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Technics st-c04 digital display not turning on

Always measure DC voltage in audio equipment with the - (black) probe on ground. Nothing past the rectifier diodes is going to be AC unless you have a seriously compromised ripple filter (the 945 transistor) or a bad capacitor, radio frequency portions of the tuner aside.

You need to locate a service manual, else we are fishing in the dark. You will need to compare the voltages on the schematic with what you actually have to figure out what's wrong.

That said, it's highly likely you have a number of bad electrolytic capacitors, they are well past their expected life. I've found more than one shorted or open, and plenty of them leaking in similar age equipment. Given that you found the D880 transistor to be hot all the time the tuner was plugged in it may have an "always on" circuit and a soft switch to power it up. The always on part of the circuit holds the station memory.

Shorted or low resistance capacitors can indeed pull the voltage down, the ones I mentioned are high on the list. The 6.3V 1000uF cap is probably the station memory cap, and the one I had in a Kenwood tuner of similiar was bad. So were most of the rest of them for that matter.
 
Can't seem to locate the manual for free anywhere else. Put all the original parts back. Capacitors, the c945 etc. Interestingly one of the zenor diodes near the c945 runs extremely hot when the unit is powered On.

I'll shall give it to a tech as I don't have the necessary professional tools to conduct this.
 
A quick update. I've not been able to find a technician who can work on the unit. The tech I usually give my electronics too has closed down his operations.

I tried to look for others . However, seems like most of them have closed down due to this pandemic.

I did measure the emitter Q701 while the system was powered up. The readings were 26.1 volts. Seems like it's higher than normal which I believe is resulting to the zenor diode getting extremely hot.(D705 on the board) I have yet to take this diode out and test it.

So on taking out capacitor located on C707 after that, the zenor diode D705 was cool.

I know we would require a service manual to help track down the voltage. But I just wanted to know, what could be the cause in most cases for the extra current 27.1 volts being drawn in the emmiter of Q701. Do note, the transistor 880D is a new one on the Q701.

I totally agree a service manual is a better option to help guide me. But right now I don't have access to one. A free one.
 
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