Need help repairing couple 'scopes (different faults)

[fezder]

Hey!

as title says, i have couple, 3 actually crt-oscilloscopes with each different faults. My first question is, is it worthwhile reparing these, as they are after all pretty old, but considering they are pretty precious for hobbiests (right term?), repairers and by green-point-of-view. Why throw away if it can be fixed?

But right, the symptoms of these are as follows: last time i tested, hopefully situation hasn't changed during ''shelf-time'' in storage:
one simply put, puts out small amount of smoke, and nothing comes to screen. Old 'lytics in PSU section/other fault in PSU section, i didn't notice any burn-marks, yet.

other two has pretty similar fault between eachother, signal is distorted, no matter what div/etc settings are, but they turn on.

sorry if this info is small/useless, but repairing 'scopes is pretty new to me but i'm aware of HV-hazard at least....

Any help, is helpful!

 

[DerStrom8]

Hi Fezder,

I have had to repair each of my 5 scopes to some extent, the oldest being from the '50s and the newest being from the early '00s. I always attempt to fix them, at least, regardless of their age. They could be useful to someone--if not you, then sell them on ebay or something like that. You can never go wrong by just trying to fix them.

For the first scope, have you been able to pinpoint exactly where the smoke is coming from? I think that would give you a good idea where the issue lies, whether it be the transformer, capacitors, or a burning resistor (or other component).

For the other two, is the signal always distorted? It could be a loose (or corroded) connection, or there could be a more serious issue. Looking inside the scope (and taking pictures) would be very helpful at this point, I think.

Good luck!
Matt

 

[fezder]

Hmm,good points. i'll take more deeper approach at these and post some photos

 

[fezder]

Update for this manner, positive also, as one came alive! It was alive before, but now signal is not distorted at all, and all i did was take cover off and turn it on. Funny thing is, that one resistor was loose inside case, rattlin' around, so would it be wise to locate it's place? or is it necessary as it works well now....
As it's in working condition, should i sell this or my previous, there is only one thing better in my older scope, and that is biggest volts/div setting. Older has 10v/div, and this new one has 5v/div. Also bandwidth is much diferent, older has 10mhz, and new one has 140mhz. Both are 2-channel. older also has adjustable triggering, whereas newer adjusts it via time/div knob. I simply dont have need for this many scopes, i already have 2 extra 4-channel scopes at shelf....

But back to topic, the scope that gives smoke, has something weird in PSU section, heat haze/smell/smoke comes from there, and the picture shows lm317, which gets pretty hot pretty fast, as does transformer, which is alarming sign, i mean that transformer is too hot touch!.

The other scope, which has distorted signal, has fuzzy signal no matter what are knob's settings, see from photos gnd-settings on both channel. it does swing and change amplitude in x-axis when frequency knob is turned, but only one sweep, sometimes not even that, sweep stays at the left corner. Quite hard to take picture..... And i did find one burnt-looking resistor, hopefully you'll see is from pictures. There seems to be other symmetrical portion, where all are same on pcb, but of course resistor isn't burned looking. If i recall right from amplifier troubleshooting, oscillation can kill resistors/transistors, but what do you think about these two(three) faulty scopes now, as there is some info in hand?

 

[fezder]

seems kikusui scope, which i managed to repair earlier without doing a thing actually, started now to malfunction....
seems like capacitor problem: scope shuts down, then turns back on like ''dims''waveform is also like it tries to draw line, but then cuts off due power loss. It doesnt blow fuse, but it's still fault.

Any other guesses than capacitors, i think maybe PSU caps?

 

[KeepItSimpleStupid]

I think you can get a repair manual direct from Kik. At least that's been by experience. I have a COS6100, I thinl

 

[fezder]

never thought finding service manual for crt scopes, worth a shot, thanks for tip!

 

[tvtech]

A bit Off-Topic here Fez...

Thanks for your ratings on /in the "Funny Pictures Thread". I try and do my best to keep things lighthearted here....kind of cleans the Soul if one can smile

Your Friend,
tvtech

 

[fezder]

off-topic is fine by me i laughed tears of my eyes minute ago in that thread...

 

[fezder]

ok small update. that kikusui which started malfunctioning couple days back, i found one resistor that was came loose due bad solder from heat of two 2w/4.7k resistors connected in parallel. I installed loose one back in its place, but problem still exsists. I noticed also that capacitor problem shouldn't be far away, as i noticed one capacitor, seems to be main filter caps, is pretty hot. There is heatsink near it, but it doesn't seem to radiate heat to cap, instead it seems heat is developed inside capacitor. Excess ripple current? is it normal for caps to heat themselves?

also i noticed strang thing. when this fault comes, screen illumination is illuminated BRIGHTER than when it's working correctly, and when brightness is adjusted quickly, screen tries no something on screen, but in bad result. Also there is dim, hazzle in screen when backlight is dimmed

 

[tvtech]

I noticed also that capacitor problem shouldn't be far away, as i noticed one capacitor, seems to be main filter caps, is pretty hot. There is heatsink near it, but it doesn't seem to radiate heat to cap, instead it seems heat is developed inside capacitor. Excess ripple current? is it normal for caps to heat themselves?

Hi Fez

This is where the whole understanding thing comes in....

If the Cap you are referring to is the Main Smoothing Cap on the Primary Side.....problems. They do not normally run warm as all it has to do is smooth Rectified AC Mains....no high frequency smoothing. So if it is getting hot......possibly going faulty.

On the Secondary Side.....they normally run warm/hottish. Lot's of work smoothing high frequency Rectified AC........from the Chopper Transformer.

That is why they are normally rated at 105 Degree Celsius as opposed to 85 Degree Celsius for the Main Smoothing Cap.

Hope this makes sense

Regards,
tvtech

 

[fezder]

hmm, made that much sense that i need to take another peek inside, i knew that mains caps usually don't run hot, but on HF-side esr comes more in play, right?

 

[tvtech]

hmm, made that much sense that i need to take another peek inside, i knew that mains caps usually don't run hot, but on HF-side esr comes more in play, right?

Hi Fez

Whats your Mains Supply in Finland.....110VAC or 220VAC?
I am asking this for a reason to help you sort out Primary smoothing problems without removing parts...

Regards,
tvtech

 

[fezder]

220, actually nowadays it's 230 to be precise

 

[tvtech]

220, actually nowadays it's 230 to be precise

OK. Here is what you do to make sure your Main Smoothing Cap is OK..
Take your Meter. And read across the it while the PSU is running.

If you read around 320VDC......it's working. No need to take it out for any testing.

Saving you time, saving you money...putting you first.

Regards,
tvtech

 

[fezder]

how come i didn't remember that measurement...oh well, can't remember all can we?

 

[tvtech]

how come i didn't remember that measurement...oh well, can't remember all can we?

Hi Fez

Handy to remember without wasting time

Tomorrow, we work on the Secondary side

All the best,
tvtech

 

[fezder]

ok measurements made here's the results, there are 3 same size, but different rating caps

mains smoother rated 200v, measured 200 volts.
35v rated, two of 'em, measured them separately, measured 9 volts when ''fault'' is on (bright backlights) and when signal comes to screen, voltages rise on both to 19v. And, the hot one is another of these 35v caps, but that is still next to heatsink.

see the photo, caps are shown, 200v rated is rightmost, and that big chung of wires comes straight from transformer. and just for satety's sake, i do use isolation transformer....

 

[tvtech]

mains smoother rated 200v, measured 200 volts.
View attachment 87241

Fez...you have it wrong

Horribly wrong.

I believe you cannot tell the difference between Primary and Secondary stuff on Power Supplies...and that really is a huge problem..

I doubt this is even SMPS...I believe it's a Linear Supply.......

Even worse.

I don't know what else to think/say......confused

 

[fezder]

what? it says 200v/470uf on cap, and i measured it across the cap terminals on dc-setting on dmm, what did i do wrong? :S...