Atari 800 display issues

[bernie]

Does the CRT work with other known good video sources?

 

 

Yes works fine with cable and I tried to change channels as well for RF

[bernie]

Given the problems you're having across multiple TVs, I'm pretty sure you've got an issue with one fo the main chips, probably ANTIC, GTIA or the 6502.

 

 

Correct. Second TV works fine for cable.

 

My understanding is that all those are on the CPU PCB- correct?

 

Looking at the field manual and looking for video out functions I found the following info:

 

The functions of the CPU PCB according the to the atari field manual are to:

 

Generates video signals to be processed by the motherboard before they are sent to the RF module on the Power Supply PCB

 

 

The relevant functions of the motherboard are:

Receives video data from the Central Processing Unit PCB, converts it into a composite video and routes it to the power supply PCB.

 

Motherboards route the Composite video signals (COMP CHROMA, COMP LUM, MOD, and COMP VIDEO) to the Power Supply to build the RF video output.

 

Powersupply:

The RF signal is generated in both the 400 and 800 Power Supplies from the MOD signal received from the motherboard. It is routed through the A203 RF Module to develop the RF output through 3203.

 

Is there a way to isolate?

Edited March 7, 2017 by bernie

[+DrVenkman]

 

Is there a way to isolate?

 

With a logic probe or oscilloscope and the field service manual, I imagine so. For the rest of us, chip-swapping is the way to do it. One thing you can do is rig the cartridge cover switch to stay closed so you can test with the case open and the RF shield off. See if any of the chips gets noticeably hotter than the others while the unit is on. Swap that one with a chip from a second system. If you can't do that or none of them are warmer than the others, swap them out one at a time until you find the culprit.

 

Without a second system or supply of spare chips, a simple thing to do is to remove and reseat all the chips on the board. Occasionally they work themselves loose over the years, or corrosion builds up. Removing and reseating the chips can sometimes improve bad connections.

[bernie]

Thanks. An oscilloscope is more than my current skill set and I currently don't have another 800 :/

 

I currently have the case off the atari with the cartridge cover switch rigged.

 

It's almost 4am in Hawaii, so I'll look into it tomorrow.

 

There is a section for "snowy screen" in the service manual (5-3) I will also change the voltages and check the ground connections of the RF module (which hopefully I will find!) But the only guidelines are "is the RF module mistuned or defective" -> Repair

[Almerian]

If you haven't doen so yet:

Open the 800 up and check if the RF cable is properly connected inside.

When you're in there anyway check for a broken RF cable with a multimeter. Do not over tighten any screws when you reassemble the case.

[bernie]

If you haven't doen so yet:

Open the 800 up and check if the RF cable is properly connected inside.

When you're in there anyway check for a broken RF cable with a multimeter. Do not over tighten any screws when you reassemble the case.

 

Thanks. The RF cable is properly seated.

 

The issue also exists via the monitor out

[bernie]

So I think I'm going to buy a new CPU board and potentially a ROM card.

 

Are the 400/800 ones the same? There may be a used 400 for sale locally.

Edited March 9, 2017 by bernie

[bernie]

Duplicate

Edited March 9, 2017 by bernie

[Mclaneinc]

Sorry to hear this little machine has put you to so much trouble and cost but at least you will have the possibility of some spares.

 

Best of luck

 

Paul.

[electronizer]

Have you removed and reseated all the chips on the CPU board as DrVenkman suggested? Also removed and reseated the CPU board itself? Removing and reseating chips worked for me on a 600XL (I also lightly sanded the chip pins with fine grit sandpaper). Usually socketed chips are a good thing, but not when it comes to decades of corrosion.

[bernie]

Have you removed and reseated all the chips on the CPU board as DrVenkman suggested? Also removed and reseated the CPU board itself? Removing and reseating chips worked for me on a 600XL (I also lightly sanded the chip pins with fine grit sandpaper). Usually socketed chips are a good thing, but not when it comes to decades of corrosion.

 

 

I just ordered a tested CPU from eBay. Will remove the chips now! I did notice that the cooper was starting to peel off the back of the PCB on the circuit board.

Edited March 11, 2017 by bernie

[bernie]

I just unseated/reseated the chips. Unfortunately, no change.

 

BTW what's the best way to unseat them? I gently pried them off with a plastic tool.

[+DrVenkman]

I just unseated/reseated the chips. Unfortunately, no change.

 

BTW what's the best way to unseat them? I gently pried them off with a plastic tool.

 

I use a flat-blade jeweler's screwdriver to gently pry up one end just enough to break tension in the socket, then repeat from the other end. Gotta be careful because it's very easy to bend or break pin which you really don't want to do.

[bernie]

It was the CPU board. Just got a replacement today and seems to work fine with the video.

 

So if you get a rolling video screen on both RF and monitor out, and your unit is not PAL on NTSC (or the other way round) and you have tested the voltages, and reseated/ swapped the RAM to test, and tried reseating CPU chips, try replacing the CPU board.

 

Thanks again for all those that helped me get my atari monitor displaying again!!!!

 

One more atari resurrected from the graveyard!

Edited March 14, 2017 by bernie