Oscilloscope Screen Dead - Page 1
Forum section: electronics / repair

author: keynaboutradios
author_rank: Contributor
timestamp: September 12, 2018, 09:13:27 am
content: Thanks for reading my questionSo I am learning electronic repairs, and have managed to repair a few items, and now trying to be a smart ass and repair a Hameg HM1005 100MHz oscilloscope for the first time.I have turned it on and all led's come on but no trace on screen, also I have taken front panel off to clean all switches, I do not have a high voltage probe so I am keeping well clear from that area and the fact that I'm a newbie better safe than sorry!!, but I do know that's for the CRT which could well be the problem!, if I turn lights off in the workshop and turn power switch on and off, I can see a trace that runs from top of screen to bottom briefly then nothing, each time I do it is the same, also I can see the heating element glowing orange in rear of CRT, so I am guessing that the high voltage is there.Could someone please advice me on possible causes, and any tips.Manual ishttp://wsw.ko4bb.com/getsimple/index.php?id=download&file=06_Misc_Test_Equipment/Hameg/Hameg_HM1005_Oscilloscope_Service_Manual.pdfMany ThanksJustin

author: tautech
author_rank: Super Contributor
timestamp: September 12, 2018, 09:25:43 am
content: First do us a favor by posting up a link to the Service manual so we can all play along at home.The LV PSU rails is where to start checking they are all exactly to spec as listed in the service manual; voltagesandripple measurements.You could also shift this topic to the Repair board using the tab only you the OP can see at the bottom left of this page.Once in the Repair board check the Repair docs thread and grab the Tek troubleshooting scopes pdf in the first post.Have some study, check those LV rails and report findings in another post.Others will jump in if I'm not online and guide you through were to look next. But first spend some good time RTFM.

author: abraxa
author_rank: Frequent Contributor
timestamp: September 12, 2018, 12:39:22 pm
content: Quote from: keynaboutradios on September 12, 2018, 09:13:27 amThe heating element isn't high voltage, though, so that conclusion is unfortunately invalid. From what I understand, the complete lack of HV on the deflection plates would yield a bright dot in the middle of the screen that doesn't move. Since that's not what you see, my guess would be that either one of the deflection circuits is broken (beam goes off-screen, either vertically or horizontally).If I was in your position, I would disconnect the vertical deflector plate from the circuit and see if the horizontal sweep works - with the vertical deflection gone, you'd see a horizontal line across the middle of the screen. However, I have no clue how to disconnect the connector in a safe manner as I assume you don't have HV protective gear.

author: keynaboutradios
author_rank: Contributor
timestamp: September 12, 2018, 01:21:13 pm
content: Thanks tautech for the link to Oscilloscope troubleshooting pdf, I'm reading it atm.And thank you to abraxa too, I am going to look at schematic and see if I can work out how to disconnect one of the deflection plates, that does seem to make sense with when I turn it on I get that trace shooting from bottom to top of screen.Wish me luck!!!

author: audio08
author_rank: Newbie
timestamp: September 12, 2018, 02:24:46 pm
content: check T602 BD237 if it seats tightly in its socket. T602 uses the metal shield that covers the high voltage part as heat sink. you can see it by taking the back cover off

author: keynaboutradios
author_rank: Contributor
timestamp: September 12, 2018, 04:50:30 pm
content: Checked T602 that fine.I think I really need the full service manual for it, all the ones online are missing all the service instructions S1-S3, if anyone has those pages would be a great help for me.Hope this might shed some light on it, as I said earlier I can see the trace moving from the top of CRT to bottom, and while I turn it on and off, I can move the X position, so does that mean it is the Y deflection is at fault?.There is a green Led on the YF-Board and that is not lighting up, nor is the one on the XY Board.

author: Martin.M
author_rank: Frequent Contributor
timestamp: September 12, 2018, 05:11:52 pm
content: https://cdn.rohde-schwarz.com/hameg-archive/HM1005_englisch.pdffirst of all read D12 and check all voltages from the PSUgreetingsMartin

author: keynaboutradios
author_rank: Contributor
timestamp: September 12, 2018, 05:17:05 pm
content: Hi MartinChecked all voltages from power supply and they are all ok, cant find any missing voltages anywhere

author: tautech
author_rank: Super Contributor
timestamp: September 13, 2018, 12:59:18 pm
content: Quote from: keynaboutradios on September 12, 2018, 05:17:05 pmOK if you have all +5, -5, +12, -12, +58 and +110 rails and all LV are within 50mV and 58 and 110 are with a couple of volts let's look further.Front panel lights.....Good.Does the Beam finder work ?What test equipment do you have ? Another scope by any chance ?1kV rated DMM ?If only a good DMM we can still look further.The EHT generation is where we look next on P42/48.We can see TR601 windings drawn in 3 parts, the feedback winding (1,2), primary (4,5) and center tapped secondary (6,7,10).I suspect the oscillator circuit for the primary is not working (but why not?)and you've checked T602.Maybe feedback is shutting it down = display flash when powered.Lets look at the LV side first.+12 and -12 to IC601 ? (power ON)Power OFFInductors L601 and L602 conduct ? (not open)All resistors and diodes measure correctly ? Resistor values near IC601 need be correct. (not drifted)Check regulation feedback resistor R611.To the EHTCheck all Mohm resistors, I see 6 of them plus R611. 7 in all.Report findings.

author: keynaboutradios
author_rank: Contributor
timestamp: September 13, 2018, 05:29:59 pm
content: Thank you so much for using up your time with me Mr tautec.Why I didn't reply straight away, is because I have been re-capping the YF-Board, then I seen your message and wanted to recap the Z-Board and do as you said.So here are my findings, all the Mega-Ohm resistors are correct or well within tolerance, also I did find that the T602 socket had a dry joint, went round all Diodes and checked them only to find very last one D615 is completely open, Will that make sense with having a brief flash of a trace on screen?.Oh and I have a 1000 Volt DMM and a Rigol DS1054 Scope.Trying to find a replacement BY509 is going to be a challenge, I can see originals on Ebay but not cheap!, any idea if there is a equivalent?.Thanks so much, once I can get one if it's ok with you will you see this through to the end?

author: Chris56000
author_rank: Frequent Contributor
timestamp: September 13, 2018, 05:47:02 pm
content: Hi!The 2CL12 diode will meet your needs, these are only a pound or two on eBay!Note that the cathode band on this series of diodes indicates the diode's type & PIV rating, the datasheet is readily available!The 2CL12 is rated 12kV 5mA and a few Members have used these excellent little high–voltage economical devices to rebuild Tek Oscilloscope Multiplier modules!Try one of these and let us know how you get on – we'll all be reading this and if you're still stuck afterwards, either myself or another Member will be back to assist!Chris Williams

author: keynaboutradios
author_rank: Contributor
timestamp: September 13, 2018, 06:17:05 pm
content: Thanks Chris!All ordered so hope it will be here in next few days!Thanks againJustin Shears

author: tautech
author_rank: Super Contributor
timestamp: September 13, 2018, 07:57:37 pm
content: Quote from: keynaboutradios on September 13, 2018, 05:29:59 pmNot likely but possible.It's a HV diode and has a high Vf, too high to measure with the vast majority of DMM's.Without the gear to check it you must setup say a 20mA load and increase supply voltage until it conducts, probably ~15V region.QuoteGood, have you a 100:1 scope probe ?I will watch how your progress as we still have more tricks to share and more things to check like the EHT oscillator for correct operation.

author: keynaboutradios
author_rank: Contributor
timestamp: September 14, 2018, 08:40:38 am
content: Ok so as far as I can tell all components check out to be good on Z Board set up jig to test D615 (BY509) that I thought was faulty and that checked out to be good, and all voltages are fine, I put a scope on T602 and I can see a 37KHz sine wave on Base & Collector.Is there any way someone can tell me if D502 on Y Amplifier Board Should be on at this point as neither D502 or D2507 on Intermediate Amplifier are turning on.How should proceed?Manual ishttp://wsw.ko4bb.com/getsimple/index.php?id=download&file=06_Misc_Test_Equipment/Hameg/Hameg_HM1005_Oscilloscope_Service_Manual.pdfThanks

author: tautech
author_rank: Super Contributor
timestamp: September 14, 2018, 09:13:39 am
content: Quote from: keynaboutradios on September 14, 2018, 08:40:38 amGood, then without a 100:1 probe to check the 1kV on pin7 TR601 we need presume the EHT supply is working.QuoteWhat is D502 indication for ?Just to the right of it is R5035, a 100K NTC thermistor. Check its values as if it's not operating correctly to pull the 58V rail low then this might explain no trace.Controls need be set to mid positions for brightness, focus, position etc. Check 58V is indeed passing the inter-board connection and correct at the middle position in the resistor dropper network between the Y outputs to the CRT plates.BTW, with V Position control set mid way, the voltages on each of the Y plates should be ~equal WRT ground.All looking/measuring OK, turn the intensity to full in a dark room and look for any signs of a trace and experiment with focus if necessary.More tricks to come as we have jumped forward some.

author: keynaboutradios
author_rank: Contributor
timestamp: September 14, 2018, 10:38:39 am
content: So there is 56.6 Volts at Pin1 of connector and Led is not on, Thermistor checks ok and voltage there is 19 volts, and to confirm I replaced with 100k resistor briefly.56.9 Volts on middle of resistor network, those resistors are getting red hot, but guessing thats ok, thats why they put vents in Pcb, voltages on Y outputs are 37.5 Volts.played around with focus intencity nothing.

author: tautech
author_rank: Super Contributor
timestamp: September 14, 2018, 08:37:53 pm
content: Ok we need confirm EHT is there.Your DMM may not measure 1kV @ 37 KHz on pin 7 but will tell us something is there. Note beCarefulwith these voltages ! Double check settings and use 'hands free' measurement methods. Take note the negative voltages !CRT's work by way of potential voltage differences to promote electron flow from the cathode to the phosphors on the display. Study and understand how this works. From -2kV on the cathode to 12kV on the anode, that's 14kV beam acceleration !Again be careful, here everything bites !There are several voltages to check on the schematic but only pin 7 is within capabilities of your DMM.To go further you need a HV probe or if you can find an AVO 8 Mk5 locally cheap they can do 3kV.If EHT voltages are present andall EHT capstest ~OK there are 2 further failures modes to provide a raster to check:Retrace Blanking (always ON) This must be explored.PDA trebler module. (broken) Build another.The trebler multiplier is C's and HV diodes so and checks for correct operation out of the scope must be from voltages higher than the Vf for the diodes for it to operate.

author: tautech
author_rank: Super Contributor
timestamp: September 14, 2018, 10:01:41 pm
content: If banned memberoldway, our former Hameg guru is watching, do drop me a PM if you can with any tips or guidance on where to proceed.If PM's are no longer usable, hunt out my website for my email address.TIA

author: keynaboutradios
author_rank: Contributor
timestamp: September 15, 2018, 11:21:49 am
content: So this is where it gets really interesting for me!, I have at long last some type of trace as seen in pictures, but have no idea how they are there for what i'm doing, I do have an old Avo-8 that was hidden in cupboard as I only use my bench DMM, anyway I thought I would put Avo on transformer side of R634 to test, then I put Avo C624 side of resistor and then I get trace on that side only, now could you explain why this is happening, like I said am new to this and think that it is loading down the circuit thats why but unsure what is happening.

author: tautech
author_rank: Super Contributor
timestamp: September 15, 2018, 07:42:17 pm
content: Great, you might have some other dicky components around the EHT area for an AVO measurement to produce a trace. Check all low value resistors and if you can check caps do so.