Joined: 19 Mar 2012
Location: Exeter, England
|Posted: Tue Mar 20, 2012 6:46 pm Post subject: Hitachi V-1150 Oscilloscope problem and cure.
|I offer this in the hope it will save someone hours of frustration!
I was delighted to be given a Hitachi V-1150 oscilloscope, a lovely 'scope, 150Mhz, 4 channels, delayed time base etc.
However, my delight was short lived. After several minutes of operation the 'scope stopped working, just leaving a single bright spot on the screen, the spot was not able to be moved either vertically or horizontally by adjusting the X or Y controls.
I switched the ‘scope off and then on again, this caused it to function normally again, but for just a few minutes. This cycle could be repeated indefinitely.
I checked out the various voltages on each PCB, but all seemed in order irrespective of the fault state.
After several faulting sessions chasing clock pulses etc around, I decided a different approach was needed.
I made the educated(?) guess that the fault was on the digital board (PEF-745) and applied heat from a hair dryer to the board; this indeed accelerated the speed at which the fault appeared. Using shielding I narrowed down the area affected by the heat to one small area.
Touching one particular IC caused the fault to appear immediately, further investigation revealed pin 15 of IC2020 was the root of the problem.
This pin according to the diagram should be tied to +5V, it was in fact floating. This is the CLEAR pin of a 40H259 8-bit addressable latch.
Applying +5V to this pin via a 3.3K pull up resistor caused the ‘scope to work normally! This chip works in parallel with another 40H259 (IC-2019), which also had a floating pin 15, this was given the same treatment.
Looking at an admittedly rather poor PCB track layout diagram in the service manual, it appears that the pad pin 15 sits on (both ICs) is isolated and goes nowhere!
A design fault? Perhaps it depends on the individual 40H259s used weather the CLEAR pin drifts naturally high or low when left unterminated.
For reference the affected ICs are located in positions 4J and 3J on the digital control board PEF-745.
In practice I joined pin 15 to the +v5 supply on pin 16 on both chips...........just 2 solder bridges needed.
Hope this saves you some time!