Troubleshooting help? 600XL Black Screen

[dkerfoot]

I have two basically identical 600XLs. Both Rev A, both have the 3-wire 64K memory upgrade and both have UAV Rev D. Oh, they both have Hias' 1.30 high speed ROM too. (The perfect compact combination for anyone that isn't into demos or actually doing a bunch of typing, IMO)

One suddenly has a black screen on boot. Power LED lights and the TV goes from blue (unconnected default) to black. I never hear the "data sound" of basic loading.

Since I have another (and happily the RAM upgrade I did doesn't require soldering to the MB) I have been able to swap all chips between the two units, 3 or 4 at a time. The bad unit remains bad and the good unit remains good.

There is no apparent damage to the MB and the contacts all look good - nothing bent, apparent corrosion, etc. I did reseat each chip a couple of times. No joy.

Any ideas on what it could possibly be? I do have reasonably good electronic troubleshooting skills and tools (including an o'scope), but I don't know enough details about the circuits to know where to start or what to look for. Does the black screen point to any particular circuit?

 

Any help would be much appreciated.

 

 

[_The Doctor__]

pictures are always a plus, did you swap mmu or ls support chips... do these upgrades have any resistors in the select lines?

[dkerfoot]

ALL chips were swapped - No exceptions. No resistors for any of the upgrades. Also, for clarity, the upgrades have all been in place for six months or more and it was working fine, until yesterday when it stopped.

I should add that none of the chips get hot.

Edited February 26, 2018 by dkerfoot

[_The Doctor__]

did you swap power supplies and uav's? the usual search for cold solder joints and bad socket wipes..

Edited February 26, 2018 by _The Doctor__

[+DrVenkman]

did you swap power supplies and uav's? the usual search for cold solder joints and bad socket wipes..

 

I don't think the UAV has anything to do with it, really. The system should still boot even with no video, yet he doesn't hear any boot noises from the machine at all.

 

However, this is not a good troubleshooting technique:

 

I have been able to swap all chips between the two units, 3 or 4 at a time. The bad unit remains bad and the good unit remains good.

 

 

Typically, the best brute-force method of troubleshooting is swapping stuff one chip at a time from the bad system into the good one and seeing if the good one still works (that way if the dead one is killing chips, you don't run the risk of a chip from the good system getting killed by the swap). The fact that whatever chip swapping was done didn't affect things means - I guess - that the problem likely doesn't involve those chips.

 

One question here: does this include RAM and the support logic chips? Those can go bad as well.

 

Personally, my guess is a cold solder joint on one of the memory upgrade jumpers, or alternately, a socket wipe on the CPU or ANTIC that's not giving good contact to the leg of the chip - a logic probe and a copy of each chip's pinout is probably the easiest way to find that kind of problem.

[dkerfoot]

Agree it isn't likely UAV but I may swap it too just to be certain. Yes swapped PS. Actually used the same one on both machines, yet swapped it too just to be 100%.

Don't disagree that one chip at a time is better, but when time is limited... I was up until 1:30 AM working on the thing as it was...

ALL chips swapped - no exceptions. ALL.

 

Because the three wires used in this RAM upgrade are all soldered to the shoulders of the pins on the chips, the entire thing can be moved from machine to machine. In other words, if I had a cold solder joint, it would have moved from bad machine to good along with the chips.

I'll have a closer look at the CPU and Antic.

I do very much appreciate the help.

[shoestring]

A change from blue to black screen indicates you're still getting video sync.

 

Things it could be. You will need a logic probe for this. Faulty reset circuit [ confirm Sally is receiving RST ( from low then held high as the system comes up ) ]. Confirm CPU address and data lines toggle... if they don't toggle then you may have a bad oscillator [ rare ] or some component in the clock circuit [ Faulty transistor, shorted capacitor ..etc. ].

 

Also make sure you're getting 5v at the chips, anything lower than 4.7v may indicate a short somewhere.

Edited February 26, 2018 by shoestring

[dkerfoot]

A change from blue to black screen indicates you're still getting video sync.

 

Things it could be. You will need a logic probe for this. Faulty reset circuit [ confirm Sally is receiving RST ( from low then held high as the system comes up ) ]. Confirm CPU address and data lines toggle... if they don't toggle then you may have a bad oscillator [ rare ] or some component in the clock circuit [ Faulty transistor, shorted capacitor ..etc. ].

 

Also make sure you're getting 5v at the chips, anything lower than 4.7v may indicate a short somewhere.

 

Thanks for the help. I am slow to reply because I couldn't find my old logic probe, so I had to order one.

 

Reset (pin 40) goes from low to high when turned on. Pins 9-20 and 22-33 all toggle at various rates.

 

5.02 VDC on pin 8 of Sally, Pin 21 Antic, Pin 27 GTIA, Pin 17 Pokey, Pin 20 PIA.

 

Swapped out UAV, no change.

 

Do you suggest checking power on the other chips too?

Any other suggestions?

[shoestring]

Weird. From your description it appears that the system is actually running properly.

 

Is the system running blind ? ( can you still load software via the SIO port ).

[dkerfoot]

I don't think it is. No "data sound" on startup. No sound if a cart is loaded. No typing sound.

[shoestring]

You wouldn't hear anything if you have no video signal but you'll should still be able determine if you can boot software via the disk drive or SIO2USB/PC.

 

What happens if you try to get into the diagnostic mode ?

[+tf_hh]

Hi,

 

5.02 VDC on pin 8 of Sally, Pin 21 Antic, Pin 27 GTIA, Pin 17 Pokey, Pin 20 PIA.

Any other suggestions?

 

First check out if there´s a video interface failure. One way without a scope is to remove the O.S. ROM (or EPROM in your case like it looks) and power on the machine. Without O.S. ROM the hardware I/O area will not be zero´ed and you should get a red/brown display (more red on NTSC, more brown at PAL). If this happens, then video interface is fine.

 

If not, the video interface MIGHT be the source of trouble, but you can test some things with a standard multimeter. Check pin 39 of the CPU, this is the PHI2 clock signal. Reading this pin with a multilmeter should show different voltages between 1.5 and 3.5 volts. If so, then the CPU generates this important signal. If you´re reading below 1 volts or over 4 volts (constant "low" or "high"), then the CPU may defect. Check pin 37 of CPU. When also read below 1 or over 4 volts, then ANTIC, GTIA or main crystal circuit is defect - no system clock frequences generated. Otherwise CPU is bad or something connected to PHI2.

 

More tests, when you really changed _all_ parts (specially the "Delay Line", CO60472), are not useful w/o a scope IMHO.

 

Good luck, Jurgen