MFJ 259B

[KJ4BAE]

Is it possible to check the input and output tuning on a transistor amp with a mfj 259B? If so how, i don't want to harm my analyzer by trying some thing that wont work.

 

[Shockwave]

While there are ways to use an antenna analyzer to check the resonance of the input and output circuits of a tube amplifier, I'm not aware of a technique to do this with transistor amps. If your transistor amplifier has an input tuner, adjust it for minimum SWR seen at the radio with an SWR meter. If your amplifier has an adjustable output capacitor, adjust it for maximum PEP output with modulation using a watt meter.

 

[W5LZ]

Why not connect the two and see what the analyzer says? Amplifier 'OFF' naturally! On second thought, may not work, the thing is RF keyed isn't it? Or just manually trip the change over relay and try it that way?
- 'Doc

 

[Shockwave]

Keying the relay manually may work to tune the input side on a solid state amp but it won't help you to tune the output. It could also cause the amp to oscillate and burn up the MFJ. The method I mentioned before is easy and more accurate since it is done at normal drive and power output. All you need is an SWR / Watt meter.

 

[W5LZ]

Shockwave,
The important part is that the amplifier is NOT powered on. That's why I said to manually close that relay. That '259 would then be seeing the output circuit/tuned circuit, right? Using an SWR/watt meter is certainly one way of doing it, but that wasn't what was asked.
- 'Doc

 

[Shockwave]

The problem is without power applied to the amplifier with at least the transistors biased up, the input and output impedances are going to look like open circuits on the transistors. If the amplifier were a tube amp with a tuned input circuit and PI output circuit, the amplifier could be roughly tuned powered off by using a resistor to replace the tubes load impedance. Because it's a solid state amp biased off, the MFJ will see the input to a broadband transformer attached to transistors that are not representing any load impedance because they are off. It's not an accurate way of tuning a broadband transistor amplifier.

 

[bob85]

you cannot test the amp tuning with the mfj by simply hooking it to either input or output of the amp and pulling the relay over for the reasons shockwave mentioned,

transistor input and load impedance changes with changes in collector voltage / drive level / bias level, a similar resistor method to the one shockwave suggested for tubes can be used to measure transformer vswr in solid state amps, transistor capacitance also needs taking into account.

 

[KJ4BAE]

Thanks for the replys.

 

[HiDef]

Keying the relay manually may work to tune the input side on a solid state amp but it won't help you to tune the output. It could also cause the amp to oscillate and burn up the MFJ. .

I wouldn't let the amp anywhere near the MFJ

If it's a simple circuit there could be nasty little transients.

 

[Captain Kilowatt]

The manual for the MFJ-269 CLEARLY warns that absolutely no RF whatsoever should ever be applied to the input terminals of the device or damage will result. I know of one broadcast engineer that had his '269 at a TX site and mearly turning it on in the presence of a high RF field was enough to induce RF currents in places it should not have been and needed to be repaired. Another I know cannot get reliable measurement at his home on anything other than the lower HF bands because he is in the shadow of a 100KW FM transmitter several miles away.The input is very susceptible to RF.