[SOLVED] Oscilloscope Tenma 72-6800 no trace

[E-design]

Is there a specific time setting where below this fault occurs?

 

[ccorona]

Yes, when I decrese the Time/Div below to 2mS, please see attached file

 

[E-design]

Unfortunately to fix a scope, you often need a working scope to measure signals and pulses.

I think it would be good to look how the unblanking pulse changes when you switch to 2mS and below.
This pulse unblanks the beam during the trace period (when you want to see a signal on the CRT) and blanks the beam during retrace or fly-back period. If there is some timing, trigger or control problem when switched to 2mS/div and lower, then it may blank the beam during the trace period as well.

 

[ccorona]

Yes of course, I was checking signals with other scope, I have a Tektronix, I checked signal where you toll me on R482 or is the same point at emitter of Q601 but doesn't have a signal, please see attached file

 

[E-design]

Do you see the same condition when set to auto trig and triggering in normal mode with external signal?

 

[ccorona]

Yes is the same condition in both options, I saw that when I set the Time/Div to X-Y I can see the trace point, but whe I change the Time/Div the trace intensity is lossing, please see attached video

https://www.youtube.com/watch?v=yG8gRRYgt7s&list=WL48812379ADCBE5B9&index=1&feature=plpp_video

 

[E-design]

From your video it seems there is an overall blanking problem. I would expect the trace to be much brighter on all sweep settings, assuming you have it on max intensity setting. There is also some chopping action visible on your video (might be due to the video capture rate). If not it may also be a clue to the problem. I will concentrate on the blanking and DC restorer circuitry in the CRT schematic.

I assume you have done all the basic checks like making sure all power supplies are correct.

 

[ccorona]

You're right, the intensity is in max setting, and and chopping actios is due to video capture, Do you know some test points on DC restorer circuit? Anyway I'm going to check voltages and I'll shared readings

 

[E-design]

The diodes and caps I marked are part of the DC restorer circuit and often the cause of low intensity problems in scopes. You should test these for any signs of leakage, open or shorts.

 

[ccorona]

I checked all caps marked and diodes but all components are good, I checked for open or short cause I don't know how to check for leakages, also I chcked voltages and I think these section is working properly. Please see attached file

 

[E-design]

What was the history leading up to this fault? Sudden loss of brightness or happened over some time?

What is the bias voltage at the point shown?
Can you alter the intensity by adjusting the sub inten control?

 

[whitspirit]

Hello.

I don´t know if this will help, but in my search of solution for reparing the tenma escope i find a nice guy named "JAMES" with similar problem in focus adjustmemt...this his the email:

" I did manage to fix my focus problem by persistance. There was a 2.2 meg ohm resisitor on the crt circuit board that was open and a 1 meg ohm in the crt circuitry that had changed value.
Hope this helps ".

Keep up the good work and share knowledge...

 

[ccorona]

Hi E-design

The fault was suddenly.

The bias voltage is with pot fully CCW 104.5V and with pot fully CW is 39.8V

Yes, I can alter the intensity but only for turn off the trace, I mean I try to adjust the sub inten control but doesn't have more intensity.

---------- Post added at 08:15 ---------- Previous post was at 08:05 ----------

Hi withspirit

Thanks a lot for your info, I was checking the resistors that you mentioned but only found 2.2meg ohm(R627), I was looking for 1 meg ohm but I can't see it, also I was checking schematics and only have R633(10 K Ohms) and R634 (100 K Ohms), I checked both resistor but are fine.

 

[E-design]

See what happens when you use an screwdriver tip (make sure the handle is well insulated as not to shock yourself) to momentarily short pin 2&3 on the CRT base socket. If the tube is OK with enough emission left, then you should see a very bright trace on the screen. Don't do this for long as it may burn the the tube phosphor. This is just a quick check to see if the tube is still usable. If that is the case, we are still dealing with a blanking/bias issue. If not and the heater voltage checks out correct on H1/2 then the tube itself may have a problem.

 

[E-design]

The fault was suddenly.

The bias voltage is with pot fully CCW 104.5V and with pot fully CW is 39.8V

Yes, I can alter the intensity but only for turn off the trace, I mean I try to adjust the sub inten control but doesn't have more intensity.

This looks wrong. You indicate that on the anode side +390V to +580V. The diode D606 appears to be open circuit if that is the case.

 

[ccorona]

I have bad news according with your instructions ....I made a short pin 2&3 but bright trace didn't change, also I checked voltaje on H1 & H2 and there are 6.7VCA, I'm going to look for tube manufacturer, I would like to know how much is it and then take a desition

---------- Post added at 13:23 ---------- Previous post was at 11:45 ----------

Yes, is wrong the voltage, correct value is +39V to +58V

 

[E-design]

So it made no change in the intensity level at all?

 

[ccorona]

No changes in the instensity, is this the best way for ensure that the tube is failing?

 

[E-design]

Doing this check removes any influences from blanking and intensity control problems. Provided the HT supply and heater voltage is correct you would expect to see the tube at maximum brightness. What seems a bit strange is that the fault according to you happened suddenly. Although this is always possible, I would have expected to see a gradual loss of brightness with a CRT going soft. Was the scope perhaps dropped or bumped hard prior to this fault?

 

[bobdob]

Bumping this thread because I had the same symptoms on an Instek GOS-620, which is pretty much identical. In my case, Pin 3 of the HV Transformer (T601) had a bad trace to R623. It wasn't visible, but tested open. No initial symptoms, just one day, no display. A little cleaning and a solder bridge onto the trace was enough to repair it successfully. Thanks to BradtheRad!