IWATSU SS-5705 Oscilloscope Tear Down/Repair - Help me repair this thing please! - Page 1
Forum section: electronics / repair

author: mianchen
author_rank: Regular Contributor
timestamp: August 06, 2012, 11:02:45 pm
content: I have this scope for about 4 months, and it died a very sudden death this Sunday (https://www.eevblog.com/forum/beginners/my-oscilloscope-is-dead/). A few people suggested that I should give it a go and try to repair it. Well I am an absolute noob, but I'd like to give it a go anyway.So this thread is to show some pictures of the tear down/repair of this scope as I go along. Hopefully I can eventually repair this thing. Any suggestions/ideas/helps are very welcome.The service manual is too big (41MB) to be attached here, please ask me for a copy if you are interested.

author: mianchen
author_rank: Regular Contributor
timestamp: August 06, 2012, 11:24:07 pm
content: Quote from: mianchen on August 05, 2012, 11:33:42 pmThe scope is relatively clean inside. I've done some visual checks, could not see any bulged caps, burnt resistors, burn mark, loosen connectors etc. nor could I smell anything burning or hear any unusual noise when it's turned on.Unfortunately there is no traces showing at all, no flashing on the screen when turned on/off, not a glimpse of any trace even in complete darkness. Power LED is on when turned on. When a signal in connected to an input, the TRIG'D LED is turned on.The filament of the CRT didn't seem to turn red (I'm not sure if it is supposed to), not open circuit, measured about 7ohm...I've also tested the secondary DC supply voltage as per instructions in the service manual. All voltages (+120V, +50V, +/-10V, +5V) seemed to be ok. Also the calibration signal generator (1KHz, 0.3v square wave) is working.My next step would be following the troubleshooting flow chart to test everything else and report back here.

author: Psi
author_rank: Super Contributor
timestamp: August 07, 2012, 12:42:31 am
content: I'm fairly sure the problem is with the high voltage to the CRT.It's probably a dead flyback transformer, tripler or HV driver.Be careful when working on the HV section and the CRT, there are important safety issues you need to know.The CRT can "charge up" and hold that charge for days. Google CRT high voltage safety before doing any work on the HV area.

author: nctnico
author_rank: Super Contributor
timestamp: August 07, 2012, 01:07:18 am
content: I'd start with checking the power supply. After all thats where it all begins.

author: lowimpedance
author_rank: Super Contributor
timestamp: August 07, 2012, 01:30:36 am
content: Have you checked the 70v rail via fuse F01 (power board) 1A slow. If blown check the components around the HV transformer /oscillator ie q03, q01 d09, d21 the opamp! etc. Could be a shorted HV transformer or the potted multiplier (that would be a bummer). Any way as stated BE VERY CAREFUL If you have recently powered the CRO and the HV section is actually working there will be high capacity charges for sometime after switch off (particularly the tube).Nice looking little CRO, definitely worth trying to fix it. Take your time and think before touching!.Good luck.

author: David_AVD
author_rank: Super Contributor
timestamp: August 07, 2012, 02:13:15 am
content: Quote from: mianchen on August 06, 2012, 11:24:07 pmCan you (carefully) measure the voltage applied to the heater pins of the CRT? That would be a logical place to start. Make sure you do this with the CRT connected of course.

author: lowimpedance
author_rank: Super Contributor
timestamp: August 07, 2012, 03:10:22 am
content: Check the fuse first , this needs to be intact and the thus the HV oscillator running to generate all the tube potentials including the filament.If the fuse is okay it would be the next step to confirm the AC filament voltage (frequency ?? also note r34 worth checking ) is present.BE CAREFULYes I am looking at the service manual for the part numbers!.

author: lowimpedance
author_rank: Super Contributor
timestamp: August 07, 2012, 03:37:52 am
content: Just a thought after looking at your original thread about it taking longer, maybe there is a fault in the vertical or horizontal amps where a component has been slowly drifting out of spec.! thus biasing the deflection plates 'hard' to one direction which prevents any scan being visible.Have you tried pushing the beam find (push the intensity knob)?.

author: mianchen
author_rank: Regular Contributor
timestamp: August 07, 2012, 09:05:42 pm
content: Thanks for all the replies. I haven't managed to do much so far due to lack of time after the normal day job...What I have found out was that this thing is quite dangerous! Exposed mains wire with no protection at all (see pic). Was this a common practice back in the time of this CRO? I have to be extra careful with this thing.

author: Monkeh
author_rank: Super Contributor
timestamp: August 07, 2012, 09:08:39 pm
content: Yeah, that's not uncommon.Back in those days, they assumed anybody poking around in such a device knew what they were doing. Perhaps a silly assumption, but also one you should be able to make.

author: grumpydoc
author_rank: Super Contributor
timestamp: August 07, 2012, 09:10:39 pm
content: QuoteBe aware that although the heater itself is only going to have about 6.3V across it both ends will be a couple of kilovolts negative with respect to the chassis. the 6.3V is often fed from a winding on the HV transformer so it's also usually highish frequency AC.

author: grumpydoc
author_rank: Super Contributor
timestamp: August 07, 2012, 09:25:27 pm
content: As to fixing the 'scope do you have, or can you borrow another 'scope, it's a lot easier to fix a dead one if you have a spare.

author: Lukas
author_rank: Frequent Contributor
timestamp: August 07, 2012, 09:31:59 pm
content: BTW: The front panel looks quite similar to the tek 475 or so. Is this 'scopea) a blatant rip-offb) a rebadged tekc) something completely different?

author: grumpydoc
author_rank: Super Contributor
timestamp: August 07, 2012, 09:43:31 pm
content: I think it's only intended to look vaguely like the 475 - they're completely different 'scopes (for one thing the Iwatsu is 40MHz and the Tek 200MHz) - also as you can see inthese photosof the insides of a Tek 475 the construction is actually totally different and the Iwatsu actually looks much more modern.

author: mianchen
author_rank: Regular Contributor
timestamp: August 07, 2012, 09:50:13 pm
content: Quote from: Psi on August 07, 2012, 12:42:31 amI don't have a high voltage probe, how can I test the HV part of the circuit? I have poked around all low voltage parts and the voltages at test points are within specs - not brave enough to go anywhere near the high voltage parts yet.Quote from: nctnico on August 07, 2012, 01:07:18 amDone that, all voltage test points seem to be ok.Quote from: lowimpedance on August 07, 2012, 01:30:36 amF01 fuse is not blown. Have not been anywhere near the "DANGER HIGH VOLTAGE" signs yet.Quote from: lowimpedance on August 07, 2012, 03:37:52 amI have push the intensity/beam find knob - nothing.Quote from: David_AVD on August 07, 2012, 02:13:15 amI've connected an el cheapo DMM to the pins then turned on the main power. Reading is 0 volt.I think you guys are right - something in the high voltage driver circuit/transformer etc is dead. As a noob, I am a bit scared of working on the high voltage stuff at the moment - may be I should give up and come back when I have more experience?

author: mianchen
author_rank: Regular Contributor
timestamp: August 07, 2012, 09:51:15 pm
content: Quote from: grumpydoc on August 07, 2012, 09:25:27 pmThinking of getting a Rigol 1052 or 1102 when this thing is confirmed dead, might get it soon..

author: Monkeh
author_rank: Super Contributor
timestamp: August 07, 2012, 09:58:30 pm
content: Quote from: mianchen on August 07, 2012, 09:50:13 pmOn AC, right?

author: mianchen
author_rank: Regular Contributor
timestamp: August 07, 2012, 10:05:45 pm
content: Quote from: Monkeh on August 07, 2012, 09:58:30 pmYes, both AC/DC readings are 0 and I have made sure the DMM is working.

author: grumpydoc
author_rank: Super Contributor
timestamp: August 07, 2012, 10:20:11 pm
content: QuoteOK, so the HV inverter/transformer is looking suspect, although the inverter oscillator may be running fast enough that it's beyond a cheapo DMM's bandwidth.Is F702 intact?Check Q608 and D605.If you can borrow a 'scope short term check to see whether there's any input to the PRIMARY of T601 (the HV transformer).

author: grumpydoc
author_rank: Super Contributor
timestamp: August 07, 2012, 10:30:52 pm
content: Apologies - looking at the 5702 circuit diagram. The 5705 is a bit more complex but the oscillator is Q03 and Q01 - check those plus the primary of the HV transformer T01

author: mianchen
author_rank: Regular Contributor
timestamp: August 07, 2012, 10:36:37 pm
content: Quote from: grumpydoc on August 07, 2012, 10:20:11 pmI think you are looking at the SS-5702/5703 schematic, my one is 5705 which is about 20 pages later (if we are looking at the same pdf - SM for SS_5702/03/05/06)...Thank you (grumpydoc and everyone else) for the help so far - I have to go to bed now and will work on it again tomorrow after work...

author: mianchen
author_rank: Regular Contributor
timestamp: August 07, 2012, 10:37:24 pm
content: Quote from: grumpydoc on August 07, 2012, 10:30:52 pm

author: lowimpedance
author_rank: Super Contributor
timestamp: August 07, 2012, 11:40:53 pm
content: When you measured the 0v of the filament was that measured before or after the resistor R34?? (12 ohm). This component may have been slowly drifting to an effective open cct. (perhaps !).Does your cheapo DMM have a decent diode check function ? , have you checked the transistors and diodes in the HV transformer oscillator ie D09, 21 and Q01 and 03 ??.Also check for some resistance of the HV transformer windings to eliminate open windings. If you have an open winding that would be the end of the road for this CRO.Using the ohms range try to measure the resistor values in the oscillator section ie R06 , R12 to R19 (Should get something close given you will be measuring in cct!.)One last thing, the opamp 4558 are not the most reliable (have had problems with these in some old Wavetek generators), its nothing special basically a dual version of the LM741, (google lm741 and you will get more info than you can poke a stick at!, unless you already know of course :)If you do give up for the moment and get a Rigol or similar , put this beastie away in the cupboard and get back to it when you have more experience or inspiration!.Don't give up yet.

author: mianchen
author_rank: Regular Contributor
timestamp: August 09, 2012, 12:24:44 am
content: Quote from: lowimpedance on August 07, 2012, 11:40:53 pmR34 is exactly 12.1 ohms (I desoldered one lead and measured, so it's not in circuit value.Quote from: lowimpedance on August 07, 2012, 11:40:53 pmI connected the cheapo DMM (DT-830B) to check the high voltage part cuz I don't want to blown my less cheapo DMMs. Anyway, both transistors Q01 and 15Q03 are fine. D09, 21 are ok too. I have also checked all the high voltage diodes using an external 12v supply and a current limiting resistor because my DMMs with diode function (VC97/VC99/DT830B) don't work on these diodes. All these high voltage diodes seem ok.Quote from: lowimpedance on August 07, 2012, 11:40:53 pmI have measured the HV transformer using HP3468A 4 wire Ohm measurement. Please see pic for the value. There don't seem to be any open (or short?) windings.Quote from: lowimpedance on August 07, 2012, 11:40:53 pmin circuit measurement: R06=673R (680), R12=525R (820), R13=816R (1.1K), R14=24.5K (47K), R15=1.25K (2.2K), R16=22.34k (22K), R17=1.26K (2.2K), R18=9.49K (20K trim pot), R19=44K (47K). () = value in schematic.Quote from: lowimpedance on August 07, 2012, 11:40:53 pmI will check the op amp tomorrow. It's 1.25am in the morning here...have to go to work in the morning....Quote from: lowimpedance on August 07, 2012, 11:40:53 pmI'm not giving up yet! Thanks for your help!!!

author: T4P
author_rank: Super Contributor
timestamp: August 09, 2012, 04:16:08 am
content: Isn't the 830B like the cheapest meter out there

author: lowimpedance
author_rank: Super Contributor
timestamp: August 09, 2012, 06:34:44 am
content: Okay the transformer winding resistances don't seem unreasonable I guess!.If you do not get any voltage readings at R34 relative to pin 5 of the hv transformer then something is stopping the oscillator from starting.The resistance values measured don't point to any obvious problem with them.You mentioned that you used dt-830 to measure what exactly? CCT powered on or turned off, voltages etc?.Maybe replacing the 4558 opamp anyway, Just a thought is the 10v rail present on pin 8 of this 4558?.

author: David_AVD
author_rank: Super Contributor
timestamp: August 09, 2012, 06:56:04 am
content: Maybe also worth checking / replacing any electrolytic capacitors located next to heatsinks.

author: mianchen
author_rank: Regular Contributor
timestamp: August 11, 2012, 10:48:48 am
content: Quote from: lowimpedance on August 09, 2012, 06:34:44 amI have no idea how to test the 4558 in circuit so I desoldered and tested it on a bread board with some basic op amp configurations. It seems that it is dead!I've soldered an IC socket in the place of the op amp and odered a few 4558 and some LM358s today, hopefully that is the ONLY problem!!!Quote from: David_AVD on August 09, 2012, 06:56:04 amAll caps seemed ok when tested using my home made ESR meter...

author: djsb
author_rank: Super Contributor
timestamp: August 11, 2012, 01:36:40 pm
content: This may be helpfulhttp://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdfIt helps to diagnose the problem at the front panel level before removing the cover. The PDF explains the technique.

author: lowimpedance
author_rank: Super Contributor
timestamp: August 12, 2012, 03:50:04 am
content: The opamps are cheap and easy to replace so its an easy test to replace with a new one. Lets hope that's it.Probably would have not suspected the caps so much (sure check if any are near a heat sink but where not talking switch mode environments here).Keep us posted if the opamp fixes or otherwise.

author: mianchen
author_rank: Regular Contributor
timestamp: August 14, 2012, 06:12:47 pm
content: Quote from: djsb on August 11, 2012, 01:36:40 pmThank you, it's very helpful.Quote from: lowimpedance on August 12, 2012, 03:50:04 amPut the op amp into the IC socket, powered up the scope - nothing happened... Too bad I don't have an other scope to poke around the circuit with. I used a frequency counter to measure the output pin of the op amp, it read about 14KHz. I guess this is the indication that the oscillator is working?When I accidentally poked the lead of R13 connected to pin 4 of the HV transformer with a frequency counter probe. I could hear very high pitched squeaky noise coming out from inside the "Danger High Voltage" box (the transformer?). After the squeaky noise, I saw a green dot on the CRT, when the Beam Find/Intensity knob was pushed. The dot then quickly faded away when the noise completely disappeared.I think there is hope! The CRT is still alive. Did the frequency counter work as a cap (measured 10nf with a DMM) and brought the circuit back to life temporarily?

author: mianchen
author_rank: Regular Contributor
timestamp: August 14, 2012, 06:23:05 pm
content: Ok, I became braver and kept the frequency counter probe on the 'squeaky' spot (pin 4 of HV) for longer. Now I could see some traces appear on the screen. It's moving slowly on the screen and the positions seemed random. I tried to take photos but they didn't come out very well (see pic):

author: mianchen
author_rank: Regular Contributor
timestamp: August 14, 2012, 06:30:14 pm
content: To make it easier for people to help, here is the schematic:

author: David_AVD
author_rank: Super Contributor
timestamp: August 14, 2012, 09:32:28 pm
content: As well as replacing electrolytic capacitors in the suspect area, also check the value of higher wattage resistors. Power resistors going high in value is reasonably common in enclosed spaces. Also check for cracked solder joints. Make sure you discharge any of the larger / higher voltage capacitors before poking around.

author: mianchen
author_rank: Regular Contributor
timestamp: August 14, 2012, 10:36:20 pm
content: Strangely, I can't replicate making the squeaky noise any more...Poked the output pin of the op amp with FC reads 50Hz (was 14kHz) and pin 4 of HV transformer reads ~400kHz and fluctuating.Oh, well, that's it for today, try again tomorrow.Quote from: David_AVD on August 14, 2012, 09:32:28 pmAll the caps and resistors in HV area have been checked, could not find anything...

author: lowimpedance
author_rank: Super Contributor
timestamp: August 27, 2012, 12:06:30 am
content: Mianchen, I have noted that you have a shiny new Rigol from the 'multimeter thread'. Perhaps you could use that to look at the waveforms around the Iwastsu HV oscillator Primary and the opamp output to see if there is something sensible happening!. (make sure you use the x10 probe of course!, to reduce loading and to watch out for the 70v rail on the transformer and the switching transistor).Nice HP multimeter for the price BTW

author: mianchen
author_rank: Regular Contributor
timestamp: August 27, 2012, 06:10:10 pm
content: QuoteThanks, I really want to get a more modern DMM maybe 87V or something, but this one isn't too badQuote from: lowimpedance on August 27, 2012, 12:06:30 amI was busy playing with the Rigol last week didn't have a chance to look at the IwatsuSo I probed the oscillator opamp output pin one, all I have was noise (so I believe), please see pic 1.Probing pin 1 and pin 2 of the HV transformer gave me some interesting waveforms (pic 2 and 3).I think the opamp oscillator is not working at all? I have swapped the opamp with a few new 4558Ds, everything remained the same. These 4558Ds were actually working in the oscillator test circuit I built on a breadboard - so the IC can be ruled out?I re-measured all power rails in the 'scope, all spot on except the +70V rail read ~65-66V.I don't really have much of a clue what to do next :<One thing I have found was that C46 in the schematic is missing on the PCB...

author: mianchen
author_rank: Regular Contributor
timestamp: August 27, 2012, 06:26:55 pm
content: It was very difficult to probe around the 'scope because there was no silkscreen or any marking on the solder side of the PCB. I have drawn the components in Photoshop to help me quickly identify them. I think I'll just post it here in case anyone out there also have one of these 'scopes with faulty HV circuit

author: lowimpedance
author_rank: Super Contributor
timestamp: August 27, 2012, 11:34:13 pm
content: The 70v rail is an unregulated one (nominal value),so it could be around the measured value and the waveforms at the transformer are the main PS ripple.Is the '70v' dc rail present at pin 2 of the HV transformer?. Also could you measure the dc values on the pins of the op amp. DC on Q03 and Q01 C,B,E 'sDouble check the HV transformer windings and be sure the other semi's in the oscillator circuit are in fact ok. (are you sure?). If yes recheck the passive components.No cracked pcb traces?.Just one other part to check, test if R41,42, and pot R43 are ok!. (all meg ohm values!) which are part of the Focus cct.

author: mianchen
author_rank: Regular Contributor
timestamp: August 30, 2012, 08:38:50 pm
content: Quote from: lowimpedance on August 27, 2012, 11:34:13 pmYesQuoteAll DC measurements are in the picture below.QuoteI've desoldered the HV transformer and measure the windings, it seemed ok - all resistance of the windings were similar to what I have measured in circuit and there were no shorts and opens.QuoteI could not see any visual cracksQuoteR42 and R43 are what they meant to be in the schematic, R41 measured 2.2M (6.2M in schematic), however when I opened the metal can covering the HV circuit, R41 is indeed a 2.2M resistor according to its colour bands.I've replaced most of the resistors just in case... the ones that were removed from the PCB were actually fineI'm beginning to loose patiencewell I think I don't have enough knowledge to tackle this as a beginner .... I keep wondering that maybe time is better spent to learn more basic stuff first.

author: lowimpedance
author_rank: Super Contributor
timestamp: August 31, 2012, 12:22:48 am
content: OK, maybe its down to stab in the dark approach now!. Have you replaced q1 and 3 with new parts?, if not do that (if available and not outrageous in cost!). The other possibility is the secondary is loading cct. , killing the HV oscillator from starting (wild guess!!). Try to measure the diodes on the tube bias cct's. and the resistors (note your multimeter may not be able to forward bias the HV diodes!). Bit hard to tell if the x12 multiplier Mo1 is OK (given you got some sort of "spot" when probing with the counter, maybe its fine, as was getting some heater drive voltage).It is getting a bit hard to guess the problem remotely (nothing beats getting your hands on it to try some new idea if the previous one failed!!!).As I said earlier put it away for the time being, but remember you have gained some useful experience which you can apply to something else in the future.Good luck.