Hi everybody,
I'm troubleshooting a Hameg HM203-7 oscilloscope (see
https://www.eevblog.com/forum/testgear/hm203-7-service-manual/ for the manual) with a problem in the vertical stage.
The problem shows up in the following way: the trace is either
a) heavily blurred especially going far away from the center and presents a "pillow" distorsion inwards
b) or jumps upwards randomly at higher intensity.
Timebase seems to work ok and also it triggers correctly.
In X-Y mode you can see the the dot jumping upwards similarly to the trace.
I checked power rails and they seem fine. For High Voltage I checked the potential across R3015 and reads ca. 480V and the trace is actually showing up but distorted, so I'm ruling out (for now, there might be a bad cap somewhere though) the HV stuff.
I checked also the voltages in the final Y stage and they seem quite close to what reported in the manual. BTW I think there is an error on the voltage reported at the node connecting R2038 and R2039. I am pretty sure it should be +5.88V instead of -5.88V.
So I read in the TEK troubleshooting PDF (posted in
https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/) about "common moding" technique and I'd like to use it to isolate the bad stage in the vertical final stage. So this should be similar to shorting the signal path to ground and the input to isolate the noisy stage in non-differential amplifiers. There is not much though in the TEK PDF about this technique and I'm wondering how should this be execute properly (at least in my circuit) and what can go wrong.
Let's assume I short the collectors of T206 and T207. So the way I understand it is that the differential part of the signal should not pose a problem when the output of a differential stage gets shorted because the current produced by one of the devices should be equal to the current absorbed by the other device.
Though if there is a heavy common mode signal could it happen that this procedure toasts the output devices which I'm shorting? In other words what's a safe way of doing this? Should I rather short the bases of the transistors in the next stage (in my case the bases of T208 and T209) instead of the collectors of the previous stage?
I also was thinking to put the multimeter in mA mode instead of abruptly shorting. At least it's fused and I can see if there are concerning currents going through the short. Does it make sense?
Thanks in advance for any reply