Apple //c won't boot

[wyngnut]

Hi,

I have a Apple //c, board 820-0115-C. I am getting a video signal (see picture), but no boot.

I have replaced MRD7 RAM, 80 DIR, Video Latch, tried an alternate CHAR GEN and burned a new MON chip.

I have tried my spare CPU, MMU, and IOU chips.

Any suggestions of what to try next?

Thanks!
Chris

 

[desertrout]

Have you inspected / reseated all socketed chips? Have you tried booting without the keyboard and disk drive attached?

 

[wyngnut]

Have you inspected / reseated all socketed chips? Have you tried booting without the keyboard and disk drive attached?

Yes, yes, and yes. Just ordered some deoxit to clean the contacts as well (its not here yet). Thanks for the suggestion.

 

[desertrout]

Yes, yes, and yes. Just ordered some deoxit to clean the contacts as well (its not here yet). Thanks for the suggestion.

Had to ask. When you say no boot, does that means no beep or drive seek?

 

[wyngnut]

Had to ask. When you say no boot, does that means no beep or drive seek?

No beep or drive seek.

I hooked up a logic analyzer to RWB and RDY on the 65C02, they are both toggling. I also see the address lines and the data lines toggling at the 65C02 as well.

 

[desertrout]

What does the reset line do on power on / control-reset? On both the CPU and IOU (schematic, just in case: http://www.applelogic.org/files/IICSCHEMATIC.pdf)

Edit - I'm seeing that you already swapped the IOU, but I guess check it again anyway. Otherwise, there are a bunch of clock checks... obviously pin 1 of the GLU, then it gets divided down... I'd have to check

 

[wyngnut]

I see RESB go high at power on and RESB toggle low/high on Control-Reset.

 

[desertrout]

I see RESB go high at power on and RESB toggle low/high on Control-Reset.

OK, as it should. Clocks then... as I wrote above in an edit to my last post (while you were responded, sorry about that).

 

[desertrout]

When looking for the divided signals, I came across a Sam's manual that has the readings and locations as part of the troubleshooting guide (starting page 8) --- probably your best resource at this point: https://mirrors.apple2.org.za/ftp.a...re/misc/sams/Sams ComputerFacts Apple IIc.pdf

Given that you have what looks like a decent video signal, the oscillator itself and the 7.2 colour reference signals are probably good, but there could be an issue with RAS/CAS timing or CPU timing / traces?

 

[wyngnut]

14.71 Mhz at the GLU pin 1, 7.25 Mhz at GLU pin 20, 3.57 Mhz at TMG Pin 3, and 1.02 Mhz at PHI2 at the 6502.

AFA and CAS/RAS at the 4164, I see RAS strobes at 2.05 Mhz and CAS at 1.02 Mhz.

 

[desertrout]

hm, afaik CAS should be the same as RAS (i.e. both should be 2.040968)

 

[wyngnut]

Looks like I was on the wrong pin for CAS, it is 2.05

 

[desertrout]

Darn, I thought you had found something there, like a bad TMG (though a 1.02 doesn't seem like a bad output)... one last clock to check is Q3 on pin 16 of the TMG should also be 2.05.

If that's good, then you need to check for RDY, IRQ, and NMI on the CPU - they should all be high. Pins 2, 4, and 6 respectively. If any are low, trace those lines back.

 

[wyngnut]

Darn, I thought you had found something there, like a bad TMG (though a 1.02 doesn't seem like a bad output)... one last clock to check is Q3 on pin 16 of the TMG should also be 2.05.

If that's good, then you need to check for RDY, IRQ, and NMI on the CPU - they should all be high. Pins 2, 4, and 6 respectively. If any are low, trace those lines back.

All are high. And TMG 16 is 2.05 Mhz

 

[desertrout]

Well... assuming your CPU, MON, MMU, and IOU are all good, that only leaves the RAM afaik. Do you know which ROM version it has? Can you attempt the self diagnostic?

When you replaced MRD7, did it have any effect?

 

[wyngnut]

I replaced the 4164 because I was seeing some low output voltages on Q. Its possible I wasn't on the pin correctly. Regardless before and after all the three chips I replaced the results on the screen was basically the same.

Interestingly I am hearing speaker do a short crack noise (nowhere close to a beep though) on boot probably 1 out of 20 boots. Not sure if this is new or just a function of me turning the power on/off so much now.

 

[bibilit]

that only leaves the RAM afaik

I agree, you can try piggybacking

 

[desertrout]

And - I'm kind of joking but also not - if they're MT branded chips, just do a complete swap out (with sockets)

 

[wyngnut]

I ended up putting sockets in MRD0 - MRD7.

My machine has TMS4164-Z20NL. I got 5 of the same off eBay, after seeing the voltage issue on MRD7. Turns out 2 of the 5 "new" ones I bought don't work. I assume one of the broken ones is the one I solder into to MRD7 (sadly didn't mark it). Strangely, after putting sockets in and putting back the original 8 4164s chips it now boots.

Regardless, my Apple //c from 1984 is back working.

Thanks for your help!!!!

 

[desertrout]

Hey that's great!! Congrats!
Be sure to run some diagnostics to put the RAM etc through its paces and make sure everything is cool. The XPS one is pretty good: https://mirrors.apple2.org.za/ftp.apple.asimov.net/images/disk_utils/diagnostics/XPS Diagnostic IIe 1.0.5.dsk
This one is the dealer diagnostic: https://mirrors.apple2.org.za/ftp.apple.asimov.net/images/disk_utils/diagnostics/Apple IIc - Diagnostic diskette Program service 077-8125-A.dsk

Yup - unless you're getting 4164's from known-reliable places like Jameco, it's a gamble. But I'm very glad the process came back full circle - sometimes machines just need to be fussed with a little to get working again.