2465 was working fine, but now it won't power on after I replaced all the caps - Page 1
Forum section: electronics / repair

author: 3w_049
author_rank: Newbie
timestamp: December 16, 2014, 08:54:49 am
content: HI, I bought a 2465 from ebay. The scope works at the beginning, but I decided to replace the electrolytic capacitors anyways, after I done all of that, now the power supply won't even turn on.Following the troubleshooting procedure, I determined there are some problems with inverter. In the inverter troubleshooting procedure, there's a step asking me to "Connect +5VD Load (Described at left) to J303", unfortunately, the service manual I have doesn't have the 'left side'), so I am not sure what this 5V Load is. Please help.

author: mij59
author_rank: Frequent Contributor
timestamp: December 16, 2014, 09:27:58 am
content: Hi,The dummy load is on the previous page.Since the power supply was working check connectors, polarity and voltage rating of the capacitors.

author: SeanB
author_rank: Super Contributor
timestamp: December 16, 2014, 11:40:21 am
content: One of the new caps is either faulty or put in backwards. Check them all against the board and the schematic.

author: wraper
author_rank: Unknown
timestamp: December 17, 2014, 10:56:20 am
content: Or you messed up with soldering and shorted something.

author: grumpydoc
author_rank: Super Contributor
timestamp: December 17, 2014, 12:04:02 pm
content: I hope you get your 'scope up and running soonI confess I'm never sure whether to change the caps in an otherwise working 'scope purely as preventative maintenance - as you can see it sometimes causes problems of its own.What does the panel think?

author: Fraser
author_rank: Super Contributor
timestamp: December 17, 2014, 12:29:41 pm
content: Changing out capacitors carries risk. The following thoughts come to mind:1. Damaged track during desoldering/soldering2. Pulled out feed through during desoldering3. Incorrect type of capacitors fitted4. Incorrect wiring during reassembly5. Aggravation of a fault that was already present...i.e damaged track, dry joint or damaged component lead, such as on a transformer where weight loading is high.Do not underestimate the effect of using replacement capacitors with a different ESR spec. Tek power supplies have a habbit of not firing up if the wrong spec of capacitor is used. I believe that they should all be low ESR types but you need comment from one of the TEK Guru's on here or the Yahoo Group.Aurora

author: tautech
author_rank: Super Contributor
timestamp: December 17, 2014, 06:34:59 pm
content: And one more: the existing old rectifier bridge did not like the new low ESR caps it was asked to charge and went open.As with any major changes/service a dim buld tester should be used on power up.

author: 3w_049
author_rank: Newbie
timestamp: December 25, 2014, 12:30:18 pm
content: I double triple checked all the caps on inverter and regulator board, i didn't find any caps that are reversed.There are a few things I am not so sure about:1. When power on just the PSU, I can hear some high pitch noise coming from the supply. I don't think I heard this kind of noise before when the scope was working.2. I put two 2Ohm 25W resistors in series across 5VD and GND (J232), and power on, the voltage across the resistors is 6.2VDC. Without the 2Ohm resistors (it really is 4ohm), the voltage across J232 +5VD and GND is more than 9V. I did replace C1110 and C1111 with 50V 330uF caps(see attached screen shot), that shouldn't have an effect on the voltage, should it? Where should I check from here on?

author: Nermash
author_rank: Frequent Contributor
timestamp: December 25, 2014, 01:56:17 pm
content: High pitch noise from SMPS and not starting usually means short on one or more rails.If you are not familiar with psu repair, then I recommend reading Sam's repairFAQ on switched mode power supplies, that should give you some ideas on how to proceed.

author: 4cx10000
author_rank: Regular Contributor
timestamp: December 26, 2014, 02:09:26 am
content: What about the other voltages, are they also high with the 4ohm load on +5VD? Is it possible to show both primary and secondary side of your PSU and lastly, did you use low ESR caps?

author: guido
author_rank: Regular Contributor
timestamp: December 26, 2014, 02:28:32 am
content: Quote from: 3w_049 on December 25, 2014, 12:30:18 pmDid you check against the silkscreen or the schematic? Ithinki remember seeing a silk screen error for one of the caps (reversed)... But i'm not sure now and i also recapped a number of scopes at that time. Only one of them being a 2465.

author: 3w_049
author_rank: Newbie
timestamp: December 26, 2014, 05:50:43 am
content: Quote from: 4cx10000 on December 26, 2014, 02:09:26 amI bought mostly Nichicon KL series, that's about what I could find at the moment. KL caps are advertised as having Low Leakage current, but there's no mention of impedance.I have attached a page of the A2 and A3 board. I am testing the PSU only with all the connectors detached. An isolation transformer is used. With 110VAC output, and shorted J206 and J207 on A2 PCB for 115V input.With P231 unplugged, voltage across Pin 1 and Pin3, is +55V, Pin2 and pin3 is -55V, which makes sense.But with P231 plugged in, Pin1 and Pin2 voltage is almost 160V and -160V respectively.Also, on the schematic where it says +264V* (on T1020 pin1), the actual voltage is measured to almost 350V (across R1020 and R1064 REF). Could a short circuit somewhere causing it?Quote from: guido on December 26, 2014, 02:28:32 amI checked the polarity against silkscreen, but when I was recapping them, I matched the original cap polarity.

author: 4cx10000
author_rank: Regular Contributor
timestamp: December 26, 2014, 11:47:39 am
content: Hi again!Well, I wouldn't be sure that your psu will run properly without a feedback - FB PIN1 (with P233 disconnected?) - to the very common TL494 PWM-controller. So, just to put an load on one of the rails, without FB, wont put the voltages to specified values, that's my best guess but I could be wrong...

author: RogerMc
author_rank: Contributor
timestamp: December 27, 2014, 08:04:30 pm
content: A wise man once told me ... Well, read for yourselfhttp://conradhoffman.com/capchecktut.htm

author: crystal
author_rank: Contributor
timestamp: December 28, 2014, 01:34:45 am
content: Did you test the new caps with some good LCR/ESR meter before mounting them?

author: rf-loop
author_rank: Super Contributor
timestamp: December 28, 2014, 02:23:44 pm
content: Quote from: 3w_049 on December 26, 2014, 05:50:43 amDid you never read service manual? There is chapter "Theory of operation".Inside there is very deep description how this power supply works.Read it before do anything more. (... and hope you have not yest destroyed it) Without enough knowledge how this very very clever whole PSU works you are like blind elephant in art porcelain shop.2465 service manual pdf and there it starts from page 3-40.Specially read how this Low voltage PSU pre regulation works, including you fully understand how it is controlled and including also important FB signal. (also it is good to understand how start up sequence and shut down sequence works)Without FB (feed back) it no not work right at all.

author: 3w_049
author_rank: Newbie
timestamp: December 29, 2014, 12:04:42 am
content: Thanks everyone. The PSU is finally up and running. There was a short on C1066, also the regulators needs some load in order to work properly, it was in the service manual and I missed it initially.Putting it back together, and measured all voltage on J119, all is within tolerance. Hooray, thanks everybody.

author: c4757p
author_rank: Super Contributor
timestamp: December 29, 2014, 12:07:35 am
content: Quote from: 3w_049 on December 29, 2014, 12:04:42 amFairly common for in-device power supplies in general - sometimes they can be damaged by unloaded operation, so be careful.