Tenma 72-6802 - UK Vintage Radio Repair and Restoration Discussion Forum

See_Mos · 16th Nov 2018, 3:55 pm

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.

kevinaston1 · 16th Nov 2018, 7:19 pm

I found this online, hope it helps.

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

Kevin

Diabolical Artificer · 17th Nov 2018, 8:00 am

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.

See_Mos · 17th Nov 2018, 11:38 am

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.

Diabolical Artificer · 18th Nov 2018, 7:15 am

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

Andy.

See_Mos · 22nd Nov 2018, 12:16 pm

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.

McMurdo · 22nd Nov 2018, 1:31 pm

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

16th Nov 2018, 3:55 pm	#1
See_Mos Pentode Join Date: Nov 2003 Location: Ossett, West Yorkshire, UK. Posts: 240	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.

16th Nov 2018, 7:19 pm	#2
kevinaston1 Pentode Join Date: Aug 2014 Location: Featherstone, West Yorkshire, UK. Posts: 166	Re: Tenma 72-6802 I found this online, hope it helps. https://www.element14.com/community/...-channel-20mhz Kevin

17th Nov 2018, 8:00 am	#3
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 3,982	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. __________________ Curiosity hasn't killed this cat...so far.

17th Nov 2018, 11:38 am	#4
See_Mos Pentode Join Date: Nov 2003 Location: Ossett, West Yorkshire, UK. Posts: 240	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.

18th Nov 2018, 7:15 am	#5
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 3,982	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 Andy. __________________ Curiosity hasn't killed this cat...so far.

22nd Nov 2018, 12:16 pm	#6
See_Mos Pentode Join Date: Nov 2003 Location: Ossett, West Yorkshire, UK. Posts: 240	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.

22nd Nov 2018, 1:31 pm	#7
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 3,329	Re: Tenma 72-6802 I'm going to guess it's made by Good Will Instrument in Taiwan __________________ Kevin