oscilloscope question

[mdanh2002]

Hi,

I just purchased a GW INSTEK GOS-6101 100 MHz oscilloscope from ebay (see eBay.com.sg: GW GOS-6101 100 MHz OSCILLOSCOPE (item 310304963347 end time Mar 25, 2011 17:19:51 SGT)) . The price was very good (26 USD) and the item seems to be working. However, when operating, it seems very noisy. The culprit turns out to be the cooling fan at the back of the oscilloscope. The fan is small but generates more noises than that of a computer power supply and as noisy as the fan in my room. Is this normal?

 

[Reloadron]

I am not familiar with that specific scope but looking at the images you may want to open it up and give it a good blowing out / cleaning. The fan is small and for a small fan to move a lot of air it can get noisey but I wouldn't think to where it was annoying. Clean it all up and see if that helps. Fans are generally inexpensive so you could try a replacement.

Ron

 

[Nigel Goodwin]

Usually cleaning isn't the problem, it's bearing wear (cheap fans just use bushes and not real bearings), so a replacement fan sounds like the best idea.

 

[mdanh2002]

Thanks. I opened it up (after discharging the CRT), cleaned the dust around the fan and the oscilloscope is now less noisy. Just curious, why do they need a fan in an oscilloscope? My other INSTEK 20Mhz oscilloscope does not have a fan.

Another question, if I configure the oscilloscope in dual channel (CH1 & CH2) mode with the on screen display (cursor readout) on, then I could not get a stable display using the calibration signal (1kHz, 2 Vp-p). No matter what trigger settings I use, the traces always flicker slightly. If the readout is turned off, then the trace is stable. Does this indicate any problem, or it's just the way it is?

 

[Reloadron]

My guess would be that at design time for that scope the determination was made to use a fan for moving air through the case for cooling. All the components generate heat and heat removal was a factor. Possibly the fan was added to help the unit maintain its accuracy specifications. If it is just a common fan, you may want to look at replacing it.

As to the flicker? I really don't know why it does it only when using the on screen display. Possibly because the beam is a little more busy but I would think that was considered during design. Someone else with a similar scope can likely answer that better.

Ron

 

[mdanh2002]

I think the scope is less noisy now and I have no intention to replace the fan until something more serious happens. Although I think it's just a normal DC fan (having 2 wires connected the power supply), finding a replacement fan with the same size and similar power consumption is nearly impossible I believe.

I think you are right that the beam is busy when the OSD is on. I tried with a function generator, the trace stops flickering when the frequency is around 3kHz. At 1kHz and with 1 channel only, the trace also flickers but less. So perhaps with the OSD on, part of the beam time is dedicated to the display of the OSD. At high frequency retinal persistence sets in and flickering stops.

And I just found out that INTEN, FOCUS and ILLUM to adjust the CRT display have virtually no effects. Well, at least my eyes are not sensitive enough to notice any little change. Do you know how to troubleshoot this, or it can be left as-is since the display is still fine at the moment?

 

[Reloadron]

This is getting to where a good maintenance manual would be nice to have.

Intensity and Focus are going to be functions of the CRT HV circuit(s). Generally the graticule illumination is a different animal. Most of my scopes allow the front bezel over the CRT face to be removed. Under the bezel frame there are usually several tiny incandescent lamps that vary in intensity. Those are the graticule illumination. The pot simply varies the voltage to the lamps. I have seen older scopes where all the lamps simply failed over the years or the pot has failed.

Getting into the high voltage for the intensity and focus in another story and where a manual is just about a must.

Ron

 

[Dean Huster]

The heat in most scopes is generated by power supply regulator pass transistors and the vertical and horizontal output amplifiers. No coolie, no workie. Hewlett-Packard had lousy cooling in their 180-series mainframes and the vertical and horizontal output transistors went up in smoke left and right.

 

[JuliaKhanam]

It would zoom in and out of the displayed wave form on the oscilloscope screen on the horizontal axis

i.e. if you wanted to see a 40KHz wave form for example, you would have to change the time base to zoom right into the wave form so that you didn't just see a solid block on the screen.