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Posted Sat Jan 12, 2019 5:29 PM
Posted Sat Jan 12, 2019 9:15 PM
Posted Sat Jan 12, 2019 10:35 PM
Output voltage on the Z80 is allowed to dip as low as 2.4V for a high. This is still TTL compatible, which defines voltages from 2.1+ as definitely high. The 74LS chips typically have high outputs around 3.2V. So to answer your first question, 3.3 is perfectly serviceable.
Do you get the "insert a cartridge before turning it on" screen when you turn it on without a cartridge?
Posted Sun Jan 13, 2019 6:51 AM
Posted Sun Jan 13, 2019 1:55 PM
If you're not sure about power usage, you should probably unplug the system when it doesn't need power. By the way, I assume you've already checked the capacitors for leakage? Rather than bridging the +5V across the switch, connect your meter across in amps mode, to see how many it's pulling.
If you have an Arduino or similar board that is 5V-tolerant, you can use it to check the SRAM chips from the expansion port. It's easiest if you have a 60-pin card edge connector.
1) Connect BUSRQ to ground.
2) Connect A15 to ground, and A14 and A13 to +5.
3) Connect MREQ to ground.
4) Connect A9-A0 to the Arduino, along with WR. If you have enough pins left, you can either connect all the D lines or just a subset to test.
5) Now write a program to loop through SRAM memory writing values then reading them back.
Set the address, and data, set WR low. Do a NOP instruction. Set WR high. Next.
Leave WR high, set the address, do a NOP instruction, read the data and compare with expected value. Next.
Posted Sun Jan 13, 2019 2:33 PM
Clean with alcohol the crystal area and surrounding chips. Maybe you'll need to change the U22 74LS04.
If after cleaning you don't see improvements it's very probable the video chip is failing, the video is always rock steady in a good chip even if it shows trash.
Posted Sun Jan 13, 2019 3:38 PM
Posted Sun Jan 13, 2019 4:40 PM
Posted Sun Jan 13, 2019 7:32 PM
Ben did some unnecessary engineering there...
Composite video can be read off of pin 6 on the RF to PCB connector row. Sound can be read from pin 2, both with respect to ground.
If you scope the 9928's pins 35, 36, and 38 (B-Y, Y, R-Y), triggering on Y's low pulse, you should see waveforms matching the 9928 technical manual on page 5-13. See http://www.bitsavers...anual_Nov82.pdf
Note that the manual is wrong about B-Y back porch voltage sync. It DOES do that on the 9928, to create the color burst signal. Just not on R-Y.
Posted Sun Jan 13, 2019 9:18 PM
Posted Sun Jan 13, 2019 10:56 PM
Hmmm... someone who knows the TMS9928A better should chime in on this, but just after reset, according to the manual, it may not be outputting anything. Hmmm... on second thought, it probably DOES need to at least output sync signals, or the TV would just run wild. Well anyway, just after reset, it's at least blank until the CPU programs it. So unfortunately, I can't tell you whether this definitely is or isn't a problem.
The INT output on the 9928 is triggered during retrace, so if there's no retrace there will be no INT either. The data bus (17-24) is common to all of the CV--SRAM, CPU, sound, ROM, inputs, expansion, etc. The VRAM is connected to pins 25-32 and 1-11.
Definitely check that the clock input is 10.738MHz though. As you might have noticed, it uses an LC circuit to get close, but relies on the 3.58MHz clock to keep it honest.
Edited by ChildOfCv, Sun Jan 13, 2019 11:03 PM.
Posted Tue Jan 15, 2019 12:36 PM
Posted Tue Jan 15, 2019 3:15 PM
Did you watch for activity on CS pin on the ROM? The first thing the Z80 does after reset is start running instructions at address 0000. CV placed the ROM at that location, for obvious reasons.
Posted Wed Jan 16, 2019 9:28 PM
Posted Wed Jan 16, 2019 11:51 PM
Well, since it begins its life by making function calls, it's important to have a working stack. And guess where the stack is? In RAM. So yeah I'd look for a way to test the RAM, or if you happen to have working chips already, try putting them in.
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