Analog Scope Vertical Drift - Page 1
Forum section: electronics / testgear

author: rexxar
author_rank: Frequent Contributor
timestamp: May 30, 2013, 03:24:17 am
content: Last year sometime, I picked up a used Leader LBO 502 analog scope on ebay for $17(!) After I'd used it for a while, I noticed that the trace drifted upward very slowly as the scope warmed up. This really didn't bother me, because I just got this for looking at waveforms, I didn't really need to take measurements. I did a cursory search online for service manuals, and didn't find anything.Since I just started school to get my EE, I figured I'd have a go at fixing this thing. I've looked everywhere, and the service manual for this scope just doesn't exist online. Since the main board also goes to this scope's big brother LBO 505, I tried to find that manual as well. I managed to find it, but they had only uploaded the first ten pages... and the servicing started on page 40.Does anyone here happen to have a copy of this manual they could scan and upload?Without a service manual, or even a schematic, I don't know where to start looking inside the scope. Absolutely nothing is labelled. Since everything else works fine, I didn't expect to see any blown up components, and of course there aren't any. (As an aside, though,allof the caps are Nippon Chemi-Con, except the HV ones, which are Marcon)Not sure how to progress from here.

author: c4757p
author_rank: Super Contributor
timestamp: May 30, 2013, 03:34:50 am
content: Start tracing back from the vertical deflection inputs from the CRT. Stick a voltmeter on things and see which voltages drift with the trace. When you've traced it back to a point where it no longer drifts you know you've found it.Doesn't sound unusual for a cheapass scope to me, though. Some things are going to run hot, so you're going to be subject to temperature coefficients. I wouldn't use the thing for measurements anyway...

author: rexxar
author_rank: Frequent Contributor
timestamp: May 30, 2013, 03:45:36 am
content: Quote from: c4757p on May 30, 2013, 03:34:50 amSounds reasonable, but how do I know which pin is which?Quote from: c4757p on May 30, 2013, 03:34:50 amHeh, no I plan to get a 'real' DSO once I save up the money.This oscilloscope definitely wasn't built with servicing in mind. To get the main board out, I'd have to de-solder the connections from the front paneledit: Would the deflection inputs be in the kV range? I wouldn't trust my cheap DMM with high voltages.

author: c4757p
author_rank: Super Contributor
timestamp: May 30, 2013, 04:21:27 am
content: Quote from: rexxar on May 30, 2013, 03:45:36 amCan you see through the glass? A lot of cheaper tubes have some clear sections where you can see the plates. Otherwise, find a manual for a similar model, they probably used the same CRT in all or most of their scopes at the time. I wouldn't go probing around, some of the voltages you find may make you sad.QuoteHundreds of volts but not thousands. Use proper probing technique and you shouldn't die.

author: rexxar
author_rank: Frequent Contributor
timestamp: May 30, 2013, 05:11:14 am
content: Quote from: c4757p on May 30, 2013, 04:21:27 amOkay, the CRT was under a shield, and when I pulled it out, I can see something through the glass. I'm not sure what I'm looking at, though. I googled the part number (toshiba 130bhb1) and didn't immediately come up with anything. I'll attach some pics. The pics are sideways, the big white parts are at the top and bottom.

author: edavid
author_rank: Super Contributor
timestamp: May 30, 2013, 05:32:22 am
content: Usually this happens right at the front end, since later stages are differential. I would start with the input JFET and see what its source is doing. It may not really be possible to fix it.Also, the Leader LBO-324 manual is on BAMA, can't hurt to look at it:http://bama.edebris.com/manuals/leader/lbo324/

author: c4757p
author_rank: Super Contributor
timestamp: May 30, 2013, 05:33:03 am
content: Not of much use, unless you can look down the tube. I'll have a look around tomorrow. It's pretty late where I am though so I am going to bed now...

author: rexxar
author_rank: Frequent Contributor
timestamp: May 31, 2013, 05:51:39 am
content: Quote from: edavid on May 30, 2013, 05:32:22 amGood call, that seems to be where it is.I took measurements of the source voltage over about an hour and got some interesting results. It started out at 7.23V, and after 30 seconds it was up to 7.27V, and the trace had already gone down 1/5cm. At 10 minutes it was 7.41V and -1.5cm. At 30, it was 7.46V and -2cm, and at 1 hour it was 1.5V and -2.5cm.The JFET is on a little V-Amp board which has a plus and minus 27 volt rail, both of which were rock steady the whole time.Does this sound like a fixable problem?

author: dfmischler
author_rank: Frequent Contributor
timestamp: May 31, 2013, 10:00:26 am
content: Quote from: rexxar on May 31, 2013, 05:51:39 amMaybe. Got any circuit freeze spray? It's useful for locating temperature sensitive components. See if there is a component that either works correctly or fails completely when you freeze it. If so, see if you can get a replacement for it.

author: edavid
author_rank: Super Contributor
timestamp: May 31, 2013, 05:08:07 pm
content: Quote from: rexxar on May 31, 2013, 05:51:39 amIt depends on the rest of the circuit. Does it use a dual JFET, 2 discrete JFETs, or just 1 JFET? Does it have a DC balance adjustment like the LBO-324?

author: rexxar
author_rank: Frequent Contributor
timestamp: May 31, 2013, 06:30:34 pm
content: Quote from: edavid on May 31, 2013, 05:08:07 pmThere are two identical JFETs strapped together in a socket (I guess to keep them at the same temp?). I've not looked to closely at how the second one is connected. There is a DC balance adjustment, but I think I've found the problem.I neglected the first law of troubleshooting: thou shalt check voltagesThere's a handy testpoint here that's supposed to be at 12V. It's closer to 14. It's attached to a TO-220 device, which I'm guessing should be a regulator. The device is really hot, 40C with the case open and the air on in my apartment.I happened to have some heatsinks lying around, so I put one in the freezer, and attached it to the regulator. Keeping the temperature down keeps the voltage-- and the trace-- stable, but the voltage is still wrong.I'm assuming it's a regulator, just based on the location, but I can't find any concrete information about it. I think it's Toshiba branded, and the part number is D235. Some places say it's an audio transistor, some it's an NPN amp, but I swear I saw somewhere call it a 12V regulator, but I can't find that site again.

author: edavid
author_rank: Super Contributor
timestamp: May 31, 2013, 06:35:57 pm
content: Quote from: rexxar on May 31, 2013, 06:30:34 pmIf it's really marked D235, that would be a 2SD235 transistor, but I wonder if there's another digit in there that you missed. They wouldn't mark a regulator with a transistor-like code like that.

author: c4757p
author_rank: Super Contributor
timestamp: May 31, 2013, 06:36:30 pm
content: Quote from: rexxar on May 31, 2013, 06:30:34 pmProbably matched JFETs. You'll need a big batch to pick from and a curve tracer (I suppose you could do without the curve tracer but you'll have to take a tedious series of measurements) to match a pair.Quote"Closer to 14" may be close enough, if it's just an auxiliary 12V rail, and 40C isn't really hot. The more important part is stability, both long term and short term.QuoteThe trace stabilizes if you cool the regulator? Try replacing it.QuoteIt's probably an NPN transistor acting as a regulator. Only takes a small handful of discrete parts to make a regulator (resistor, Zener diode, and one or two transistors), so it used to be quite common. Any old TO-220 NPN transistor should do as a replacement.

author: rexxar
author_rank: Frequent Contributor
timestamp: June 01, 2013, 05:41:36 am
content: Well, I dropped a new transistor in, and nothing's changed except the 12V rail is a little lower and the transistor runs cooler. I think the improvement I was seeing earlier was because I had left the case open; when I put it back on and let it sit, the trace crept back to where it was before.So, something's overheating. A few of these big electrolytic caps look a bit sick, could they be the culprit?

author: edavid
author_rank: Super Contributor
timestamp: June 01, 2013, 04:39:10 pm
content: Quote from: rexxar on June 01, 2013, 05:41:36 amWhy do you think it's overheating? Try adjusting the DC balance. Be sure to mark the original position in case you make it worse.

author: rexxar
author_rank: Frequent Contributor
timestamp: June 01, 2013, 11:17:14 pm
content: Quote from: edavid on June 01, 2013, 04:39:10 pmBecause when the case is open, and the scope has good ventilation, it behaves normally. When I put the case on, and things heat up, the trace wanders away.Quote from: edavid on June 01, 2013, 04:39:10 pmOkay, what's it supposed to do?

author: c4757p
author_rank: Super Contributor
timestamp: June 02, 2013, 12:15:16 am
content: Quote from: rexxar on June 01, 2013, 11:17:14 pmThat is a possibility. Shitty capacitors and semiconductors often become shittier as they heat up. You could try changing some of the sick capacitors.Interestingly, many items run hotter when the case is open because you've disrupted planned ventilation paths. My very tightly packed Tektronix 2445A is an example. I doubt this one is like that - I don't think it even has a fan - but it's something to keep in mind.QuoteThe DC balance has to do with the vertical offset between scales on each channel. I don't think it'll cause drift like that, unless I'm missing something?

author: rexxar
author_rank: Frequent Contributor
timestamp: June 09, 2013, 01:17:51 am
content: I can't seem to find the right value caps on Mouser or Digikey. The caps are 4.7uf at 500v, and I can't find those as axial caps. This is in the HV rectifier section, so how critical are the actual values?edit: There's also four huge oil capacitors on the same board. How prone are these to failure? Should they be replaced after the 30 years they've been around?

author: vk6zgo
author_rank: Super Contributor
timestamp: June 09, 2013, 05:08:24 am
content: Quote from: rexxar on June 09, 2013, 01:17:51 amOil filled capacitors ,unless they are leaking,are usually good for 30+ years,so I wouldn't worry a lot about them.Older test equipment,especially it originally was inexpensive, usually needs to warm up for a while before it stabilises.if you are taking the position of the trace when cold as correct,then youwillsee drift--does it ever stop drifting?

author: GK
author_rank: Super Contributor
timestamp: June 09, 2013, 05:22:37 am
content: Quote from: rexxar on May 30, 2013, 03:24:17 amThis is 100% normal behavior for any old solid state 'scope; particularly for designs that have discrete component amplifiers and/or jfet buffers at the front-end. Trust me - I have and maintain quite a collection. Tube scopes are even worse. My "hybrid" Tek 422 is the version with a nuvistor front-end and that is positively awful for both short term an long term drift. The drift generally settles down almost completely after five for ten minutes, but there almost always is some degree of long term drift that requires periodic re-calibration of the ground reference position when taking accurate measurements.

author: rexxar
author_rank: Frequent Contributor
timestamp: June 09, 2013, 05:41:34 am
content: Quote from: GK on June 09, 2013, 05:22:37 amIt might stop eventually, I've never really let it run for a long time to see. Earlier in this thread I monitored the scope for an hour, and it was mostly stable then at almost 3cm below the starting point, but it was still drifting. I would think that is a significant amount of drift, and indicative of something gone wrong.This scope might be a bit older than I thought. the date code on these transistors seems to be 1976.

author: GK
author_rank: Super Contributor
timestamp: June 09, 2013, 05:51:57 am
content: Quote from: rexxar on June 09, 2013, 05:41:34 amThat is a significant amount of drift but by no meansnecessarilyindicative of anything actually wrong with the instrument - especially so for the "state of the art" of a design as ancient as <=1976. You can only do good for it by replacing all of the old electrolytic capacitors but don't waste too much time chasing ghosts out of lack of experience with the performance limitations of such old gear. Just reminisce the good old days of drifty scopes!

author: vk6zgo
author_rank: Super Contributor
timestamp: June 09, 2013, 06:07:08 am
content: With respect,a Leader LBO 502 wasn't anything like "state of the art" for 1976.It was a reasonably inexpensive 'scope,& was appropriately,built to a budget.

author: rexxar
author_rank: Frequent Contributor
timestamp: June 09, 2013, 06:11:19 am
content: Quote from: GK on June 09, 2013, 05:51:57 amI think replacing the caps is about as far as I'll go with this thing. It's not a huge deal that the trace wanders away, as long as the thing still works. I'm actually going to get a Rigol DS1052E soon, which I'll use for actual measurements.Since I can't find all the right value caps, how close is 'close enough' for the HV rectifier circuit?

author: GK
author_rank: Super Contributor
timestamp: June 09, 2013, 06:15:38 am
content: QuoteWell that just further makes my point. The cheap/simplistic designs generally had worse drift performance than the expensive scopes. Just saying that the drift issue as reported isn't out of the ordinary for something that old even if it is in pristine working condition.

author: GK
author_rank: Super Contributor
timestamp: June 09, 2013, 06:22:21 am
content: Quote from: rexxar on June 09, 2013, 06:11:19 amIf you can replace all of the old electros and actually establish that the regulated supply rails are within specification and stable then you are pretty much 95% of the way there.As for the uF values of psu caps, one size up (or even more) generally isn't a problem at all and almost always preferable than going less in value.

author: rexxar
author_rank: Frequent Contributor
timestamp: June 09, 2013, 06:34:58 am
content: Quote from: GK on June 09, 2013, 06:22:21 amOh, that's another thing. How can I probe the HV stuff without blowing up my cheap DMM? I think the highest rail is 2kV

author: vk6zgo
author_rank: Super Contributor
timestamp: June 09, 2013, 06:59:07 am
content: Some of the older 'scopes had a voltage divider off the EHT,so you could read a lower voltage proportional to the high one.The test point was marked "HV test" or something like that.