Superstar 6900N V6 - Fault with SSB mode

[crackle]

Here is the file, it was too large to fit on this site as a PDF in reasonable quality, so I have placed it on my website.
http://www.kbmuseum.org.uk/other_ser...vice_notes.pdf
Please help yourself and save to your PC.

Mike

[SiriusHardware]

Will do. Enjoy your radio.

[Radio Wrangler]

Originally Posted by Mike. Watterson

That's poor alignment advice. With the varied Vt., setting a particular gate volts is roulette. It's the current that counts.

I very strongly agree with these two statements, and I wouldn't have just called it poor.

You set whatever bias volts it takes to get the right current.

That's why two devices in parallel is a big no-no.

David

[SiriusHardware]

Sometimes the 'wrong' action leads to the right outcome, as seems to have happened here. In the ideal world we would know exactly the current required and would be able to advise how to go about adjusting the bias for that current.

Here, by a matter of luck, the bias voltage values crackle found online have turned out a reasonable match for his selected MOSFETs and put his radio into the zone where it works, and that was the outcome he was looking for. I think it best to leave it at that.

[crackle]

The problem I see is the circuit is not designed to enable you to check the current easily with standard domestic type DMM. I may be wrong but would the long leads to the meter in series with the supply to the OP pair cause problems.

OH, and nobody knows what the current should be set at.

Mike

[SiriusHardware]

Mike W did cover the use of a meter to measure the current in some detail...

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.

But you're right, we still don't know what exactly what the current should be anyway. As I said, arriving at your destination is usually more important than how you get there.

Wonder if the skip will be running today as well? Stuck at work, unfortunately, so I won't have much of a chance to find out.

[Radio Wrangler]

If it was designed to have current sampling places, they would be decoupled so that test cables wouldn't create RF problems. You'd still need short thick cables because of the current involved and the need to not disturb things with voltage drop, so special cables would need to be made up. DMMs would be poor choices, most go loopy in the presence of RF power, so you'd be looking at something expensive from Fluke or HP which get carefully filtered - or just use a classic passive meter like an AVO!

As for the currents, it would be nice to know the design values, but from knowing the supply voltage and the output power, a pretty good guess can be made, and anyone using it will get a working transmitter. It's a matter of re-doing the design calculations, and not particularly difficult.

Trying to get other people to just set a gate voltage introduces the variation of the transistors in terms of Vgs versus Id. That transistor has a data sheet stating there is 2 volts of variation in the threshold voltage (2 to 4 volts) and that's only at 250uA in the drain. It gets worse at higher currents!

So, setting the bias by just the gate voltage is very easy to do and very easy to describe, but it will only work for a fraction of the transistors of that type out there. Some people will be left with too low bias and very little output power, others will take enough current to blow fuses. You've been lucky that a figure suggested to you worked for yours. Not everyone will be so lucky.

1) Designing something is one thing.
2) Designing it so that it can use any part that is within spec for that type of transistor is harder.
3) Designing it so that you can adjust what needs to be adjusted is harder still.
4) And finally, writing a procedure that other people can follow to set one up properly is a right sod of a job!

That radio is OK on 1) above.
It's a fail on 2) because it needs matched pairs. Can't be relied to work with randomly chosen transistors
as for 3) it's not too bad, there is a sequence which will get it there, given matched transistors. Not matched on a hand-held tester or a multimeter, but matched at high current as they will be used.
number 4) is a complete wipeout. There is no info available on how to select transistors and set it up.

So, some of us can work out what current to set it up to, and a process for setting it up, but there's as much work involved as there was in designing these aspects in the first place. There is a class of circuits which can only be fixed with any certainty by someone that could have designed it right in the first place!

Someone I used to know had a radio and TV repair business. He kept getting asked to repair guitar amplifiers in the early sixties. He lost his rag with some of the junk that came his way. "I could do better than that!" was his thought. So he did. His name was Matt Mathias, a jewish escapee from Nazi Germany. His amplifiers were branded "Matamp" but a guy with a boutique music shop in London wanted to buy lots with his own branding on them and bright orange tolex covering. So when you see famous groups fronting "ORANGE' speakers and amps, it all came out of the thought 'I could do better than that!'. Better build quality than Marshall, too. Matt was a nice guy, and the people who called in at his place were a who's who of rock and pop.

So, you learn more from coming across a piece of iffy design than you do from an honest part failure in a good design. Ironic, really.

I've been enjoying a few days of sporadic-E propagation on 50MHz. 20metres is coming alive, I think the next sunspot cycle is starting.... yippee!

David

[crackle]

Just before I put this radio back in its case I did a few more measurements.
I dont know if these will help.
The following currents were measured on the Volts Amps meter on my 12.3v bench PSU.

With the driver bias J4 off and the PA bias J3 on,
AM 1.37A
FM 1.87A

With J4 and J3 off the currents were
AM 0.8A
FM 1.09A

SSB current no audio TX with J4 & J3 on 0.5A
SSB current no audio TX with J4 & J3 off 0.48A hardly any difference.

Receive current 0.34A

Can the Output pair idle current be determined from the above readings?

Dont forget that these figures were all obtained with a bench supply of 12.3v.

Mike

[SiriusHardware]

This figure in particular:-

SSB current no audio TX with J4 & J3 on 0.5A
SSB current no audio TX with J4 & J3 off 0.48A hardly any difference.

...makes it sound to me as though there is not much bias. I'll leave it up to others with better knowledge to take up the heavy lifting on this one.

A couple of questions to start things off, though. Which FETs from your original 'tested' list did you fit?

Can you also state the maximum power that the radio should have on AM / FM / SSB according to the user manual / specifications?

[Radio Wrangler]

In lieu of a two-tone audio intermodulation test into a spectrum analyser, getting reports of how much distortion and adjacent channel splatter there is on SSB might be the only check

David

[crackle]

Here are the specs from the CRT SS6900N v6 user manual.

Mike

[SiriusHardware]

OK, so

12W AM
30W SSB
40W FM

I think Mike W and David both felt that it might be possible to estimate the likely required driver and output bias current if they knew these figures.

And sorry, do you know which pair of FETs you fitted? It might be useful to know what the VT figure was for them.

[crackle]

Although the radio was capable of achieving those power readings, everyone you speak to says it is not a good idea, for long at least not on FM.

Here is the full user manual, with a few comments by me. http://www.kbmuseum.org.uk/ss6900nv6...ser_manual.pdf
You may notice on the second page it says duty cycle TX 10%

[SiriusHardware]

Originally Posted by crackle

You may notice on the second page it says duty cycle TX 10%

I can't imagine it lasting long on 10m, then (we amateurs are notorious wafflers).

Even if rarely used on full power it needs to be set up as though it is going to be so that it works right across the full variable power range. That's why I asked what the maximum figures were.

I had a look at the CRS SS6900 on Knights and I notice they offer it either in a full power version (yours) or a power-limited version (still quite powerful) in which the transmitter is dialled back a bit, probably to enhance its own survival.

[crackle]

Sorry I forgot to mention, I fitted the two IRF520s that you thought were best matched, E & H
Incidentally the 2 that were in the radio when it was not working on SSB were A & B. I don't know why I chose such a badly matched pair, I suppose I just didn't realise the significance.

Mike

[SiriusHardware]

Mike W and I came up with a theory about how the manufacturer actually sets up the bias on these. Given that there are no access points where you can easily break into the power feeds to the driver and output MOSFETs to measure the bias current, how or at what stage does the manufacturer do that?

This is what we came up with. It's not so different to what you tried yourself a few posts back, actually.

With J3 and J4 removed, SSB TX mode, no audio drive, measure the exact current drawn by the radio. This is the TX current drawn by everything else in the radio EXCEPT the driver and the output stage. Call that 'TX current with no bias' and note it down.

Fit J4 and adjust the driver bias until the total current drawn by the radio is ("TX current with no bias" plus "Driver Bias Current").

Remove J4 and fit J3, adjust the output bias until the total current drawn by the radio is "TX current with no bias" plus "Output Bias Current"

Then with both links refitted, check for normal operation.

In case the above is not clear, here's an example with some COMPLETELY MADE UP figures using round numbers to illustrate the principle.

Suppose you are the manufacturer and you know that the driver bias current should be 500mA, and the output bias current should be 1000mA.

With both links out, you measure the TX current drawn by the radio and it is 750mA.

Fit J4 only and adjust the driver bias preset until the total current is (750mA+500mA) = 1250mA.
Fit J3 only and adjust the output bias preset until the total current is (750mA+1000mA) = 1750mA.

Now the bad news: We still, unfortunately, don't really know what the driver current and the output current should actually be set to.

If you know a friendly person with a working example, you can ask them to do this:

Remove both links and note how much current the radio draws in SSB TX mode, no audio drive.
Fit only link J4 and note how much the current increases by. The amount it increases by is the driver bias current.
Fit only link J3 and note how much the current increases by. The amount it increases by is the output bias current.
Refit both links and resume normal operation.

Needless to say, none of this is ideal but this is the sort of approach you sometimes have to take when you don't have a proper suite of RF test equipment and when, as in this case, the manufacturer does not even bother to give basic setup information.

[crackle]

I can probably do that as I have a neighbour down the road who has one of these radios.
But to be honest I actually wonder how much care the factory put into setting the current correctly.
The other thing maybe I ought to do is invest in a decent 13.8v bench supply.
I suspect if I try and readjust the bias by setting the current with 12.3 volt supply it may upset things if I put the radio on a 13.8v supply.

Mike

[SiriusHardware]

Yes, that would actually make a difference, both radios would need to be on the exact same supply voltage and set for maximum output power. Just use your neighbour's PSU to both make the measurements from his and set the measurements on yours?

When you measure the no-bias current, even the smallest things such as being set on a different channel will alter the readings from the two sets - on some channels (like 38) more of the channel LED segments are lit than on channel 1, for example. So, identical control settings on both sets will help you to have a similar starting point.

[Radio Wrangler]

Originally Posted by crackle

But to be honest I actually wonder how much care the factory put into setting the current correctly.
The other thing maybe I ought to do is invest in a decent 13.8v bench supply.
Mike

You never know how diligent people are at manufacturers and it will vary from individual to individual.

13.8v power supplies.... Well there are plenty out there. I'm not sure I'd call that many of them good.

Switch mode ones can be extremely noisy unless they are very well screened and filtered which costs a lot of money.

Linear regulator ones use rectifiers and a reservoir capacitor to make a higher voltage than wanted, typically 18-22V and then use power transistors to regulate it down to the wanted 13.8V

A common fault in power transistors is them going short-circuit. This immediately dumps 18-22 volts onto your 13.8v radio. Not good. I've had a number of radios in the >£1000 bracket come my way that have been overvoltaged by a power supply failure.

In the amateur world it's common for people to save up and fork out a couple of thousand on the best transceiver they can afford, and then buy the cheapest power supply they can find. Very trusting, to put it mildly.

What's needed is a circuit called a 'crowbar' If the output goes too high, the crowbar triggers and shorts the power supply output, protecting the radio. Well the power supply already has a problem to cause the excessive voltage, the crowbar might pull a fuse or might do more damage, but the power supply would need fixing anyway and the radio is an awful lot more expensive to fix.

Yaesu, Trio/Kenwood, Icom and all that lot made power supplies for theur 13.8v rigs, but having fixed a number of them, I'd say they were lacking in protection and a shorted power transistor would damage the radio being powered. Do you think they wanted to sell more radios

If you fancy a good power supply, have a look for the 'Marchwood' published in PW. It was fairly thoroughly done (it does have a couple of loopholes) but the article is well explained, and it's better than most you can buy.

David

[SiriusHardware]

I'm wondering what sort of PSU it is that crackle has at the moment. At 30 Amps capable, it's unusually powerful.

Crackle, as that power supply is obviously regulated it is almost certainly adjustable, internally if not externally. What is it exactly?