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Hi all

Having just resurrected my G6LBQ Multiband BITX transceiver I discovered a problem with the PA. Historically I had issues with a splurge of carrier and stuff as I went from TX to RX. Now fixed.

However the PA stage is playing up, and taking it of the case and testing results in the BC640 transistor giving up as soon as I go to TX. I cannot measure the idle current (35mA). Has anyone else had problems with the Mk 1 PA ?

I notice there is a MK2 and a MK3 so am wondering if there was a common problem, hence the new versions ?

Cheers

Adrian M1LCR

Hi Adrian

Sorry to hear you are having problems with your MK1 Linear amplifier.

I am not aware of any issues with the MK1 linear apart from people over driving it when used with the MK2 multiband.

It sounds like the BC640 is passing excessive current so it may be worth checking the 5 volt regulator in the bias circuit, also please ensure when you first power up the amplifier the three bias presets are set so the wiper is at the ground position as this will prevent the RF driver and output transistors from being biased.

With the RF transistor bias presets all set to zero or ground the BC640 should switch and power the 5 volt regulator with minimum load/current being passed, if the BC640 still fails there must be a short or faulty component in the bias circuit.

I hope this helps and you can resolve the problem.

Regards

Andy G6LBQ

Hi Andy
Thanks for the reply.
I have been using the instructions dated 10 April 2013 to set up the PA.?

Could you confirm a few things please ? Referring to the instructions above, my measurements are:

• Test 1: I measure no standby idle (well I measure a few micro amps like 5 or 6uA)
• Test 2: I measure around 20mA
• Driver BIAS adjustment: I can set the BIAS to 145mA (20mA plus 125mA
• I can then BIAS each of the output devices to 250mA each. So a total current consumption of 645mA.

I am not sure about the standby idle current, this is really weird.?
Could you also advise typically how much attenuation is used between the MK2 Exciter and the amp. ?

Cheers

Adrian M1LCR

Hi Adrian

Unfortunately I do not have a MK1 linear board to hand I can test though I may have one packed away in my attic and if I get time over the weekend I will have a look.

To use the Mk1 linear with the MK2 exciter the RF output ideally needs reducing by 15 to 17dB, this will take the MK2 exciters RF output down from around 500mW to 10mw and produce 10 watts of RF output from the MK1 linear.

You could drive the MK1 linear a little harder and get up-to 20 Watts RF output but the linearity will start to reduce.

The MK1 linear was originally produced to match the MK1 exciter which only had an RF output of a few mW which is much less than the MK2 Exciters 500mW so the excess power must be reduced via the attenuator.

Regards

Andy G6LBQ