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Old 25th May 2020, 1:02 pm   #128
crackle
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Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 4,100
Default Re: Superstar 6900N V6 - Fault with SSB mode

Quote:
Originally Posted by SiriusHardware View Post
I am beginning to think that the original matched pair fitted in this radio had a VT towards the very high end and the bias was set accordingly, so whenever you fit a pair with average or low VT the original bias setting is turning the outputs on too hard.

I would imagine the 'right' bias voltage for any single device is somewhere above its 'VT' because otherwise the device would not be turned on (biased) at all, but the higher it is above VT, the harder the devices are turned on and the greater the standing current.

How about this for an approach: While the devices are out / disconnected, turn the bias down to the lowest it will go (and verify that that is the case by measuring the gate pad voltages, so you know you have turned it all the way down and not all the way up - sometimes controls work in the opposite direction to the way you might expect).

Take an output pair which are well matched according to the tester, fit only -one- of them, remove L10 and insert a multimeter on its 10 amps range in place of L10.

Put the radio into SSB TX, power turned fully up, no audio drive. With the bias set to minimum the fitted device should not be turned on, no current flowing. (There may be a tiny current being drawn by Q12 and its surrounding components).

Carefully increase the bias preset until the current first starts to increase and then keep going until the current being drawn is 1 Amp. Try to do this in a fairly short space of time because you need the device to stay cool. Let it it cool down for a while, check that the current is still 1A, readjust if necessary.

Then, fit the second output device in the second position and measure the current again. If the current being drawn is now about 2 Amps, the devices are as well matched as the tester says they are. If the total current is noticeably less than 2A or noticeably more than 2A, they are not well matched.

If they do seem to be well matched, you can then go on to set the final combined current to whatever everyone agrees is a good likely value.
OK Ill have a go later at this process.

But if someone can come up with an easy to follow numbered step by step guide, without too much background information and history that would make it much easier for me to follow.

Thanks
Mike
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