Tenma 72-6802 oscilloscope very dim display

[idm0_3]

Hi I have a Tenma oscilloscope that was working fine, than after not using for a few months all i could find was a very dim spot that disappears as soon as I switched on the time-base. I tried following this thread https://www.edaboard.com/threads/oscilloscope-tenma-72-6800-no-trace.228452/ . One of the suggested things was to short pin2 and 3 of the tube to see if the spot became bright. This had no effect for me.

When I switch off the scope there is a brief pulse of light from each of the neon tubes in the HT circuit. Does anybody have any insights or thoughts on what components are most likely to have failed and should be replaced or what to test.

Many thanks
Ian

 

[BradtheRad]

If the stationary beam is dim then it's harder to see when in motion. It's almost certainly a failing high voltage circuit.

* Does making the room dark allow you to see it better?
* What happens when you change Intensity?
* Can you shift the beam's position manually either horizontal or vertical?

It's not easy to measure the high voltage since it's in the area of negative 1-3 kV.

There's a chance the tube's heater is faulty. Perhaps it's not getting the typical 6V. (I only have experience with my Tektronix scope which joins heater voltage and high voltage.)

dim spot that disappears as soon as I switched on the time-base

Can you slow the sweep? There's a chance an abnormally fast sweep makes the beam move too fast to see.

When I switch off the scope there is a brief pulse of light from each of the neon tubes in the HT circuit.

There's a chance your high voltage is diverted where it ought not, while the scope is turned on. Can you see any sign of arcing around any component when you observe in darkness? Is there a high-voltage transformer? Is there carbon dust buildup around its wiring? It helps if you find out as much as possible about the high-voltage circuit.

 

[idm0_3]

* Does making the room dark allow you to see it better?
* What happens when you change Intensity?
* Can you shift the beam's position manually either horizontal or vertical?

The only way I can see the spot is by turning down the room lights.
Turning the intensity potentiometer reduces the spot intensity, in fact a slight turn of the dial and the spot is gone.
The spot can be moved using the vertical and horizontal adjust

Can you slow the sweep? There's a chance an abnormally fast sweep makes the beam move too fast to see.

I tried different sweep speeds and once I thought I could see it track slowly across but cannot repeat the observation.

Can you see any sign of arcing around any component when you observe in darkness? Is there a high-voltage transformer? Is there carbon dust buildup around its wiring? It helps if you find out as much as possible about the high-voltage circuit.

I have tried to have a really good look round the HV circuit and there is no obvious discolouration of any of the caps or resistors. There is also no sounding or arcing or clicking during the scope warm up. There is a small HV transformer under a metal screen. I removed the screen but again could not see any sign of damage. I also tried cleaning the HT cable which runs from the PCB to the CRT base. Again, this has had no effect.

One thing I do notice is that I can only see the spot from the B trace. Whatever, I try I cannot see a spot from the A trace (obviously its a dual scope). I will have another look at the PCB tonight and look for any signs of damage. The link I put up in the first post has a pdf of virtually the same scope with the HV PSU that seems identical. Maybe that might help someone spot the likely issue.

Thanks in advance
Ian

 

[idm0_3]

As an update played with the scope and can now get both channels to work and can see the calibration square wave. However, the trace is unusably dim. So I am pretty sure its the EHT supply. This is the only circuit diagram i can find https://www.edaboard.com/attachments/hv_section-gif.63458/

Any ideas on which are the most likely components to fail?

 

[Relayer]

Hello idm0_3,
Try adjusting the screen voltage via the flyback transformer.
You can locate the transformer by following the EHT ultor cap
that comes from the bell of the tube. It looks like a suction cap.
You should have a screen and focus adjustment on it.
You should also check the heater voltage for 6.3 volts. But
you'll have to check it on AC, and not DC.
There is a possibility that the CRT itself is sick. How old is the
scope?
Regards,
Relayer

 

[BradtheRad]

Looking at your schematic it appears your HV comes from transformer 601.
Can you locate the HV circuit on your scope? Suspect capacitors have the 2kV rating. It's worth testing each one out-of-circuit with the scope unplugged. Likewise the diodes and resistors.

Weak HV could occur if:
a) a parallel component has become low resistance (diverting current where it ought not), or
b) a series component has become high resistance (inhibiting current).

It's not necessarily easy to measure 2kV directly. Current may be so tiny that it's unable to drive a D'Arsonval meter. Probably same with an inexpensive DMM.

As for the neon bulbs I suppose they indicate whether HV is present. It's hard to figure out what it means when they brighten when you shut off the scope.

Notice the CRT diagram shows the heater wire at the bottom. If it's similar to my Tektronix then that same wire also emits the electron beam. 6.3V combined with high voltage. Anything you try calls for caution so you don't ruin either circuit.

Your aim is to figure out if one or the other voltage isn't normal. Find a wire you can disconnect so you can measure 6.3 VAC safely.

My Tektronix scope developed a problem of arcing in the transformer windings. The trace was a row of ditto marks across the screen. The rapid sparking creating a note singing inside the transformer. I ended up installing a separate small 6.3 V transformer.