I may have a few of the originals. If you’re still looking, let me know and I’ll look.
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ERF 7530
- Thread starter n4dxx
- Start date
Radioman56
Motorola bench-tech
Yes it (the 24N40F) can be subbed-in., but one MUST be aware (in some cases) of the LARGE differences in "VGS(th) Gate Threshold (turn-on) Voltage" between differing MOSFET devices.!
The "Palomar" ERF-7530s have suffix designators (A through J) with Gate voltages ranging from 2 to 4 volts.
The 24N40F has a Gate turn-on voltage of 3 to 5 volts.
I had a problem with a customer's "Rocket-Box-250-HD" that was "DOA" right-out-of-the-box from the manufacturer., in that it was producing just barely a few watts of VERY distorted (AM) audio, regardless of "drive-power" going into it, and / or the setting of the "low, med, high" switch :-(
When I searched for the problem, I found that instead of having the "advertised" ERF9530 devices, it had the 24N40F devices (which when removed, also tested just perfect).
After doing the research of the 2 different devices, and seeing the "stated" Gate-voltage differences, I then went about reverse-engineering of the amp's bias circuitry, and drew a schematic with their parts values.
What I found, was that the component values of the factory, resistor "voltage-divider", calculated-out to about 2.1 volts (and was what I had also measured in-circuit).
So., after adding a small trim-pot (of the appropriate value, in the appropriate circuit location), and slowly brought the bias-voltage up (while watching the resulting RF envelope on my scope), was able to get that "Rocket-Box" to come-alive with nice clear audio.!
Moral of the story is., is that although pretty much all BJT transistors have about the same turn-on voltage (~0.6x volts)., a typical MOSFET (whether small or large) can vary GREATLY in their turn-on voltage (under 2 volts to over 5 volts).!!
Hope the above "mini-novel" will be of some help to others who may read it.....
i have something like 20 24n40s and on the transistor tester the rdson figure definetly varies between them and the rocketbox fets. its higher on the rb fets. so my suspicion is that they pick these oddballs out of large lots of 24n40s and charge for it. the part number IS correct however because i bought some rocketbox fets that were unlabeled with the original lettering sanded off, BUT NOT GOOD ENOUGH FOR ME TO NOT SEE THE NUMBERS IN GOOD LIGHT. wild stuffYes it (the 24N40F) can be subbed-in., but one MUST be aware (in some cases) of the LARGE differences in "VGS(th) Gate Threshold (turn-on) Voltage" between differing MOSFET devices.!
The "Palomar" ERF-7530s have suffix designators (A through J) with Gate voltages ranging from 2 to 4 volts.
The 24N40F has a Gate turn-on voltage of 3 to 5 volts.
I had a problem with a customer's "Rocket-Box-250-HD" that was "DOA" right-out-of-the-box from the manufacturer., in that it was producing just barely a few watts of VERY distorted (AM) audio, regardless of "drive-power" going into it, and / or the setting of the "low, med, high" switch :-(
When I searched for the problem, I found that instead of having the "advertised" ERF9530 devices, it had the 24N40F devices (which when removed, also tested just perfect).
After doing the research of the 2 different devices, and seeing the "stated" Gate-voltage differences, I then went about reverse-engineering of the amp's bias circuitry, and drew a schematic with their parts values.
What I found, was that the component values of the factory, resistor "voltage-divider", calculated-out to about 2.1 volts (and was what I had also measured in-circuit).
So., after adding a small trim-pot (of the appropriate value, in the appropriate circuit location), and slowly brought the bias-voltage up (while watching the resulting RF envelope on my scope), was able to get that "Rocket-Box" to come-alive with nice clear audio.!
Moral of the story is., is that although pretty much all BJT transistors have about the same turn-on voltage (~0.6x volts)., a typical MOSFET (whether small or large) can vary GREATLY in their turn-on voltage (under 2 volts to over 5 volts).!!
Hope the above "mini-novel" will be of some help to others who may read it.....
Radioman56
Motorola bench-tech
As shown by the two attached "screen-shots"., the ERF-9530 states a gate-threshold of 2v-4v (using an improper nomenclature of "V/TH"), whereas the 24N40 states a gate-threshold of 3v-5v (using the proper nomenclature of "V/GS(th)"
What get's me "blown-away" / amazed., is how someone / anyone would design ANY type of MOSFET amplifier (for RF use) that does NOT have at least one (or more, depending on the number of "pairs") of some type of bias-voltage, adjustment trim-pot/s.!
As not only does the turn-on voltage of a MOSFET vary GREATLY (voltage / percentage-wise) when compared to any BJT transistor., but also that a MOSFET (having virtually infinite input impedance) can NOT generate its own "forward turn-on bias" with increased input "drive"...
While even in a BJT, class-C amplifier (ie: no external bias voltage applied), to generate a sufficient bias, turn-on voltage., can be easily accomplished within the BJT's Base-Emitter (diode-like) junction., in which, when a sufficient amount of carrier / signal is applied to its input (low impedance) will "generate" its own nominal 0.6x volt, turn-on bias.!
This is why I've had to add my own trim-pot/s into a number of those "RB" boxes in the past (whether they contained ERF-9530s or 24N40s)., even though, myself and others have been told by the supplier of RBs: "we hardly ever get any of our RB boxes back (for service) that have failed in any given year"
What get's me "blown-away" / amazed., is how someone / anyone would design ANY type of MOSFET amplifier (for RF use) that does NOT have at least one (or more, depending on the number of "pairs") of some type of bias-voltage, adjustment trim-pot/s.!
As not only does the turn-on voltage of a MOSFET vary GREATLY (voltage / percentage-wise) when compared to any BJT transistor., but also that a MOSFET (having virtually infinite input impedance) can NOT generate its own "forward turn-on bias" with increased input "drive"...
While even in a BJT, class-C amplifier (ie: no external bias voltage applied), to generate a sufficient bias, turn-on voltage., can be easily accomplished within the BJT's Base-Emitter (diode-like) junction., in which, when a sufficient amount of carrier / signal is applied to its input (low impedance) will "generate" its own nominal 0.6x volt, turn-on bias.!
This is why I've had to add my own trim-pot/s into a number of those "RB" boxes in the past (whether they contained ERF-9530s or 24N40s)., even though, myself and others have been told by the supplier of RBs: "we hardly ever get any of our RB boxes back (for service) that have failed in any given year"
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