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Atari 400 - faulty 48K RAM cartridge, but no mod wires used.


davemac
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Hello everyone,

 

I bought my Atari 400 in February 1983. I thought it had been thrown out many years ago, but I recently found it in my mother's cupboards!

I tried powering it up, but it won't work. I tried a known-good 16K RAM card from my old Atari 800 and Star Raiders plays fine.

 

I remember spending every cent on upgrading the RAM from 16K to 48K back in '83, which worked fine up until I last used it, which was probably around 1989/90.

 

The odd thing is that the motherboard needed no mod wires to make it work. I've looked at many threads on here and on other sites and all upgrade kits seem to have needed the 4 mod wires from the RAM connector to the ROM cartridge connector.

 

My board is a 'Mapsoft' one, so I'm wondering if anyone else has seen one, and perhaps they can advise me as to how to go about troubleshooting faults on it. I've socketed every IC, and have replaced all 74LSxxx ICs, followed by the RAM ICs themselves. I substituted the Mostek 4564-20 ones for 4164-15 chips, which I'm told will work. The originals were 200mS, but the new ones are 150mS. All chips replaced are known-good and I've tried more than one of each just in case. I even tried the 4-wire mod in case I was going mad!

 

Any help would be greatly appreciated. I have an oscillocope and a multimeter!

Many thanks,

Dave.

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Edited by davemac
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Just plug-in a BASIC cart and type in "? FRE(0)". When the computer responds "37902 bytes", then the whole 48 KByte are recognised - and, any kind of four wires must be attached. Most solutions are at the bottom of the 400 mainboard or using a mini-PCB plugged into the 74LS42´s socket, but at least two wires to RD4 and RD5 are needed. Without any wires the full 48 KByte can´t be ued nor a catridge will work fine.

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Thank you again everyone. I made some progress today, but I still have issues! I dont know what RD4 and RD5 are referring to. I can't see these identifiers on the cct diagram - please help!

 

I have connected mod wires as follows:-

 

R to pin 14 of the 74LS42 (A14)

T to Pin 13 of the 74LS42 (A13)

M to Pin A of J108 (Cartridge slot)

N to Pin 14 of J108 (Cartridge slot)

 

Now when I power up the 400, I get a plain black screen (better than brown I guess!) Oddly though Star Raiders works fine! Donkey Kong does not.

I'm going to borrow a BASIC cartridge tomorrow and try ?FRE(0), assuming it will boot at all!

 

 

The image below is from the standard 16K to 48K mod instructions. Stargunner - I think this is what you thought was correct, but perhaps my card is different to the modded ones because I don't see any reference to RD4/5.

 

 

 

Thanks, Dave.

 

 

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RD4 and RD5 are A and 14 of the cart slot (look at the 800 left cart slot schematic). Also you need A15 instead of A13 from the LS42.

 

IMHO Dave only make a typo, he wrotes:

 

> R to pin 14 of the 74LS42 (A14)

> T to Pin 13 of the 74LS42 (A13)

 

Correct is:

 

R to pin 14 of the 74LS42 (A14)

T to Pin 13 of the 74LS42 (A15)

 

But the pictures show that all wires are ok.

 

Dave, did you ever change ANTIC and/or GTIA? It´s quite rare that one of them is faulty, but I have had very strange issues in the past.

 

2nd try... remove the A105 ROM (MATH_ROM, C012399). Don´t know exactly why, but I have over 6 defect MATH-ROMs in the last years. Most times the computer won´t start, if you detach the MATH-ROM, it will. Of course Atari BASIC and all other programs using the Floating Point Math-Routines in that ROM won´t run, but normal boot of a DOS should work. In some rare cases I had defect ROMs disturbing the buss, best seen with a logic analyzer.

 

If CPU, ANTIC and GTIA is not the source of trouble, then check the chips using in the DRAM / buss logic circuits (74LS10, Z104 - 74LS42, Z105 - 74LS138, Z106).

 

Good luck!

 

Jurgen

 

P.S.: Just for the records... here´s the "manual" for Atari´s 48K expansion.

 

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Star Raiders works so CPU and ANTIC and GTIA are OK and at least 8K RAM are working. This board is different from the Atari board so those wires won't work. Look at his photos of the board and see that 18, U, S, and P do not connect to anything. That is why he needs A15 and A14.

 

Dave, with an ohmmeter, check the resistance between ground and each of M, N, R, and T on the board. Two of them will read 1K or less - those two connect to RD4 and RD5. That should narrow down the possible wirings.

Edited by ClausB
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As its well known I'm ex Maplin and have istalled many of those cards at the time and my memory is there was soldering, I'm 100% sure, just not how many wires. I had to do a soldering course (Heathkit) to learn how to solder specifically to install those boards in 400's and from memory we only did one Mapsoft version of the board.

 

Never heard of a solderless one tho.....

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Star Raiders works so CPU and ANTIC and GTIA are OK and at least 8K RAM are working. This board is different from the Atari board so those wires won't work. Look at his photos of the board and see that 18, U, S, and P do not connect to anything. That is why he needs A15 and A14.

 

Dave, with an ohmmeter, check the resistance between ground and each of M, N, R, and T on the board. Two of them will read 1K or less - those two connect to RD4 and RD5. That should narrow down the possible wirings.

 

Yes, I know that this board needs a different wiring - never told something like that. As I wrote... just for the records.

 

This kind of 48K memory expansion uses the most complicated way (and most # of TTL chips) I´ve ever seen for the 400. The most expansions incl. the german Compy-Shop one uses RD4, RD5, S4 and S5 - which is enough to do the same thing.

 

And, of course this could be an defective ANTIC for example - I have ANTICs in my defects drawer with strange issues. One of them has "holes" in the video display, only the right third part of the screen, the other can´t display some graphics modes without errors (flickering lines, randomized data etc.). GTIAs I have some with collision defects, PMG defects (player didn´t displayed or with errors) and so on. Most of these failures are detected by Super SALT, but not all.

 

Handling >30 years old stuff - comfortable in sockets - should be exchanged first with well-known working parts before hours of hours of investigation is spent - IMHO

 

Jurgen

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Star Raiders works so CPU and ANTIC and GTIA are OK and at least 8K RAM are working. This board is different from the Atari board so those wires won't work. Look at his photos of the board and see that 18, U, S, and P do not connect to anything. That is why he needs A15 and A14.

 

Dave, with an ohmmeter, check the resistance between ground and each of M, N, R, and T on the board. Two of them will read 1K or less - those two connect to RD4 and RD5. That should narrow down the possible wirings.

Thanks everyone once again. I discovered that RAM slot Pins M and T have 1K resistances to ground, so, assuming they are RD4 & RD5, I have connected everything as below:

 

RAM slot Pin M to Pin A of cartridge slot (RD4).

RAM slot Pin T to Pin 14 of cartridge slot (RD5).

 

RAM slot Pin N to Pin 13 of 74LS42 (A15) sorry about earlier typo!

RAM slot Pin R to Pin 14 of 74LS42 (A14).

 

Unfortunately still no luck. In fact Star Raiders no longer works. I tried reversing the address and RD lines just in case too. The computer is good because everything is fine when back on 16K. I suspect the RAM slot pin assignments on the Maplin board are quite different to all others?

 

Later, I traced:-

 

RAM slot Pin N to the RAM board's 74LS42 - Pin 13 (C input of decoder).

RAM slot Pin R to the RAM board's 74LS42 - Pin 14 (B input of decoder).

 

Not sure if this makes sense!

Thanks guys.

Edited by davemac
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Yes, this board is odd. But it sounds like you're getting close. Did you try all four remaining wiring combinations?

Hi.Thank you for your post.

 

I tried reversing M & T, but left N & R in their current config initially. Next I reversed N & R and tried M & T reversed and normal, so that's the remaining three combinations. I'm assuming that's what you meant, but I'm not 100% sure.

 

I'm guessing that N & R were probably already correct because both 74LS42 ICs (RAM board and motherboard) have pins 13 & 14 commoned, thus connecting A14 and A15 to both of these address decoders?

 

Thanks again,

Dave.

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Hi everyone,

 

Success at last!

 

I thought I'd tried everything, but I was wrong. The wiring shown below is correct for the Mapsoft board. Thanks to the ex-Maplin man for remembering the four wire modification, and the very technical information from some clearly very knowledgable people. I replaced all RAM ICs for 4164s earlier just in case, so that may have helped.

 

Again, thank you all for your input. I was amazed as to how much help I've received on here.

 

Dave.

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