My oscilloscope is dead - Page 1
Forum section: electronics / beginners

author: mianchen
author_rank: Regular Contributor
timestamp: August 05, 2012, 11:33:42 pm
content: I bought an IWATSU SS-5705 40Mhz 3 channel oscilloscope for £50 from a local guy a few months ago. It was ok for the first couple of months, however I'd noticed recently that it took longer to show the traces after turned on. Today, I was working on some audio project, but left it on (no traces were being shown) and went to watch the Olympics for a good few hours. When I returned, I could not make it to show any traces any more.My real question is - is it worth trying to repair it? My guess is that the CRT has just run out of steam and can't be helped.

author: Astroplio
author_rank: Regular Contributor
timestamp: August 06, 2012, 12:03:37 am
content: Does it have too much dust inside? Have you opened for cleaning since you bought it?The monitor appears to be completely dead now or it shows the flash when it is turned off?Well, in any case, if I were in your shoes I would crack it open anyway...Hey it could just be a cap fail, who knows...BTW when I googled the model, first link it pops up is the repair manual! That's nice

author: Monkeh
author_rank: Super Contributor
timestamp: August 06, 2012, 04:10:51 am
content: Could easily just be a failed cap or resistor. Crack it open and see if you can fix it. Alternatively, send it my way, I'll try and fix it.

author: Psi
author_rank: Super Contributor
timestamp: August 06, 2012, 04:33:36 am
content: Try and turn it on from cold in a 100% dark room and check for any glow from the CRT.Also, if your scope has any LEDs for various things, check if they are working.This may prove that the scope is actually running fine internally and just not displaying anything.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 06, 2012, 06:11:09 am
content: Quote from: mianchen on August 05, 2012, 11:33:42 pmCRTs don't normally fail in that manner.They usually start up OK,but over but fairly long time,(months) the display just gets duller until you can't see anything.Hove you tried looking at it in a dark room?If it is working at all,you may see a faint trace.

author: mianchen
author_rank: Regular Contributor
timestamp: August 06, 2012, 07:35:57 am
content: WOW! Thanks everyone for the reply! I am going to open it this evening when I finish my day job (I am a hobbyist and work in a complete unrelated field).Quote from: Astroplio on August 06, 2012, 12:03:37 amI've opened this scope before, it's clean inside, not very dusty at all. No flash when turned on or off.QuoteI have the repair manual, just not sure if it's repairable or not. Now after everyone's reply, it seems encouraging.Quote from: Monkeh on August 06, 2012, 04:10:51 amLet's hope so and thanks for the kind offerI will try to fix it, so I can learn from it. I was asking just in case it's beyond repair so I don't waste my timeQuote from: Psi on August 06, 2012, 04:33:36 amEverything seems to work except the display. The TRIG'D LED turn on when I feed in some signals.Quote from: vk6zgo on August 06, 2012, 06:11:09 amCannot see any trace at all, but it's a gorgeous morning here (rare in Britain), not dark enough even with the curtains down. Will have a look again later. I will post some pics later. Thanks again!

author: BravoV
author_rank: Super Contributor
timestamp: August 06, 2012, 07:50:10 am
content: If you're going to do teardown, please, post a teardown thread with lotsa photo shoots of it.

author: mianchen
author_rank: Regular Contributor
timestamp: August 06, 2012, 11:08:05 pm
content: Quote from: BravoV on August 06, 2012, 07:50:10 amTear down is here [https://www.eevblog.com/forum/product-reviews-photos-and-discussion/iwatsu-ss-5705-40mhz-oscilloscope-tear-downrepair/]

author: Psi
author_rank: Super Contributor
timestamp: August 07, 2012, 12:40:35 am
content: Some part of the high voltage section is probably dead.

author: Things
author_rank: Regular Contributor
timestamp: August 07, 2012, 01:54:23 am
content: I'm not familiar with how exactly CRT scopes acquire their voltage. I know they use a typical flyback transformer, however if it has something like a HV multiplier on the output, they can break down over time due to the corona eating away at the insulation, as well as the flyback itself.You could try carefully removing the HV anode connection from the CRT and arcing it to something grounded, to see if you're still getting HV. I have seen spare flyback transformers for TV's available, maybe you could get lucky and find one for that scope if it's an issue.On the other hand, it could also be the electron guns themselves worn out, in which case it's basically a door stop.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 07, 2012, 04:11:03 am
content: Be very cautious,it is not unusual for the CRT in an Oscilloscope to have what you could call a "back to front" EHT circuit,where the cathode is held at say -2500V,& the anode/s are at close to earth potential.To the tube,it still looks like the anode/s are 2500V positive w.r.t the cathode.This can be a trap for people who are used to TV sets,where the cathode is close to earth potential.From the picture,it look like the anode connection is heavily insulated ,so it either a similar setup to a TV,OR a hybrid type with both types of EHT system.This may be done to get a higher effective value of HT.

author: grumpydoc
author_rank: Super Contributor
timestamp: August 07, 2012, 09:22:04 pm
content: QuoteMy understanding is that the anode is held close to earth potential with the cathode a couple of kV negative because 'scopes use electrostatic deflection and this means that the deflection plates are also at 0V and the final X & final Y amplifiers do not need to operate with a couple of kV DC offset on their outputs.In TV's, which use magnetic deflection, there is no need to worry about this.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 08, 2012, 12:07:45 am
content: Yes,this is the most common (& logical ) way of doing it,but I have come across a few devices which use the "hybrid" set up I referred to,where all the elements are effectively "floating" for DC.The main thing is not to take it for granted that either end is at earth potential.