Lynx repair question - blank white screen

[eggplant_casserole]

Firstly, I have to say that I've checked the forum for threads that might help, and yes I've read the threads on shorting the grounds to diagnose a faulty switch, and it's not that!

 

I have two model II Lynx units, both with similar issues.

 

The first would not turn on without using the ground short method. The power pin has snapped off, so I'm only able to use batteries at present. Upon shorting the grounds, the machine comes on, but only with a blank white screen. Shorting the cart pins does not result in the insert cartridge screen, and none of the buttons or controls (including brightness) do anything. 5V points are reading around 8 or 9 volts. This unit has a Rev. A [1] board.

 

The second unit doesn't require the ground short, but still displays a white screen. The on/off buttons work, but the brightness control does not. Again, the pin short does nothing and 5V points read around 8 or 9 volts. This unit has a different board - Rev. 2 [6], which didn't have any shielding. Some components differed, too.

 

Unfortunately, the only circuit diagrams I've been able to obtain haven't been very useful for fault-finding, with components having different numbers or being in completely different places. All components I've tested so far appear OK.

 

I've seen people mention that a higher voltage suggests that some chips have been fried. Could anyone elaborate on what is the cause of this higher voltage? I don't see any regulator in the circuit, and 9V is obviously the voltage the batteries would be giving out, so it would seem that something isn't doing its job of regulating the voltage down to 5V... but what?

 

Any assistance would be greatly appreciated! Thanks in advance!

[+karri]

Again, the pin short does nothing and 5V points read around 8 or 9 volts.

 

The Lynx cannot take 8V. If regulation of power has failed the chips are burned.

 

In my opinion you cannot fix it.

--

Karri

[eggplant_casserole]

What's regulating the power, though?

 

It's fixable - anything made by man can be fixed by man. Whether it's worth the time, effort and cost is another matter, though! It's worth a try, anyway.

 

Was there a service manual for the Lynx?

[+karri]

What's regulating the power, though?

 

It's fixable - anything made by man can be fixed by man. Whether it's worth the time, effort and cost is another matter, though! It's worth a try, anyway.

 

Was there a service manual for the Lynx?

 

The "power supply" is a joke. They just have a 120 ohm resistor R56 and a Zener diode D12. Either one of these are broken. You might find the broken one by looking at where the PCB looks black and smells charcoal.

 

I would replace it with a modern 7805 chip instead.

 

The schematics are available here at AtariAge.

 

 

At home I have a separate regulated +5V supply. The easiest way to see how the Lynx behaves with correct power is to apply a regulated +5V directly to the +5V power trace. If the Lynx start working normally then it is worth fixing the power supply. If not then you need to go through some heavy troubleshooting.

 

- does the clock work

- does sending a reset start the CPU

- do the address/data lines have some activity after giving a reset

 

If there is no sensible operations after power is applied you are pretty much lost.

 

It is most likely that you have to replace custom chips Suzy or Mikey. Perhaps also all other digital IC's on the board.

 

--

Good luck with fixing it.

 

Karri

Edited April 5, 2011 by karri

[eggplant_casserole]

Whilst the 7805 would have been available when the Lynx was designed, the problem with it is that it requires at least 7 volts to operate. In theory, that would mean a shorter operating time on batteries.

 

That's helpful, thanks. The diagram I have (same as the one on the site) is not correct for the board I have... at least, not the first one I attempted. I've yet to check it against the later revision. It was quite frustrating working with a diagram that wasn't the same as the circuit!

 

Unfortunately, my bench supply is in need of repair itself, or I would have tried it on 5 volts direct. I might just knock one up with a spare PSU and a 7805 just for this purpose, though. I'm not that hopeful that it'll work straight away, but it's worth a try! I do indeed fear that Suzy or Mikey might be damaged. If any other IC is gone, I wouldn't mind!

 

I'll certainly give interrogating the CPU a go, though.

 

Thanks for the info and suggestions! I'll report how it goes!