EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: gigipaplayed on August 08, 2022, 11:55:39 pm
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Hi everyone,
i am an electronics engineering student, humbly asking for someone's help. Here i will put ALL the resources i have (and those that you ask me) to repair this old scope with some informations i gathered probing things here and there. I do not have any ESR meter so i cannot tell if any capacitor has ESR issues (but i do have a four channel scope and 2 multimeters to probe things with). The ONE AND ONLY low voltage power supply that does not work properly is the 120V, reading about 7V with HV horizontal and vertical scale, and 53V when nothing is connected, so i assume that the problem it's not elsewhere other than A16. Everything else is pretty much spot on other than the 43V being 42,8V, which is pretty okay i guess.
CR1 looks to be okay on every diode, C3 capacitor is reading quite high, almost 180DC. I tested all diodes related to BJT's but only Q3 B-C junction and Q4 B-E junction seem to not be readable by my Voltmeter diode test. Could this really be the explanation of my problem? If so, could you help me understand why? (if those transistors are broken why should my voltage drop so low...)
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If Q4 and ext. Q2 are closed the ground connection goes only through R2 and R8.
To change that the base current of Q4 must be enough to open the transistor and then open ext. Q2.
Local Q2 is the one for the task and Q3 is denying it, if needed.
Lift a leg or remove a component to be sure what the situation really is.
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Do you have the 147.5 volts at the output of the bridge rectifier CR1? If not, isolate the load side and make sure it is there and go from there.