Sea Wolf picket fence issue: UPDATE: FIXED!

[keithlee]

So many Sea Wolf tech threads, so few resolutions. Hopefully not this time. With the help of the knowledgeable and generous KLOv'ers I have faith that this will get fixed and I will be sure to document it here for posterity.

After recapping the power supply, monitor, and sound board; replacing some bad transistors in the lamp circuitry almost everything is working perfectly. Almost.

So, my Sea Wolf has the issue with the picket fence lines on the moving objects, both the ships and the mines. my understanding is that the issue is likely with the shifter circuitry, but there is a lot going on there and I'm not sure where to start. I poked around with a logic probe just to see if there was anything obvious and I even tried piggybacking a 25S10 on the 4 chips on the board but there was no discernible change in the behavior. At this point I'm at a loss.

What is the process for narrowing down the fault?

thanks!

 

[mecha]

there's 74153s in that bitshifter circuit, I think. I mean that was 2 years/2852 missions ago for me, but look deeper at it. there was a Sea Wolf at my current job with picket fence and I was able to get it to mostly go away replacing those multiplexer chips. when you turned the game on there would be picket fence the first game, but when the game ended the fence would go away. nothing further was done on it and they apparently sold it while I was in Texas last year.

 

[advans13]

this will be a fun one. you need the manual and tshooting guide

 

[keithlee]

this will be a fun one. you need the manual and tshooting guide

I have that but I'm not sure how much help it will be. All I get from this is that I might have missing or incorrect data bits going to the shifter.

 

[wugly]

look for missing data bits with a logic probe near the shifter. could be bad data that enables shifter

 

[keithlee]

look for missing data bits with a logic probe near the shifter. could be bad data that enables shifter

Thanks, but how would I be able to tell if there were missing bits?

 

[wugly]

logic probe. look for stuck data lines or floating connections

 

[keithlee]

So, it ultimately was something simple, a pin stuck HIGH on one of the 7417 buffer outputs. Unfortunately that was basically the last step in the shifter circuit so it took a lot of crouching and probing before I finally worked my way to that chip.

I will document my process and findings in the event that it helps someone down the road.

All testing done in attract mode.

So the data bus from the mother board enters the game board at pins 28, f, e, 27, 26, h, d, and 25 (D0-D7). Probe showed continuous pulsing on all 8 lines. The data lines then fanned out to the five 74175 flip-flops A5, B5, C5, D5, and E5. (as well as to the output circuitry but I didn't have issues there) All inputs were present here. All outputs were pulsing while the ship was on screen and LOW otherwise.

The flip-flop outputs then arrived to the four 25S10 bit shifters at A3, A4, B3, and B4. All inputs were present. The outputs probed essentially the same, as did the SELECT pins. Of note, the ENABLE pins on A3 and B4 would pulse then HIGH, while the ones on A4 and B3 would pulse and then go LOW. I assume that is correct.

The shifter outputs then went to the four 74153 multiplexers at D3, D4, E3, and E4. All inputs were present. There were some discrepancies in the outputs here that caused me some concern but evidently are normal. Most of the outputs pulsed continuously, however the outputs at pin 7 on E4 and pin 9 on D4 would pulse and then go LOW similar to the input signal.

The multiplexer outputs then proceed to the 7417 buffers at C3 and C4 before returning to the motherboard via the multiplexer data bus. All inputs were present. The outputs all matched the inputs with the exception of output pin 4 on C3 which was stuck HIGH as mentioned above.

I swapped in a new 7417 and the picket fence was GONE!

I was fortunate that the solution required only observation rather than actual experience/knowledge/brainpower that I am lacking when it comes to solid state electronics.

Anyway hopefully this will prove useful to someone at some point.