no traces on a GW GOS-643 scope

[kragon]

i've recently purchased a  GW GOS-643 scope ,in working order.
the biggest issue is that the intensity was set far too high.

ive tried fixing this by adjusting the sub int trimmer pot in the z-axis amp

the first time i adjusted it ,it became too dim.
adjusting it the other way caused it to stop displaying anything.
i now cannot adjust it to display anything....oops

no magic smoke has escaped
the power still seams fine,(i haven't checked all the rails)
the pot is still fine

any advice?
i have a copy of the manual and the schematics (possibly) if they are helpful

[Andy Watson]

i have a copy of the manual and the schematics (possibly) if they are helpful

A schematic of the section that contains the CRT would be helpful. My initial suspicions would be towards the circuit that translates the relatively low voltage Z-blanking signals up to the 1.8kV (negative) voltage level that occurs on the CRT pins.

[kragon]

here's the schematic for the crt part,sorry about the quality its the best copy i've found

ive marked the pot i adjusted,and thanks for taking a look

[tautech]

Did you mark the trimpots so they can be returned to previous settings?

Normal procedure for the Sub Intensity adjustment would call for the front panel Intensity to be set to ~10 o'clock and then the trimmer be adjusted until the trace is barely visible. Then you will find "normal" viewing Intensity setting will be ~12 o'clock position.
When adjusted as such any departure from the normal 12 o'clock position indicates something is "not quite right".


Be sure to check all voltage rails are to spec.

[Andy Watson]

With the exception of the final level translation (C623,624625) the Z-control stage is DC coupled. It should be possible to verify correct operation of the low voltage Z-amplifier (Q606 through Q613) by measuring the steady-state voltages with a DVM. It should also be possible to change the state (blanked - unblanked) by manipulation of the timebase, or by direct control via the "EXT INTEN MOD Z" input.

[kragon]

i didn't mark the pot's original position.although a slow sweep hasn't returned functionality.

voltage rails are within tolerance.

unfortunately the fuse appears to have mechanically failed from all the handling , so i need to source a new 0.3A fuse.so I'm going to have to put this on hold until then. sorry

[kragon]

ok,got a new fuse ,so powers back

power rails are all good

160V :  159.2V
5V     :  5.2V
-12V  :-11.2V
12V   :  12.0V

[tautech]

ok,got a new fuse ,so powers back

power rails are all good

160V :  159.2V
5V     :  5.2V
-12V  :-11.2V
12V   :  12.0V

The -12 needs attention. All need also checking for ripple, all except 160V should be less than 10 mA.
You can check ripple with a DMM set to AC MV.

I have seen the high value resistors like those R638 to 648 drift in value and they should also be checked that they are still within tolerance.

[kragon]

Sorry for the delay.
The -12v reading was from the wrong set of measurements (making all the rookie mistakes on this one).
It's stable at -12.02v

The two resisters in question are within tolerance.same with the ripple voltage.


As an option I was considering stripping the triggering/crt amplifier  pcb and the crt ,and feeding the formatted inputs into a high speed adc. Process it with a micro-controller or sbc

Any opinions on this?