MegaST ROM adapter - black screen

[arti040]

Hi,
I bought dedicated PCB from our, well known store and also asked @Badwolf to burn some ROMs for me, so I'm 100% sure, they are OK. However, when I put all together and turned my Atari on, I saw a black screen only.

Two questions:
- did I put the chip correctly?
- does that small capacitor soldered "backwards" could cause such issue?

megast-roms.png (2.98 MiB) Viewed 2726 times

[rubber_jonnie]

Hi,
I bought dedicated PCB from our, well known store and also asked @Badwolf to burn some ROMs for me, so I'm 100% sure, they are OK. However, when I put all together and turned my Atari on, I saw a black screen only.

Two questions:
- did I put the chip correctly?
- does that small capacitor soldered "backwards" could cause such issue?

megast-roms.png

Normally ROM issues present as a white screen, not black, unless you are using HIRES mode, in which case it'll be black.

First thing I'd do is pop the original ROMs back in to see if the machine boots OK, that way you've narrowed it down to the ROM adapter.

I can't tell from the pic if that cap is a tantalum, in which case they don't like being connected backwards, but if it's a standard SMD cap with no polarity, then being backwards doesn't matter.

One thing I would suggest is that the jumper would work better with square pins. Most jumpers are made for the square pin type headers and the round ones you're using may not give a good contact.

Other than that, check all your solder joints for bridges and that the board is making a good connection at the sockets.

[arti040]

Original ROMs work fine. Computer boots normally.

[HigashiJun]

Isn't you EPROM erased ?

There's no cover on it, and if you left it exposed at direct sunlight too long, it might be blank.

[arti040]

It reacts with UV light only, I guess.

[rubber_jonnie]

Isn't you EPROM erased ?

There's no cover on it, and if you left it exposed at direct sunlight too long, it might be blank.

They can last quite a while without cover, so whilst I'd agree and suggest a piece of tape to cover it is a good idea, it probably isn't erased yet. A well lit top down photo would help check this.
I suspect the selection jumper isn't making good contact and/or there is a similar problem with the adapter board pins and the original sockets. There really isn't a huge amount to go wrong with these.

One other possibility is that the socket is incorrectly oriented on the PCB, or the chip isn't sitting in the socket correctly, but it's difficult to check as I can't see where pin 1 is supposed to be or it's position on the ROM.

I would add that I have some identical Hitachi ROMs and the pins on them are terrible. Unlike the ROMs with a plastic chip carrier, where the pins are better aligned in the plastic moulding, the pins on the ceramic ROMs can move very easily and get out of alignment.

It's for that reason I only use them for testing ROM imaged before burning to an OTP ROM for permanent usage.

[coonsgm]

Does the ROM switcher need the ROM jumpers (just links) shifted to the 2 ROM or 6 ROM config? Or does that even matter? There are a few links 0 ohm resistors on the front left side of the motherboard. I remember having to make some simple mods to change from 2 ROM's to 6.

[rubber_jonnie]

Does the ROM switcher need the ROM jumpers (just links) shifted to the 2 ROM or 6 ROM config? Or does that even matter? There are a few links 0 ohm resistors on the front left side of the motherboard. I remember having to make some simple mods to change from 2 ROM's to 6.

I don't think it's required for the dual TOS board, only if you're changing to a 6 ROM set from a 2 ROM set and vice versa.

[HigashiJun]

They can last quite a while without cover [...]

Yes, but how long his EPROM was left uncovered ?

[HigashiJun]

I had the same Dual TOS board in my Mega ST:

viewtopic.php?f=29&t=4608&hilit=Mega+ST

and everything was OK.

Pin 1 of the ROM should face the capacitor.