EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: PoDuck on March 04, 2018, 02:49:56 am
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I have a switch mode power supply on a TRS-80, and I had some other issues with it, such as the SCR (Q6) blowing for an unknown reason, and a 30V zener (VR1) blowing because apparently this power supply must have a load on it at all times, but now that those issues are sorted, I have another problem, which is that I am getting double what I should be getting for power. The 12 volt rail reads 24 volts, the -12 volt rail reads -24 volts, and the 5 volt rail reads 10 volts. It is pretty much exactly double what it should be. I am thinking that should be a clue, like maybe a shorted resistor in a voltage divider somewhere, but for the life of me, I'm not sure where it would be.
The schematic for it is here: at https://ia600901.us.archive.org/14/items/TRS80Model4PServiceManual/TRS80%20Model%204P%20Service%20Manual.pdf (http://at https://ia600901.us.archive.org/14/items/TRS80Model4PServiceManual/TRS80%20Model%204P%20Service%20Manual.pdf)
The schematic for the board I'm working on is on PDF page 164, which is page 161 of the manual.
When I run this board, there are a couple resistors that get extremely hot, but I'm certain that it is because they are dissapating more power than the circuit was designed for. I only run it for a short time to do tests, then turn it off so it can cool down. I am sure that even this is not good for things.
I have checked the test points D and E with the oscilloscope, and all the wave forms look like the pictures say they should, except that I have to use a higher volts per division setting to see it, which is likely because things are running at about twice the voltage they should. The sawtooth from pin 5 of the PWM is around 36Khz, but I don't know what it should be at. I also don't know what duty cycle I'm supposed to be finding. The reference voltage on pin 12 is 4.95 volts, which I am assuming is supposed to be 5 volts and within specs, but I don't know.
The odd thing is that I am not detecting DC voltage on pins 1 and 2, but I do detect an AC voltage of between 4.5 and 6.3 volts, depending on when I test them. I am assuming that this is showing as AC because it is a square wave signal, but I don't know.
So, it appears that the PWM is probably okay, but likely getting the wrong inputs. I just don't know where to look, and I really don't want to start taking every resistor off the board to test it. Am I even correct in thinking it could be a bad resistor, or am I completely stupid?
Anyway, any help would be appreciated.
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Hi PoDuck,
this seems to be the common failure of the optocupler (in your case is a 4n35 labeled as U2).
Hope this helps.
Best,
0xfede