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Old 16th Jul 2019, 3:39 pm   #1
WaveyDipole
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Default Diode behaviour

I am working on a Farnell DTC12 oscilloscope and the Y final amp seems to have a fault. Plate Y2 seems tuck as +250vDC. So far I have been unable to locale a faulty part and have re-flowed all the joints. However while probing I noticed a peculiar behaviour with the diodes D301 to D304, which are 1N4148 signal diodes.

I measured the voltages at each point with reference to ground and was expecting to see approximately +12VDC at the anode of D304 and roughly 0.6v increments at each junction until D301 cathode where the voltage would be whatever was appropriate for the circuit at that point - which could potentially rise to +254VDC at the emitter of VT309.

These are the readings I obtained:

-> 252v
D301
-> 123V
D302
-> 20.20V
D303
-> 20.43V
D304
-> 20.74V

The regulated +12VDC supply is derived from the 20-0-20 transformer winding so this probably explains the unregulated voltage at the anode of D304.

I replaced D301, but I still get similar readings. Why such a big difference across D301 and D302? The voltage difference across diode D301 is about 130V which is in excess of its stated reverse DC voltage which is 100V.

The readings across D303 and D304 appear to show a 0.2V drop across each diode which is less than expected for a silicon semiconductor.
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Last edited by WaveyDipole; 16th Jul 2019 at 3:51 pm.
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Old 16th Jul 2019, 4:08 pm   #2
julie_m
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Default Re: Diode behaviour

D301 and D302 probably are going into reverse breakdown (the PIV isn't precisely specified for ordinary non-Zener diodes, so 129V and 103V are not outrageous). The reverse leakage of D303-304 will be limiting the power dissipation. As long as the applied voltage is not sufficient to push all four diodes out of the normal, reverse-blocking mode, the series combination will retain rectifying behaviour.
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Old 16th Jul 2019, 4:52 pm   #3
Trigon.
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Default Re: Diode behaviour

The figures show the middle junction of D302 and D303 to be at a voltage lower than either end of the chain !!

Assuming there is no leakage at that junction toward 0v dragging it down, the small voltage drops across D304 and D303 suggest they're just resulting from the meter current, implying D302 is open circuit.

If D302 is open, then the 123V reading is simply resulting from the reverse leakage of D301.

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Old 16th Jul 2019, 8:00 pm   #4
WaveyDipole
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Default Re: Diode behaviour

I have just taken voltage readings across the individual diodes - a bit tricky given the position of a couple of them underneath the rear of the CRT

This is what the meter read:

Reverse:
D301 : 0.93v
D302 : 0.24V
D303 : 0.25V
D304 : 0.42V

Forward:
D301 : -0.52v
D302 : -0.56V
D303 : -0.48V
D304 : -1V

With the exception of D304 perhaps, there is evidently insufficient forward voltage to properly turn on the diodes and therefore insufficient current for the readings to make any sense. In the circumstances they may well have been affected by meter current as well. Whatever the case, I guess that the initial readings ought to be considered unreliable.

In any case, I need to solve the problem with VT309 E being jammed at 254V first, then, given the cost of 1N4148's, I think I will just replace all of them to be sure.

Thanks for the replies.
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