Micro-Kim Repair

[SirWhy]

Hey guys,

I'm an old school DIY computer lover, pouring a lot of time into Vince Briel's Replica-1 and Micro Kim kits, the software side anyway.

I recently picked up my Micro Kim again with the hopes of running some Microchess (because a 1k chess program is pretty cool) but after plugging in and resetting the board, no real signs of life. The power LED is on but the display (6 seven segment LEDs) won't light up. I then checked to see if serial communications were working but no joy. At this point I said it was time to pick up the oscilloscope and check some things out. Everything seems in order from what I can tell (troubleshooting isn't my strong suit) other than certain address lines not being pulled low on reset, although I can't trace where or why this is happening.

My best guess is the 6532 RIOT chip, however I thought I would check in with you guys first before dropping some cash on a potentially expensive replacement.

Normally this would go on Vince's forums but I can't seem to access them, so sorry for any unfamiliarity 

Thanks in advance!

[sanman]

Hit the R/S button and see if the displays light up. There is usually nothing on the displays @ power-up.

[PA0PBZ]

Did you check the clock (pin 37 on the CPU)?
Any sign of life on PHI2 (pin 39 of the CPU)?
Does the EPROM still have valid data?
Is the address decoder working (U5)?
Do you see activity on the address  and data lines?

[SirWhy]

Clock and PHI2 are good. No real way to test if the eprom has valid data at the moment.
U5 however has a little problem, pins 7 & 9 are meant to be high but aren't. A little tracing back and I find that at R20, we have 5V on one side but not the other, it may also be a cold solder joint that never gave trouble until now. Only little discrepancy is that even the lead on the other side doesn't seem to read 5V. Will heat it up anyway, I doubt the resistor has blown or anything, not with this system

EDIT: Pins 7 and 9 read 0.2V sorry, so next to nothing

[PA0PBZ]

U5 however has a little problem, pins 7 & 9 are meant to be high but aren't.

U5 is a BCD to decimal decoder, depending on the input ABCD one of the outputs /Y0 - /Y9 will be low.
Pin 7 will be low when the input is 0110, and pin 9 for 1110, so it could be normal that that the output is low: check the input or pull the jumper JP1, they should be high then.
K7 (pin 7 & 9) is the chip select for the EPROM, so if they are still low after pulling the jumper remove the EPROM. If still low it could be a bad U5 of course.

[SirWhy]

Ok, took off JP1, all of them read high., with the jumper on the input is 0111 with K7 still low. Correct me if I'm wrong, but surely it should read high as that's a binary seven, pin 9 should read high off of that.

[PA0PBZ]

Ok, took off JP1, all of them read high., with the jumper on the input is 0111 with K7 still low. Correct me if I'm wrong, but surely it should read high as that's a binary seven, pin 9 should read high off of that.

Pin 9 is /Y7, binary 7.  Did you notice that one of the outputs goes LOW while the rest stays HIGH?

[stmdude]

While I've never repaired (or even seen) a Micro-Kim, I do repair quite a few vintage computers and video games.

Things from this era usually hold together pretty well if they're stored somewhat decently.

However, we have reached the point where RAM and EEPROMs are dying of old age.  My troubleshooting list usually goes like this:
1. "Thou shalt check voltages"
2. "Thou shalt check your oscillator"
3. Probe CE/OE on the RAM+ROM, make sure they have flanks. Also, check one of the low address-lines on the ROM (not A0 in 16bit machines though), to make sure the CPU is trying to read something.
4. Probe every data-line on the ROM. Make sure there's no dead ones.
5. Same for the data-lines on the RAM.

If all that checks out, _actual_ troubleshooting starts.

[PA0PBZ]

Other words, same list 

[SirWhy]

Pin 9 is /Y7, binary 7.  Did you notice that one of the outputs goes LOW while the rest stays HIGH?

Yep, it does that, even removed the chip and tested it on a breadboard. It's working fine.
So the problem may be a corrupted or bricked EPROM?

[PA0PBZ]

So U5 was a red herring? Back to the drawing board then.
Do you have a scope? check all address and data lines for activity.

[SirWhy]

Probed the lines and there's no activity on them just the one reading with no changes.

D0 - Low
D1 - Low
D2 - High
D3 - High
D4 - High
D5 - Low
D6 - Low
D7 - Low

A0 -> A15 are all high with no change

High in the case of the data lines is 5V but for the address lines it's only about 3.6V

[PA0PBZ]

How about pins 2,4,6 and 40 of the CPU? They should all be high.

[stmdude]

High in the case of the data lines is 5V but for the address lines it's only about 3.6V

That's not right..  Lift the EEPROM + RAM + RIOT + U5 chips, and probe the address lines again.  If you still have no activity on the address-lines, I would presume that the 6502 have gone to the big ALU in the sky..

If the address-lines start twiddling with the EEPROM+RAM+RIOT+U5 removed, add them back one at a time, and probe in-between. That should give you an indication of which chip is messing with the address-bus.

( Or, do it the other way around. Remove one chip, test, remove next if it still doesn't look right )

Edit: My thinking behind this, so we're educating as we go along as well, is that the 6502 should be the only device that's _driving_ the address lines. Since we're not seeing a proper 5V on the address lines, something is preventing the CPU from driving the address-lines to their proper level.
By eliminating everything that's connected to the address-bus, we should be able to deduce which device is preventing this.
 

[SirWhy]

How about pins 2,4,6 and 40 of the CPU? They should all be high.

All High

That's not right..  Lift the EEPROM + RAM + RIOT + U5 chips, and probe the address lines again.  If you still have no activity on the address-lines, I would presume that the 6502 have gone to the big ALU in the sky..
 

Took out the chips and the address lines now have activity (albeit at around 4V) so I think the 6502 is alright (for now). Will start putting chips in one by one

UPDATE: The activity on the address lines stop as soon as the RAM or ROM are put in their sockets. Also I notice that the address line activity isn't very clean or predictable, there is also some weird activity on the data lines (even when all chips are removed)

Edit: Ok, so adding them in again and the activity on the data and address lines looks good only at around 3.7 - 4V. Adding in the RIOT chip puts an end to this, bringing me back to the idea of the RIOT chip being the problem.

[stmdude]

Edit: Ok, so adding them in again and the activity on the data and address lines looks good only at around 3.7 - 4V. Adding in the RIOT chip puts an end to this, bringing me back to the idea of the RIOT chip being the problem.

That would have been my conclusion as well.

Jameco seems to have them in stock for $7 each.  http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&productId=43297&catalogId=10001&freeText=6532&app.products.maxperpage=15&storeId=10001&search_type=jamecoall&ddkey=http:StoreCatalogDrillDownView