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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 16th Nov 2018, 3:55 pm   #1
See_Mos
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Default Tenma 72-6802

A friend has two of these 'scopes with brightness faults.

I have not been able to find a circuit diagram online so I will put a request in the wanted section.

Meanwhile, if anybody knows of an equivalent model or maker etc I would be pleased to hear from them.

One has reduced brightness and the other you have to turn the brightness control down to increase intensity, the trace does not bloom so I don't think the tube supply is the problem, probably resistors in the high voltage chain.
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Old 16th Nov 2018, 7:19 pm   #2
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Default Re: Tenma 72-6802

I found this online, hope it helps.

https://www.element14.com/community/...-channel-20mhz


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Old 17th Nov 2018, 8:00 am   #3
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Default Re: Tenma 72-6802

Doubted you'd find a schematic, I wrote to Tenma for a meter schematic, heard nowt back. Looks like there is one though.

"probably resistors in the high voltage chain." worth having a look, brightness and intensity on CRT scopes is one of the easiest things to fix.

Looked at an EEVblog video's yesterday of a modern Keysight scope fault, as they say in the best mafia films " Forget about it" An old CRT scope can't tell you the time or the latest Yen to $ exchange rate, but you can at least fix them when they go wrong : )

Andy.
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Old 17th Nov 2018, 11:38 am   #4
See_Mos
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Default Re: Tenma 72-6802

Thanks Kevin, that's great.

I had a look at the circuit and there is no high voltage chain as such but it does have a 'bright-up' circuit similar to another thread on here recently, I think it was a Telequipment or possibly HP so I am now looking for that thread just to see what was said.

We have a spare scope so fault finding will not be too difficult.
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Old 18th Nov 2018, 7:15 am   #5
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Default Re: Tenma 72-6802

Brightness/intensity and other CRT voltages are usually derived from a simple potential divider, IE a load of resistors and pots in series with a positive voltage at one end, and ground or a negative voltage at tother end.

So the brightness pot should adjust the cathode - grid voltage, grid should be at - something volts in relation to the cathode, adjusting the pot you should get - 20/30v or whatever to - 5v say, something like that.

To fault find see if you can find a pinout for your CRT, find the cathode pin, black probe of DMM on that, red probe on the grid.

Here's some bumf to give you an idea - https://www.robkalmeijer.nl/techniek...e45/index.html

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Old 22nd Nov 2018, 12:16 pm   #6
See_Mos
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Default Re: Tenma 72-6802

I haven't had a chance to look at these myself but my friend could not find anything amiss so he swapped the tubes over and the faults followed so it looks like both tubes are faulty.
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Old 22nd Nov 2018, 1:31 pm   #7
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Default Re: Tenma 72-6802

I'm going to guess it's made by Good Will Instrument in Taiwan
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