TMNT board repair (sound) FIXED

[lilshawn]

I have the "Cowabunga" coin up sound but no other sound effects audio or music. so I'm getting digitized "voice" sound but no music or noise generator sounds. (otherwise the game operates fine.)

with a logic probe....I found was getting no data action in the mask rom, yamaha synth chips or anything having to do with the system voice circuit.

I initially found a few components broken on the board (bypass capacitor and a 640 khz oscillator that where physically damaged) connected to (D16 - UPD7759C voice synth chip), and I replaced both of those items with new components...but still no luck.

I've traced this down further to a failed (F10 (3.5795 crystal oscillator) that had no output to the Z80 and other sound systems. (crystal drives into a 74LS04 to invert then out to the sound systems) with no clock pulses at all being generated.

after replacing the crystal in position F10 I now have an oscillator output... but i'm still not getting a proper clock to the Z80. (just reads solid low with no pulsing) if I lift out the pin2 on the inverter (D8 - 74LS04) driving the clock of the Z80 and check an undriven clock pulse right from the 74LS04, it oscillates a normal looking clock pulse. I've changed the 74LS04 just to be sure thinking it might be damaged, but still no luck.

I've also isolated the Z80 clock (output tees off at R15 so I lifted out the resistor for the time being as it drives another circuit) so that the clock output is just driving the z80 and it's still holding low. I went ahead and pulled a Z80 from another board with a different failure (99% sure this Z80 works) and it exhibits the same behavior.

Check the reset line of the Z80 and it's being held high so everything should just go.

any idea what is causing this clock input to get sucked low with no pulse? I have not noted any other board damage.

been staring at this for a week now and i'm starting to become blind to it. i'm probably overlooking something obvious. I wouldn't think the Z80 would be able to effect it's own clock input... but isolating the clock doesn't seem to "fix" the low clock line issue.

EDIT ADDITIONAL INFO AND SWAPPED Z80

[lilshawn]

I should have known about pulling up the clock signal. The scan of the schematic I have is damaged or corrupted a bit at the top of that page in the exact area where it shows 5 volts going in (of course it makes sense now, i can kind of see it there)

I did reconnect and check the resistor, it's within spec. @ 0.998K

But still no oscillation. I'll have to dig out the Oscilloscope to be extra sure, but my logic probe is still showing straight low.

I checked the INT output from U115 and it's high, both at the Z80 and the U115.

I also checked the SNDON signal from pin 11 and it's currently low.

I went ahead and resoldered the components i've messed with or replaced. I normally solder about 700 so i've given it another blast up higher. this board has a pretty brutal ground plane in it.

unfortunately still no luck.

[lilshawn]

after some more futzing around and resoldering of some things I finally have action on the Z80 data in and out. This ground plane in the board is a real bugger. sucks up the heat big time. It makes getting things out connected to ground a pain in the ass...but also to get a good ground when new parts are installed.

anywho, after replacing a few bypass caps on the 640khz for the speech synth IC and some on the LM358's (used as mixers) I now have sound. 

[baritonomarchetto]

Nice log, thanks for sharing!

[DeLuSioNal29]

Glad you got it fixed!

I'm looking to on graduating myself to another level of fixing PCB's with a logic probe.  Any recommendations on what to buy (with links)?

DeL

[lilshawn]

I have a super generic high speed logic probe "LP-3" straight out of 1980 they (for obvious reasons) don't make them anymore. It's good up to 35mhz (15ns pulse capture which is plenty fast for all 74 series TTL and CMOS) it was quite expensive when I bought it. It's performance is quite good for what it is.

I've long since lost the grounding clip for it, but it still works just fine getting ground through the power supply circuit, so I just make sure i have a common ground rail power supply when I use it. it get's a little sketchy above 10mhz due to the long ground acting like an antenna...but if need be, i'd just make a clip for it.

https://www.tequipment.net/GlobalSpecialtiesLP-3.html

I've often dreamed of getting a radioshack probescope but in the last 15 years i've done electronics I've had to bust out the logic probe maybe a half dozen times... the oscilloscope, probably less than 3.

If I were to suggest a modern logic probe and If i were replacing my probe, I'd probably buy this one.

https://www.tequipment.net/ElencoLP-900.html?v=54851

being able to manually inject a pulse would be super handy... or this one...since it appears to be a direct descendant of mine...and I already know how to use this one.

https://www.tequipment.net/ElencoLP-525K.html?v=54851

otherwise something in the 25 to 50mhz range will cover you for 99% of the speeds running in arcade boards... short of PC based systems.