Tektronix 2235A PSU sound - Page 1
Forum section: electronics / testgear

author: Mars
author_rank: Contributor
timestamp: October 27, 2013, 12:54:34 pm
content: I picked up a Tektronix 2235A for free because it seemingly only had a beam when the "beam find" button was pushed in. The owner/company hadn't been using it for years as I understood. I brought it home, opened it up and probed aimlessly around, and after a while the beam magically appeared and both channels worked perfectly. I've rebooted several times now during the last couple of days and it starts up normally and everything looks fine.There are, however, two light-gray clouds on the horizon:1) At one point, the beam on CH1 seemed to fade in and out a little bit to the point where it almost disappeared. I switched to CH2 when this was ongoing and it was bright and clear. I have not been able to reproduce this behavior since, but it bothers me nonetheless.2) The PSU makes a high-pitch sound during operation. It's not alarmingly loud, but still audible from say a meter's distance. I guess this isn't all that uncommon from a SMPS but I'd like to know if this is normal, and if not what actions I can take to correct it.I'd greatly appreciate any comments on these two issues, and also on why the scope went from having no beam to suddenly having one. Thanks.

author: SeanB
author_rank: Super Contributor
timestamp: October 27, 2013, 01:26:03 pm
content: 1 dirty switches need cleaning with Deoxit and 2 is dying caps in the power supply that needs to be changed.

author: Mars
author_rank: Contributor
timestamp: October 27, 2013, 04:43:08 pm
content: Quote from: SeanB on October 27, 2013, 01:26:03 pmThanks. I've done a visual inspection of the PSU board without finding any clues. Next planned step is to get a Mastech MS5308 LCR Tester for around 200$ on Ebay and check all the electrolytic caps on the PSU. Sounds good?

author: oldway
author_rank: Super Contributor
timestamp: October 27, 2013, 05:44:54 pm
content: It is a well known problem of this scope: a bad transformer T906 (120.1439-01) in the pre-regulator (chopper). (internal shortcircuit)This happens because it runs too hot ...I installed a fan in my 2235 as in 2236. (see the Service Manual 2236)Unfortunately, this spare transformer is very hard to find

author: Mars
author_rank: Contributor
timestamp: October 27, 2013, 09:07:13 pm
content: Quote from: Mars on October 27, 2013, 04:43:08 pmScratch that. I'm not going near that thing after seeing the pictures in post #231 in this thread:https://www.eevblog.com/forum/testgear/mastech-ms5308-lcr-meter-with-esr-measurement-on-discount-at-the-moment/225/Ok, I'm a being a bit dramatic here, but I won't sleep well knowing that so little care went into something I have paid money for. Instead, I'm grabbing the DE-5000 from a japanese seller on ebay for 120$:http://www.ebay.com/itm/DER-EE-DE-5000-High-Accuracy-Handheld-LCR-Meter-TL-21-TL-22-New-Japan-/161104294237Cheaper and the spec is better too if I'm not mistaken after a quick glance at the respective datasheets!http://akizukidenshi.com/download/ds/deree/DE-5000_manu_en2p.pdfhttp://micromir.ucoz.ru/RLC/MS5308_english_manual.pdfQuote from: oldway on October 27, 2013, 05:44:54 pmAre you saying that all 2235's have a internal short-circuit in transformer T906? The sound from the PSU is present from the moment I switch the power on—I'm not sure how that resonates with what you say about the xfmr running too hot. Is there a test I can perform to verify your claim?

author: oldway
author_rank: Super Contributor
timestamp: October 27, 2013, 09:28:17 pm
content: My English is not good and I think you did not understand what I meaned.I never said that all the 2235 had the fault but that between the failures of this model, it is a common and known failure.QuoteWhat is happening is a loss of insulation between turns and shorten turns, causing a sharp reduction in the inductance.The preregulator then operate with very high peak current and is no longer able to provide the full load current (approximately 42V and 1A)

author: Mars
author_rank: Contributor
timestamp: November 17, 2013, 09:20:29 pm
content: Re: bad electrolytic PSU capsI received the DE-5000 LCR meter yesterday and have now measured most of the PSU electrolytics:RefDesRated CMeasured CESR (ohm)Test frequencyC90675uF246uF0.171 kHzC9401000uF975uF0.031 kHzC94210 uF11.1uF0.4710 kHzC94310 uF11.3uF0.4810 kHzC95433uF34.2uF0.481 kHzC956270uF508uF0.191 kHzC960840uF2770uF0120 HzC961840uF2930uF0120 HzC962840uF2770uF0120 HzC963840uF2930uF0120 HzC968840uF6790uF0120 HzC970840uF6600uF0120 HzSome of these ESR numbers look a bit on the large side to me, but I haven't been able to find the datasheets for these old caps so I'm not sure what to make of it. Also the 120Hz test frequency seems a bit meaningless (surely I'm measuring half the board in parallel at such a low frequency?), but that's the best the meter could do if I'm not mistaken. Any comments on these numbers?Re: Transformer T906I also made some crude inductance measurements on the windings of T906. I'm not sure if it makes sense to measure the winding inductance like I've done here, but I'm also not sure what else I can do to probe the T906 transformer to see if it in fact is working as it should. Any pointers here would be appreciated. If nothing else, perhaps someone has the same unit and we can compare notes.PinPinMeasured LMeasured DCR123.36mH2.07 ohm453.36mH1.60 ohm67431uH2.13 ohm9104.7uH0.26 ohmRe: fading CH1 signal:I've cleaned the Volt/Div rotary switches on both channels with Deoxit—hopefully that settles the issue.Tektronix 2235 PSU Schematics:http://www.ko4bb.com/Manuals/Tektronix/Tektronix_-_2235/foldouts_lg.pdf(21 MB)

author: Unknown
author_rank: Unknown
timestamp: November 17, 2013, 09:36:26 pm
content: In-circuit capacitance measurements are extremely unreliable due to parallel components as you mention. You can get a better ESR reading in Rs mode and a higher test frequency, but you'd still be measuring the caps in parallel, so a high reading indicates a problem but a low reading does not ensure that all caps are fine. None of the ESR reading strike me as particularly high, but I haven't checked the schematics.

author: markce
author_rank: Regular Contributor
timestamp: November 17, 2013, 09:59:20 pm
content: What frequency are you measuring ESR at? Zero for 840uF 105C Mallory's is unlikely.I did a complete recap of a 2236 after checking ERS of original cap's.Most of them where still good, but not superb. As most supply voltages are unregulated in this scope, and the age, I decidedto replace all (with modern Nichicon's). Silenced the unit as well. Then replaced the Tek installed fan with a low power Papst fan,which even further reduced noise.

author: Mars
author_rank: Contributor
timestamp: November 19, 2013, 10:52:30 pm
content: Quote from: alm on November 17, 2013, 09:36:26 pmRight, that makes sense. Maybe I was overly optimistic regarding what I would be able to do with a 120$ LCR meter from ebay. The exercise has been disappointingly inconclusive.Quote from: markce on November 17, 2013, 09:59:20 pmI measured all the caps in the above table at the highest frequency that gave a reading on the meter. I'm not sure why it reports zero ohm—the measurements on the caps that are listed with zero ESR were done with the meter set to series-mode. But, again, at 120Hz I don't really know what the hell it is I'm measuring. The meter does report a non-zero ESR when the same measurements are done in parallel-mode, but the considering that the reactance of those 840uF caps @ 120Hz is 1.58 ohms, I don't think the non-zero parallel-mode numbers are informative. All in all, I don't think it was a very useful experiment. With some practice I should be able to make better use of the instrument in the future.Quote from: markce on November 17, 2013, 09:59:20 pmWhat type of noise did you experience prior to changing the caps? Are you talking about the same kind of high-pitch--but not very loud--hissing that I mention in the OP? If that's the case, I don't understand (or believe) that a new fan would solve the issue. My problem is not likely heat-related; the PSU sound is present immediately after power ON.