Repair of a 8052 computer

[ChristofferB]

So I've bought this 8052 computer, and it worked briefly over serial (had some trouble, but I did get it working for a while). Today it does nothing. There's no life at all. No data, no clock, nothing.
It's a fairly simple board with just the 8052 BASIC microcontroller, 2x 8255 ports, an EPROM, some sram, some misc. logic, a 74LS373 for data/address latching (I assume), and the MC1488/1489 rs232 line drivers.

It requires 5V, + and - 12V and ground for power. I ran it on 5V only, and removed the 1488/89 (presumably the only parts needing the +-12V), and it did work in that config.

I've reseated all socketed IC's, backed up the EPROM, 5V to all chips are good, but nothing moves.
This could be because the clock doesn't do anything, but there's no real clock circuitry, just a crystal and some caps hooked up to the 8052. I've also tested all socketed logic and ram, works fine.

I've measured noice, and there's about 0.2V Peak-Peak noise on the logic LO lines.
What would the next logical move be?
I fear that the 8052 has died, which would be a problem, they aren't that easy to come by.

Any help would be much appreciated,
Thanks in advance,

-- Christoffer

[ChristofferB]

Update: It most likely isn't the CPU. Just tried with another 8052 (non BASIC, as opposed to the processor used on the board) and it exhibited exactly the same behavior.  Do crystals go bad? Never seen it happen, though.

Unfortunately I don't have the schematics, only some random notes on overhead transparencies.

--Christoffer

[brabus]

Hello Christoffer,

Do you have an oscilloscope? I assume yes, since you mentioned 0.2Vpp noise.

Let's start with some basic stuff: is the RST line OK? Maybe the start-up reset (maybe just an RC) is stuck.

If RST is OK, I would directly poke the crystal pins. Which 'scope do you have?

Keep us updated! Good luck!

[ChristofferB]

I found the trouble! some not-so-great soldering on the back of the board shorted some pins on the processor  fixed now, and working great with a:
*MCS-51(tm) BASIC V1.1*
READY
>

Photo of the board attached. Pretty cool board, actually. Although who designs a single board computer without a reset button?

Thanks for the help, anyway!
--Christoffer