Tektronix 7904 high-pitched whistling noise - Page 1
Forum section: electronics / repair

author: FlexibleMammoth
author_rank: Regular Contributor
timestamp: October 28, 2017, 12:14:15 pm
content: Hi all,I pulled my old 7904 out of storage and brought it up on the transformer. Everything looks fine, illumination and traces all there. I let it run for about three hours and it got pretty warm (no wonder at ~200W constant power consumption).When i came back, i heared a very annoying 10kHz high-pitched constant whistling sound from the back of the scope (PSU i suppose). Before I dive in there, any suggestions where that may come from?PS: Does the 7904 have a fan? Because mine is completely quiet.Cheers,Andreas

author: bd139
author_rank: Super Contributor
timestamp: October 28, 2017, 12:28:53 pm
content: 7904 doesn't have a fan but does have a switching power supply. Probably that. The HT and main supplies are derived from the main power supply on these. I'd check it out against the service manual performance parameters (very damn carefully as most of the supply is mains potential) and go from there. Audio range is generally avoided but if there's a loading problem anywhere on the primary or secondary it might drag the supply into the audible range. Not 100% sure what class of supply this is so that is only a hypothesis.Also might be dry caps, duff tants, all sorts.I sat in front of a 7904 for about a three months of my life and didn't notice any such noise!

author: Fraser
author_rank: Super Contributor
timestamp: October 28, 2017, 12:53:25 pm
content: I have met Tektronix scopes that had a whistling power supply. The frequency chosen for the SMPSU seemed to be too close to the humans audible range. We sent the scopes for repair and the PSU's were rebuilt/replaced.As has been stated, whistling of a SMPSU can also be an early sign of overloading due to failing capacitors. It is well worth fitting new electrolytic capacitors in older test equipment power supplies as sadly they do tend to degrade with age. Just ensure that the new dapacitirs are similar ESR and a quality brand. Poor quality capacitors can spell doom for an SMPSU, especially in Tektronix power supplies where low ESR capacitors are used.Fraser

author: FlexibleMammoth
author_rank: Regular Contributor
timestamp: October 28, 2017, 06:22:29 pm
content: Thanks for the ideas. I'll shotgun all electrolytics from the PSU, just to be sure.Question: i've seen pictures of the PSU with huge-ass electrolytics in the post quoted below, similar to those used in power amplifiers. should I change those too? the last time i changed stuff like that they were €15/pc.---Quote from: MadTux on October 16, 2016, 11:04:49 pm

author: bd139
author_rank: Super Contributor
timestamp: October 28, 2017, 06:39:15 pm
content: At the time of construction they were quite large and expensive. They have significantly reduced in size and cost now fortunately.It’s worth checking this isn’t something outside of the power supply first. Sometimes these issues can be as silly as a knackered tantalum capacitor. You’ll find that it still does it after you’ve replaced all the electrolytics and been annoyed (been there!)

author: chris_leyson
author_rank: Super Contributor
timestamp: October 28, 2017, 07:37:50 pm
content: 7000 series power supplies operate at around 30kHz. The HV section is fed from a 110V winding on the inverter transformer and is then multiplied by a x7 multiplier for the crt. They all use a custom controller chip for the inverter with the exception of the 7104 which uses a discrete design. The hugh-ass electrolytics just after the line rectifier may need replacing but they have screw terminals and it may be difficult to find a drop in replacement. There is a voltage summing circuit that checks if all the supply rails are within spec and a fault like a shorted tantalum cap will cause the power supply to go into hick-up mode.

author: Armadillo
author_rank: Super Contributor
timestamp: October 28, 2017, 08:00:58 pm
content: Clean and blow away all the dust and dirt from the circuit boards and tube from such long storage.These dust and dirt may form conductive paths and all the weird stuff.

author: David Hess
author_rank: Super Contributor
timestamp: October 28, 2017, 09:11:21 pm
content: Quote from: chris_leyson on October 28, 2017, 07:37:50 pmThe 7704 has the discrete switching controller and was the first of the 7000 mainframes to have the high efficiency power supply. The 7704A and 7904 which were released at the same time replaced the discrete controller with an integrated controller which was used in all of the following models.Quote from: FlexibleMammoth on October 28, 2017, 12:14:15 pmMy 7904 does not have a fan either but have been told that late 7904s do have fans.

author: FlexibleMammoth
author_rank: Regular Contributor
timestamp: October 28, 2017, 09:20:42 pm
content: Quote from: David Hess on October 28, 2017, 09:11:21 pmThis is actually quite interesting. Maybe i'll hack one in just while I'm at it, there must be a reason why tek suddenly decided that's a good idea...EDIT: Removed silly double-post due to bad internet connection.

author: David Hess
author_rank: Super Contributor
timestamp: October 28, 2017, 10:08:51 pm
content: Quote from: FlexibleMammoth on October 28, 2017, 09:20:42 pmSome of the later plug-ins drew more power and the 7904 was marginal in powering them.I have not seen any documentation on the late 7904 fan. My guess is that it is similar to the fan in the 7844 which also uses a one piece chassis and there is plenty of space at the top/right/back of the 7904 chassis to add a tubeaxial fan exhausting to the rear. The 7834 uses a two piece chassis and its fan exhausts to the right.Note that there separate 7904 service manuals for early (serial number B259999 and lower) and late (serial number B260000 and higher) model 7904s and the 7904 with fan was on even later. My late model manual does not mention or show the fan.

author: FlexibleMammoth
author_rank: Regular Contributor
timestamp: December 09, 2017, 11:39:00 pm
content: Hi,yeah I know you shouldn't resurrect the dead, but I'm doing my Master Thesis right now and there may be some months between a good evening of scope repair (and thus me posting new stuff here), so bear with me.I just opened the scope and poked around on the PSU rails, which can be measured on the Z-Axis board as described in this PDF:http://w140.com/Tek_7904_Power_Supply.pdf.While they seem fine voltage-wise, their resistance is a bit low (5-10%) on some rails, and incredibly high (2,8kOhm instead of 250Ohm) on -50V rail.I was quite baffeled and went on a hunt for an early (i.e. low serial number) service manual, but I can only find the higher one (Tagged B260000-UP on Title page and top page corners).Does anyone have a low-serial manual, so that I can actually check against what MY scope should do, instead of a ten-year-later model?Quoting David here, in the hope he has one (does quoting give you a notification?).Thanks,AndreasQuote from: David Hess on October 28, 2017, 10:08:51 pmEDIT: sleep-deprived brain shut off during typing.

author: chris_leyson
author_rank: Super Contributor
timestamp: December 10, 2017, 12:13:04 am
content: QuoteThanks David, I probably got it wrong, I thought it was the 7104. After poking about in a few 7854 power supplies I was going to have a stab at building a discrete controller to replace the IC version as they are unobtainable. I still think it's nice design for a self oscillating LLC resonant supply, it's got proportional base drive and doesn't stress the switching transistors, it's a classic for a 200W discrete LLC. Had to sell all of my 7000 scopes and plugins to survive so I don't think I will be reverse engineering any of the magnetics any time soon. Anyway I was lucky and had chirping supplies that wouldn't start up and that's a lot easier to fix than whistling ones.

author: David Hess
author_rank: Super Contributor
timestamp: December 10, 2017, 12:21:46 am
content: Quote from: FlexibleMammoth on December 09, 2017, 11:39:00 pmWeb forums are not that sophisticated.QuoteI have a paper manual which is missing the power supply schematic but it is also for the late model.Artek Manuals has the early and late model 7904 manuals:http://artekmanuals.com/manuals/tektronix-manuals/Quote from: chris_leyson on December 10, 2017, 12:13:04 amIt is a great power supply design. The portable 485 also used this controller IC. The Tektronix datasheet for the controller gives a lot of details about the input and output signal requirements but unfortunately lacks a transistor level schematic.I have been assembling notes on the design variations between the different models but only have a 7904, 7934, and 7854 to worry about.