Electronics > Repair

Hitach V-1150 repair help needed

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phil:
Hello,

I recently scored a hitachi V-1150 on ebay. It arrived today and did even work for about 3 minutes  :-+
The focus went crazy in that time, drifting all over the place. No way to get the trace focused for more than a few seconds.

After a few minutes the trace vanished and was never seen again. Tracefinder does not help, XY mode does not help either. I checked all voltages mentioned in the service manual. They are spot on. Also, the horizontal and vertical deflection seems to work as far as I can tell by looking at the plate's waveforms.
HV is there, too. I could discharge the tube via 10mm air gap. However, I lack the equipment to measure this voltage directly.

There is no response on the screen when I switch the unit off. But there is a short, wide beam on the screen when I switch the unit on while the cathode is still hot. This lasts about 0.5s or so.

I *think* that the problem is either with the Z blanking or with the focus voltage. Has anyone had a similar problem with an analog scope?
Got any hints for me?

- Phil

phil:
Well, I checked the service manual again and I think I found the problem.
Page 199 shows the Z-axis amp.
TR1746 and TR1747 control the Z blanking. TR1747 is dead and TR1746 behaves stragely and seems to be damaged. I will try to source these two transistors and see if that fixes my problems. Blanking occures when R1747 is pulled low.

Disconnecting the blanking circuit (lifting R1747) from the HV output stage restores the beam as expected. Of course, the use of the scope without is severely limited due to the beam not blanking on return.
However, this does not really explain the fuzzy focus. While the focus behaves normal when the blanking stage is disconnected, the question remains why it failed in the first place.

Maybe some voltage drop on the supply lines due to the dying transistors?

Phil

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