Buying a Goldstar Analog Oscilloscope - Page 1
Forum section: electronics / repair

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: January 06, 2019, 04:33:52 pm
content: A local online site (Mercado Pago) has a used Goldstar for sale which is not working correctly on one of its two channels. The seller says the square wave is distorted and shows a picture which confirms this. The other channel works correctly. The price seems reasonable and I wonder if the distorted channel is a fault which can be repaired easily?Here is a link:https://http2.mlstatic.com/osciloscopio-20-mhz-analogo-2-canales-goldstar-D_NQ_NP_601577-MLM29083213107_122018-F.webpThe unit uses discrete Goldstar ic's and the wave appears to be an inverse of a square wave. Thus my suspicions is it's some kind of a logic fault making it possibly beyond economical repair.Any suggestions?

author: Armadillo
author_rank: Super Contributor
timestamp: January 06, 2019, 04:43:25 pm
content: More than likely, mere cleaning of those contacts and rotary switches would restore the square waves.This however is my opinion based on my experience.Unless it's dirt cheap, otherwise reconsider it [e.g. less than ~US$25]

author: coromonadalix
author_rank: Super Contributor
timestamp: January 06, 2019, 05:14:42 pm
content: Normally you use this pin with an scope probe, not an jumper, you dont have the right impedance for a start.It is used to check the probe compensation and adjust the small trimmer on the probe bnc to have perfect square rise time and fall time edges, you dont put anything there other than a scope probe.

author: rqsall
author_rank: Regular Contributor
timestamp: January 06, 2019, 08:05:33 pm
content: Quote from: joseph nicholas on January 06, 2019, 04:33:52 pmi have the exact same one and it had a similar looking problem when I bought it, except on both channels. It turned out the jfets Q201 and Q301 at the beginning of the vertical amplifiers had failed (shorted if I remember correctly). There are no unobtanium ICs in there as far as I know. I have a schematic if you need it. I replaced them with 2SK30 jfets. (bought from futurlec.com).Hope this helps.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: January 06, 2019, 09:09:00 pm
content: Thanks for all your replies. The seller wants 750 pesos so 750/20=37.50. The advice here suggests the upper limit for something like this is about $25USD working properly.I will pass on this since it's not a pressing need for me to buy a 25 year old analog scope with some serious component problems. The cause of this is probably someone used it to test the output of a high voltage device without knowing what they were doing.

author: Cyberdragon
author_rank: Super Contributor
timestamp: January 08, 2019, 06:40:17 am
content: There is most likely nothing wrong with it. That seems normal behavior for a coiled clip lead attached to what's clearly labled "probe adjust".However $37 is steep for one that simple, so yeah, hard pass.

author: james_s
author_rank: Super Contributor
timestamp: January 08, 2019, 07:17:19 am
content: Price depends heavily on location, I have no idea what the market is like in Mexico but it's not necessarily the same as in the US.

author: Armadillo
author_rank: Super Contributor
timestamp: January 08, 2019, 08:43:58 am
content: Talk is CHEAP!, why don't you demonstrate with a coiled clip using same? No such effective 1 turn coil exist over such low frequency.And,He said, one channel is working correctly, apparently using the same method.it's available, you can buy it instead.Quote from: Cyberdragon on January 08, 2019, 06:40:17 am

author: Ian.M
author_rank: Super Contributor
timestamp: January 08, 2019, 09:01:32 am
content: Without x10 probes in usable condition, its price is a bit on the steep side for what's currently a single channel scope. Make an offer that's realistic for a single channel 20MHz scope in your country, if its close enough to collect, and its then up to the owner to get back to you if they don't get the price they want. OTOH if its not in easy pickup range its not worth shipping it, so unless you can get a friend to pick it up and bring it on their next visit, I wouldn't make an offer . . .

author: Armadillo
author_rank: Super Contributor
timestamp: January 08, 2019, 09:31:52 am
content: This should bury the dead!Quote from: Armadillo on January 08, 2019, 08:43:58 am

author: tautech
author_rank: Super Contributor
timestamp: January 09, 2019, 05:24:04 pm
content: Anyone notice the CRO in the OP's link is set to AC coupling....waveform as expected.

author: Ian.M
author_rank: Super Contributor
timestamp: January 09, 2019, 05:46:21 pm
content: Nope. That's not the issue. AC coupled mode should have a bandwidth from something like 10Hz up to the scope's rated bandwidth, and the calibrator signal is probably 1KHz and is certainly *NOT* below 100Hz. Also, *BOTH* channels are set to AC and its reported working on the other one.

author: Armadillo
author_rank: Super Contributor
timestamp: January 09, 2019, 05:52:19 pm
content: Yeap!, wave is not centered => more sickness inside.AT AC can clearly see input capacitor lead-thru, charged and discharged with bad/poor contact but front end should be fine.

author: Ian.M
author_rank: Super Contributor
timestamp: January 09, 2019, 06:18:15 pm
content: Hang on a mo. I think pull x10 mode is on on CH1 and on many CROs that's AC coupled, and may have more limited bandwidth than the input coupling. There is a slight difference in the inner knob position (height) of the two Volts/div controls and the giveaway is the leading edge is 2 divs decaying to a 0.2 divs level relative to the level immediately before the step.If viewing it in person, you'd probably have an opportunity to feel if the knob clicks in before making your offer.

author: bd139
author_rank: Super Contributor
timestamp: January 09, 2019, 06:32:45 pm
content: I paid £5 (around $7) for one of these a few years ago fully working with probes and manuals as new. That's about going rate. They aren't bad scopes if they're working TBH but very basic. Most of it is quite repairable apart from the switches so I'd run away if I were you.

author: djacobow
author_rank: Super Contributor
timestamp: January 09, 2019, 06:36:26 pm
content: I've owned this scope's slightly upgraded brother, the OS-9020G, with built-in function generator, for almost thirty years. Got it in high school. I have the manual, too, with schematics, if that is useful to the OP.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 09, 2019, 08:55:34 pm
content: The signal does not need to be centered in AC mode - there is an x-position adjustment. :+The AC coupling should not effect the usually 1 kHz test signal. As the same probe was likely used for the other channel too, it likely OK.The peak like signal looks like an AC coupling with way to little capacitance or divider that is way out of compensation.Chances are one could fix it, though it may be tricky to get good performance if it is not the AC coupling cap that is bad.I would consider some $35 too high for such a broken scope - maybe if new and with a probe.

author: Armadillo
author_rank: Super Contributor
timestamp: January 09, 2019, 09:20:40 pm
content: We are all talking about the Y direction.Quote from: Kleinstein on January 09, 2019, 08:55:34 pm

author: Kleinstein
author_rank: Super Contributor
timestamp: January 10, 2019, 09:55:22 pm
content: Sorry for confusing with the x-position, but there is an y position adjustment too, so one can position the trace wherever one want's it to.For the faulty curve I can not read the gain setting - this could give another hint on the defect. A simple test would be to look at the same signal with DC coupling - this would exclude the input cap (and the switch contact). These 2 would be relatively easy to fix.

author: Armadillo
author_rank: Super Contributor
timestamp: January 10, 2019, 10:12:31 pm
content: Its been a long while since I used the analogue scope. Have been using digital ever since. But if I remember correctly, in AC mode , it stays at the center, Yes No?? not sure now confused. You can see from my old scope, the trace is not even properly rotated.Maybe someone can advice this. thanksQuote from: Kleinstein on January 10, 2019, 09:55:22 pm

author: Kleinstein
author_rank: Super Contributor
timestamp: January 11, 2019, 07:38:28 am
content: The Y position regulator also effects the AC coupled mode. It's also used to position the traces in dual mode - so its also active in AC mode.I still use an old style scope - so it's nothing from the distant past to me.There usually is a trimmer for trace rotation, that is accessible from the outside. This could be somewhere in the back / from the side. Over the years it may need readjustment.

author: rqsall
author_rank: Regular Contributor
timestamp: January 11, 2019, 09:47:24 am
content: Quote from: Kleinstein on January 11, 2019, 07:38:28 amThe trace rotation trimmer is on the front panel right next to the screen.