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Oscilloscope GW Instek GOS-602

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unseen

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Hello.
I have a GOS 602 scope, (same as the Tenma 72-6800 which already has a thread here on forum) which works fine. I only got it a few days ago.
My concern is, that the -8V regulator (U601) and the 4 vertical power transistors (Q313, Q314, Q315, Q316) get extremely hot. 80+ deg C, or maybe more. Measured with a simple K-type probe. The Q313, Q315 slightly hotter. I noticed, that the ADD BAL pot is a bit off center.
A couple of times when after pushing the power switch, a loud buzzing noise came from inside. (the green LED was on)
After turning off and back on again, it worked ok.
So rare, that I could not track down the source.
Inside looks very clean, no burning marks.
Any thoughts?
Thank you.
 

Hello unseen,
What is the voltage going into the -8V regulator?
With the back off the unit, get yourself a small blanket and cover yourself
with it and enclose the scope as well, then turn it on to see if you're getting
any arching. Look closely at the ultor cap and the neck board PCB's spark gaps.
Regards,
Relayer
 

loud buzzing noise came from inside. (the green LED was on)
After turning off and back on again, it worked ok.

Your intermittent problem might resemble mine years ago. My aged scope would make a rapid snapping noise (or buzzing). The beam looked like a row of commas (or apostrophes) across the screen. Once the problem appeared it took a few seconds to begin after I switched the scope on.

It was caused by arcing inside the transformer. One 6.3V winding was also carrying 1kV from the high voltage circuit.

I could hardly believe it was deliberately designed this way although the manufacturer was reputable Tektronix. I solved the problem by installing a separate 6.3V transformer.
 

What is the voltage going into the -8V regulator?
The 8V and -8V regulator input voltage is around 15.3V, and -15.3V
Yes it is a lot of extra to dissipate, but they come directly from the transformer and bridge rectifier, so cannot be changed. Based on the schematic, they should be +14V and -13.5V
The +5V and +8V reg. heatsinks are much cooler.
Datasheet:
**broken link removed**
Look closely at the ultor cap and the neck board PCB's spark gaps.
As far as I can see, this scope has no ultor cap.
It was caused by arcing inside the transformer. One 6.3V winding was also carrying 1kV from the high voltage circuit.
The secondary of the main transformer has only a 220V and a 120V winding. There is a small separate HV transformer.
If I touch the core and the white plastic simultaneously with my finger of the HV transformer, the buzzing noise comes on (not sure if the same I've heard at the beginning when the case was still on) Now if I touch the core only, there is a HV spark between my finger and the core. The line on the display becomes like dashed. So there is an arcover inside the transformer.
Don't know however if it is the same noise. (The original noise seemed more like if the main power transformer had too much load) Couldn't make it happen again. I did some resoldering on the J607, J608 power connectors. The joints had some cracks.
That large heating of the 4 vertical output transistors are normal?
HV transformer.jpg
vericalt transistors.jpg
 

I only got it a few days ago.

Did you purchase the unit used? It's possible the previous owner started seeing the same things. Perhaps he dropped the scope, or in any case could not fix it, so he decided to unload it.

noise seemed more like if the main power transformer had too much load

The joints had some cracks.

This could indicate multiple problems. Examine the entire board for further obvious signs.

It's not easy to track down what's really the root cause. A problem is able to cause a second problem.

Noise which is at mains frequency (or maybe 2x) could be due to extreme current draw. Maybe it's in the power supply, or in the scope circuitry. It's a good idea to test the power supply by itself.
 

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