I have a TP84 with no sound. The audio amp seems fine, and buzzes when I rub my fingers on the pins of it, and volume pot is good.
I've checked with a logic probe, the audio board z80 CPU is not in reset (reset pin starts low then stays high).
I believe that the problem is no valid clock signal. On pin 6 of the CPU (which I believe according to the schematics is supposed to be the clock signal), it just reads high on the probe. Nothing is pulsing on the CPU or audio ROM. On a good board set, I get a pulsing signal on CPU pin 6.
Starting back at the crystal, the clock goes through some caps/resistors and first ends up on pin 3 of the LS04 at G9, then to pin 4, then to 5 and 6, then to pin 1 of LS393 at F9, and then from pin 4 of F9 to the clock pin 6 of the CPU.
On a working board, I get pulsing signals on the first resistors, and immediately at pin 3 of the LS04 at G9. On the bad board, I don't get any pulses anywhere, not on the resistors near the crystal or anywhere on the LS04. I tried piggybacking a good LS04 over the top of the existing one and it didn't change anything.
Does this mean I have a bad crystal? Do crystals really go bad, or is this possibly a problem with another chip down the line that is shorting and pulling the signal down? Maybe the LS393? There are only two chips between the crystal and CPU clock pin, the LS04 and the LS393.
I've checked with a logic probe, the audio board z80 CPU is not in reset (reset pin starts low then stays high).
I believe that the problem is no valid clock signal. On pin 6 of the CPU (which I believe according to the schematics is supposed to be the clock signal), it just reads high on the probe. Nothing is pulsing on the CPU or audio ROM. On a good board set, I get a pulsing signal on CPU pin 6.
Starting back at the crystal, the clock goes through some caps/resistors and first ends up on pin 3 of the LS04 at G9, then to pin 4, then to 5 and 6, then to pin 1 of LS393 at F9, and then from pin 4 of F9 to the clock pin 6 of the CPU.
On a working board, I get pulsing signals on the first resistors, and immediately at pin 3 of the LS04 at G9. On the bad board, I don't get any pulses anywhere, not on the resistors near the crystal or anywhere on the LS04. I tried piggybacking a good LS04 over the top of the existing one and it didn't change anything.
Does this mean I have a bad crystal? Do crystals really go bad, or is this possibly a problem with another chip down the line that is shorting and pulling the signal down? Maybe the LS393? There are only two chips between the crystal and CPU clock pin, the LS04 and the LS393.
