Weird Psu Diagram (Can't figure it out)

Thread Starter

Dead1y_Wolf

Joined Nov 14, 2019
17
Hello guys,

i am currently repairing an old Oscilloscope that was gifted to me.

I have the circuit diagram and all but i can't really figure it out how the whole system works.

I am new to this kind of circuit and wasted already two weeks troubleshooting this one board and replaced every transistor.
I also built it in a simulator, but it didn't work as intendet.

It would be awesome if someone could explain how this circuit functions.


Thanks in advance for your feedback
Mike
 

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Marley

Joined Apr 4, 2016
294
It's a fairly conventional circuit containing s number of linear series regulators. Some of the output rails are unregulated.
Before changing anything, disconnect all outputs from the loads (the rest of the scope) and check that the voltages are correct. If not, trace back through the circuit. Example: are the AC outputs of the transformer correct? Are the bridge rectifiers working?
Some of the linear regulators depend on other voltages to be correct first.
Logical steps!
 

SteveSh

Joined Nov 5, 2019
99
Good advice from Marley. Check all the output voltages first. Are all of them bad, or only some? Divide and conquer.
Lucky you - you have the schematic. That's 90% of the battle :)
 

Marley

Joined Apr 4, 2016
294
Last thing you want to do is change components. Will do more harm than good! Only Change a component when you are fairly sure it is faulty.
 

atferrari

Joined Jan 6, 2004
3,489
I have the circuit diagram and all but i can't really figure it out how the whole system works.

I am new to this kind of circuit and wasted already two weeks troubleshooting this one board and replaced every transistor.
Rhetorical question: why?

No surprise if you later find out that you created an additional trouble...
 

Thread Starter

Dead1y_Wolf

Joined Nov 14, 2019
17
Okay i got updates because i wrote the first post on my phone at work because ist bothered me :)

It was faulty from the beginning on because when I first got the Oscilloscope it blew a Cap at another board.
So i checked the +/- 30V line and discovered that all the voltages were wayyy beyond the specs.

All capacitors on the psu board are okay, so are the diodes i checked (except the zener because i have no specs other than the zener voltage from the diagram)
 

Thread Starter

Dead1y_Wolf

Joined Nov 14, 2019
17
I have to excuse myself for the sloppy first post.
I got updates because i wrote the first post on my phone at work because ist bothered me :)

It was faulty from the beginning on because when I first got the Oscilloscope it blew a Cap at another board.
So i checked the +/- 30V line and discovered that all the voltages were wayyy beyond the specs.
I already disconnected the psu from everything else and visually inspected all the lines on the board.

All capacitors and resistors on the psu board are okay, so are the diodes i checked (except the zener because i have no specs other than the zener voltage from the diagram).
Also with the new transistors the voltage on the -15V side is too high.

Input Voltage AC on "high" plug: 96V -> should be: min 108V AC max 132V AC
Output on Rectifier 30V AC: 49.7V DC: 66.6V
Output on Rectifier 15V AC: 27.2V DC: 28.1V
Output on Rectifier 05V AC: 07.3V DC: 08.4V
Output on Rectifier 200V AC: 114.0V DC: 154.5V
Testpoint +5V: 5.1V
Testpoint +15V: 8.5V
Testpoint -15V: -27.6V
Testpoint +30V: Tuned for +30V
Testpoint -30V: Tuned for -30
Testpoint +200V: 185V

Tolerances:
-30V: -29.925 to -30.075
-15V: -14.85 to -15.15
+5V: 4.9 to 5.1
+200V: 180 to 240
+15V and +30V: Same as -15V and -30V but +xV


So according to the measurements the -15V side has the fault in it but it does not really explain (at least for me it does not) why the main input voltage is under the mandatory regulating range.
And changing the line selector block from "high" to "low" pushes the voltages over the specs (obviously from the coil therefore more voltages on the rectifier).

I still have no clue :(
 

AlbertHall

Joined Jun 4, 2014
8,543
The output voltages are reasonable except for the -15V. Check the transistor on the heatsink (Q880? - difficult to read) and if that's OK check the other transistors in the -15V supply.
 

AlbertHall

Joined Jun 4, 2014
8,543
Hang on a minute.
When you measured the output voltages the supply was completely disconnected from the rest of the 'scope?
If so then the -15V supply may be OK as there is a resistor (51 ohm) across the series pass transistor which will raise the output voltage with no load. As post #11 check that series pass transistor and if OK then you will have to connect the supply to the 'scope and check the voltages again.
 

Thread Starter

Dead1y_Wolf

Joined Nov 14, 2019
17
The output voltages are reasonable except for the -15V. Check the transistor on the heatsink (Q880? - difficult to read) and if that's OK check the other transistors in the -15V supply.
To measure them i have to solder them out.
But they have to be OK because they are brand new.

Voltage wise what voltages should i expect on the transistors ?
I could also measure them with another crt oscilloscope
 

Thread Starter

Dead1y_Wolf

Joined Nov 14, 2019
17
Hang on a minute.
When you measured the output voltages the supply was completely disconnected from the rest of the 'scope?
If so then the -15V supply may be OK as there is a resistor (51 ohm) across the series pass transistor which will raise the output voltage with no load. As post #11 check that series pass transistor and if OK then you will have to connect the supply to the 'scope and check the voltages again.
Okay but could i use a dummy load?
I am afraid of fucking the whole oscilloscope over.
Because a cap on another board already blew out
 

Thread Starter

Dead1y_Wolf

Joined Nov 14, 2019
17
Hang on a minute.
When you measured the output voltages the supply was completely disconnected from the rest of the 'scope?
If so then the -15V supply may be OK as there is a resistor (51 ohm) across the series pass transistor which will raise the output voltage with no load. As post #11 check that series pass transistor and if OK then you will have to connect the supply to the 'scope and check the voltages again.
Using a 70ohm 300W resistor (only one i could find) the AC input voltage is still only at a low 92ish V.
The +/- 30V are there too and now the -15V are actually -15V.
BUT... :( the +15V is at +6V and the +5V is -0.6V and the 200V/185V is still at 182V

Still can't make out what this causes
 

AlbertHall

Joined Jun 4, 2014
8,543
OK, so the -15V supply will be OK when connected to the 'scope.
The +5V supply relies on the +15V so that's the next to get sorted.
The +15V supply also has a resistor across the series pass transistor so the voltage should be high not low.
I would start by checking the diode and capacitor across the +15V output (C880 and CR850 I think).
 

Thread Starter

Dead1y_Wolf

Joined Nov 14, 2019
17
CR850 has a forward voltage of 0.618V
and C880 is a tantalum capacitor: Capacitance is in range and at the ohms check it moves to infinity
 
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