Iwatsu 7606 power failure

[smallfreak]

Hi folks,
I recently found that my trustful Iwatsu 7606 does not power on again. Simply nothing. Plain dead. Last time I stowed it away, it was fine, but that was two years ago, while it rested on the shelf in the meantime. The need for an oscilloscope to repair some (analog) stuff gets lower each year.

Since it is 25+ years old now, chances are good that it might "only" be due to aged capacitors in the power supply. I don't like the idea to throw it away, just because it's not all digital and eventually might easily got fixed. Even if I would get a brand new DSO for the price of a spare power supply alone, I'd like to give it a try, offering the item some more years to live.

I DO have the service manual that is floating around the internet, but the images are quite bad.
http://www.download-service-manuals.com/download.php?file=Iwatsu-6700.pdf&SID=7q61n637itv79p3cmeern41230

I already tried to disassemble the parts, but postponed the whole procedure until I have some more equipment and knowledge how to start. It is not quite easy to repair an oscilloscope without an oscilloscope, isn't it? 

Does anybody have experience with this Scope and Problem and could give a hint or two where I could start to check? The internal fuses are OK  but I'd like to avoid taking out every Capacitor just in case. A first sight check did not show any obviously dead component. Since it's not showing ANY life, I assume a pretty basic problem.

For now I ordered a small Pocket Scope (DSO112A) so I at least can see anything that's going on. It seems that this cheapy thing can do quite a lot of the basic tasks without the need of a full flagged Scope. If all goes wrong, maybe that will become my new cutie. 

Thanks in advance.

[orbanp]

Hi smallfreak,

You could do a couple of measurements with just a DMM before you dig into it.

The schematics is on pg. 208, the brief description of the circuit on pg. 55 of the manual you referred to.
Do check the rectified/filtered mains voltage on C10, C11.
If you have 220VAC mains voltage you should see over 300VDC voltage on those two series connected caps.

Moving on, the basic switching power supply is controlled by IC1, an Iwatsu hybrid IC.
The IC is controlled (feed back) from the +5VDC output stage, that is the bottom most circuit on the right side of the switcher transformer T1, fed from the winding with terminals marked 1 and 2.
At the output of that circuit you should see +5VDC, for the digital circuits.
You can measure this voltage at the W6 connector, going to the CPU board (top right corner on the page), at pins 1, 2, GND, and pins 3, 4, the +5V.

You should have this +5V before anything else, then you can check the other supply voltages.

There is quite a long thread talking about repairing such a PS.
Here is the thread, though it is in Hungarian:
http://hun.lists.elektro.narkive.com/EcUwcO6N/flyback-tap-melegedes
I guess you can run it through some translator...

The PS was basically working for the guy, but he felt it was overheating.
He experimented with replacing that Iwatsu hybrid IC, B365N, with a more widely available switching regulator IC.

PS: I would rather get another older analogue scope instead of that DSO112A, unless you need the small size!

Good luck, Peter

[IanMacdonald]

R7 390k between rectified mains and Q1 base would be a prime suspect. It's a startup resistor.

The PSU is a  relatively simple self-oscillating flyback converter.  When Q1 gets a tickle of current from R7, it conducts enough to start positive feedback via the two hot-side windings and C61. IC1 then takes over and regulates (we hope..)

Bear in mind that if R7 is o/c the reservoir caps C10/C11 will remain charged and hazardous. It's supposed to double as a bleed resistor.

Also check R1 which provides mains-present indication to the logic via an opto. Another high value high voltage job so might be o/c.   

Other than that, bulged, leaking or otherwise worn out secondary caps (C 26, 19, 22, 15) are the usual culprits. Surprisingly it's unusual for the mains reservoir caps to fail.

[smallfreak]

Thank you for your very detailed suggestions. I will check that.

Is it supposed that this circuit powers on without load? I certainly have to remove the board from the scope to test it offline and I have to remove a lot of cabeling. Otherwise I have to organize a dummy load on at least one of the secondary outputs.

600V charged power capacitors are a thing to avoid touching  Is shortening the contacts with a power resistor a safe way to discharge them?

Maybe I find that once the oscillator starts going, something else will get blown.

@orbanp
I will keep the DSO112A. $50 is something I can spend on a pocket tool that can be useful for a couple of measurements where it is unsuitable to cary an "old analog scope" with me. Like the camera on the mobile phone would not replace the DSLR, but it is the one I always have with me.

The voltage on the W6 connector is to be tested in assembled state? If there is nothing, then the oscillator is down and I have to check that one? I assume there is no output, as not a single light goes on and the fan is off. I will check that.

[orbanp]

Yes, you do need a load for such PS to start up.

You will not see 600VDC, only about half of it, the peak voltage of the 220VAC mains voltage, but do be careful none the less!

Keep in mind that the primary side of the PS (the Iwatsu hybrid IC and its components) are directly connected to the mains, so if you test that part of the circuitry while under power, do use an isolation transformer!
This is especially true if you use a mains powered scope to measure in that part of the PS, the ground of those scopes are usually tied to the mains neutral line!

The architecture of this PS is very similar to earlier PC PSs, so do read up on the repair and debugging of those!

Good luck, Peter