[Amps] RM Italy BLA350
hs0zed at gmail.com
Tue Jun 23 05:36:22 EDT 2020
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.
On 23/6/63 14:23, Steve Thompson wrote:
> There's been almost 300V p-p around the input circuits so I'd check
> D1-D3 as well, maybe other diodes too.
> Attenuator - it's difficult to predict what to expect depending on its
> internal configuration. Can I suggest you measure resistances in-gnd,
> out-gnd and in-out and post the results, hopefully we can help figure
> it out from those. Also, patch in 50ohms (or something close, say
> 47ohm resistor) from output to ground and measure the input resistance.
> With the feedback resistors burned out I fear for the FETs, you say
> resistance measurements look 'sensible' - what are you seeing?
> Steve G8GSQ
>> I have been asked to look at the subject amplifier which the owner
>> accidentally applied 200 watts to the input of. Specification states
>> 12-14 watts maximum drive.
>> All 4 of the 18 ohm resistors in the drain to gate feedback are
>> burned up as are 2 resistors on the secondary of the input power
>> sense coupler.
>> Have unsoldered the drain and gate tabs and lifted them. Resistance
>> measurements look sensible
>> So it looks like a handful of resistors so far
>> There is a 3dB input attenuator, monolithic device just marked 3dB,
>> not sure what sort of resistance measurement I should see across it
>> or to ground.
>> What else might need to be looked at?
>> I have the schematics but there is no circuit description. Does
>> anyone here have experience with these RM amplifiers? It looks fairly
>> well built if a little lightweight but not bad for a 300 watt amplifier.
>> Martin, HS0ZED
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