Troubleshooting CPU board on a 400

[johndias]

Hi all,

 

I am a complete newbie to this hobby, so forgive me if I missed the obvious. I think I've done my homework but you can tell me if not!

 

I am following the troubleshooting in the field service manual, my workflow begins with a system that powers up but has no video. The power has tested fine on the PSU board, so now I'm on to the CPU. Here I ran into a test failure and I'm not sure what to do.

 

The docs have me checking clock circuit and I am not getting any waveform for pin 8 of Z302. From the docs, it just says it will tell me which component is failing but I can't see how to figure that out.

 

Any ideas?

 

Thanks in advance.

[Rybags]

There's 2 versions of the CPU board for 400/800. Early one has the plain 6502B but later one is the Sally 6502C variant (Atari custom).

 

So, diag docs might refer to one or the other.

The code on the chip is the easist reference to see what yours is: CO14377 is the original 6502B, CO14806 is the 6502C Sally version which is also shared with XL, XE and 7800.

 

The pinout varies between the two chips - use this one for Sally https://www.atarimax.com/jindroush.atari.org/achsally.html

 

If you've got a 'scope then it makes things a whole bunch easier. Various ways to test for signs of life.

On all 6502 variants there should be a waveform from the SYNC pin. It goes high every time the CPU reads an instruction so should be a semi-random pulse every 2-7 cycles or so of the 1.79 MHz system clock.

[ACML]

Sorry to sound obvious, but have you disassembled the case and re-seated the CPU and memory boards? On 400s and 800s, sometimes you just need to re-seat the cards and Voila, it works again.

[johndias]

Thanks to both of you for the help.

 

@rybags it appears I have the "legacy" CPU, so I'll follow your suggestion on scoping the SYNC pin.

 

@ACML no worries, never hurts to state the obvious when helping someone troubleshoot! But, I did re-seat the CPU board chips. However, I didn't re-seat the chips on the memory board so I will try that as well.

[Rybags]

Other stuff - do you get a video signal? Black screen, white screen, red/brown screen, blue text window?

 

Also listen to the internal speaker when you power on (turn TV volume and other noise down). A click means the CPU is at least able to start the OS.

Press Reset, should also generate a click (note it's not like a keyclick but a barely noticable thing).

[johndias]

I get a black screen, but I do hear key clicks from the internal speaker. I was following these pages from the field manual (attached)

 

 

 

 

[+tf_hh]

I get a black screen, but I do hear key clicks from the internal speaker. I was following these pages from the field manual (attached)

 

Don´t call me silly, but which chip you tried to look at pin 8? Maybe you were influenced by the "CPU" on this page, but this meaning the whole CPU board, not the 6502/6502C. pin 8 on both CPUs is +5 volts power.

 

Pin 8 of Z302 (aka 74LS74) is only existing on the old CPU board variant with 6502 standard CPU, it´s the "PHI1" signal and has the frequency listed in the document.

 

When key click appears, the O.S. must be running. So I would assume that the CPU is working fine. So it´s a display issue. The 400 has only RF output (antenna), here´s the first way to check if your TV really supports these old VHF channels. Some newer flatscreen TVs (if you use such) doesn´t support this old stuff any more. So make troubleshooting easier, remove the memory cartridge. After power-on you must now get a whole red/brown screen.

 

Good luck, Jurgen

[johndias]

 

Don´t call me silly

 

Never! And thanks for the help.

 

So I was testing from the board perspective, not the CPU itself. However, it seems that I've gotten things to work somewhat (maybe by re-seating every chip I could find). The RF looked snowy on my display, so I went ahead and installed the UAV I bought recently and presto - I get video... but black and white, no color. Tonight I investigate that issue, although I'm certain I tapped the correct place on the mobo for color in.

 

Good news is, a quick keyboard check in memo pad shows all keys working.

[ACML]

 

I went ahead and installed the UAV I bought recently and presto - I get video... but black and white, no color.

No color from UAV could mean that you didn't have enough of the wire in the green terminal block on the UAV to make good contact (been there, done that). I just stripped a little more of the wire and it made solid contact. Try wiggling the chroma and see if it comes in and out. See photo for proper pick off of the color signal for the UAV. If you stick with the UAV, I highly recommend Orpheouswaking's 400 sound board. I combine the video signals (composite, chroma and luma) with the blended (POKEY, SIO) sound from Orpheouswaking's board and run it to a single 5 pin DIN Atari monitor cable jack.

Edited June 14, 2018 by ACML

[johndias]

No color from UAV could mean that you didn't have enough of the wire in the green terminal block on the UAV to make good contact (been there, done that). I just stripped a little more of the wire and it made solid contact. Try wiggling the chroma and see if it comes in and out. See photo for proper pick off of the color signal for the UAV.

 

Rechecked the connections, and wiggles didn't change anything. I wonder... I'm attaching to a 4 pin s-video cable I had laying around.

 

S-Video

1 - no connection

2 - ground to UAV 2

3 - Chroma on UAV

4 - Luma on UAV

 

And I'm picking the C187 on my motherboard for color. Anything else I can check?

[JR>]

[+tf_hh]

Rechecked the connections, and wiggles didn't change anything. I wonder... I'm attaching to a 4 pin s-video cable I had laying around.

And I'm picking the C187 on my motherboard for color. Anything else I can check?

 

Did you check the color pot on the CPU card? Maybe you just need to turn it around to get the color back.

 

If this doesn´t help, some hints:

 

- Check if there are 12V attached to one pin of the color pot

- Check voltage of "color pot output" at pin 17 of the GTIA - should be between 5.2 and 7.0 volts DC for correct colors

- If this was an used system (bought from eBay etc.), check if the system crystal on the main board fits to the TV system (NTSC or PAL)

[johndias]

Yep, I documented wrong but installed correctly! Good catch.

 

[johndias]

 

Did you check the color pot on the CPU card? Maybe you just need to turn it around to get the color back.

 

If this doesn´t help, some hints:

 

- Check if there are 12V attached to one pin of the color pot

- Check voltage of "color pot output" at pin 17 of the GTIA - should be between 5.2 and 7.0 volts DC for correct colors

- If this was an used system (bought from eBay etc.), check if the system crystal on the main board fits to the TV system (NTSC or PAL)

 

So, I adjusted the pot but only got some "noise" on the screen - colorful noise, but noise. When I checked, the pot only got around 5.5v if I turned it all the way in one direction.

 

Voltage on pin 17 of the GTIA was around 5.5v.

 

The GTIA has a part number for NTSC. I don't know how to confirm the crystal?

[+tf_hh]

So, I adjusted the pot but only got some "noise" on the screen - colorful noise, but noise. When I checked, the pot only got around 5.5v if I turned it all the way in one direction.

The GTIA has a part number for NTSC. I don't know how to confirm the crystal?

 

In this thread I list the atari part numbers in relation to the frequencies.

 

Did you measure always 5.5v even if the pot is turned to the most left or most right position? Then there´s something wrong. You should get up to 12v at one end or nearly 0v at the other.

[johndias]

 

In this thread I list the atari part numbers in relation to the frequencies.

 

Did you measure always 5.5v even if the pot is turned to the most left or most right position? Then there´s something wrong. You should get up to 12v at one end or nearly 0v at the other.

 

OK thanks, then that's the right crystal.

 

Yep, when I turn all the way left I get 5.5v - back to the right it starts to decrease to zero when it stops. So, I think I have to trace back to my power supply. I'll pour over the schematics to see what I can see.

[BillC]

 

OK thanks, then that's the right crystal.

 

Yep, when I turn all the way left I get 5.5v - back to the right it starts to decrease to zero when it stops. So, I think I have to trace back to my power supply. I'll pour over the schematics to see what I can see.

From the attached image(clipped from the Sobola schematic of the CPU board) +12V connects to the adjustment potentiometer(R309) thru a 330k resistor(R308).

 

[johndias]

From the attached image(clipped from the Sobola schematic of the CPU board) +12V connects to the adjustment potentiometer(R309) thru a 330k resistor(R308).

 

400-800-color-adj.jpg

 

So, this schematic matches what I am seeing. And basically I should NOT be getting +12V on the pot, but rather +5V so there's no problem there.

 

At this point, I think the GTIA chip is bad - I'm going to see if I can find a replacement.

[BillC]

 

So, this schematic matches what I am seeing. And basically I should NOT be getting +12V on the pot, but rather +5V so there's no problem there.

 

At this point, I think the GTIA chip is bad - I'm going to see if I can find a replacement.

R308/R309 form a voltage divider, the resulting voltage between them should be +7.23VDC(+12VDC*500/830). The output voltage of R309 should be adjustable to anywhere between this and ground.

[johndias]

R308/R309 form a voltage divider, the resulting voltage between them should be +7.23VDC(+12VDC*500/830). The output voltage of R309 should be adjustable to anywhere between this and ground.

 

OK, I see. I can't figure why I'm only getting +5.5VDC maximum. I pulled R308 and it tested at 330K OHM. However, with power the voltage out from R308 is only +5.5. Should I just replace R308?

[+Nezgar]

 

OK, I see. I can't figure why I'm only getting +5.5VDC maximum. I pulled R308 and it tested at 330K OHM. However, with power the voltage out from R308 is only +5.5. Should I just replace R308?

Whats the voltage going IN to R308/309?

Edited June 17, 2018 by Nezgar

[BillC]

 

OK, I see. I can't figure why I'm only getting +5.5VDC maximum. I pulled R308 and it tested at 330K OHM. However, with power the voltage out from R308 is only +5.5. Should I just replace R308?

There are a few possibilites for low voltage between R308/R309.

1. The +12V power supplied is too low, with the resistance values given it would be about +9.1V if they're accurate.

2. The resistance across the potentiometer is too low, if the +12V supply is good I calculate it to be about 280k instead of 500k.

3. The capacitor(C305) is failing, allowing DC current to ground which increases the voltage drop across R308. I believe it would only take 5 micro-amps current thru C305 to cause the lower voltage. (This would make it equivalent to a 1 to 1.1 meg ohm resistor)

 

If the symptoms are caused by item 2 or 3 replacing R308 with a lower value(150k or 180k) may resolve the issue for now, but the capacitor or potentiometer may degrade further causing the problem to re-occur.

 

I think R309 is the most likely culprit, with C305 being the second most likely. The +12VDC supply being low is less likely as this would probably cause other problems(4116 RAM chips require +12VDC, but if the RAM has been upgraded to 4164 chips this wouldn't be an issue)

Edited June 17, 2018 by BillC

[johndias]

Whats the voltage going IN to R308/309?

Just under +12V

[johndias]

There are a few possibilites for low voltage between R308/R309.

1. The +12V power supplied is too low, with the resistance values given it would be about +9.1V if they're accurate.

2. The resistance across the potentiometer is too low, if the +12V supply is good I calculate it to be about 280k instead of 500k.

3. The capacitor(C305) is failing, allowing DC current to ground which increases the voltage drop across R308. I believe it would only take 5 micro-amps current thru C305 to cause the lower voltage. (This would make it equivalent to a 1 to 1.1 meg ohm resistor)

 

If the symptoms are caused by item 2 or 3 replacing R308 with a lower value(150k or 180k) may resolve the issue for now, but the capacitor or potentiometer may degrade further causing the problem to re-occur.

 

I think R309 is the most likely culprit, with C305 being the second most likely. The +12VDC supply being low is less likely as this would probably cause other problems(4116 RAM chips require +12VDC, but if the RAM has been upgraded to 4164 chips this wouldn't bean issue)

 

So as I was desoldering C305 it disintegrated on me. I'll have to replace it, but I guess it was so fragile because it was failing?

Edited June 17, 2018 by johndias

[Rybags]

Ceramic caps are supposed to have a good lifetime but still degrade over time. But of course approaching 40 years it's to be expected that the odd one might fail though your failure mode seems to be that it's just too brittle.

 

Luckily those things are cheap. Well really every component individually is fairly cheap, your advantage is that this failure is a blatant obvious one. But whether replacing will fix the video is yet to be known.