Why won't this power scaler circuit work?

[Randall]

I have a Morgan AC40 in, stuck in 1/2 watt mode. Morgan is no help, after a couple of emails promising to send me parts, he has stopped responding. There is a little, nearly impossible to get at board with a small amount of parts that has three wires, B+ in, +out, and ground. It has a NTE 2973 N-Ch mosfet, two 100K resistors, a 1n4742 zener, a 10ohm 5 watt resistor and a volume pot. I replaced the 10 ohm because it was broken off the board when I got it. The 100Ks measure good. The pot is good, I replaced the mosfet thinking it tested bad, but no luck. That leaves the zener. I'm pretty sure it is a 1N4742, but my eyes aren't good, and man it is really small print.

So here is my best idea of what is going on. B+ is fed to the mosfet drain. + out comes off the mosfet source thru the 10 ohm 5watt on to the OT. The zener is cathode to gate, anode to +out. Shorting B+ to +out gives full power output.

I've been fooled by zeners before. This one measures 0.4v in one direction, and 0.5 in the other. Also, in series with a 1K resistor it drops 6.1 volts. A 1N4742 is supposed to be a 12v 1 watt zener. Do I have a partially bad zener?

[The Dude]

If the zener measures .4V one direction and .5V the other out of circuit, it's leaky/shorted.

[Randall]

OK, I can accept that. Can someone explain why a defective zener would disable this circuit? I realize not having a schematic is problematic, but I am not adept at drawing one.

[Dave H]

Can someone explain why a defective zener would disable this circuit?

The zener is a gate source protection diode (or it's for current limiting as below). If it's shorted it will turn the FET off.

Edit: The schematic will be something like this.

[Mick Bailey]

The components and description sound very similar to the VHT 12/20 special circuit - maybe even identical.

[alexradium]

its the same old Dana Hall VVR that everybody use with different fancy name,because they "engineered" a special way to control the amazing power of their cathode bias amps,in house,in the full moon day.

[mac dillard]

its the same old Dana Hall VVR that everybody use with different fancy name,because they "engineered" a special way to control the amazing power of their cathode bias amps,in house,in the full moon day.

According to your description this is correct. l have the Hall VVR package with the drawings and schematic. Will share if you need and I can figure out how to post them..

[Randall]

It sounds like this zener is the problem, so I will get it ordered. Thanks guys! I will report back when it arrives. I had not heard of the Hall VVR. Pretty disappointed in Morgan for dropping the ball. It wouldn't have put him off too much to share the info.

[xtian]

Here are a couple looks at the cathode-bias VVR circuit.

[Chuck H]

Morgan is no help, after a couple of emails promising to send me parts, he has stopped responding.

Pretty disappointed in Morgan for dropping the ball.

Wow! Can "I" do this? Sell a product that breaks, tell the customer I'll take care of it and then just let time suck away the liability!?! I can think of all kinds of ways to make money with this scenario.

[Randall]

I am kind of concerned that both those drawings show 6v 1 watt zeners, and the labeling on the one I have I THINK makes it a 12v 1 watt. And the one I have drops ~6v. Should I be worried?

[nosaj]

I am kind of concerned that both those drawings show 6v 1 watt zeners, and the labeling on the one I have I THINK makes it a 12v 1 watt. And the one I have drops ~6v. Should I be worried?

Do you have a second set of eyeballs to verify the part number or a clear pic that can be blown up?

nosaj

[The Dude]

We're not entirely sure what's going on in the circuit without a schematic. Does the zener go from G-S on the FET (check with meter). If so and it's for protection, the NTE2973 has a typical on state max Vgs of around 6V. So, a 12V zener isn't going to offer much protection. As long as you're replacing it anyway, I'd probably go with a 6V zener. Maybe the NTE is not an original part?

[The Dude]

Also, in post #1, you said the diode, "measures 0.4v in one direction, and 0.5 in the other". In post #11 you said, "the one I have drops ~6v". I'm confused. Which is it?

[Randall]

Both. 0.4v in one direction, 0.5v in the other out of circuit on diode mode. In series with a 1K resistor on a power supply, it measures 6.1v across itself.

I am now confident saying the zener is a 1N4742A. Do I need to order another 6v one? I think this one is original. The mosfet, I don't know, and Morgan ain't saying.