Baffled by Rev 3 Motherboard

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Baffled by Rev 3 Motherboard

I purchased a Rev 3 motherboard on eBay with the hopes of installing it in my original Apple II. I currently have a working Rev 4 motherboard in the Apple II. I started by testing all the ICs that came with the Rev 3 row by row by placing them into the Rev 4 motherboard. I was able to find one or two bad ICs this way. During this process I had placed the ICs removed from the Rev 4 MB into the Rev 3 MB so I had known good ICs in the Rev 3. I installed the Rev 3 MB into an Apple II bottom chassis with power supply and speaker (which I had previously checked with a different MB so I know the power supply and speaker are good). I did not connect a keyboard. I connected the video output to a known good monitor. I powered on the Rev 3 MB and go no beep and no video output. Using the SAMS ComputerFacts for the Apple II I found that the 14M oscillator and transistors Q1 and Q2 were a problem. I replaced these components from a known good MB (not the Rev 4 MB) and I was able to get a working 14M signal on the Rev 3 MB. I decided to go all the way to the video output and with the help of my RIGOL oscilloscope I was able to set the video output level to give a good character display, but still no beep. I also did not get an asterisk prompt at the bottom of the monitor screen. I have Integer BASIC ROMs installed. I was thinking that the ROMs are not getting started so I began to check the 6502 processor. I checked the Phase 0 input clock at pin 37 of the 6502. It looked good with a 1 MHz frequency clock. Next, I checked the output Phase 1 (pin 3) and Phase 2 (pin 39) signals and also get a good 1 MHz signal. I next checked the ADDR lines starting with A15, A14, A13 and A2 at pins 25, 24, 23 and 22, respectively. This is where I see something different when compared to the Rev 4 MB. On the Rev 3 MB I observed a very short time of a pulsed signal on the ADDR lines followed by a final value of a TTL High output level. On the Rev 4 MB I observe a continuous pulsed signal for as long as I have the power on. I decided to remove the chip at H5 position, 8T97 part number, and on initial power on saw the same a very short time of a pulsed signal on the ADDR lines followed by a final value of a TTL High. On subsequent powers on I observe a continuous pulsed signal for as long as I have the power on. I know the 8T97 IC is good so I decided to clean the socket with contact cleaner and a tooth brush to get into the pin sockets. I did this for all 3 8T97 chip sockets and the 6502 socket. I re-inserted the 8T97 ICs and 6502 and was greeted with the original problem again - a very short time of a pulsed signal on the ADDR lines followed by a final value of a TTL High. I removed the H5 8T97 IC and probing the 6502 A14 line at pin 24, again, on initial power on saw the same very short time of a pulsed signal on the ADDR lines followed by a final value of a TTL High. But on subsequent power on's I observe a continuous pulsed signal for as long as I have the power on. I re-installed the 8T97 and  checked Vcc (+5V) and GND levels (they are good) and pins 1 and 15 of the 8T97 (I call these the Enable lines) and observed a TTL logic 0 level (this is correct).

 

I believe the ADDR lines not staying at pulsed signal level is the reason why I do not get ROM programming to start and give an asterisk prompt at the bottom of the screen, but I do not know what to do next to get the ADDR lines to work continuously like my Rev 4 MB. Anyone have any ideas?

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Just random thought, I didn't

Just random thought, I didn't see you mention if you checked the reset line.  The address line pulsing is a running CPU accessing memory (reading ROM on boot, and then all the other read/write memory accesses). I suspect if reset is being pulled low address lines could be pulled high as the reset vector is about to be read which is in high memory at 0xFFFC/D which would mean all address lines but the low ones are high. 

 

I also seem to recall, there are a couple versions of the II systems which won't boot without the keyboard. Can you try the keyboard?

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Results of RESET\ timing and keyboard

Jeff d,

Thanks for your suggestions.  I did check the RESET\ signal on pin 40 of the 6502. I used the timing diagram in Wiston Gayler's Apple II book (see below/attached). 

I measured the RESET\ timing on the Rev 3 MB from power on  and it measures about 318 ms (see below/attached oscope picture). I also measured the Rev4 MB timing and it measured a little less at 268 ms. I'm not sure if the time difference is significant. Both the Rev 3 and Rev 4 MB stayed a TTL High for as long as the computer was powered on. 

I connected a keyboard to the Rev 3 MB, but, unfortunately, it did not change the power up state. I get a screen full of characters, but no beep from the speaker and no asterisk prompt at the bottom of the screen (see below/attached). One thing I did notice about the screen of characters is that a lot of characters would flash (Not sure if this is problem or normal/expected behavior - the Rev 4 MB does not flash characters on power on), but on the Rev 3 MB some of the characters, for example, the '=' character would change to '<' character along with the flash. I do not think that is normal. I'll see if I can find the reason why the character is changing.

 

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As I understand reset needs

As I understand reset needs to just be held low for two clocks, then back up. LOL yeah looks like the diagram you have shows the reset vector fetch as mentioned.Guess it's fair to ask what ROMs are installed on the REV3 board? These masked ROMs or EPROMs? I don't remember if you did swap the ROMs or not.Sorry for not much help and basic questions, there's a lot of potential problems. I'm sure if it was on my bench I'd be tooling through some of the same things your doing.

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I have Integer BASIC ROMs installed

The Rev 3 MB has Integer BASIC ROMs installed. They are the original ROMs from Apple. I do have another set of Integer BASIC ROMs in the Rev 4 MB and I have swapped them between MBs. Each set of ROMs work great in the Rev 4 MB and each set does not work or get a chance to work in the Rev 3 MB. The next thing I'm going to do is read more from Wiston Gayler's book and see if something triggers a possible problem/issue to check on the Rev 3 MB.

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POR

It sounds like it could be an issue with the power-on-reset circuit?

The Rev. 0 didn't have this circuit (the user must always press RESET to get to BASIC), but a Rev. 3 board should have it.

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robespierre wrote:It sounds
robespierre wrote:

It sounds like it could be an issue with the power-on-reset circuit?

The Rev. 0 didn't have this circuit (the user must always press RESET to get to BASIC), but a Rev. 3 board should have it.

I was thinking something similar when I asked about the ROMs, I thought autostart was a special F8 ROM, but at this point I can't recall if that right, it's been way too long since I last needed that info.

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Sounds like you could have an

Sounds like you could have an issue with one or more IC sockets. Perhaps you could try pressing on each chip and hitting Reset to see if you can identify one.

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I will try cleaning the IC sockets

I have tried known good Applesoft ROMs (including the Autostart ROM)  on the Rev3 MB and I get the same result at power-on. Screen full of characters . No  'Apple II" at top of cleared screen.

I have tried pressing on the top of each of the ICs and do not see a change in power-on state. 

The Rev 3 MB had areas with a lot of dust that I cleaned off with contact cleaner and a small toothbrush.  I have even used Deoxit on the 8T97 chips sockets, the 6502 socket and the ROM sockets thinking that, maybe, one of the pin sockets is oxidized and needs to be 'de-oxidized', but that has not improved the power-on results. But I may, first use contact cleaner  a clean each IC socket and re-install each IC and see if that improves the power-on state. If that doesn't help I may go a little further and apply Deoxit on all the IC sockets.

ggb
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Using your oscilloscope can
Using your oscilloscope can you trigger from RESET going high and capture output of  pin 7 of 74LS138 at F12 to see access to F8XX ROM over 30 uS. Regards GeoffB
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Will get this done next week

Thanks for the suggestion. We have our family vacation this week, but I will get this scope measurement/picture next week.

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