| Posted By
Imperious
on 2014-03-28
22:25:39
|  Suspected PLUS/4 Ram fault
Hi Everyone,
Been lurking here for about 2 months or so, about time I said Hello, need some help too.
Initially I was only really into my A500 Amiga, and c64 I got last year and repaired. I have now
gone a bit nuts and have 11 retro computers. 3 64's, one an immaculate c64c, a Vic20 with
Arfon Micro expansion, 3 TI99/4a's (my first computer), and finally added 3 plus/4's.
I have a bit of an obsession with spares, hence why I have some of these in 3's
The 3 PLUS/4's are from the States, I'm in Australia. The 1st one I got works perfectly and I have made
custom Joystick adaptors as well as modifying my SD2IEC with a 7 pin Mini Din as well as the standard c64
cassette connector. I also converted it to PAL and have heatsinked the TED and 8501.
The 3rd one has an annoying Ram fault, but I would like to figure it out myself.
Problem Is I do not understand how to figure out what DATA BIT corresponds to a character symbol.
I can find no explanation of this anywhere on the Internet.
There is another thread here about a ram fault and it is explained that characters with a hex code of
$20 and $22 are data bit 1. If I can underdstand how to work this out it will help myself and others
with similar problems. I have used the Kernal DIAG264 image, it gives 3 flashes for U13, but I have
replaced that and all is still the same. I need this final piece of the puzzle explained so I can get PLUS/4 number 3 up and running correctly.
Any help would be much appreciated Thanks.
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Posted By
MIK
on 2014-03-29
06:37:12
 | Re: Suspected PLUS/4 Ram fault
Welcome and worry not about having spares as your not alone. 
In most cases C16/Plus4's can be fixed from a simple chip change as these fail long before anything else soldered to the mother board.
Just some extra pointers as I'm not good on ram...
A black screen can be a dead CPU, sometimes when a CPU dies it also takes TED with it, but not always. A black screen can also be a dead TED but more than likely your actually see something on screen before they totally go, that being a perfect clue TED is failing when you see it. Indeed TED likes to hang-on before it totally dies.
Random characters on screen, normally the TED is failing. Ramdom characters via joystick port, TED is damaged but only related to the joy ports.
Faulty 3+1 roms can lead to the machine locking up as it boots, it may hang on the Basic screen as you switch on and will not allow you to do anything. These roms if faulty can be completely removed for good working or not as they are only software.
When they are plugged in, the Plus/4 is told to look for them, but removed it will ignore the fact they were ever part of the system. "3-PLUS-1 ON KEY F1" will be removed from the BASIC screen if removed and the machine will act like a Commodore 16, but with 64k of memory.
As for RAM I've yet to see a machine with failed RAM so other than changing them for working ones I'm useless. Hopefully the guy Crock will pass by as I'm sure he's good on RAM related issues.
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Posted By
crock
on 2014-03-29
07:56:23
 | Re: Suspected PLUS/4 Ram fault
Hi there Imperious, what is the background colour of the screen, can you take a screenshot? If you've already tried the RAM there's another couple of things which could cause the same symptom, including the multiplexers (74LS257's) or a ROM holding one of the datalines high or low.
If you've programmed a kernal version of Diag264, try taking all the other ROM's out, but not the PLA (251641-02), as this is necessary for the machine to run. I have never seen a flaky CPU or TED create that symptom, but it is possible. If you have a working machine, do some chip swapping to prove all the removable chips are good.
Rob
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Posted By
Imperious
on 2014-03-29
09:59:30
| Re: Suspected PLUS/4 Ram fault
Thanks heaps Guy's, I've actually fixed it now, it's just about 12am here so I'll do a proper
detailed post of how I figured it out with pics too in the Morning.
Just quickly though, I had already swapped out all Rom chips and the PLA before I even tried
DIAG264. 4 RAM chips faulty all up, but as said, more detail coming tomorrow Australia time.
Had to do some Brain rewiring to get my head around it too.
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Posted By
Imperious
on 2014-03-29
20:58:10
| Re: Suspected PLUS/4 Ram fault
I eventually figured out how to work out what data bit was faulty when I realised that the Data bit system worked the same as address lines on a ROM chip, 2,4,8,16,32,64, etc. Then I wrote out a table of sorts, but it was when I shifted the even numbers back one place that it all made sense.
U11 U12 U13 U14 U15 U16 U17 U18
D0 D1 D3 D4 D5 D6 D7 D8
1 3 7 15 31 63 127 255
On the boot screen everything was fine except these random characters on the screen, which were $, !, and a 0, where there should have been a space. Using the numerical (decimal) values for the characters and subtracting the value for space, which is 32, gave me 1 for the exclamation mark, 4 for the $, and 16 for the zero. That told me U11, U13, U15 were faulty.
DIAG264 gave me exactly the same results as far as the subracted values were concerned, as I had D, A, P on the screen where @ should have been.
Initilally, as DIAG264
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Posted By
Imperious
on 2014-03-29
21:16:41
| Re: Suspected PLUS/4 Ram fault
Sorry, hit the key for post by mistake (and cannot see a way to edit)
DIAG264 was giving 3 flashes for U13, so that was changed 1st which got rid of the D on the screen, the photo I'll post is after U13 was replaced. After replacing U11 and U15 DIAG264 moved onto the high ram test which gave me a BAD at $430A-08 MUX-UP. The 08 is $08 which is decimal 8 which corresponds to U14.
After changing that all RAM, ROM tests passed with flying colours.
I plugged in joystick, sd2iec, and audio and ran a game and demo, all working very nicely.
Here is links to the pics I took.
http://tinypic.com/r/23saes4/8
http://i57.tinypic.com/29llriu.jpg
http://i60.tinypic.com/16h8114.jpg
http://i61.tinypic.com/59uejn.jpg
http://i62.tinypic.com/21ozrlk.jpg
A BIG thanks to Crock for DIAG264, no doubt I would have replaced all the ram chips without that and the documentation that I have read. Once I got my head around how the data bit system worked it all became a lot clearer. Another Plus/4 working nicely.
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Posted By
MIK
on 2014-03-30
00:14:37
| Re: Suspected PLUS/4 Ram fault
Very cool. I'm glad you know what your doing and got another Plus/4 running. Congrats on that, fine achievement!
In so many posts time, about 7-8 it should ask if you wish to be a member, once that's done your be able to edit your posts, rate software ect...
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Posted By
Retro76
on 2014-03-30
05:44:26
 | Re: Suspected PLUS/4 Ram fault
Hey Great Technical Advice & Support Guys
This is why I am now back as a registered member !
It is great to hear there are now more of these amazing machines brought back to life, particularly in Australia
I have now brought back 2 c16's to life & will use them sparingly & preserve them for my lifetime.
Just interested Imperious on knowing what you will do with them now; sell or preserve?
As you know the 16's & plus4's are rare these days in OZ & I would love to get my hands on the 116 model which refuses to show up for sale anywhere in the world.
Oh BTW! Just bought a working PLUS4 computer online & it should arrive within a week!
Yaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhooooooooooooooooooo ;) Cant contain my excitement LOL
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Posted By
Imperious
on 2014-03-30
07:24:24
| Re: Suspected PLUS/4 Ram fault
Thanks Mik and Reto76.
At the moment I'll preserve them, all 3 are about as close to immaculate as they come, and all are working now.The only trick here in Australia was getting a step-down transformer as the power supplies are 110v AC, but that was only $23.50 for a 200 watt unit.
The 3 are all from the USA, and I have noticed that quite often they pop up in original boxes with all original documentation inside. 2 of mine are boxed with packaging intact, the other was the console only, but is still in great condition. I payed just over AUS $100 each for the boxed units, $65 for the other one.
I found out pretty quickly that there is hardly any NTSC games, 90%+ are PAL. This is likely why there are quite a few from the USA popping up on ebay. You have to wait for one with reasonable postage though, as it is all over the place, anywhere from $40 to over $100 which is too much. The plus/4 is easy enough to convert them to PAL, just replace the crystal with the correct 17mhz part and burn a PAL Kernal ROM, i bought a cheap clone of a Willem eprom burner for Christmas so that has proved an asset, was only $37 too.
I will be using my plus/4 sparingly as I am aware of how fragile the TED and CPU is. I came across a circuit here which is a diagram for making a 8501 replacement with a 6502, 6522, and some 74ls chips, so might give that a go at some point.
My work history is as an Electronics tech, but have very little engineering knowledge. I really enjoyed getting my head around this problem though, and same with the 2 c64's that I repaired last year.
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Posted By
Retro76
on 2014-03-31
05:25:42
| Re: Suspected PLUS/4 Ram fault
Mate you really know your stuff
My three machines are all PAL; the 16's from Victoria one had faulty cpu & the other a boxed version I got earlier this year had faulty TED. Both the chips were sourced from Germany for a hefty price. The Plus4 on its way & was well looked after from an old guy in Sydney.
My first imported computer from the states was a Vic20 with 117 rated voltage. Back then I didn't know about step-down transformers & got a nasty surprise when I plugged her into the 240v mains Oz outlet. Boom, smoke, crash, opera...............phew the smell was unforgettable.
Gotta say vintage computer experts & repairers like you are pretty hard to come by in OZ.
You would make a killing if you are in Victoria (Australia''s Vintage Capital) & advertise your services.
There was one guy called Space Taxi, a UK dude in Melbournes CBD that diagnosed & repaired everything in vintage computing but im not sure if he is still around?
If you can reproduce anything PLus4 Related (re: 8501 replacments etc) there will be many interested members here.
I know MIK would love to get his hands on the elusive 64k ram memory chips!
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