Posted By
TechMusic
on 2019-12-02
11:34:07
|  Commodore 16 Troubleshooting
Recently, I've obtained a nice condition C16 with its original box, for a decent price. Sadly, all it'll do is produce a flicker, then a black/blank screen with faint vertical lines on my Commodore 1702 monitor.
Today, I've gotten in a second C16, this one listed as faulty, and in fairly poor condition. However, it does a little bit more than the good condition one: It produces either garbage on screen, displays a solid color, or the blank screen with vertical lines like my other C16. Sometimes it'll also make hanging squarewave beeps, varying in pitch. Resetting often does affect both the display and audio.
I've swapped pretty much all chips across the both boards, and have come to the conclusion that the PLAs, ROMs, TEDs, and the boards themselves, do not make a difference in this behavior. The CPU from the good condition C16 still only produces the blank screen (and the TEDs stay cold), whereas the other CPU will still produce garbage video and audio (with the TEDs becoming slightly warm, but not hot).
Testing the voltages at the regulator show that the 9V is a bit low (8.89V or so), and the 5V pretty much being bang on (5.06V) on both boards.
I've also tried piggybacking RAM on both boards to rule that out. I've used known good TI 4416 DRAMs from an Atari 600XL that received a 64K RAM upgrade. This didn't make a difference on both boards, but it should be noted that none of the supporting logic chips, or the DRAMs, get hot.
I've recorded some footage of the behavior, you can hear me hit the reset button or occasionally flip the power switch in these videos, however I can not seem to post these YouTube links without hitting the spam filter.
This behavior gives me some hope that both TED chips are still working, given audio and video are still produced, albeit garbage, and that something else is causing garbage data to go to the TED. A logic probe has been ordered a few weeks back from China, and should arrive in the coming days, but I've never used one and am not 100% sure what to look out for, any advice on this would be appreciated, as well as thoughts as to what could be causing these issues.
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Posted By
SVS
on 2019-12-03
02:08:29
 | Re: Commodore 16 Troubleshooting
Hi,
it's hard to guess what is the problem(s). But IMO I bet on a 8501 fault. The best try you can do is to have a working C16 and to substitute its surely working chips with the other ones in order to know which is good and which is bad.
Many years ago I had a similar behavior on my just bought Plus4 and I had to substitute the 8501.
If you can see, inside the garbage on screen, some alphabetic characters then it is probable that the TED and the ROM are good.
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Posted By
candle
on 2019-12-03
07:16:29
| Re: Commodore 16 Troubleshooting
quick and dirty way would be 8510 swap, you don't need eprom for that, serial transmission won't work, but for the test, just if it boots it will suffice
missing sync is bad thing, and it seems it first case, but having excluded TED as you did makes it repairable
i would start with clock circuit of that first board you got yourself, some kind of oscilloscope will help
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