shoestring

Check the power supply. I've had similar issues with my Amiga 3000. Expansion cards can over stress an already failing power supply and cause random crashes like that. I would measure your voltages ( under load ) with and without the expansion card and see if there is a significant voltage drop.

I wouldn't give up hope yet. If you're still getting a black screen then check black/grey screen symptom in section 3 of the field service manual. http://classiccomputers.info/down/Atari/Atari_800XL_Field_Service_Manual.pdf

Brilliant!

My 800xl was doing the same thing recently. A logic probe revealed the CPU ( pin 40 ) wasn't getting the typical low to high reset behaviour even though it was present everywhere else. Reseating Sally didn't help. It ended up being a bad socket, so I just replaced it and machine now boots on power up every time.

Of all the 1084 models, the S model ( Philips ) was the best. They do have a bad reputation for poor solder joints , poor quality caps in the power supply and an on/off switch that will fail. In the 20+ years that I've had mine, I've only had to reflow some of the solder joints, replace a flyback and 1 bad cap. Not bad for 20+ years of continuous use. Most of the folks over on the arcade forums who have both seem to prefer the Sony monitor, particularly the BVM series which apparently has superior video quality.

As I understand it the Amiga was supposed to be a games machine / console from the beginning. However, due to the video game crash they decided to build a computer system instead. Most of the hardware for a computer already existed on the breadboards, so they made minor adjustments such as adding line drawing mode and I/O ports.

When you say dark keyboard I'm thinking the earlier // & //+ models. I recall that the //e key board seemed to travel further. I liked the clicky feel of those key switches. The c64 keyboard I had back in the day was like hitting bricks in comparison.

The type 4 keyboard has the same issue. Are there any plans for a Mylar replacement as well? I've managed to repair several broken connections using thin cut strips of adhesive copper tape but looking ahead, I think it's going to get worse over time.

The Apple //e had a nicer keyboard, it feels great, wouldn't you agree ?

I did just that yesterday. I've modded my Sigma 9000TB to connect to Atari joystick style port. It works great but I'm wondering if it's worth adding a second fire button the same way it's done for the 2600 in the following link. http://atariage.com/forums/index.php?app=core&module=attach&section=attach&attach_id=171299

Its simultaneous 2 player.

Very useful if someone is willing to do the work.

Never played the Vic 20 version. My friend and I spent hours playing the c64 version after school, it was his favourite c64 game and always asked to play it. Leonard Bertoni did a great job porting it over to the c64.

It would definitely be a good start since you've already eliminated the power source as the potential problem. You could go ahead and use a shotgun approach but you could potentially waste a lot of money if you don't get lucky. The best and cheapest option may be acquiring a working a8 system and substituting known working parts.

You should at least verify the contents of the EPROM, get it re-programmed if it doesn't check out or just get a replacement if you don't have a programmer to do the verification yourself. EPROMs have a tendency to forget their data after 10-15 years or so ( this is called bit rot ).

It would fail to gain a presence outside of Japan, too expensive and the market was already flooded with 16/32 bit machines. Tramiel was really good at keeping the Japanese out of the 8bit market and to some extent, he also succeeded in keeping them out of the 16 bit market as well with the ST. That was until the Nintendo NES systems came out, they sold a lot of those.

I'm guilty of using cheap knock off chargers myself.I've replaced a MacBook charger with a cheap one, eventually the charger failed catastrophically. Being curious I broke opened up the case and one of the electrolytic caps had a decent sized hole in it. Scary to think that I used that thing for a year and half. Apart from being dangerous to use, the manufacturers remove necessary components from the design to bring down costs which result in under-performance (they often don't deliver the necessary performance as claimed in their specs & suffer a lot of problems ( AC ripple, overheating causing components to exceed their max ratings & eventually fail )).

I would never cheap out on power supplies, especially avoid cheap iPhone / iPad counterfeit chargers. There's a reason why these are so cheap, much less components. They are house fires waiting to happen. http://www.righto.com/2014/05/a-look-inside-ipad-chargers-pricey.html?m=1

Any closeup of that EPROM ?

I just looked at the gut shot above and it appears your OS rom was replaced at some point. There's an EPROM (The chip with the glass window ) there instead of the original chip. What are the markings on that chip? So I would definitely suspect that repair and get someone to verify the OS rom and basic rom whilst they are at it. You can also get an original OS rom from Arcade components. He is great to deal with. http://www.arcadecomponents.com/catalog/item/3054735/5262650.htm

If you've measured 5v at the DRAMs with the other PSU then that's a good sign. It sounds like from your description that the CPU is good but may be getting lost and is crashing due to bad rom, bad stack ram or some other logic fault. Does the computer appear to do something when you press the reset button ? What happens if you hold the option key whilst turning the machine on ? Does that bring you into the test mode ?

Hard to speculate without replacing the PSU first & re-testing. I doubt you've damaged the PCB if the load imposed is dragging the voltage down from 6v to way below 5v. The danger is when the voltage at the chips is excessively high.

If you've measured correctly then I would suspect a bad PSU. I don't know which country you're in but this is the PSU used in Australia. I've used this for both my 800xl & 600xl. Mine is outputting 5.1v at the connector with no load ( computer switched off ). Under load I'm measuring 5.05v which is acceptable. You'll get some drop in voltage which is normal. But if you're starting off by measuring 6v then measuring a big drop with the machine switched on then there's definitely a problem with the PSU and I wouldn't be using that power supply again. Get yourself a known working power supply.

4-4.5 volts from inside seems a bit too low. ttl chips start to play up with voltages below 4.8v

The PLA tends to run hotter than all the other chips so perhaps in some factories they elected to solder them into the boards. It's due to chip creep, which is a process caused by thermal contraction & expansion which tends to push the chip out of its socket as the computer is turned on / off.