SM 124 black screen BUT power LED ok and sound

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Captain Atari
Captain Atari
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Location: UK

Re: SM 124 black screen BUT power LED ok and sound

Postby siriushardware » Tue Jul 02, 2019 9:10 pm

And the voltages on both ends of those two resistors when the monitor is powered?

You should have 12V (more or less) in each case, assuming you readjusted the B+ adjustment to bring the +12V supply back to +12.0V. Also please check the voltages on both ends of R712, and also check the value of that resistor with the power off.

If all that is OK please check the voltage on C305 positive end (should be 12V) and C302 positive end (I would guess that should be about 6V or a little less). If the last voltage is either 12V or 0V instead of ~6V, power off and check resistors R305 and R302 (should both be 220R).

I note you already said you had checked the voltage on C714 = the same voltage as your 27V rail, so power is getting to the line output stage. With the power off, can you check very carefully for ultrafine ring-shaped cracks in the solder around the pins of the Line Output Transformer (T701). The best way to do that is to look carefully at the solder joints on the track side of the PCB while you gently rock the body of the LOPT back and forth with your hand. If you see any of the pins moving into and out of the surrounding solder when you do this, resolder them.

I know you said you didn't have a scope, but there is only so far we can get by checking voltages and components and doing visual checks like this. Is there anything else that you do have? Frequency meter? Signal generator?

Edit: Sorry, I see right back at the beginning you said you already tried another LOPT, so I'm sure the soldering on the LOPT will be OK.

Atari God
Atari God
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Joined: Tue Feb 14, 2006 12:15 am

Re: SM 124 black screen BUT power LED ok and sound

Postby Chandler » Sun Jul 07, 2019 10:13 am

R712 value is just under 120 ohm so ok

As for powering up i'll have to re install the PCB again so will be back sometime soon...

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