I have a Smiths tachometer out of a vintage Volvo P1800 that is reading very high at hot temperatures.
The tachometer is implemented using a blocking oscillator as a one-shot that is triggered with each ignition pulse. It is triggered by the car's coil current via the transformer used for the blocking oscillator. Normally, an ignition current pulse generates a nice, wide one-shot current pulse through the tachometer's ammeter to indicate the engine RPMs.
Here are the schematics for the tachometer:
The transistor and the cap have been replaced with new units and the tachometer functions well under about 124 degrees F. But at and above 124 degrees F, the one-shot appears to oscillate after the one-shot pulse is done and before the end of the calibration pulse. Here is a scope photo with the calibration signal on top and the one-shot signal on the bottom. Below about 124 degrees F, there is simply a one shot pulse on the bottom trace, but above about 124 degrees F, the signal seems to oscillate after the one-shot pulse ends:
I do not have an intuitive understanding of how blocking oscillators work and I have no clue as to what might happen to an old tachometer that could cause the signal to oscillate like this.
I would appreciate any help you all might have in helping me to intuitively understand the operation of this one-shot circuit and to try to diagnose what might be causing this oscillation at higher temperatures.
Thanks in advance.
The tachometer is implemented using a blocking oscillator as a one-shot that is triggered with each ignition pulse. It is triggered by the car's coil current via the transformer used for the blocking oscillator. Normally, an ignition current pulse generates a nice, wide one-shot current pulse through the tachometer's ammeter to indicate the engine RPMs.
Here are the schematics for the tachometer:
The transistor and the cap have been replaced with new units and the tachometer functions well under about 124 degrees F. But at and above 124 degrees F, the one-shot appears to oscillate after the one-shot pulse is done and before the end of the calibration pulse. Here is a scope photo with the calibration signal on top and the one-shot signal on the bottom. Below about 124 degrees F, there is simply a one shot pulse on the bottom trace, but above about 124 degrees F, the signal seems to oscillate after the one-shot pulse ends:
I do not have an intuitive understanding of how blocking oscillators work and I have no clue as to what might happen to an old tachometer that could cause the signal to oscillate like this.
I would appreciate any help you all might have in helping me to intuitively understand the operation of this one-shot circuit and to try to diagnose what might be causing this oscillation at higher temperatures.
Thanks in advance.
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