[Amps] RM Italy BLA350
g8gsq72 at gmail.com
Wed Jun 24 05:59:29 EDT 2020
Sadly you're right on both counts there. I can see there's some
overdrive protection on the input which switches in 10ohms across
the input to the transistors if the drive goes too high - but it
doesn't latch and when the input sampler evaporated the trigger
signal was probably lost so it would have dropped out.
Hopefully the TL084 has survived and it didn't get into the low
voltage supply rails.
Quick tests for FETs using a DVM:
D-G and G-S should be very high resistance any way round. I'd be
suspicious of <10M
Connect G-S and look at D-S in diode check setting. -ve to D
should show a diode drop, +ve to D open cct and should stay that
way for at least a few seconds if you remove the G-S connection
and let G float.
Apply +5V to the gate. D-S should show close to short cct, and
stay that way for at least a few seconds after removing the gate
volts and let G float.
> Thank you Steve,
> I have lifted the gate and drain of both devices to measure them. One device shows a short from drain to source so I suspect they are indeed blown. Other one measures 300k S-D but open D-S. But with one showing a short I think both have to go regardless.
> For the 3dB pad, In to Out 2.5k, In to Gnd 2.5k, Out to Gnd 92 Ohms. I think that's not so good either.
> Martin, HS0ZED
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