Newly empassioned-Good Will GOS-955 oscilloscope

[Coiljunkie]

Hello my name is James and i have just discovered my passion for electronics. I have decided to make a career change as a result and I am currently undertaking a electronics engineering degree at Deakin university. I have been tearing down electronics while competing a tafe pre-voc course. I understand the very basics of electricity and magnetism. I look forward to being able to geek out with people who share my interest.

That out of the way i am hoping for some help with my first oscilloscope. I was given this as a result of an add in gumtree asking for broken of no longer wanted electronics. Any way it seems to work, i mean it turns on and displays a line. However i am pretty sure there is something setting with it. There is an expanded cap inside and possibly an afters market addition of two banana jacks on the back. Also there is an intermittent popping sound that corresponds with a signal spike on the screen, as well as a "squiggly line " all when no probes are attached.

When i attach a pair of multi meter leads the squiggle disappears but the spike continues. Last thing the screen is very faint.

My questions are can i replace the metal cap with a similar value ceramic cap. And odd the bodge standard poor should i remove them. Also is the machine itself any good or am i wasting  my time. Sorry for the long post. Thabks in advance.

[james_s]

That's an electrolytic capacitor, you won't find a ceramic cap that you can replace that with. It looks like it may just be the label that has shrunken with age and split, may not hurt to replace it but I can't say just by looking at it that it's bad.

The pop sounds like HV arcing from the CRT, you might run it with the cover off in a dark room and see if you can spot the arc. It may be at the CRT itself, or in the HV circuit that supplies the anode.

The squiggly line may be normal depending on your volts/div setting, scopes are sensitive and can pick up radio waves and such out of the air.

[Coiljunkie]

Thanks for that. I'll try the dark room. I think I have a ceramic cap that has the same values. 1.5wKV .1MicroFarad ofcourse I could be reading it incorrectly. I'll double check.

[james_s]

Oh you're right, that is a HV capacitor, it looked like an aluminum electrolytic at a glance. That could actually be the problem.

[Coiljunkie]

So do you think a ceramic capacitor will suite?

[Coiljunkie]

Replaced cap and popping sound is gone as well as stray signal.

[james_s]

Nice, I love it when a fix turns out to be something easy.

[P90]

that looks a lot like an old heathkit scope I had...

[Electro Detective]

WOW! that's the same model I had back in the day, 

my first scope till I could afford a used 'wish list' dual channel

The rear sockets may be standard as an option, or a mod perhaps, I can't remember.
If they look ok and secure, just cover them up with electrical tape in the meantime in case they are a useable item which I suspect they are.

The GW was a good reliable AFFORDABLE scope with decent sized screen and performance, and a better entry level than others like the smaller Dick Smith model,
only sold it to upgrade to a dual channel Trio-Kenwood 15mhz, still going to this day 

[Coiljunkie]

That's awesome. I really like it, though I'm not entirely sure it is working correctly as some of the readings I'm getting are pretty crazy also I cant seem to get even the smallest of waves to fit on the screen at once. Meaning I have to move the position up and down to see the top and bottom of the wave. Cant complain really though as it was free and I had no way of seeing any kind of wave before, AND I got to fault find and fix while learning, so pretty dang sweet!!

[james_s]

What is your volts/div setting, what kind of probe are you using and what are you measuring? You should be able to adjust the signal all the way down to nearly a flat line, if it's going off the edges of the screen that's clipping, you don't want it doing that.

[Electro Detective]

The controls may need a good workout, especially the trigger and attenuators

Try a 9 volt battery as your test reference, or better still, hit the Youtube highway   

[Coiljunkie]

Yeah sorry I should have specified I can get a flat line and I am using multi-meter probes so not at all what is intended  Also the cap I am using has far less capacity as in 10 times less .01micro compared to the .1 micro the previous cap had. So id assume the machine is still not running as intended. I will wait till I can get a proper cap before I use it too much. as far as the voltage/div setting I honestly don't know I am still figuring that out. The analog dial is on 1/100 vertical attn. and the lowesr hz setting which is 10 I think, and I am measuring the output of a stepdown transformer 14v ac. I tried aa 6 volt large flashlight battery and got nothing, but that wasa before I replaced the cap.

[wasyoungonce]

Hi James.

best you replace the cap with a correct capacitance.   You can get them from:
WES components  WES you need to become a member, its free, they are ok for good for parts like this.  members can see prices etc.
RS components  Free shipping
Element14 free shipping over $45 order.

I reckon WES will have a suitable replacement.  Good old WES. 

Anyway congrats and welcome.

Brendan

[danadak]

That cap looks like it is same value of immediate adjacent one, 2000 uF ?

When a cap blows short it draws a lot current, usually taking out other
components. If this cap is a part of power supply then it may have taken
out one or more diodes. Although I must admit if scope still working,
has correct voltages and ripple on supply rails, then I would be at a loss
to describe why other components were not also damaged.

Another frequent failure in old scope are carbon composition resistors that
change their value. So you might go thru scope and measure them as well.

Lastly controls get very noisy, use a good cleaner on the switches and pots.
Deoxit one of the best.

The scope vertical attenuation/gain switch is in volts/div I am thinking, so
you should be on 1, and using a 10X probe, so that you can see the 14
Vac signal. A 10 x probe would give you 1.4 divisions of observable
waveform, if your 14 V is peak-peak.


Regards, Dana.

[wasyoungonce]

That cap looks like it is same value of immediate adjacent one, 2000 uF ?.....Regards, Dana.

Yes I agree...it cannot be .1uf!  The other sticker 2000 sounds about right for the size...no idea on voltage though.

[neko efecktz]

I have the same scope,
It did the same thing working ok then a popping noise from inside the case then nothing.
On later investigation I found that the high voltage winding had gone open circuit.

I rigged up a couple of transformers to give me the 600 odd volts and switched it on.
The popping noise started again.
It sounded to be coming from the same caps that have shrunken plastic wrap in the pictures.
It was going to cost quite a few dollars for a new transformer so I scrapped the idea of fixing the scope.

Maybe someday I will make a switch mode supply for the HT supply and get it going.
In the mean while I now have a 1965 model Tektronix-422.
It has miniature valves that are replaceable with FETs or so the web tells me.

Goes like a dream. 2 channel up to 15MEG Hz

Sounds like you were lucky that you didn't blow the transformer.
The scope got a lot of work  for a few years doing audio testing.
Not much good over a couple of megs.

Good luck with the scope.
It would have to be over 30 years old by now.

BILL.

[danadak]

Look at the good cap, should have  a V rating on it.

Otherwise use your voltmeter and measure V. Or use
make a 100x simple probe using two resistors, and
look at the cap voltages.


http://how-to.wikia.com/wiki/How_to_make_a_100X_oscilloscope_probe


Be careful with probe, you are looking at killer voltages. Take the time to read this.


http://projects-web.engr.colostate.edu/ece-sr-design/AY13/measurement/High_Voltage_Safety_Manual.pdf


Might be smart to power scope from a power strip, and have a friend man the power strip switch
to flip in case you start getting fried. Just a thought. Friend of course need not be hugging / touching
you when you start to vibrate. Otherwise you will both be frying. Even better is friend an experienced
tech or engineer to learn from on safety.


Regards, Dana.

[james_s]

That cap looks like it is same value of immediate adjacent one, 2000 uF ?

When a cap blows short it draws a lot current, usually taking out other
components. If this cap is a part of power supply then it may have taken
out one or more diodes. Although I must admit if scope still working,
has correct voltages and ripple on supply rails, then I would be at a loss
to describe why other components were not also damaged.

Another frequent failure in old scope are carbon composition resistors that
change their value. So you might go thru scope and measure them as well.

Lastly controls get very noisy, use a good cleaner on the switches and pots.
Deoxit one of the best.

The scope vertical attenuation/gain switch is in volts/div I am thinking, so
you should be on 1, and using a 10X probe, so that you can see the 14
Vac signal. A 10 x probe would give you 1.4 divisions of observable
waveform, if your 14 V is peak-peak.


Regards, Dana.

Look closely at the capacitor, it clearly says 0.1uF, the 2000 sticker is something else. Something like these ought to work:
http://www.ebay.com/itm/5pcs-dip-High-Voltage-Ceramic-Disc-Capacitors-104PF-2KV-100nF-0-1uF-2000V-sku330-/171308486063?

I suspect these are part of a voltage multiplier to power the CRT. Given the way one has failed and the reports of failing HV transformers I would suggest replacing all of those HV capacitors.

[Coiljunkie]

Thank you all for the great information. The caps are 1.5KV .1uF  I will get some proper probes from ebay, maybe from Wun Hung-Lo. I will also def check out  those sites and get a proper cap asap. Thanks for the cleaning tips and carbon resistor tips Dana. Very helpful forum.

[wasyoungonce]

The PCB has markings for value and voltages for capacitors.