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SMS Garbled Video
Posted: Wed Aug 18, 2021 11:21 pm
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I recently acquired a model 2 SMS that seems to be having video issues. When powered on i sometimes get a black screen or a screen with garbled colored squares & there is no audio except for a static hiss when i turn the power switch off.
There board appears to be in good shape and there is no leaking caps/stains anywhere. I tried reflowing the solder on the RF box & the voltage regulator/power brick seem to be ok when probed with a multimeter. Im wondering if this is a problem related to the ram chips. Anyone else have this issue? |
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SMS Garbled Video
Posted: Thu Aug 19, 2021 5:17 pm
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Looking at the screenshot, it could be an issue with the ROM or RAM or CPU because the screen looks like uninitialised VRAM which can mean no CPU execution or a crash. It could also be a faulty VDP.
The screen has colour so, going by that, I think that the XTAL and VDP CLK are OK. The service manual for the SMS 2 is available in the Development section. The manual does contain schematics and fault finding. Fault finding starts with checking the power rails, which you have done at the Voltage Regulator. It's also good to check for 5 volts steady power at each chip. Then check the Reset line to make sure it has come up to 5 volts after the start-up. Then check the CLK signals at the CPU and VDP. This can be done with a multimeter to at least check for activity. Then check pins 20 and 22 (CE and OE) lines of the ROM and RAM chips for activity (i.e. pulses, etc) After these checks, it's a deep dive. :) |
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Posted: Sat Aug 21, 2021 10:17 pm |
Hmm, well i tested all of the chips on the VCC pins and they are all reading at 5V. From what ive read the reset is supposed to be pin 33 on I/O chip, but my SMS has a different revision. Not sure how to do CLK with a multi, can i use DC setting for that? Unfortunately i don't have a scope to test pulses. I did notice i was getting continuity on C68, not sure if its something im doing wrong as i dont think ive ever heard of a ceramic failing. Dont know what the value is as there is no marking on it. |
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Posted: Sat Aug 21, 2021 11:39 pm |
That board is probably closer to the master system 1 schematic. The component numbers might not match up but you should be able to follow it through.
https://www.smspower.org/Development/SegaMasterSystemServiceManual#Schematics The reset circuit is the cap + resistor + diode in the bottom right of your picture. The chip next to it is the io chip. If your multimeter has a frequency counter setting you should be able to use that to check the clocks |
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SMS Garbled Video
Posted: Sun Aug 22, 2021 11:58 am
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C68 is connected in parallel to inductor L1 right above it, which is why you’re measuring a short circuit on C68.
You have an NTSC SMS 2 so on your board the reset line goes to these pins: IC1 (CPU) pin 26 IC4 (315-5216) pin 41 IC5 (315-5246) pin 23 The fact you have colour on the screen leads me to believe the clock frequency is OK because colour encoding is at a very precise frequency, otherwise the screen would be monochrome. We’re just checking that every chip that needs a clock is receiving a clock. Yes, life is easier with a scope, but if your multimeter doesn’t have a frequency setting, or you don’t have a logic probe either, then we can “rough it out” and measure the DC voltage. The MM will measure the average voltage, so a working clock signal will measure somewhere in the middle between 0V and 5V, like 2.5V. If you measure 0V or 5V, then the line is not oscillating. On your SMS, the CPU clock is generated by IC 5 and goes to the CPU (IC1, pin 6) |
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Posted: Sun Aug 22, 2021 11:55 pm |
Voltages seem ok & reset lines are reading at 5V. I also tested some of the RAM data lines as well, some of them seem low but hard to say without a scope. Its my suspicion there is probably a dead cap or dry joint somewhere. I may try and recap the board as there aren't that many caps to swap anyway. Some readings: IC1 6:2.19 26:4.55 IC2 20/22-4.99/2.5.8V IC3 1:2.62 20:4.98 22:2.5 27:3.48 IC4 41:4.50 IC5 23:4.50 IC6 13:2.27, 12:2.28, 11:0.92 15:2.29, 16:2.28 17:0.94, 18:2.27, 19:2.27 20:1.65 IC7 13:1.72 12:2.72 11:2.29 15:2.65 16:2.68 17:0.51 18:0.51 19:0.52 20:1.64 |
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SMS Garbled Video
Posted: Mon Aug 23, 2021 3:27 am
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It looks like there is no activity on the ROM and RAM /CE lines (pin 20).
This could mean your IC4 (315-5216) which generates these signals is dead, or being inhibited somehow. The reset line is a little low at 4.5V. I'm not sure if this is a problem or for IC4 and the CPU. From memory* I believe reset always rises to close to 5.0V but I would need to double-check. |
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