an amiga 500+ board.

For the Serial port issue:
Please be aware that the output pins 6, 8, and 11 of U38 (the MC1488 RS232 quad line driver) can be between +5 and +15 volts for a logic high and between -5 and - 15 volts for a logic low as per RS232 signal standards. To operate correctly IC U38 needs -12v on pin1, +12v on pin 14, 0V (gnd) on pin 7.

The excessive +/- voltage levels on U38 output pins can damage inputs on cheap logic probes or oscilloscopes. Check your logic probe or oscilloscope manual and confirm that it can handle those input voltage ranges.

For both the serial an parallel ports you can remove the covers if any on your testing dongles plug them in and use continuity tests from the exposed port pins to the motherboard pins for the affected socket; that will tell you if the motherboard socket end is the issue.
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Old 13 August 2021, 01:57   #19
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That’s great, thanks very much for the advice and testing suggestion. My board is an 8A.
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Old 13 August 2021, 01:57   #20
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Quote:
Originally Posted by rjnorthrow View Post
... My multimeter probes do not fit in the socket holes but I think I can rule that out too.

unravel a paperclip and wrap it around your probe. The other end of the paperclip is a good fit for female D-sub holes.
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