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Oscilloscope issue

I

IDM

pfm Member
I have been happily using a Tenma 72-6802 oscilloscope which I bought second hand for about the last 18 months. However, last weekend I went to use it and found that the screen trace has suddenly become incredibly feint. Indeed if I turn the time-base up you cannot see the line at all. So it now basically unusable.

Are there any obvious things to check or ways of fixing it before I get rid of it?

Thanks,
Ian
 
Thanks David, I will have a look. Would I be right in assuming that I should leave it alone for a good few days for any high voltages to dissipate or is the EHT generated in a different manner. I am used to dealing with valve amps, but am a little nervous with the kind of voltage I assume are required for a CRT.

Thanks
Ian
 
if you’re not confident working on something I’d leave it alone.

a crt acts like a capacitor and can store quite a kick.

easily discharged with the right gear.
 
Hi,

Having no other projects on the go at the moment I thought I would have a look at my scope with very dim display.

I have removed the HV half wave rectifier diode which is a Y10GA. when tested it appears to be failing however, I think that these HV diodes may behave a little strangely its specifications are:
Silicon Rectifier
V(RRM)(V) Rep.Pk.Rev. Voltage=6.0k
I(RM) Max.(A) Reverse Current=4.0u
@V(R) (V)(Test Condition)=6.0k
I(FSM) Max.(A) Pk.Fwd.Sur.Cur.=500m
V(FM) Max.(V) Forward Voltage=22
@I(FM) (A) (Test Condition)=5.0m
I(O) Max.(A) Output Current=5.0

So a forward drop of 22V, both my multi-meter and component tester say its broken, but I don't know if this is due to the high voltage drop. So a couple of questions:
1. What would be the best way to test the diode or is it likely broken?
2. The HT is a bout 1.9kV, could I replace the diode with a string of 1N4007's as each of these can handle 1kV, so 3 or 4 would be 3 or 4kV. Would that work or is it dangerous? The only issue I can see is the HT would be slightly higher as the diode drop would be somewhere between 4.5-6V rather than 22V. However, is this difference inconsequential compared to HT of 1900V?

Grateful for any thoughts

Cheers
Ian
 
As a general rule you can`t measure EHT diodes on an ordinary multimeter due to the high forward voltage drop, you would need to rig up something with a much higher voltage and a suitable resistor and mesure the current in both directions.

It is usually easier to measure the EHT although a lot of DVMs won`t go anywhere near 1.9kV.

I wouldn`t use IN4007s in a string, they are probably too slow for the PSU oscillator, UF4007s would be better and you need at least enough to add up to three times the voltage you are rectifying.
 
three times the EHT that's interesting I guess that's why the original diode is rated at 6kV for a 1.9kV EHT.

I was actually going to use UF4007's because that's all I have in the spares box :)

I did know about the issue of actually measuring the EHT due to the high impedance of the EHT supply, so when I said 1.9kV, I meant that's apparently what is should be.

I just tried the original diode hooked up to a PSU at voltages between 0-30V and the V drop varies between 1.8-2.8V so nowhere near the specification of 22V. Reversing the diode did stop conduction so it is behaving of sorts. However, if the V drop is out by such a huge margin then maybe that is the root of the scopes problem. Maybe I should stack the UF4007's. Hmmm
 
Something like the Tenma should be available fairly cheap on ebay etc and I'd just buy another one. The questions being asked by the OP show a lack of the knowledge required to safely repair it.
 
Arkless Electronics said:
Something like the Tenma should be available fairly cheap on ebay etc and I'd just buy another one. The questions being asked by the OP show a lack of the knowledge required to safely repair it.
Click to expand...

In principle I agree but as a thirteen year old I learnt a lot from sorting out the mains derived 2 kV EHT fault on a Cossor 1035 and lived to tell the tale......
 
Barrymagrec said:
In principle I agree but as a thirteen year old I learnt a lot from sorting out the mains derived 2 kV EHT fault on a Cossor 1035 and lived to tell the tale......
Click to expand...

OK.... Well first the OP needs to get a schematic for it, which shouldn't be too difficult (it may well be available under a different brand name and model number as is typical of gear like Tenma) and then put it all back together as it was and either buy or make an EHT probe.... time and cost mounting up already...
 
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