EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: azu_ on July 05, 2018, 07:39:06 pm
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Hi,
I have a problem with my Tektronix 2235 analog oscilloscope, it hasn't started since few days. When I turn it on, it makes noise (like click).
I open it and tested voltage regulators out (5V, 30V...) and for all it 0V. So I had tested diode bridge out and there is no problem I think (around 250V in C906)
I also measure voltage from pins 2, 3, 4 and 5 of the T948 and is around -100V. In output I had tested few pins of the T948 and is 0V.
I take measurements when powered.
Schematics : http://www.ko4bb.com/manuals/77.133.21.23/foldouts_lg.pdf (http://www.ko4bb.com/manuals/77.133.21.23/foldouts_lg.pdf)
Suggestions? Thanks
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Do you measure voltage regulators inputs? :-DMM
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On the schematic, I had measured voltages form T948 transformer (in input -100V) and on output pin 19, 20 and 22 used for 5.2V and 8.6V is 0V.
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Hello azu_,
Apart from the ticking noise, is the scope working normally?
If it is working, does the ticking affect any of the traces
shown on the screen?
Regards,
Relayer
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On the schematic, I had measured voltages form T948 transformer (in input -100V) and on output pin 19, 20 and 22 used for 5.2V and 8.6V is 0V.
Just two weeks ago I repaired a 2230 with identical symptoms. I believe their power supplies are identical.
The two outputs of T948 are recitified and then C-L-C filtered. The first filter capacitor (820uF) on the 8.6V side was shorted. Very unusual for a 'lytic to fail by shorting. I replaced it with 1000uF @ 25V low ESR and it started up fine. I then replaced all the other 820uF caps. may as well while you're in there. Since then, I have been using it onthe bench and really like it. It may become my main scope for troubleshooting/repair work. :-+
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Hello azu_,
Apart from the ticking noise, is the scope working normally?
If it is working, does the ticking affect any of the traces
shown on the screen?
Regards,
Relayer
It doesn't work, nothing on the screen and the LED is off.
On the schematic, I had measured voltages form T948 transformer (in input -100V) and on output pin 19, 20 and 22 used for 5.2V and 8.6V is 0V.
Just two weeks ago I repaired a 2230 with identical symptoms. I believe their power supplies are identical.
The two outputs of T948 are recitified and then C-L-C filtered. The first filter capacitor (820uF) on the 8.6V side was shorted. Very unusual for a 'lytic to fail by shorting. I replaced it with 1000uF @ 25V low ESR and it started up fine. I then replaced all the other 820uF caps. may as well while you're in there. Since then, I have been using it onthe bench and really like it. It may become my main scope for troubleshooting/repair work. :-+
I had tested all C-L-C filter capacitors for 5.2V, +-8.6V (C960 - C964, C968, C970) and capacitors seems good (measured capacity around 1000uF). So I don't think the problem is here.
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Hello azu_,
I had tested all C-L-C filter capacitors for 5.2V, +-8.6V (C960 - C964, C968, C970) and capacitors seems good (measured capacity around 1000uF). So I don't think the problem is here.
Did you test them for ESR?
Caps can measure perfectly for their capacitance, but fall
down due to higher ESR.
Regards,
Relayer
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Did you test them for ESR?
Caps can measure perfectly for their capacitance, but fall
down due to higher ESR.
Regards,
Relayer
T948 is very low impedance (< 0.1 Ohm DCR) and will read in parallel to the cap's ESR. So you'll need to remove them to get a good ESR reading.
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How I can measure ESR? I only have two multimeters.
Else when I measured capacity, I also measured resistance (around 5Mohm).
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It's sounding like the transformer may be bad. Check the winding resistances. Check all the solder joints.
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Winding resistance of L960 and L961, is around 0.30ohm.
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Hello azu_,
How I can measure ESR? I only have two multimeters.
You need to try and find an electronics outlet that sell the ESR meters in kit form.
To buy one pre-made one can be expensive.
Regards,
Relayer
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Finally, I had change caps (C960 - C963, C968, C970) with low ESR 1000uF 35V but the oscilloscope doesn't start and the clicks doesn't disappear...
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Did you change C940?
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Why I need to change it, this cap is before the transformer and here everything seems good. The problem is after the transformer (short circuit on one supply line I think)
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Apparently your SMPS is not oscillating correctly. Does not help that some DC on the primary is correct. Either you create a full test setup with a support supply or try a few obvious cheap options first. That's where I tried to help you: exchange the ESR critical cap's on the primary, of which C940 is the most obvious.