IWATSU SS-5705 Oscilloscope Tear Down/Repair - Help me repair this thing please!

[mianchen]

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.

[mianchen]

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.

 

The 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.

[Psi]

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.

[nctnico]

I'd start with checking the power supply. After all thats where it all begins.

[lowimpedance]

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.

[David_AVD]

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...

Can 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.

[lowimpedance]

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 CAREFUL
 
Yes I am looking at the service manual for the part numbers!.

[lowimpedance]

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)?.

[mianchen]

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.

[Monkeh]

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.

[grumpydoc]

Can 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.

Be 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.

[grumpydoc]

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.

[Lukas]

BTW: The front panel looks quite similar to the tek 475 or so. Is this 'scope
a) a blatant rip-off
b) a rebadged tek
c) something completely different?

[grumpydoc]

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 in these photos of the insides of a Tek 475 the construction is actually totally different and the Iwatsu actually looks  much more modern.

[mianchen]

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.

I 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.

I'd start with checking the power supply. After all thats where it all begins.

Done that, all voltage test points seem to be ok.

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.

F01 fuse is not blown. Have not been anywhere near the "DANGER HIGH VOLTAGE" signs yet.

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)?.

             

I have push the intensity/beam find knob - nothing.

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...

Can 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.

I'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?

[mianchen]

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.

Thinking of getting a Rigol 1052 or 1102 when this thing is confirmed dead, might get it soon..

[Monkeh]

I've connected an el cheapo DMM to the pins then turned on the main power. Reading is 0 volt.

On AC, right?

[mianchen]

I've connected an el cheapo DMM to the pins then turned on the main power. Reading is 0 volt.

On AC, right?

Yes, both AC/DC readings are 0 and I have made sure the DMM is working.

[grumpydoc]

Yes, both AC/DC readings are 0 and I have made sure the DMM is working.

OK, 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).

[grumpydoc]

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

[mianchen]

Yes, both AC/DC readings are 0 and I have made sure the DMM is working.

OK, 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).

I 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...

[mianchen]

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

 

[lowimpedance]

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.

 

[mianchen]

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 !).

R34 is exactly 12.1 ohms (I desoldered one lead and measured, so it's not in circuit value.

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 ??.

I 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.

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.

I 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.

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!.)

in 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.

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 : )

I will check the op amp tomorrow. It's 1.25am in the morning here...have to go to work in the morning....

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.

I'm not giving up yet! Thanks for your help!!!

[T4P]

Isn't the 830B like the cheapest meter out there