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-   -   Superstar 6900N V6 - Fault with SSB mode (https://www.vintage-radio.net/forum/showthread.php?t=166981)

crackle 20th May 2020 4:07 pm

Superstar 6900N V6 - Fault with SSB mode
 
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. (V3 is available on the web but it is a different board layout.)
Mike

SiriusHardware 20th May 2020 4:38 pm

Re: Superstar 6900N V6 - Fault with SB mode
 
Can you show us a good overhead and underside photo of the PCB? I may possibly be able to lay hands on a diagram, but I don't recognise yours from the model number. What identifying features does it have - what type are the MOSFETs, what type is the power control / modulation transistor, etc.

The idea that anyone can rule out semiconductors from the warranty sounds extremely dubious - I could understand it more if they meant the output MOSFETs -only- because they are vulnerable to misuse / abuse (transmitting with no aerial connected, etc), but I still would not find it acceptable.

SiriusHardware 20th May 2020 4:54 pm

Re: Superstar 6900N V6 - Fault with SB mode
 
Forgot to mention, if it's the sort of chassis I am thinking of then the output stage does run on the regulator / modulator transistor even in SSB mode. It normally just passes on the full 13.8V supply voltage in that mode, dropping to roughly ~7-8V in AM mode, probably FM mode as well.

crackle 20th May 2020 5:10 pm

Re: Superstar 6900N V6 - Fault with SB mode
 
Hi
Thanks for your reply, I will get something together and upload to my website where they can be seen in high res.
thanks
Mike

crackle 20th May 2020 8:46 pm

Re: Superstar 6900N V6 - Fault with SB mode
 
Here are some photos of the radio in question.
http://www.kbmuseum.org.uk/ss6900nv6/ss6900_n_v6.htm
Also included is an image of the Anytone AT5555 v6 circuit board layout which appears to be identical.

If you cam make anything out of the circuit board and what may be causing the problem I would really appreciate it.

Thanks
Mike

SiriusHardware 20th May 2020 9:14 pm

Re: Superstar 6900N V6 - Fault with SB mode
 
Crackle, you have a PM.

SiriusHardware 20th May 2020 10:29 pm

Re: Superstar 6900N V6 - Fault with SB mode
 
Having thought about this for a while:

The modulator / regulator transistors on these are a common cause of problems, often failing in unconventional ways, such as open-circuit from base-emitter. This fault doesn't fit the usual profile but it may be worth desoldering two of three legs of the modulator / regulator transistor (Q55, TIP36, if it is the chassis I am thinking of) and just checking that the junctions look normal in forward / reverse modes.

At the transmitter driver/PA end of things the only difference between AM/FM and SSB modes is that the PA finals run on 13.8V in SSB mode and about half that in AM/FM. The supply voltage to the RF PA stage is controlled and, in the case of A.M, also modulated by Q55.

I'm wondering whether, instead of the suppressed carrier USB or LSB signal you should have going to the input of the transmitter, you have a flat-out full-duty-cycle carrier wave going to the input of the transmitter instead. With the RF PA stage running on 13.8V the RF output and current drawn would be quite hefty.

Techman 21st May 2020 12:36 am

Re: Superstar 6900N V6 - Fault with SB mode
 
Yes, I think we've been here before with this one, but as said, probably a slightly different failure mode this time. I've only just seen the OP's post regarding this radio on here, but had previously seen he'd posted regarding it on another forum earlier and thought exactly the same as has been said. I'm not a member of that other forum, just an occasional reader, so was unable to comment and was wondering if he'd post it on here instead. I think that this type of fault is likely to be way over the heads of most of them on there these days (although famous last words perhaps - we will see). There's only one or two with good radio knowledge left over there and I've noticed that they only post occasionally nowadays. I think the OP was worried about the radio not being regarded as particularly vintage enough for this forum, but he could have posted in the 'new technology section' as I did.

SiriusHardware 21st May 2020 8:51 am

Re: Superstar 6900N V6 - Fault with SB mode
 
I've pointed crackle to a source for a probable circuit diagram for the V6, the same place you found yours, I think. Hopefully we can make some headway on it here.

SiriusHardware 21st May 2020 12:16 pm

Re: Superstar 6900N V6 - Fault with SSB mode
 
Crackle, I get the impression you do a fair bit of repairing and fixing up. Can we assume that you have a scope, ideally working up to 40MHz?

crackle 21st May 2020 12:17 pm

Re: Superstar 6900N V6 - Fault with SSB mode
 
My previous comment that AM was OK was not a proper test as I had no one to confirm the modulation was OK, but the carrier was certainly there. and the current shown on the PSU was stable at about 7 amps. I have just done another quick test to confirm that AM is Ok, my modulated signal was received. But I noticed 2 things, the RF power control did not seem to function fully in AM mode, i.e. it would only go down to about 5 watts.
I made some voltage checks on the supply to the finals, these were only done on receive mode.
FM mode, lowest power setting 1.9v, highest power setting 11v
AM mode, lowest power setting 3.21v, highest power setting 6.33v
SB mode, lowest power setting 2.52v, highest power setting 11.67v

Q55 the TIP36 is getting extremely hot in AM tx mode.
Would this indicate the TIP36 is working correctly.

There are various links on the supply to the final mosfets.
L27 should shut down power to both the driver and the finals
L11 should just shut down power to just the driver Q47
L10 should shut down the power to the 2 final mosfets. Q45, 46
I forgot to mention that on the last occasion the finals blew, I also found L27 had melted its solder and dropped out. (like a very crude fusible link)
At that time I replaced all 3 of the IRF520 mosfets and refitted the L27 link.
It all seemed to be working OK as I had not tried AM or FM.

I propose to try disconnecting L27 then test to see what is happening, whilst in SB tx mode.
Then reconnect L27 and disconnect L11, then disconnect L10.

Just seen the last post come in whilst typing this. I do have a scope but I think it is only a 10MHz one, I will check the spec.


Mike

SiriusHardware 21st May 2020 1:06 pm

Re: Superstar 6900N V6 - Fault with SSB mode
 
Well, even a 10MHz scope will be better than nothing, it will allow you to see a 28MHz RF signal as a solid blue or green 'band' and to see whether it is modulated or constant, etc.

At work at the moment, will have a look through what you've said and make further suggestions, but my initial suggestion is to disable the RF PA and driver stages and then look with the scope to see what is being sent to the transmitter input when in SSB TX mode.

In SSB mode the RF input signal to the transmitter should obviously only 'appear' in sympathy with any audio going into the microphone, it should not be present all the time.

If you have a full constant carrier there then we'll look further to see why that might be.

I would expect that regulator / modulator transistor to get hotter in AM and FM TX modes than in SSB TX mode, the reason being that in SSB mode there is almost no voltage drop from E to C, but in the other modes there is a difference between those points of roughly half of the supply volts. (I'm assuming the transmitter runs on half supply in both AM and FM TX modes).

If you've owned this from new can we assume that none of the adjustments have been moved from their factory settings without good reason?

crackle 21st May 2020 1:30 pm

Re: Superstar 6900N V6 - Fault with SSB mode
 
Hi
I can confirm I have had this transceiver from new and I have not made any adjustments.
My scope is a BK 1435 which is 15MHz.

It is too hot at the moment to do anything so I will test the radio with power to outputs removed later this evening.
Thanks for your help
Mike

Jon_G4MDC 21st May 2020 4:24 pm

Re: Superstar 6900N V6 - Fault with SSB mode
 
Something is nagging me that the cause of this happening several times might be runaway RF feedback. Are you testing into an antenna or a dummy load?

crackle 21st May 2020 5:55 pm

Re: Superstar 6900N V6 - Fault with SSB mode
 
Of course I cant rule out an intermittent fault on the antenna or the coax feed to it, but the radio has a SWR alarm cut of currently set to 3.
The alarm cuts all signal to the output.
Mike

SiriusHardware 21st May 2020 6:39 pm

Re: Superstar 6900N V6 - Fault with SSB mode
 
If the PA was 'taking off' then I would expect to see RF output indicated on a power meter placed inline with the dummy load. Lord knows what frequenc(ies) it would be on, but the power meter wouldn't really care about that.

The original symptom described is that if the radio is keyed in SSB mode with everything connected up as normal, the radio draws 11 Amps even in the absence of any audio input. Is that still the case? (Or at least was that still the case when everything was still connected up)

If so, was there any indicated RF output on a power meter placed inline with the dummy load when this was happening? We need to work out whether this is an 'RF' fault or a 'DC' fault, ie, transmitter being driven by a large input signal when it should not be or 'taking off' (self oscillating), or alternatively, whether the MOSFETs are somehow being driven hard on by DC conditions only in SSB mode?

As said earlier, the only change that I can see to the DC conditions going from AM/FM to SSB is that in AM mode the output stage runs on half supply which is then modulated +/- from that halfway point. Judging by crackle's measurements stated in #11 it looks like the RF PA runs on more or less full supply volts in FM TX mode just as it does in SSB TX mode.

Jon_G4MDC 21st May 2020 8:12 pm

Re: Superstar 6900N V6 - Fault with SSB mode
 
I haven't yet been sure there was a dummy load.
Please can we clear that up?

In a bad case of RF feedback no protection circuit is fast enough to stop a semiconductor blowing through. Once that has happened all the DC melting stuff is surely just consequences.

crackle 21st May 2020 8:45 pm

Re: Superstar 6900N V6 - Fault with SSB mode
 
I plan to do some tests later when it is cool in my shack.
With L10 & L11 disconnected to remove power from the driver and finals, where is the best place to probe with an oscilloscope to look for a fault, is it the gate of the driver Q47, i.e. junction of D57 & C262 ?
And can you please explain what I am looking for, is it a large DC condition or a strong RF signal on the gate?
Thanks
Mike

SiriusHardware 21st May 2020 9:03 pm

Re: Superstar 6900N V6 - Fault with SSB mode
 
If you have the power to the outputs and driver disconnected then I would suggest going a bit further back to the collector of of the transmitter pre-driver Q51 which is powered in all TX modes from the '8T' rail, which is shorthand for 'eight volts, all transmit modes'.

In FM TX mode you should see a constant-height carrier there whether the signal is modulated or not.

In AM TX mode, same as above because the AM modulation is done later, in the driver / PA stage

In SSB TX mode (upper and lower) you should see no, or very little RF when there is no audio input to the microphone. When you talk into the microphone, the RF signal should appear in 'spurts' which coincide with the audio input.

If all is as above then everything up to the beginning of the transmitter looks normal, so far.

SiriusHardware 21st May 2020 9:14 pm

Re: Superstar 6900N V6 - Fault with SSB mode
 
Forgot to say: If the radio somehow still draws 11A in SSB TX mode with the power to the PAs and driver disconnected, then we'll be looking somewhere else for the problem.


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