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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old Today, 9:28 am   #121
SiriusHardware
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Default Re: Superstar 6900N V6 - Fault with SSB mode

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.
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Old Today, 10:08 am   #122
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Edit: I've just read what mike w said in #111, for some reason that post was not visible to me earlier. Maybe it was edited after I originally read it. Agree that devices with VT of ~1.9 or whatever would suffer if put in the radio if the bias was not then readjusted from its current rather high level of about 3.7V.
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Old Today, 10:31 am   #123
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Found this thread on Tranmission1 https://www.transmission1.net/viewtopic.php?t=30718

What do you think of the process in post https://www.transmission1.net/viewto...434a00#p267180

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Old Today, 10:45 am   #124
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Default Re: Superstar 6900N V6 - Fault with SSB mode

If you mean the one where it says just set the bias voltages to certain fixed values, I don't think that is good enough because there is such wide variation in the VT figures between individual devices. You really will have to set the bias specifically to suit whichever pair of matched devices you fit. Consider trying the approach in #120?
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Old Today, 12:07 pm   #125
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Default Re: Superstar 6900N V6 - Fault with SSB mode

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Originally Posted by SiriusHardware View Post
If you mean the one where it says just set the bias voltages to certain fixed values
That's a rubbish idea. If possible they'd not have wasted money on presets.
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Old Today, 12:20 pm   #126
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Default Re: Superstar 6900N V6 - Fault with SSB mode

100mA sounds low for a pair, maybe that's a 1W rig, or it is meant to be a lot closer to Class C than Class A bias.
Certainly the IRF510 is stable at about 100 mA.

In the absence of any definitive current figure there are only 2 options:
1) Calculate what seems like a reasonable DC standing current for AM high power.
2) Start at minimum and increase till 2nd Harmonics or distortion is just gone, but in any case less than the current in (1). You have to do something like this for the Driver anyway as we don't know what power it should be. Likely it's the same or slightly lower current.

Setting gate to a particular voltage is never going to be right.
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Old Today, 12:59 pm   #127
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Quote:
Originally Posted by Mike. Watterson View Post
Setting gate to a particular voltage is never going to be right.
Yes, agreed.

As you seem to prefer to set the bias in AM TX mode, can you outline the process, step by step, in terms that crackle can follow, bearing in mind the limitations in terms of available test gear?

I don't think he has a spectrum analyser, for example, but he might be able to observe the shape of the RF envelope on his scope (to look for distortion on AM).
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Old Today, 1:02 pm   #128
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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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Old Today, 1:07 pm   #129
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Default Re: Superstar 6900N V6 - Fault with SSB mode

I understand that. If only we knew someone who had a working one of these we could ask them to make measurements. Anyone got one, with a V6 chassis?
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Old Today, 1:26 pm   #130
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Default Re: Superstar 6900N V6 - Fault with SSB mode

The scope will filter off distortion. I thought of that. You need at least a 100MHz scope that's still good at 120MHz to see much distortion of 28MHz. It's a 15 MHz scope.

I suggest AM because:
1) Gross distortion due to 2nd harmonic (clipping, think full wave rectifier gives lots of 100Hz from 50), will be obvious on an Air band receiver, (carrier x 4).
2) We know AM is 50% efficient at best, in Class A. Thus for a recommend power and 6V supply (the series regulator is AM modulator so must sit at about 1/2 supply), we know the maximum ideal current is an upper limit.
3) FM high power is logically x4 the AM power.
4) SSB by default has no carrier, no drive to FETs, this is more unstable for parasitic oscillation, fortunately when we were restoring the 200W Racal military HF sets we had a good few spare transistors included with all the other spares.

For O/P or Driver current measuring:
Lift one end of the RF chokes that are wire links in ferrite tubes, judging from the photos. Tack 1nF ceramic there to a nearby earth with short leads. We don't want RF in the test meter! Twist test meter leads.

The calculated (2 x AM Power)/(PA stage supply volts) is an absolute maximum. The driver stage maximum is 1/2 this, likely less.
Start at minimum gate volts, no you can't measure it without a probe that blocks RF!, max resistance of pot. Increase till the harmonics reduce, but not more than the calculated maximum.
I'd guess the real current setting is maybe 50% to 80% of the theory for Class A AM at that power and voltage. Do they even claim any level of out of channel emission?
The driver maximum current has to be no more than (output stage current)/2. Again start at minimum and increase to reduce distortion / harmonics, up to the maximum.

Maybe GDO/ Dip Meter in passive receive only mode set to 56 MHz would help. Or a 56 MHz tuned circuit feeding an RF peak detector. Adjust either of those for maximum when o/p currents are at minimum.
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Old Today, 1:34 pm   #131
Mike. Watterson
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Quote:
Originally Posted by SiriusHardware View Post
I understand that. If only we knew someone who had a working one of these we could ask them to make measurements. Anyone got one, with a V6 chassis?
I'll look in the attic. I've box load, but most are bipolar transistor era and may not work. Basically ANY CB or 10M rig or power amp with a single ended output using one or two mosfets with a known AM power. Even if only 1W. If it's one FET, we double it. If it's only 1W we take the current as a minimum, the standing DC current might not be higher, or not much more for 4W, as stability and running cool may be more important to the purveyors than clean RF.

Also there is no assurance that a one owner from new was correctly set in the factory or hasn't drifted, or a S/H one wasn't twiddled earlier. Measuring one example of something isn't as reliable as theory.
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