LG OS-5100 100mhz Analog Oscilloscope - Page 1
Forum section: electronics / testgear

author: Seg
author_rank: Regular Contributor
timestamp: September 18, 2013, 12:43:45 am
content: So I ended up with a dodgy LeCroy 9400A, but I'm still itching for an analog X-Y capable scope. Ran across this LG OS-5100 on craigslist, from a local electronics recycler. $30 local pickup. Turns out they had it listed on eBay as well:http://www.ebay.com/itm/LG-0S-5100-100MHz-Oscilloscope-OO11-/331006307947?orig_cvip=trueReally beat up and grungy on the outside, remarkably clean and un-damaged on the inside. Produces a trace, but it's really out of whack. Traces will only get about 1 div high. Adjusting volts/div and position results in clipping at fixed places on the CRT, equally on both channels. So something is clipping near the CRT end of the signal chain.Horizontal and trigger seems to work okay, though the trigger delay seems to be doing nothing. The "B" timebase seems to not work either, I just get a blank screen. Outer "B" knob is broken off.Power rails look spot on as far as I can see, though one is marked both +120V and +130V on different boards, and is at 135V...The beam focus astigmatism was way off. I managed to find that adjustment inside and get a nice focused spot. As beat up as this thing is, the CRT seems very bright and free of burn-in.Found the power button pushrod broken in half inside...I found the operation guide, but no luck on a service manual. It seems LG Precision became EZ Digital, then disappeared off the internet. I can't even find a web site for them. Does anyone have a copy? At the very least, anyone know a good generic analog scope adjustment guide? I've adjusted CRTs before but scopes are precision and I don't want to just start messing up all the trimmers.

author: fluxcapacitor
author_rank: Frequent Contributor
timestamp: September 18, 2013, 01:42:58 am
content: This might help with calibration,i can`t find a schematic though.Calibrating the Oscilloscope for UseBefore the Oscilloscope is Turned “On”1. Center these control dialsIntensity (B)Focus (C)Horizontal Position inner & outer (M)Vertical Position (H)Trigger Level inner & outer (K & L)2. Turn the CH1 & CH2 inner Volts/Div knobs fully clockwise until they click/indent into position. (G)3. Turn the Variable knob all the way clockwise until it clicks/indents into position. (N)4. Make sure the Horizontal Display setting is in position “A”, or “B” if needed, but not “X-Y”. (R)5. Turn the Vertical Mode switch to CH1. (Between two H’s)6. Push the CH1 INT TRIG button so the trace line will be triggered from CH1’s input. (O)7. Flip the trigger Mode switch to Auto and the trigger Source switch to INT. (J & I)8. Turn on the Power. (A)9. Put the CH1 GND switch in the GND position to center the trace line in Step 10. (F)10. When the trace line appears, make sure it is centered. To center it, adjust it with the proper Position knob (H: CH 1 or CH2) so it is centered with the middle X-axis graticule line. Once the trace line is centered, press the GND switch again to take it out of grounded mode. Begin using the oscilloscope when it is calibrated properly.

author: Seg
author_rank: Regular Contributor
timestamp: March 28, 2014, 07:55:10 am
content: Okay, so after 6 months I decided to take another look at this thing. I made up an ESR meter using a signal generator and my Tektronix 465B and have been going through all my equipment checking caps. Found a couple bad caps on the horizontal board, but the vertical board looks good. Still broken.Plugged in a signal generator with a triangle wave and spent all day following the signal across the vertical board, across the delay line, and to the CRT board. Signal looks good all the way until it hits the final Sanyo 2SC4271 driver transistors. Hmmm. Check them with a meter, and what do you know, one of them is shorted across the emitter and base! Should have started on that end. Oh well I learned a lot about how this thing works. Who needs a service manual?Now where to find a replacement 2SC4271, doesn't look like anyone has it in stock anymore. Can't find any cross references. Oh well I've had enough of this thing for one day...

author: tautech
author_rank: Super Contributor
timestamp: March 28, 2014, 08:22:39 am
content: Good on ya. That's right, just follow the signal.From my x ref book;2SC4271 : Silicon NPN Vid-E, hi-def, 30V, 0.3A 5W, 2200MHz Comparable to 2SC3595Hope that helps.

author: Seg
author_rank: Regular Contributor
timestamp: March 28, 2014, 11:02:38 pm
content: With the death of CRTs, the entire 2SC line seems to be mostly unobtainable nowadays.Spent some time digging through parametric searches. I'm guessing the frequency and, judging by the large heatsink on it, power dissipation are the important bits. Not much luck. An SMD version of the 2SC4271 exists as the Sanyo CPH6006. Various exotic 4ghz+ parts from NXP and whatnot that are expensive and mostly un-buyable in singles.Ended up looking through NTE's entire transistor list (no parametric search??) and looked at every 1ghz+ part. Bingo, the NTE2510 looks to be a suitable replacement, and can be bought in singles from a number of places.It is listed as equivalent to the 2SC3950. The only significant difference I can see between the 3950 and 4271 is the output capacitance. Some Sanyo sales fluff lists the 3950 as "up to 80mhz" and the 4271 as "up to 300mhz". (FAA/Medical graphics??) I don't understand where they are getting those numbers, how do they relate to transition frequency? Is this going to kill the bandwidth of the scope?http://home.arcor.de/thomasfetzer/Sanjo/SY103A_e.pdf

author: Seg
author_rank: Regular Contributor
timestamp: April 08, 2014, 09:37:40 am
content: Can't find any 2SC4271's but I've got a 2SC3595 coming in from the UK, about $6 USD off ebay. Seems to be the next step down from the 2SC4271, "up to 200mhz" in the marketing fluff, should be plenty good enough for a beater 100mhz scope?

author: Seg
author_rank: Regular Contributor
timestamp: December 02, 2014, 06:09:29 am
content: K, took a month for the part to show up then it all sat on the back-burner for a while.Soldered in the new part:After tweaking focus and trace rotation, bam a nice sharp, straight trace:Since I twiddled some pots during my circuit exploration, I ran through a calibration based on thisLG OS-9020G service manual, which seems to be a similar design with (mostly) the same pots.Fixed the power button pushrod with some shrink tube.Lookin' good. I'm going to call this a successful repair!The B timebase knob is still busted, but I see no easy way to fix it.If you're looking for a repairable scope, these 90's era LG scopes seem to be a good option. Although mostly surface mount, it is mostly discrete parts, no custom hybrid chips to worry about.

author: tautech
author_rank: Super Contributor
timestamp: December 02, 2014, 06:18:50 am
content: Congats.The giveaway is the first image in the thread that shows the amplitude is not right.It's quite a common fix for CRO's, that is a fault in the CRT plate output amps.Looks just fine now.Thanks for sharing.

author: Seg
author_rank: Regular Contributor
timestamp: December 02, 2014, 09:06:08 am
content: Checked out the B sweep, it's doing something funny. The delay position knob, instead of setting position, causes the B sweep to accelerate rapidly to the right according to the knob position.Oh well, a friend has a Tek 7854 waiting for me so I've decided to sell this one as is:http://www.ebay.com/itm/LG-OS-5100-Analog-Oscilloscope-100mhz-2-Channel-B-sweep-broken-/141492700514