help!! An old tek 7904 that power supply is "tic tic",i think is the multiplier - Page 1
Forum section: electronics / testgear

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: May 24, 2014, 04:02:58 am
content: But i have too, an old tek 475a,can i put the 475 multiplier in the tek 7904? The 7904 high voltage is 21kv.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: May 24, 2014, 04:27:36 am
content: As usual in tektronix, I found some shorted tantalum capacitors,but this time did not solved my problem!I have a good service manual.

author: Sarasir
author_rank: Regular Contributor
timestamp: May 27, 2014, 10:33:45 am
content: the 152-0202-00 is a 1N82A which is a HF mixer diode (5V/25mA) case style A-90 according to the Tek code book I have here.Are you sure about the number?

author: edavid
author_rank: Super Contributor
timestamp: May 27, 2014, 09:19:36 pm
content: Quote from: SAUL BRITTO on May 27, 2014, 03:53:41 amThe correct Tek part number is 152-0212-00.Glass package zeners are not unusual.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: May 27, 2014, 09:22:52 pm
content: Wow Thank you edavid, you are right!

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: May 27, 2014, 09:35:33 pm
content: Sorry for the mistake! 152-0212-00 this is the number!

author: Sarasir
author_rank: Regular Contributor
timestamp: May 28, 2014, 02:36:29 pm
content: 152-0212-00 is a 1N936 selected by Tek for a Noise Spec of 4mv max at Iz=7.5mACase: DO-7

author: N2IXK
author_rank: Frequent Contributor
timestamp: May 28, 2014, 03:05:37 pm
content: The 7904 power supply was unusual for its day, being an early SMPS. Tektronix published an article on troubleshooting these things, with detailed instructions on how to localize the problem when the unit is in the "tick mode". Article available here:http://www.ko4bb.com/Manuals/Tektronix/Tektronix_-_7904A/Tek_7904_Power_Supply.pdfI just went through a problem with the PSU on my 7904, and this article was a big help. My problem ended up being a defective inverter control IC, which is still available from a few dealers, but somewhat costly being a Tek custom part.The problem could be either in the PSU itself, or in the mainframe/plugins.

author: tekfan
author_rank: Frequent Contributor
timestamp: May 28, 2014, 03:12:23 pm
content: First make sure that it really isn't just another tantalum.I found a shorted axial tantalum between the multiple plugin interconnect boards.The boards are also multilayer so removing components and soldering in new ones is quite difficult.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: May 28, 2014, 08:21:38 pm
content: Quote from: tekfan on May 28, 2014, 03:12:23 pmTekfan>> I found two tantalum caps bad,two Schottky diodes and a zener 9v 0,5W, all in the same board. Look this:http://pt-br.tinypic.com/view.php?pic=2rz5b9w&s=8#.U4ZDoPv6-vw.Thisdiode is with a very high resistence,only in X10 i can see a small deflection.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: May 28, 2014, 08:27:43 pm
content: I think that this problem (for now) is on the power supply,the manual is written: 130 volts PS can't go beyond 130v or enter pulse mode.And this 9v zener is at a sensor lead of CI.The plugins was removed.Tekfan I will examine the circuit that you said.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: May 28, 2014, 09:00:01 pm
content: Quote from: Sarasir on May 28, 2014, 02:36:29 pmSarasir look this:http://pt-br.tinypic.com/view.php?pic=2rz5b9w&s=8#.U4ZDoPv6-vw

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: May 28, 2014, 09:47:11 pm
content: Quote from: N2IXK on May 28, 2014, 03:05:37 pmThis is that troubleshooting just the PSU? If yes i have one. Your link dont works.

author: tautech
author_rank: Super Contributor
timestamp: May 28, 2014, 09:55:22 pm
content: Quote from: SAUL BRITTO on May 28, 2014, 09:47:11 pmAfter 30 seconds looking:http://www.ko4bb.com/manuals/index.php?dir=Tektronix/Tektronix_-_7904A

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: May 28, 2014, 11:17:24 pm
content: tautech>> I have these manuals, all that. I have one that is 180mb,download it athttp://elektrotanya.com/tektronix_7904.pdf/download.htmli think it's complete.See "GET MANUAL".

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: May 30, 2014, 05:29:17 am
content: Anybody can said to me, the part number for power supply of tek 7904?I'm not finding in the manuall!!!

author: N2IXK
author_rank: Frequent Contributor
timestamp: May 30, 2014, 11:53:47 am
content: AFAIK, Tek didn't sell the entire PSU as an assembly. They intended it to be troubleshot to component level.

author: PaulAm
author_rank: Frequent Contributor
timestamp: May 30, 2014, 12:58:44 pm
content: Unless you get one from somebody parting out a scope, the only way to get a PS is to buy another scope. That might be cheaper than buying parts.If you look in the service manual, there will be part numbers for the circuit boards. The part numbers changed through the life of the product, so they're associated with a serial number. There are 8 different numbers used for the CRT rectifier assembly in the 7904 manual I have, for example. Look for An in the parts list where n is the number of the board you need.If you don't have a manual, google "Tektronix 7904 manual" and get one of the free ones.

author: N2IXK
author_rank: Frequent Contributor
timestamp: May 30, 2014, 03:28:45 pm
content: IIRC, the power supply consists of 5 separate boards: Inverter, rectifier/filter, LV regulator, autofocus, and HV.And most of those boards went through multiple revisions throughout the product lifecycle.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: May 30, 2014, 05:57:28 pm
content: Ok the PSU is a block of many boards,each with a P-N, right?

author: N2IXK
author_rank: Frequent Contributor
timestamp: May 30, 2014, 06:16:30 pm
content: Yes. Each board has an individual p/n. The last 2 digits of the part number are the revision number. GENERALLY, a later revision board will replace all earlier revisions.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: May 30, 2014, 08:39:30 pm
content: Quote from: N2IXK on May 30, 2014, 06:16:30 pmHere the p/n of the boards ends in -00 mainly the LV regulator

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: May 30, 2014, 10:46:47 pm
content: Just need to know how this type of PSU behaves without charge, it may already be working fine, but the fact that have no load is presented as if it had defective.I found a big short on the interconnection for plugins, as said Tekfan.

author: N2IXK
author_rank: Frequent Contributor
timestamp: May 30, 2014, 11:27:45 pm
content: Probably one of the wet slug tantalums on the main interface board. Royal PITA of a job to replace, as they are buried inside a stack of PC boards held together with long stackable (and very easily bent) header pins. BIG mechanical dis-assembly/reassembly job for 5 minutes of soldering. You need to remove the whole plugin socket panel assembly as a unit (lots of connectors and lots of screws), then disassemble it to access the caps. Replace ALL of the buggers while you have it all apart, because the others are ready to go as well. Be careful with the coaxial cable connectors, as the center pins are easy to bend/break if roughly handled.The problem can also be inside one of the plugins, so check the rails for shorts with and without plugins installed.No the 7904 power supply won't run without a load on it. The connecting cables on the scope are just long enough to operate the supply outside the mainframe.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: May 31, 2014, 12:23:06 am
content: N2IXK when I say short is really SHORT, wow!! zero ohms!! I saw,is a bit difficult to disassemblethese boards,but I can do this.

author: MadTux
author_rank: Frequent Contributor
timestamp: October 14, 2016, 12:18:12 am
content: My favourite 7904A failed. Had it for about 10 years now and liked it very much. Replaced it with another 7904A from storage, but would really like to fix it.Symptoms are: Scope starts, fan spins up, all voltages on PSU are present, but slightly low before PSU goes into ticking mode after about 2-3 seconds. It then ticks a few times, restarts and repeats the cycle at faster rate about once every 1.5-2 seconds.(+/-50V rails => +/-47V, +/-15V rails => +/-14.7V, 5V rail => 4.7V, measured simultaneously with 5x HP-3456A with max/min recall in STAT mode)Because of good voltages on main rails, I excluded a obvious short somewhere. Upon further investigation, I smelled electronic smell from HV compartment behind CRT cathode and noted that R14 (20Ohms current limit for HV inverter) gets rather hot.Now, since most parts there are floating on -3000V, I'm a bit reluctant/scared to probe in it . Unfortunately, because of insane prices, I also don't have nice HV probes like Tek 6015. I could hack some simple HV divider from 10M resistors however.Before I start to take it apart any more, is there any good strategy to further isolate faults? Some known bad parts in there? I somewhat suspect the anode HV multiplier to be bad (which would suck hard), it should be easy to disconnect it (1 wire) and I think the scope should run fine without it, in case it's bad(without screen of course). Any ideas?

author: tautech
author_rank: Super Contributor
timestamp: October 14, 2016, 12:47:10 am
content: Wrong schematic, focus on what's providing the 25 KHz HV driver and that it's oscillating. There should be waveforms and amplitudes in the SM.It should be LV, probably ~30V and if you can't find a reference point for a scope probe you should be able to confirm its operation with a probe just held near picking up the EMI, ahla near field probe style.

author: MadTux
author_rank: Frequent Contributor
timestamp: October 14, 2016, 12:52:08 am
content: Thanks, will measure the 25kHz for HV transformer tomorrow. It's late now, here in central europe

author: MadTux
author_rank: Frequent Contributor
timestamp: October 16, 2016, 12:45:58 pm
content: 25kHz from PSU for HV inverter is there, nice differential 25kHz square wave on both outer pins of P40.I removed 25kHz inverter power connector P40 and both cables that go to Z-axis & focus amplifier (P20 & P83) along with the blue connector on the bottom (P146) to isolate the HV board from the rest. The PSU started fine. So there is certainly something wrong in there.Then I de-soldered the HV multiplier to test it with a few 100V 10kHz sine input from modified audio amplifier (audio amp with small 220V=>12V transformer reversely wired to output) Strangely it seems to work fine (200V AC in yields about 1200V out, stuck a screwdriver into anode plug and increased volume on audio amp while measuring both voltages until the 3456A 1000V range maxed out)To test the HV board without HV multiplier, I reassembled it back into 7904A and connected everything except the CRT anode plug. PSU also worked this time.So either the HV multiplier only fails at really high voltage or something is wrong with the CRT anode. (something shorted inside CRTor bad HV cable). Everything was covered with that nasty HV related soot, which I cleaned. Now I will put HV multiplier back onto the board and test it in circuit with audio amp. Then I will probably put the the board back into 7904A with anode plug disconnected and see wether that makes a difference.

author: chris_leyson
author_rank: Super Contributor
timestamp: October 16, 2016, 02:02:10 pm
content: Off topic but I think the 7904 used discrete transistors in the power supply whereas all of the other 7000 series scopes used an unobtainable controller IC so a least you don't have the problem of trying to find a replacement IC. I had (still have) a few problems with a 7854 power supply, all the rails checked OK so I assumed it had to be the multiplier, warmed it up with a hair dryer to get rid of any moisture and gave everything a good clean with IPA, it seemed to be OK for a while and then failed again. Turned out to be bad solder joints where someone had soldered PCB mount caps to the aluminium screw terminals salvaged from the original primary smoothing caps. Anyway, fixed that problem but it still won't start up, think I might have to re-cap the low voltage regulator section because ifanyrail goes out of spec in voltage or current the PSU shuts down, I think there may even be a link or jumper on the PSU PCB to override the shutdown. Overall I think the 7000 series PSU is a nice design and pretty quiet for a switcher but there are a lot of possible faults that will shut it down, it might be a primary current sense problem and the HV supply could be pushing it over the primary current limit because of a problem in the low voltage section. At least you get schematics and waveforms in the service manual, thanks Tektronix.

author: MadTux
author_rank: Frequent Contributor
timestamp: October 16, 2016, 02:34:24 pm
content: That poor HV multiplier is sparking at places, where it shouldn'tPut HV multiplier back onto board, feed it some 100V's AC from audio amp and no HV => Very weird.Since it worked in desoldered state, I removed both nuts that hold it to board and HV was back again. So I started probing the threads with a grounded screwdiver and saw some small sparksOnly problem, how do I make that HV multiplier happy again? PET foil has some high dielectric strength, so I thought about cutting that thread with a dremel and put several layers of clear PET foil (packaging tape) over it. Any opinions?

author: MadTux
author_rank: Frequent Contributor
timestamp: October 16, 2016, 11:04:49 pm
content: 7904A and me are happy againWrapped lots of PET packaging tape around that HV multiplier. Then glued a PMMA plate to the bottom for extra isolation.Unfortunately the space inside the HV compartment is quite limited, so the multiplier with added plate was too high and I had to rip the plate off again. Zip-tied the multiplier to the board. Space is still pretty limited and I had to slightly bend the PCB to get everything back together.Scope works fine, get a good trace on CRT. Will put current backup 7904A into storage again and use favourite and now repaired 7904A instead to see, if and how long the repair will last.

author: tautech
author_rank: Super Contributor
timestamp: October 17, 2016, 12:20:18 am
content: That'll keep you going for now however if you're feeling flush you might be able to get a replacement from Sphere.http://www.sphere.bc.ca/test/tek-parts/tekparts5.htmlTheir 79** multipliers seem out of stock but it'd be a good idea to send them an email anyway.