Kiotto G-5100 Oscilloscope gets dimmed after 20 minutes of use - Page 1
Forum section: electronics / repair

author: carbono.silicio
author_rank: Contributor
timestamp: June 03, 2018, 03:04:52 pm
content: Hello All,My OscilloscopeKiotto G-5100 100 Mhzworks well in the first 10 minutes after turning on but then it starts to gradually losing brightness. I adjust the knob up to compensate but after 20 minutes it looses the trace almost completely.If i turn it off and on again it recovers but the same happens again.I've checked a few components (knobs, solder joints, resistors, diodes, caps, transistors), even in the high voltage side and everything seems ok.I don't have an HV probe.Does anyone have a clue/suggestion of what is happening here?Here's a link to download the only schematic I've found online, it's for a 40 Mhz one but the circuitry seems the same.https://drive.google.com/file/d/1CQwz1qD6gglyKya4AgUdmxZj2uahRh49/view?usp=sharingThank you all

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 05, 2018, 08:53:36 am
content: A number of things could be going on, a little more detail could help shorten the list.Do you have a second working oscilloscope for troubleshooting?You can get away with the 2kV measurements adding adequate series resistance to your multimeter. (I.E: >30Mohm)A few ideas:Bad solders at CRT socket, drooping LV-PS voltages (numerous possible causes), failing HV multiplier.Could you measure CRT voltages with and without fault for comparison?

author: carbono.silicio
author_rank: Contributor
timestamp: June 05, 2018, 10:25:17 am
content: Quote from: shakalnokturn on June 05, 2018, 08:53:36 amHello! Thank you for your reply!Yes a digital one Rigol DS1052E, 50 Mhz.I'l try to find and buy a >30 Mohm resistor and check them with multimeter.As seen in the photo CRT Socket (P605), don't really know wish red wires but i'l measure all of them K1, H, H, G, F.

author: LateLesley
author_rank: Frequent Contributor
timestamp: June 05, 2018, 10:30:12 pm
content: I know nothing about this scope, but looking at your pics there, there's some crusty solder joints that look like they need attention.*** Safety warning for any newbies - Make sure it's all unplugged, and all the capacitors discharged, before working in side. ***I'd clean and resolder a few joints there, especially R816 R815, C695 doesn't look good, and while cleaning it, it may be worth popping the cap out and checking it. Connection points 7 and 8 definitely need a resolder, The top joint of R786 is questionable, so i'd do both ends of that. C696 might be questionable, and worth redoing, and maybe testing while it's desoldered. C694, same deal. R788, doesn't look bad, but I think you should redo it's joints. I know I mentioned connections 7 and 8, but looking at it, 1-6 and 9 need redoing really. C700 needs checking and resoldered. D642 and D643, it would do no harm to resolder them. And C697 - C699, I'd redo them, testing them as well while they're desoldered. R812 needs a resolder, and I think D644 and D645 would benefit too.It reminds me of the days of old CRT TVs, and their many dry joints.I'd just go around that board, and anything that looks crusty, clean off the solder, and put some fresh stuff in it's place. I think it might make the world of difference.It would do no harm to also do the same on the CRT neck PCB, but nothing really looks bad there. Again, make sure it's all discharged before working on it.I'm not up on all the safe ways of working on these things, can someone else pitch in here? (just to save anyone reading from blowing themselves across the room).

author: floobydust
author_rank: Super Contributor
timestamp: June 06, 2018, 12:53:15 am
content: Check the power supply voltages and see if they are drifting out of spec. The -12V is the reference for the +12V and +140VDC rails, so it must be solid.If the PSU rails are in spec and steady, the CRT bias could be drifting towards cutoff or perhaps the HV is fading. Is the focus also going fuzzy?You would need a way to measure high-voltage with a poor man's HV probe (bunch of series 1/4W resistors in a pen good to 2kV)ORmonitor the HV DC-DC converter's feedback control voltage, say at Q646 base. Q647 base is high impedance from the HV divider and loading it with a multimeter will upset things.See if the HV control voltage moves up (trying to increase HV) as the scope dims or it goes the other direction.Use a q-tip and IPA to clean flux on the HV section of the PCB, then let it dry a long time.

author: carbono.silicio
author_rank: Contributor
timestamp: June 06, 2018, 10:56:11 am
content: Hello,thank you very much for your precious analysis. Yes you are right, i've made a few resoldering on the board were there was very dried orange resin, i'll check the components and resolder all the HV points you mentioned.More news soonQuote from: LateLesley on June 05, 2018, 10:30:12 pm

author: carbono.silicio
author_rank: Contributor
timestamp: June 06, 2018, 11:01:39 am
content: Hello and thank you,i'm now making the HV probe using 9 x 10Mohm 1 watt resistors in series.I'll check the low voltages you indicated and de high ones after the probe is done.More news soonThank you againQuote from: floobydust on June 06, 2018, 12:53:15 am

author: carbono.silicio
author_rank: Contributor
timestamp: June 08, 2018, 04:36:19 pm
content: Hello all,i followed all your reply help and this is what i did and measured:— All pins in high voltage circuit board section were resoldered, resistors and caps tested ok.— All pins in CRT socket board were resoldered.— Voltage pins 130V, 55V, 12V, 5V, -12V tested ok immediately after turning on and 15 minutes later already with crt completely dimmed.— Vertical board tested, caps and transistors were all resoldered.— HV measuments with HV probe, relative to ground:(while hot it's about 10 minutes after CRO being turned on and trace completely disappears even with intensity knob at maximum)Pinwhile coldwhile hotK-1.281V-1.100VH-1.371V-1.080VH-1.330V-1.080VG-1.330V-1.150VF-790V-640VOther 3 contact pins (they go to Focus Potentiometer):Pinwhile coldwhile hotside pin-850V-720VWhiper-780V-670Vside pin-850V-720VTurning Focus knob works well and does not alter the brightness of trace (while visible).After all resoldering, the problem persists.Thank you for any feedback you might have.Nelson

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 08, 2018, 05:06:26 pm
content: It looks like your whole HVPS is sagging, hopefully the multiplier is not the cause.Check electrolytics in the area including the 2 on the primary side of the HV transformer, monitor primary DC voltage during warm-up.You may also want to watch out for hot diodes on the secondary side.If all else fails disconnect the input to the multiplier, and confirm the other outputs remain stable.

author: carbono.silicio
author_rank: Contributor
timestamp: June 08, 2018, 06:42:59 pm
content: Quote from: shakalnokturn on June 08, 2018, 05:06:26 pmI've checked the caps you mentioned, they are ok. Going to test the rest...Where's the multiplier?Thank you

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 10, 2018, 07:59:45 pm
content: You've checked C664, C668, C692 ?The voltage multiplier (M601F) is the large plastic thing next to T601, it will have a biggish (thick insulation) wire coming out of it and leading to the CRT.I wouldn't recommend just desoldering the input from pin9 of T601, as if the gap is not large enough you may get arcs, discharge the CRT, then completely remove M601F from the PCB before checking behaviour on warm-up again. Of course you will have no display at all, just confirm the voltages previously measured after the same warm-up time.

author: carbono.silicio
author_rank: Contributor
timestamp: June 11, 2018, 01:38:19 pm
content: Yes, removed all three, checked capacitance and esr they are ok.Ok, got itThank you very muchReport soon!Quote from: shakalnokturn on June 10, 2018, 07:59:45 pm

author: carbono.silicio
author_rank: Contributor
timestamp: June 12, 2018, 06:33:19 pm
content: Hello shakalnokturn,Shouldn't pin 9 have High Voltage AC?As i understand it now from pin 9 comes HV AC from secondary of T601 and is multiplied by M601F to HV DC to CRT Anode. Is This right?I haven't yet disconnected M601F as you suggested because there's no AC voltage on pin 9 (just a few mV)Quote from: carbono.silicio on June 11, 2018, 01:38:19 pm

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 13, 2018, 12:35:52 pm
content: Hello,Yes you're right on how that works, you should have high voltage AC. (And there certainly is if you have a display on the CRT...)Is it possible your measurement is misleading you? DMM not capable of dealing with the switching frequency or DMM input impedance too low causing all the voltage to be across your series resistors?

author: carbono.silicio
author_rank: Contributor
timestamp: June 13, 2018, 01:46:00 pm
content: Hello,I've used two DMM, both 10 Mohm internal R and reading is practically the same.Did what you suggested, took out Multiplier M601F but readings on output negative voltage still the same hot or cold. Also, unplugged crt socket from the tube itself and nothing changed. I believe that by doing this we can conclude that it's not the tube that is faulty? It must be an in circuit voltage malfunction that gets worse as it heats up?Here's a print of the HV CRT DRV with annotations of voltages and checked components.Quote from: shakalnokturn on June 13, 2018, 12:35:52 pm

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 13, 2018, 04:19:13 pm
content: There are a couple of points I do not understand:You said previously there was no voltage on T601-pin9 but your notes show 2600VAC.The schematic shows pin6 as the intermediate tap on T601 HV winding, this is contradictory with your winding resistance measurements that suggest pin5 is the intermediate tap.Did you try measuring T601-pin6 without the resistive divider on your DMM, I would be surprised if there was nothing at all...Well, at least your voltage multiplier and CRT should be OK.What values does your 100MHz scope use for R719 and R721?The next step would be to check the feedback loop from the cathode point (K) to Q646, after that things start to close in on T601.Testing the transformer itself isn't the easiest thing to do, it could be at fault only when warm. Does Q645 get very hot?

author: carbono.silicio
author_rank: Contributor
timestamp: June 13, 2018, 05:48:54 pm
content: Hello— You are right, i'm sorry, that day i measured it between Pin 9 and Ground only.— Ok we have voltage!Here's measure with DMM true RMS, from PIN 6 to PIN 5: 72 VacI just learned that i cannot use DIY HV probe on relatively low voltages, at least with this DMM.— Yes you have a good point! i checked resistance again on T601 pins and they are exactly as in previous notes. This schematic is for model 40 Mhz, maybe there is a diference...— In R719 it's a 5 band 330K 0,5% tolerance 1/4 watt resistorTheres no R721 but a VR905 trimmer instead, connecting to +12V with whipper positioned at 5.9K— Q645 is attached to a big (10x7x3 cms) aluminium heatsink, i don't think it's heating up, but i will check it.— I will analise the entire circuit between K and Q646 and take notes.— I don't feel T601 heating up while on.Thank you very much!Quote from: shakalnokturn on June 13, 2018, 04:19:13 pm