Hi everyone

Trying to diagnose issues with a System 3 sound board. I got this board with a bunch of spare boards in totally unknown condition. I placed it into my Tee'd Off and it did not boot. The main LED was locked on and pressing reset did nothing.

So, I connected the board to my test bench with Marco's test ROMs installed. With the test LED connected to pin 17 of T3, when power is turned on, the test LED locks on and the main LED stays off. Pressing reset makes the test LED dim momentarily before it locks on again. The main LED doesn't change.

With the test LED connected to pin 17 of N1, when power is turned on, the test LED starts to flash and the main LED stays off. This looks to me like the board is waiting for input/reset. When I press reset, the test LED dims and stays that way. No change to the main LED.

So it seems to me that when hooked into N1, the test ROMs attempt to boot the board, while they do nothing when hooked up to T3. I have already replaced H2 and H3. I have also tested A3, which appears to be good. It seems to me that the 6502 at T3 is the most likely suspect. If I need to replace this, can I replace it with a faster processor or do I need to stick with 2 Mhz? Faster processors are more readily available, and cheaper.

What do you guys think? Thanks in advance for any advice.

I have already replaced both RAM chips at H2 and H3.

I don't have a tester but I tested A3 and G5 with my meter in diode test. No issues - results are the same as on my working board.

Good point. I'll try and figure out how to use my logic probe and see if that gets me anywhere.

OK, I did some investigation with my logic probe and found a couple of things. Can someone help me interpret these results?

A3 gives the same readings as my working board. However I get different readings for G5 pins 11-13 (bottom left of schematic). These are a gate that output to G4 and then to the 6502 at T3. I get an unusual reading with my probe at pin 6 on T3 (where G5 outputs to) as well as odd readings at pin 20 (A11). Plus the output voltages at T3 are almost 0 whereas these were 2 volts on my working board.

So from this I am beginning to suspect G5 is bad. Is that a reasonable conclusion?

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OK, I replaced it and verified all traces. Still no improvement.

I am starting to wonder if G3 is involved here. G3 sits between A3 and G5. It gives me different probe results to the working board, too.

G3 is an 74LS377 but I don't have any in stock. Could I substitute a 74LS374?

Another interesting note: if I touch the logic probe to pin 13 of G3, the LED turns off and the board is dead until I press reset. Not sure what the significance of that is. Pin 13 is an input.

I'll order another then. I think the only other potential culprit is the 6502 processor. In particular the lower voltages at pin 37 and 39 make me think it's got issues. Is there a way to test these processors?

Alright, I've ordered another LS377. It'll take a while to arrive.

At this point I feel like I'm basically shotgunning the board which is what I wanted to avoid in the first place. Are there any other targeted tests I can do? Maybe the behaviour of the test ROM is a clue? MarAlb
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I finally had some time today to do some more work on this board. I replaced the LS377 at G3. No improvement.

Will take another look at the schematics and figure out where to go from there. I'll get this bastard eventually.

Did some more research into this today. I have replaced H2, H3, G3, A3 and G5. According to the theory of operation in the manual, A3 and B2 are the input latches. I have replaced A3 so this is working. A4 is then clocked which causes the IRQ input of each processor to go low, until A4 is cleared. This seems to work as the IRQ input goes high, so I think A4 is working as it should. However, I get different logic at G4 compared to the working board. Inputs and outputs high or low when they should be the opposite. G4 seems to be a hex inverter associated with both processors. I will replace it and see if there is any change.

I have no idea whether anyone else is reading this thread or if I am talking to myself, but hopefully when I get this board fixed someone will be able to follow my logic train...

OK, so G4 was replaced but still getting a locked on LED. No progress.

At this stage I am just going to replace A4, then the processor at T3. Just need to wait for some more sockets to arrive.

Another update. I have replaced A4, and after that made no difference, I replaced the processor at T3. Still no change. LED stays locked on.

I have now replaced basically every part of the YROM circuit. Now I can either start replacing parts on the DROM side of the board, or attack the CPU timer circuitry. I will have another go with my logic probe and compare to my working board and see if any other abnormalities come to light.
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Having some issues finding 6502 CPUs in case I need to replace the other processor. Can I use a 65C02P4? Or a W65C02S8P?

Unfortunately, nothing new to report. I moved house recently, so I've had a lot of other things on my mind! My plan was to replace the other processor and see if that made a difference. No sure yet when I'll have time to do it, but thanks for checking in!