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Re: [Amps] RM Italy BLA350

To: amps@contesting.com
Subject: Re: [Amps] RM Italy BLA350
From: Martin Sole <hs0zed@gmail.com>
Date: Tue, 23 Jun 2020 16:36:22 +0700
List-post: <mailto:amps@contesting.com>
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



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

Hi,

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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