What components can be damaged in an Italy mosfet amplifer by ALC overshoot? How can you test or determine which components are damaged? Thanks for any help here. Everyone on this forum has been extremely helpful to me. You are great radio people to have for help and giving advice.
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ALC OVERSHOOT IN RM ITALY AMPLIFIER
- Thread starter long night
- Start date
Excessive drive level tends to clobber the RF power-amplifier MOSFET transistors.
Depends on the model and the decade in which it was made. Older amplifiers used bipolar junction transistors. They were a little more forgiving of RF spikes than the more-modern MOSFET transistors. That's what you'll find in a more-recent model.
73
Depends on the model and the decade in which it was made. Older amplifiers used bipolar junction transistors. They were a little more forgiving of RF spikes than the more-modern MOSFET transistors. That's what you'll find in a more-recent model.
73
As Nomad said the parts most susceptible to damage from brief conditions of overdrive, would be the output transistors. ALC overshoot on most HF rigs, makes them completely incompatible when driving a low drive amplifier. Every new word peak, places the transistors in an overdrive condition.
Therefore, you cannot rely on just turning the power setting down and letting the radios ALC circuit clamp the maximum power at any particular level. To safely handle a situation like this, requires the use of an attenuator on the amplifiers input. The attenuator should be selected so that the radio can be operated at normal power level and only the desired amount of drive passes through the attenuator.
Therefore, you cannot rely on just turning the power setting down and letting the radios ALC circuit clamp the maximum power at any particular level. To safely handle a situation like this, requires the use of an attenuator on the amplifiers input. The attenuator should be selected so that the radio can be operated at normal power level and only the desired amount of drive passes through the attenuator.
Hi Nomad. Does that mean that the mosfets need replacing in a newer amp such as a RM Italy?
Can anyone answer my question on what damage can occur to an Italy 703 amp with overshoot causing damage. What is the most likely source of the damage? The RM3 transistors?
Nomad and Shockwave posted the answer above.
Low drive cb style MOSFET amps do not play well with HF rigs exhibiting overshoot.
I don't recommend running CB style solid state amps with hf rigs.
The other danger is if the amp breaks into self oscillation, latchs the relays and when you un key it can roast the front end of your hf rig.
Texas Star can fall victim to this.
Excessive drive can cook the 10uh choke that prevents rf from getting into the bias.
The amp can oscillate latching up and feed rf back into the radio when you un key.
This can result in damage to your radio.
73
Jeff
Low drive cb style MOSFET amps do not play well with HF rigs exhibiting overshoot.
I don't recommend running CB style solid state amps with hf rigs.
The other danger is if the amp breaks into self oscillation, latchs the relays and when you un key it can roast the front end of your hf rig.
Texas Star can fall victim to this.
Excessive drive can cook the 10uh choke that prevents rf from getting into the bias.
The amp can oscillate latching up and feed rf back into the radio when you un key.
This can result in damage to your radio.
73
Jeff
Interesting points here, what about using an RM Italy amp on a HF radio that doesn't produce an ALC spike,
Would it be best to turn the relay control in the HF radios settings menu to "off" ???
Those amps have their own relays and don't need a key line to TX.
Would it be best to turn the relay control in the HF radios settings menu to "off" ???
Those amps have their own relays and don't need a key line to TX.
Just about every 100 watt HF rig will exhibit ALC overshoot, when the drive is reduced to the point required for this style amplifier.
This is usually solved by using an external ALC voltage. The timing of the internal ALC is too slow to prevent spikes.
Hi Captain K. What issues or damage to the amplifier do you see with the overshoot?
Nomad pretty much covered that in the first reply to this thread. The transistors may bot blow right away however after repeated hammering from spikes they may eventually fail. The largest boulder can be broken by the smallest hammer given enough time.
If the rest of the group has not informed you enough...
When you "spike" a MOSFET - you have two things to concern with...
Saran Wrap and Surge Arrestor.
Saran Wrap - being even thicker than what is in the devices' package.
But the concept - of using thin material to insulate the Gate from the mess it has to control underneath it you only have so much insulation and if too thick affects the devices maximum useable frequency bandwidth
Surge Arrestor part; is in the Mess that in itself is designed to self destruct and blow a fuse, itself or a whole string of parts connected to it That is built into the device itself.
Ever use Saran Wrap in a Microwave? It'll eventually get too hot and melt onto your food - or worse...
Keep fresh batteries for the Smoke detector handy...
The Surge Arrestor is in the substrate itself - too much current is one thing - too much voltage - you can cause it to "backfire" - the power generated by the circuit - even if it's fed by less than 50% of it's maximum RATING for Reverse Breakdown - the EMF and EMP caused by overdriven Gates and the pulse generated by the mess on the opposite side - will cause the "surge arrestor" the design is, to fail - in a conducting condition - and either blow a fuse - melt a wire, burn a trace - melt lead or see above...
As you search for MOSFET ratings - one thing you do need to pay attention to is the Datasheet...
Too much of a good thing - usually is...
Some power supplies come with "overload protection" but are only as good as the ability to sense such conditions.
The problem with MOSFET's are their inherited "Zener" ACROSS (it's built this way) and how long it can stay "triggered" - a recovery issue - so as you put RF out there - any VSWR issues reflect their condition onto the leads of this device - and if excessive - will begin to clamp and keep clamping until it self destructs.
When you "spike" a MOSFET - you have two things to concern with...
Saran Wrap and Surge Arrestor.
Saran Wrap - being even thicker than what is in the devices' package.
But the concept - of using thin material to insulate the Gate from the mess it has to control underneath it you only have so much insulation and if too thick affects the devices maximum useable frequency bandwidth
Surge Arrestor part; is in the Mess that in itself is designed to self destruct and blow a fuse, itself or a whole string of parts connected to it That is built into the device itself.
Ever use Saran Wrap in a Microwave? It'll eventually get too hot and melt onto your food - or worse...
Keep fresh batteries for the Smoke detector handy...
As you search for MOSFET ratings - one thing you do need to pay attention to is the Datasheet...
Too much of a good thing - usually is...
The problem with MOSFET's are their inherited "Zener" ACROSS (it's built this way) and how long it can stay "triggered" - a recovery issue - so as you put RF out there - any VSWR issues reflect their condition onto the leads of this device - and if excessive - will begin to clamp and keep clamping until it self destructs.