A600 no video...Help!

Hi all,
I just got given two 600's and I spent some time today checking them out. One is dead, and I mean 'really dead'. Acid rot from a leaked battery has killed it. Not nice.

The other seems to power up, I hear some very promising HD activity. Caps lock works. Power LED is on, HD LED blinks...looks very good, however I can not get any video out of RF or Composite. Just a blank screen.

So I spent a little time tracing the video signal(s) with the schematic and a logic probe. I traced pulsing signals leaving 8373 (DENISE) IC, then through the two 74244's, and into a set of resistors banks (R214, R215, R216). Now this is where the pulsing signal seems to stop. The output of the resistors are all tied together, into three sets and feed into three 2N3904 transistors. Using my logic probe I get no pulsing (and only a dim glow on the probe's red (Positive) LED) at the inputs of the three transistors. I'm no expert at electronics, and I just can't understand what's going on here. I've also looked at the +VID signal here too, can anyone confirm that this is just the +5v line.

Hoping someone can shed some light, or point me in the right direction for a possible fix.

Much thanks

Philip

[lesta_smsc]

I didn't know A600 had battery?

It was a leak from a battery from the RAM board, that then magically moved through the connector and over 1/4 of the board. Traces pealing off all over the place. Terrible.

[rkidd7952]

Is there any sign of leaking electrolytic capacitors on the board? A leaking capacitor damaged some traces around Q212 on my A600 (NTSC), disabling the green channel.

I'd look for corrosion in the neighborhood of U12 and check continuity between pins on the transistors and the components to which they connect.

Robert

[Mrz]

remove ram board and hardisk and power on the computer

Mrz,
Yep done that already. Cheers

rkidd7952,
Will look at that area carefully. Thanks

[Daedalus]

Originally Posted by nama

So I spent a little time tracing the video signal(s) with the schematic and a logic probe. I traced pulsing signals leaving 8373 (DENISE) IC, then through the two 74244's, and into a set of resistors banks (R214, R215, R216). Now this is where the pulsing signal seems to stop. The output of the resistors are all tied together, into three sets and feed into three 2N3904 transistors. Using my logic probe I get no pulsing (and only a dim glow on the probe's red (Positive) LED) at the inputs of the three transistors. I'm no expert at electronics, and I just can't understand what's going on here.

What's happening here is that the bank of resistors are effectively a rudimentary digital-analogue converter. The output of the resistor banks will be an analogue signal, proportional to the 4-bit digital input into them. Each bank corresponds to the red, green or blue signal. This means your logic probe won't pick it up as a pulse unless the display is showing 100% of the relevant colour. To see what's really going on there you need an oscilloscope.

Because of this, I doubt that is where your issue is, as you're unlikely to see the same failure across all 3 banks of resistors. The video encoder is more likely to be causing issues, and is in a position where it's susceptible to damage from leaking capacitors. As suggested above, check for dull solder or other signs of corrosion in the area to the left of the IDE port, where a lot of the video signals are processed. The entire video signal can be lost if one of the important traces to the video encoder is damaged, or if the +Vid power supply is damaged, or if the sync line is damaged (which will stop most monitors from displaying anything).

I've also looked at the +VID signal here too, can anyone confirm that this is just the +5v line.

Yep, more or less. It should measure 5V, and is connected to the 5V supply via a resistor.

[supaduper]

Have you tried the RGB port ? because if you have not the, as a quick check see if it works if it thendoes generally the Video encoder chip is good but the Delay line and its components have a fault..normally the small square cans on the left side of the board near the encoder chip.
If you are handy with a soldering iron you can do a Video fix with this kit which takes out the Delay line, here
http://www.amibay.com/showthread.php...for-A600-A1200

[lesta_smsc]

Originally Posted by nama

It was a leak from a battery from the RAM board, that then magically moved through the connector and over 1/4 of the board. Traces pealing off all over the place. Terrible.

Oh no! That reminds me. I better check my A601 to make sure no battery on it!

[Daedalus]

Yeah, I've come across dead A500s from the battery on the expansion board leaking and corroding the expansion connector and nearby traces. Nasty stuff!

Thanks guys,
This has been educational. Thanks especially to Deadalus, Now I understand why I'm not getting pulsing after the resistors.

Also thanks to Supaduper, after reading your post I remembered I had an RGB to composite adapter board that I purchased for another project many many years ago. I tack soldered it to the back of the RGB connector and was able to get a picture!!!! So basically the machine seems to work fine, I've tested the Disk Drive, the HD, Audio, and even loaded a game.

So I can assume the Video encoder is good?

Maybe I should invest in that video fix kit, or should I just replace those square cans?

Cheers

[supaduper]

Well done

You will not get replacement cans they do not make them any more, best bet is a video fix kit but it really is fiddly to solder so you need to be on top of your game

Or just buy a RGB to scart adapter on Ebay and forget about composhite as no one really uses it

If your fairly sure its those square cans, maybe I could transplant some from the other acid damaged 600...what do you think?
How can you test them before removal?

Phil

[supaduper]

Originally Posted by nama

If your fairly sure its those square cans, maybe I could transplant some from the other acid damaged 600...what do you think?
How can you test them before removal?

Phil

I am just assuming its those cans as they are known to fail, someone here can perhaps give you a guide on how to test them, but changing them out can be a pain as the pads are so fragile you would need a powered solder sucker to do it really otherwise you will ruin/lift pads

[Daedalus]

With a multimeter, the cans they should read pretty low, probably less than 10 ohms in the path of the signal. Check the schematics for the pins used as the layout isn't that straightforward. Also, the signal lines should have a high resistance to ground / the shell of the cans, and because there are capacitors involved, this could wander up and down depending on your meter.

In many cases you can see if they're the actual problem by simply shorting them out. Tack a wire between the input and output signal to see if you get a picture then. The quality will be terrible because the sync is all out of place, but at least you should get something. If you still don't get a picture, it's likely something else is damaged.

To be honest, successfully removing the cans without damaging them is tricky. The shells absorb a lot of heat so desoldering isn't the easiest, and then they cook their insides as a result. The encoder upgrade may be a little fiddly, but if you've soldered surface mount component before it shouldn't be a big deal, and the results are decent - well, as good as the composite can be, anyway.