| Posted By
stRing
on 2019-02-09
16:50:02
|  Dead Plus 4 (not by TED or CPU)
Hello all from Poland.
I recently purchased Plus 4 from eBay as non working (with flipped TED/CPU upside down in their sockets - of course I did not plug it this way). I checked CPU and TED on other machine - both dead. So I put working CPU/TED from other machine to this one, solder C64 power socket (was square before) and Plus 4 did not run.
It shows black screen all the time.
What I made else:
0. Changed capacitor
1. Checked voltage - +5V on CPU line
2. Checked for short ground to 5V - no short - only ca. 300ohm between them
3. Checked Lum signals only from TED's pin 23 to TV - still black screen (as to compare - my working C16 shows pale b&w screen on this pin)
4. Changed all memory chips for 4164
5. Changed: U23, U24, U25, U19 from other working machine
6. Put new 74LS139, 74LS257 (both)
7. Checked crystal near TED with other working
8. Checked reset (switch, swap IC 555, swap IC 7406)
And I gave up... Still getting black screen. Can anybody help, what can I check more?
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Posted By
MMS
on 2019-02-10
15:04:29
 | Re: Dead Plus 4 (not by TED or CPU)
Hi,
The experts will surely help. My 2cents:
Our usual "toplist"
8501 checked
TED checked
PLA I do not see on your list.
RAM checked
PLA in C64 is a similar troublemaker, than in 264 series. So I think it worth a try. C64 version is not the same.
There are some modern retrofit versions too.
Commodore 16, 116 and Plus/4 replacement for PLA Programmable Logic Array - 251641-02
The problem:
A faulty PLA can cause: Blank screen, no border, colored screen or flashing color garbage instead of startup screen, intermittant loss of cursor, screen freeze and/or program crashes after warmup or random characters on screen.
The original chip 251641-02 runs very hot and is very poor reliable. This is the first cause of problems on Commodore computers.
Replacing the fault PLA with an original chip doesn't fix the problem definitely due to their poor reliability.
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