Flickering display on Goldstar scope at switching on. - Page 1
Forum section: electronics / testgear

author: Specmaster
author_rank: Super Contributor
timestamp: September 04, 2017, 12:39:14 pm
content: I have a OS-9100D scope that that has a display that flickers quite badly on switching on from cold but after a while settles down to become a nice stable display so I thought I'd check on the supply rails over the weekend expecting to find one or more of the supply rails following this flickering pattern, but nothing was found, all rails were within specification and rock steady and the ripple on each of them was in order of 5mV at the worst and 4mV at best so my theory was shot to shreds.I did a couple of small videos of this (MP4) but cannot attach them so I will try and get them hosted somewhere else and link to them and will come back and update this thread once I have down that.

author: bd139
author_rank: Super Contributor
timestamp: September 04, 2017, 12:53:12 pm
content: Check CRT blanking circuit on these. I've had a dodgy one on an older OS-7020A which is similar. Required a replacement transistor. Also check the HT inverter is working properly on start up and the voltages are right coming off that. That's driven off the 18v unregulated line on LG/Goldstar scopes from limited experience.

author: Specmaster
author_rank: Super Contributor
timestamp: September 04, 2017, 12:56:30 pm
content: Heres the links to the videos20170904_112909byGraham Butcher, on Flickr20170904_124348byGraham Butcher, on Flickr

author: bd139
author_rank: Super Contributor
timestamp: September 04, 2017, 01:21:12 pm
content: That looks like the inverter is stalling to me or there's some arcing going on somewhere.

author: Specmaster
author_rank: Super Contributor
timestamp: September 04, 2017, 01:56:00 pm
content: Not sure just what you mean by inverter or where this is located? There is no visual or audible evidence of any arcing taking place, it's deadly quiet all the time.

author: bd139
author_rank: Super Contributor
timestamp: September 04, 2017, 02:28:58 pm
content: The HT voltages and heater voltages are derived from a low voltage AC/DC supply in these usually as a transformer tap on the main isolation transformer. Inverter is what does this. This is converted to DC and then passed through a single BJT based oscillator and then a small transformer (on the horizontal board) to step up to the required HT voltages to a couple of Kv or so and then via the multiplier to the final acceleration voltage. Watch the secondary side of that transformer - plenty of opportunities to get a nasty shock.See:http://diagramasde.com/diagramas/otros2/132691-sp-01-en-Oszilloskop_PS_9100D.pdfpp 65 Z-AXIS & H.V. CRT DRIVE bottom half of diagram.If the multiplier is shot, this might do this silently as it's potted so that's the end of it. Making/obtaining replacements is usually a case of finding a donor scope.I'd start by monitoring the voltage at the top of the primary on that inverter and seeing if it drops out in line with the dimming events. That should isolate the HV/LV side of the scope.

author: Specmaster
author_rank: Super Contributor
timestamp: November 04, 2017, 11:45:00 am
content: Quote from: bd139 on September 04, 2017, 02:28:58 pmFinally got around to taking a fresh look at the Goldstar after the trimmer had fallen apart and was rattling around inside, not a good thing to have loose.Spare ordered and fitted and on switch on the trace seemed to be rock steady so I progressed to checking the calibration and it just about spot on so no worries in that area.I looked at the HV side but didn't go to deep as it seemed to be working. It is fed from a 24v unregulated tap on the transformer and I measured this and it was on spec, as are all the other supplies. The display did then show a flicker or two and I noted a slight variation on the DMM so I switched to an analogue meter and there was just a very slight movement of the needle butnotin step with the display. I have not yet checked it on the HV transformer primary which is where I think you wanted me to check?If it is the multiplier, I don't rate my chances of finding a donor scope unless this is unit that is common to other Goldstar scopes which will a shame if I have to junk it soon as it is cosmetically pristine condition and inside has not been touched until replaced the trimmer.

author: bd139
author_rank: Super Contributor
timestamp: November 04, 2017, 12:00:44 pm
content: I'd live with it until something else fails then. Whatever is dying will die when it's ready.It's not impossible to build your own multiplier as a replacement. It's a potted cockcroft-walton generator. If you can find the number of stages, and a small box the right size, and some HV diodes and caps and some potting compound then it's not a write-off.See:https://www.eevblog.com/forum/repair/tektronix-465/msg672083/#msg672083

author: Specmaster
author_rank: Super Contributor
timestamp: November 04, 2017, 12:35:38 pm
content: Yes, thats what I was planning on doing, its been well looked after in its life, also seems to have been used very little as the trace is still nice crisp as well very bright, the controls and fascia show no signs of wear at all and even the chrome on the handle looks as if its just been taken out of the box.The unit has now been on for a couple of hours without any signs of flickering etc on the trace and is very stable indeed, a nice unit. It brings my tally upto 5 fully working and calibrated CRO's now and hopefully soon to be 6 with 408 soon to back on the bench after I clear off the Vega 215 radio.Neat job with the potting by the way, you really had a good find with that box, just the right size.

author: bd139
author_rank: Super Contributor
timestamp: November 04, 2017, 01:57:54 pm
content: Wasn't me who did it. It'd look like shit if I did itSure I said before but I really like the LG/Goldstar scopes.