Hitachi V-252 Oscilloscope Repair

Thread Starter

IvorB

Joined Feb 10, 2017
1
My trusty Hitachi Oscilloscope suddenly ceased scanning and I was left with a dot in the centre of the screen when switched to the X_Y mode.

I have have spent ages trying to google the problem with little success, but as I have finally solved it I thought it might be helpful to others to pass on my findings!

Reseach told me that a dot indicated a Horizontal failure, and I was unable to obtain a copy of the V-252 Service Manual. However, I was able to get a service manual for the earlier models V-211/V-212/V-222 & V-422, and after looking carefully at this manual, it became clear that the PCB layout & components in my V-252 were almost identical.

I checked all the voltages and noticed that the 5 Volt line was only reading 0.92 volts, but all the other voltages were fine. I checked the 5 volt 7805A regulator out of circuit and it gave me a 5.15 volt output voltage reading so I assumed that it was OK, and looked elsewhere for the fault. Despite checking various electrolytic capacitors and resistors, including fusing resistors I was still unable to identify the problem. As a last resort I decided to use my Maplins digital bench supply, with current limiting to produce 5 volts at the output connector of the 7805, with the scope switched off and the voltage held up!. I then powered up the scope and everything started working normally again. I replaced the regulator with a new one in my spares box (L7805CV and hey preso my scope sprung back to life.

I always believed that regulators were virtulally indistructable but this obviously not the case!

I am attaching a copy of the Service Manual in case it is useful to anyone
 

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MrChips

Joined Oct 2, 2009
27,702
Thanks for the tip.
Yes, checking system power supplies is one of the first tests to perform, after a good visual and smell check.
 

LesJones

Joined Jan 8, 2017
3,905
If it is set to X Y mode you would only expect a dot with no signal on either channel. Does it work with the timebase driving the X axis. (This is the most common way an oscilloscope is used.)

Les.
 
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