Superstar 6900N v6 - Fault with SB mode

[Crackle]

I bought one of these last October and have had a load of trouble with it.
I have replaced the OP MOSFETs 3 times now, I finally used some sold as Vishay supplied by Farnell.
The radio is never run at full power, it is normally at around 20 watts (at full power it goes up to 40W) and my overs are normally fairly short.

The latest fault is with USB and LSB, they are not working. But FM and AM are working fine.
If I key up on USB, without any audio drive on the mic, my PSU registers 11 amps current draw, and the radio goes into DClo alarm.
I hasten to add the PSU is capable of over 30 amps and its OP voltage is still stable at the set volts of 12.3V.
During this fault condition the radio is still drawing 11 amps so obviously I dont try this for more than a second.
I believe 3 things are happening here.
1. There is a fault causing the excessive current being drawn by the radio.
2. Either the excessive current is causing a voltage drop in the supply cable, or
3. The radios internal voltage regulator cannot cope with the current so the radio detects low voltage.
However as far as I can tell the output MOSFETs are not fed by the internal regulator so I suspect option 3 may not be the case.
My conclusion is for some unknown reason during USB & USB mode the output MOSFETs are being turned hard ON.
During AM & FM mode the MOSFETs are being controlled correctly.

I am stumped for a solution, has anyone come across this problem, could it be a software fault, or a hardware fault.
The warranties on these radios state that semiconductors are not covered so does that make the warranty null and void?
Any advice welcome.
Or if anyone has the service information for Version 6 that would be very useful.

Mike

[Mike Watterson]

The AM & FM working and over driven on SSB suggests the SSB alignment is faulty. I'm not familiar with how SSB is done on this model. In general the carrier has to be nulled (DC offset on a double balanced mixer and then a ceramic filter blocks the unwanted side band). The FM uses class C, no DC bias on the FETS. The AM mode biases the FETS at 1/2 current. Hence power on AM is typically 1/4 of FM power. SSB only has an average power if fed a tone. Otherwise you should be seeing a peak power similar to FM. With no audio there should be almost no RF out and almost no supply current to the output stage on SSB.

It's normal that the output stage, the "finals" run straight of the supply, hence 12V gives less power than 14.2, a fully charged car battery. The 13.6 or so +/- 10% is "traditional" for a bench supply for something intended for a vehicle.

I'd remove the supply to the output FETS and see what is happening on the drive / gates with a 'scope, DC coupled to see any bias. Compare AM, FM and SSB.

These radios are sold as 10m Ham rigs to get round import controls as they are really CB rigs with more power for 11m than is licensed anywhere. They are sold under different brands. There is no comparison with Kenwood, Yaesu, Icom etc.

You need the service manual, any version, to see how the SSB works and is aligned. It depends who you bought it off as to the warranty situation, and if a qualified 3rd party says it's inherently faulty, (sounds like it's never been right), then what ever version of the SOGA applies unless you imported personally from outside the EU (UK doesn't leave till 31st Dec, just has no votes since 31st January).
The contract / warranty that counts is with the retailer that sold it. It sounds like you are describing the maker or importer's warranty.

Edit:
reasonable overview https://hamgear.wordpress.com/2013/03/04...s6900n-v6/
He says no service info or schematic.
About €195 from the Netherlands, so it's a very cheap radio really for the illegal CB market, which used to be mostly Italy due to poor enforcement, some CBers there with 1kw in the past.

[Crackle]

Hi Michael
I just wanted to say thanks for your interest and reply.
I am continuing discussing this problem on UKVRRR, where SiriusHardware and one other have experience with these types of radios. He seems very helpful and has suggested more or less the same initial tests as yourself.
I will post any causes and possible solutions on here as well, but most of the conversation will probably be on UKVRRR.

thanks
Mike

[Mike Watterson]

No problem. I do pop in there occasionally. I've only very very old CB rigs, though I modified my Superstar SS3900 to do most of 10m, which here makes it illegal to use to transmit on CB, but still legal for receive, because the Comreg rules that ANY transmitter used outside of Amateur bands must have CE (consumer stuff) or ETSI (professional broadcast gear). Ham gear can be modified, but any ETSI or CE marked transmitters lose certification if modified. Hence while many Amateur UHF rigs can do 446 MHz, it's illegal except in USA that uses a different band for Family Radio and 446 is probably in their Ham band. Even if it's a 1W only rig as it's not certified.

Additionally the UK CB band is mostly Community Radio here which must use CE marked gear and have a licence. Mostly used by churches. The lower CB band is the official CB band here.