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Plus/4 x 2 - black screens

RichH6109

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Joined
Oct 9, 2021
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I know there have been a few questions on this in the past but a Plus/4 novice would like some help in diagnosing a fault.

I obtained a mint condition Plus/4 (“untested”) a few weeks ago which outputted a black screen to TV when turned on (power LED comes on). I have since purchased a second Plus/4 in poor condition to act as a donor machine but this also has the same symptoms.

I’ve checked old threads and the suggestions seem to be to replace the chips en masse until a combination is found that works, however I’m not sure this is a good idea given that both machines have the same symptoms. I could start to buy replacement chips but I would start to incur serious expense to fix a cheap purchase.

Is there anything I can do to try and narrow down the potential fault? I’ve looked at the Diag 264 but I assume that this would no be recommended due to the lack of screen output. Is that correct?

Any help would be gratefully received!
 
a black screen can have many causes. It could be the TED, as suggested, or the PLA, but I also had a broken CPU in one of mine. There's a relatively easy mod that allows to use a 6510 microprocessor instead of the original cpu, that's quite hard (and expensive) to found; you can buy an adapter from eBay (it requires also a modified ROM) or do it for yourself. Google it, you will find a lot of info.

The PLA can be made out of an EPROM and an adapter, it's similar to the C64 PLA replacement. I made some of them using a simple multi-hole pcb, a 27512 eprom and some patience (of course it requires an eprom programmer).

With some luck you can take the chips from the donor and try them into the one you want to fix. I would try PLA, CPU, TED,ROMs and RAMs at first.... then I would try to make a replacement for PLA and CPU (the TED is quite expensive nowadays...)


BTW (almost) all the components (with only exception for the RAMs ROMs, IIRC) are the same among C16, C116 and Plus/4 (but you can use the plus/4 64k ram chips into a C16/116, and with a simple mod you can upgrade the C16/116 to 64k too).

-------------------EDIT

I was looking at my notes I wrote about the Plus/4 fix I did. Based on some info I found on the web, usually the black screen isn't associated to the TED.
Since I got 2 bad PLA, I would start from there. If you have an eprom programmer you can easily make a replacement... or you can buy one on eBay.
PLA is the weakest component in the Commodore line...
 
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The CPU was bad in mine. The CPU has to be running to initialize the TED. Of course it could be a lot of things but the TED and the CPU are the most common.

Yep, especially due to the usually poor quality of the MOS Technology products. Well, they weren't built to last so much, and honestly the modern electronic products have an higher failure rate even if compared with the weak MOS chips...

In my case the PLA was the worst component, 2 bad on 4 machines (not to mention the high rate of failure of the C64 PLA) but at least it's easy to diy a replacement. I always put an heat sinker on CPU, VIC/TED, PLA, SID, PIA, CIA, and every component that runs hot and it's prone to fail...

There is a FPGA based replacement available for the 7501/8501 CPU which is a drop in replacement with no other changes required.
Link: https://www.sellmyretro.com/offer/de...16~~%2B4-30475

Great to know, I didn't know. It's not cheap but at least it's an option.

There's a project to make an FPGA replacement for the TED: https://hackaday.io/project/11460-fpgated

and

https://hackaday.io/project/11460-fpgated/log/188130-fpgated-prototype-is-working-in-a-c16

On youtube I've seen a similar project for an FPGA replacement for the C64 VIC-II too.
 
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Hi RichH6109,

I agree with the more seasoned members of the board that a bad CPU or PLA is a likely culprit. But I did also want to mention and agree with giobbi about the TED; since I have a nonfunctioning C16, which gives a black screen. It is certainly because of a bad TED. I know this because I swapped this TED into another machine which does output to screen (my plus/4 that I am still fixing), and in doing so I then get a black screen from it.
 
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