Old Meguro MO-1251 Scope edit : MO-1252 scope!! - Page 1
Forum section: electronics / repair

author: VulcanBB18
author_rank: Contributor
timestamp: June 10, 2016, 05:21:55 am
content: Hi everyone,I was gifted this scope (Meguro MO-1251) about 10 years ago, and it didn't get much use while the kids were small, but in recent years it's seen sporadic work.Last time I turned it on, all appeared normal...except no traceTried a few power cycles, playing with the intensity, focus etc but nothing seems to help. The input amps etc appear to still be working, I can get it to trigger etc, but the CRT stays blank. I found a copy of the schematic online, so I do have some sort of guide to work from.I've pulled it down enough to start checking things, should I be able to see the heater glow in the CRT? I guess my first step will be a basic power supply voltage check, but I'm unsure how I can test that the CRT hasn't gone belly-up (which would pretty much write this scope off, IMHO).Prior to the lack of a trace, it used to take a good 5 minutes to get a visible trace on the screen, and even with focus adjustment, it was never the sharpest - is it likely the CRT has thrown in the towel?Hoping to grab a few tips from the more experienced CRO peoplecheers!edit :Whoops! It should be a MO-1252. Full Manual/schematic here :http://www.manualslib.com/manual/235946/Elenco-Electronics-Mo-1252.html- see posts below.

author: tautech
author_rank: Super Contributor
timestamp: June 10, 2016, 05:58:37 am
content: Quote from: VulcanBB18 on June 10, 2016, 05:21:55 amPlease share it with us so we're all on the same page.First thing that comes to mind....Trigger in Normal mode not Auto?Next; are all voltages present and correct and ripple low?QuoteIf you have the schematic you can check the CRT heater pins for open circuit.QuoteUnlikely but until we see some schematics I'll reserve my decision.QuoteYou might find this useful:http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

author: oldway
author_rank: Super Contributor
timestamp: June 10, 2016, 10:50:47 am
content: https://onedrive.live.com/view.aspx?resid=477358247C3BC88D!229&app=WordPdfFirst thing to do is to check all the power supplies: -200V, +120V, +12V, +5V, -8V, -15VReplace C24 1µF/250V (cursor of intensity potentiometer). It may have a leakage current.

author: VulcanBB18
author_rank: Contributor
timestamp: June 11, 2016, 12:29:49 am
content: Thanks both of you for the repliesYep that's the same manual/schematic I have.Okay, between pins 1 & 14 on the tube (should be heater) I get 6 ohms - seems reasonable to me.Measuring the voltages, I get :+5 - ok+12 - ok-8 - ok-15- ok+120 rail is 66V+200 (on the schematic, it shows -200 at the top, and +200 on the connector!) rail is 148V.Okay so both high-voltage rails are significantly down. I get the same voltage readings with tube end-cap connected or not.Now to find why both those rails are down in voltage, I haven't taken the power supply board out yet, I'll see what I can find probing around first.cheers,

author: VulcanBB18
author_rank: Contributor
timestamp: June 11, 2016, 04:03:07 am
content: Maybe I should look more closely at what I have, before jumping on PFDs online.I have a MO-1252, which is a slightly different beast. I've attached a schematic I've cut down from the PDF manual I foundIt seems all the voltages are okay based on that, heater resistance ok, hmmm now what...I can't believe I ran-on with the wrong schematicthanks again,

author: oldway
author_rank: Super Contributor
timestamp: June 11, 2016, 08:35:59 am
content: Did you replaced C? (not readable on schematics of MO-1252) 1µF/250V (cursor of intensity potentiometer)? That's the first thing to do.

author: tautech
author_rank: Super Contributor
timestamp: June 11, 2016, 08:37:41 am
content: Quote from: VulcanBB18 on June 11, 2016, 04:03:07 amYou can go back and modify your previous posts including the OP for those that find it in a search.Link this new manual please.QuoteCheck the fuse on the unreg +12V rail to the HV supply, without this -1.5kV supply there'll be no traces visible.With another scope and a probe held near this HV supply you can check if it's primary is oscillating from its EMI and normally they're quite slow ~20-40KHz.HV Failure modes. (personally experienced)Primary side componentry.Open circuit windings.Open circuit secondary DC restorer diodes.High value resistor divider OC. (M ohms)Leaking HV caps.But check oldway's suggestion....he's been to sea on old CRO's too.

author: oldway
author_rank: Super Contributor
timestamp: June 11, 2016, 08:51:59 am
content: It is a failure getting worse.For this reason, it does not seems to be a failure of a diode, nor a fuse, nor a transformer, nor a semi-conductor, nor a resistor.It seems like an electrolytic capacitor failure.

author: VulcanBB18
author_rank: Contributor
timestamp: June 11, 2016, 11:35:43 am
content: Thanks again folks. I've updated the OP. Using my$20 ESR/transistor tester (fun kit that was), that 1uF does not measure well while in-circuit.I'll get to replacing that as advised, might have to order in some as I don't think I have any 1uf 250V handy....hmm I think I have some 1uF/250V Metalised Polypropylene, that should be ok to try, yeah? (I'll get the electrolytic out and test it too).edit : yes - checked the fuse on the unreg +12V rail, it is ok, unfortunately no other scope, might bite the bullet and get a cheapie kit a-la SainSmart or JYE Tech, they would be ideal for this... ok sold, I'm getting one(need another kit, find them therapeutic and it's cheap)cheers,ps. link to MO1252 manual :http://www.manualslib.com/manual/235946/Elenco-Electronics-Mo-1252.html

author: VulcanBB18
author_rank: Contributor
timestamp: June 14, 2016, 08:13:22 am
content: Okay I managed to get some more done in between preparing for the daughters 16th...Replaced C22 (1uF 250V) - no change, and the cap I took out measured within spec. I tried a few other small caps, especially those near the isolation can they also use as a heatsink. Still no dice, and all the caps I took out measured like new!I note with the isolation can open, that when I switch off I can see the three neons light briefly (see attachment for the ones I mean). I also hear a really brief HV squeal when I first switch on, but it quickly goes silent.Checking the diodes in the "HV block" (see other attachment) which are supposed to be Y16GA, I can't get any measurement except open - meter on 20M range shows open both directions, and my $20 ESR/Transistor tester doesn't see anything....should I be able to measure something or are these truly dead??thanks again.

author: tautech
author_rank: Super Contributor
timestamp: June 14, 2016, 09:56:47 am
content: HV diodes have a high Vf and are not normally able to be checked for forward conductance with normal diode testers.A variable voltage source with the diode in series with a resistor can work and you'll find they'll conduct anywhere between 7 and 20V depending on the diode. They are only rated for a few mA so be sure to use a 100K or so.