Tektronix 2235 - Power suply problems (Analog oscilloscope) - Page 1
Forum section: electronics / repair

author: azu_
author_rank: Newbie
timestamp: July 05, 2018, 07:39:06 pm
content: 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.pdfSuggestions? Thanks

author: zztech
author_rank: Newbie
timestamp: July 05, 2018, 07:48:59 pm
content: Do you measure voltage regulators inputs?

author: azu_
author_rank: Newbie
timestamp: July 05, 2018, 08:14:24 pm
content: 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.

author: Relayer
author_rank: Contributor
timestamp: July 05, 2018, 10:06:25 pm
content: Hello azu_,Apart from the ticking noise, is the scope working normally?If it is working, does the ticking affect any of the tracesshown on the screen?Regards,Relayer

author: precaud
author_rank: Frequent Contributor
timestamp: July 05, 2018, 11:03:06 pm
content: Quote from: azu_ on July 05, 2018, 08:14:24 pmJust 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.

author: azu_
author_rank: Newbie
timestamp: July 06, 2018, 05:10:18 pm
content: Quote from: Relayer on July 05, 2018, 10:06:25 pmIt doesn't work, nothing on the screen and the LED is off.Quote from: precaud on July 05, 2018, 11:03:06 pmI 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.

author: Relayer
author_rank: Contributor
timestamp: July 06, 2018, 10:30:31 pm
content: Hello azu_,QuoteDid you test them for ESR?Caps can measure perfectly for their capacitance, but falldown due to higher ESR.Regards,Relayer

author: precaud
author_rank: Frequent Contributor
timestamp: July 07, 2018, 01:59:42 am
content: Quote from: Relayer on July 06, 2018, 10:30:31 pmT948 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.

author: azu_
author_rank: Newbie
timestamp: July 07, 2018, 10:06:01 am
content: How I can measure ESR? I only have two multimeters.Else when I measured capacity, I also measured resistance (around 5Mohm).

author: precaud
author_rank: Frequent Contributor
timestamp: July 07, 2018, 12:22:07 pm
content: It's sounding like the transformer may be bad. Check the winding resistances. Check all the solder joints.

author: azu_
author_rank: Newbie
timestamp: July 08, 2018, 06:34:35 pm
content: Winding resistance of L960 and L961, is around 0.30ohm.

author: Relayer
author_rank: Contributor
timestamp: July 08, 2018, 09:20:05 pm
content: Hello azu_,QuoteYou 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

author: azu_
author_rank: Newbie
timestamp: July 22, 2018, 09:16:29 pm
content: 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...

author: markce
author_rank: Regular Contributor
timestamp: July 22, 2018, 09:46:18 pm
content: Did you change C940?

author: azu_
author_rank: Newbie
timestamp: July 23, 2018, 10:06:42 pm
content: 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)

author: markce
author_rank: Regular Contributor
timestamp: July 23, 2018, 10:21:38 pm
content: 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.