| Posted By
ddni
on 2009-06-18
15:45:41
|  faulty plus4
Hi all first post here.
I am DDNI from the amiga forums... Mostly English Amiga Board
I had a Plus/4 as a kid and wanted to run my old Mercenary tapes again.
Got myself a boxed Plus/4 from ebay... Sadly it is faulty.
On power up the screen is filled with @ symbols and it flashes.
I read the troubleshooting guide on this site, sadly I don't have any diagnostic tools to start troubleshooting.
I have pulled, cleaned and reseated all the socketed ICs. The issue remains.
Is there anything that I can try over and above this and the guide?
Your time is appreciated.
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Posted By
Chronos
on 2009-06-19
02:28:23
 | Re: faulty plus4
once my plusi produced '!' on the screen and i cannot remove it. our experts said it produced by the ted chip, so i think its something similar and you need to replace it.
Wait for TLC coming he is a true hw expert! 
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Posted By
SVS
on 2009-06-19
02:54:54
 | Re: faulty plus4
It could be the hi ROM too, because the Font that on the Plus4 resides there. In fact the correct screen is fit by spaces ($20), your fault screen is fit by @ ($40). The problem of Chronos was $21 instead of $20.
Good luck.
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Posted By
ddni
on 2009-06-19
03:07:54
| Re: faulty plus4
Thanks for the advice chaps.
Looks like I may need to get another unit to rob of chipd...
Does anyone have any spare chips for the plus/4?
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Posted By
TLC
on 2009-06-19
06:45:45
| Re: faulty plus4
Hi!
Looks like a very interesting one .
Could you please do a test?... Turn the computer on, then press reset while holding run/stop down... let's see if you can get some valid response instead of flashing @ characters.
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Posted By
ddni
on 2009-06-19
09:37:19
| Re: faulty plus4
Hi holding Run stop and pressing reset has no effect.
Pressing reset seems to have no effect at all on the screen.
If I power on whilst holding reset, I get black stagged horizontal blocks across the screen. When I release reset, they disappear and the screen shows the flashing background filled with lines of @
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Posted By
ddni
on 2009-06-19
09:54:45
| Re: faulty plus4
Here is a screen shot of my error.
http://eab.abime.net/attachment.php?attachmentid=21832&d=1245419574
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Posted By
TLC
on 2009-06-19
11:43:01
| Re: faulty plus4
Okay.
When you turn the computer on for the first time, you'd normally see nothing (ie. black screen, or no valid videosignal at all) if there wasn't the CPU, the Kernal ROM and some ram (this latter one being important because of the organization of the Kernal; it uses subroutine calls that would never return without some stack ie. ram). You can perfectly simulate that situation by turning the computer on (I'm talking about turning it on after a long period of power-off) whilst holding reset. Upon power-up, the TED starts up from some default state -- PAL mode, blank screen, border is black -- but the rest of the init is done by the reset routine (which is in the Kernal ROM) and finished by the init of Basic 3.5 (which is in the Basic ROM).
Consequently, if (upon power-up) you can see a non-blank, non-black screen, that is perfect proof for the CPU, the Kernal ROM, and some of the RAM being at least partially correct. Moreover... if you can get some prompt by pressing run/stop + reset, that means all of the above do work correctly "enough" (TEDMON needs no Basic ROM but only the Kernal ROM + some RAM to be present... the computer is supposed to boot up with no Basic ROM at all, although in TEDMON only, which is still a nice feature). ...As we could see, this was not the case here.
The screenshot shows that wherever the TED fetches the screen data from, that area of the RAM has not been initialized by the Kernal. The screen code of @ is 0, whilst the screen code of the other typical character (code #191 here: http://www.df.lth.se/~triad/krad/recode/pet3.jpg) is $ff. The color code of black is 0, the color code of flashing bright green is $ff. If you'd check how the memory content of a Plus/4 looks like after power-up, you'd see similar patterns (mostly 00s and FFs in a row after each other + some other typical values inbetween). That may mean two things: either the Kernal ROM couldn't clear the screen (bad Kernal ROM, or bad RAM, or something around them that prevent them to work correctly), or it could indeed do that, but the TED fetches data from the wrong place (= bad Kernal ROM (wrong default values written upon TED initialization) or bad TED).
From that point and on, I would do two things: first, try booting up a ROM cartridge (game cartridge or such). If it boots, albeit with strange symptoms, that suggests a slightly faulty TED, not the Kernal. From the other hand, if it works correctly, or if it doesn't boot up at all, that might suggest a broken Kernal.
...Ultimately, I'd start testing the chips (the Kernal, the TED, and maybe the FPLA, in that order) by trying if they work in a known-to-work Plus/4... I know that's bad news... At least, you seem to have a working CPU (which is important). For the test, both a C-16, C-116 or a Plus/4 are/would be suitable.
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Posted By
TLC
on 2009-06-19
12:09:56
| Re: faulty plus4
Another (last) idea: let's suppose that the unit could boot up, we just couldn't see the screen (due to a possible fault in the TED, which prevents it to fetch screen data from $0800 which is it is specified by the Kernal).
For that's to (dis)prove I'd try moving the cursor first by using the cursor keys. ...The cursor is shown wherever it is positioned on the screen, regardless to the current screen memory bank setting. If I still couldn't judge I'd connect a floppy drive and press F3 (which is a directory command + enter)... if the floppy starts rotating that shows that the computer is still sort of alive.
...I'd suggest blindly entering commands from the keyboard to see if there's any effect, I'm just not sure if that can be done safely (ie. the keyboard is in a perfect order so we don't just get some syntax errors as the only result (that we couldn't see anyway)).
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Posted By
DDNI
on 2009-07-02
08:32:39
| Re: faulty plus4
SUCCESS!
It had a faulty U24
Thanks for your help
Now off to Targ
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Posted By
TLC
on 2009-07-02
13:19:03
| Re: faulty plus4
Glat that you could get it to work . U24. Hmmm. It's pretty rare to see CBM ROM chips to fail (I've only ever seen a single one, with one single failing bit so far)... finding a replacement (in case you purchased another fully working unit... consequently, now missing one of these chips) should be easy.
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