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Old 22nd May 2020, 10:46 pm   #41
SiriusHardware
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Default Re: Superstar 6900N V6 - Fault with SSB mode

The switching checks look good so far, although there are ever deeper layers we can still go into. However we don't want you to get too bored so, time to wield your scope again. This time I'll ask you to look at area E1-E4 on the same sheet, sheet 3.

First try looking at test point TP301, 'BFO FRQ'. You should find that there is a steady RF waveform there in all modes on TX, and on USB, LSB and CW modes in RX. The frequency of this waveform will be about 10.695MHz so it is comfortably within the range of your scope.

If all is well there, move to the collector of Q520 - this is the output from the SSB modulator. When you key the microphone you should now only see bursts of RF which coincide with audio spoken into the microphone, and if you still have the beep turned on you should see a burst of RF carrier with the same duration as the beep when you release the microphone key.
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Old 22nd May 2020, 11:04 pm   #42
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Default Re: Superstar 6900N V6 - Fault with SSB mode

I cant see E1 to E4 on that schematic.
Found TP301 about C.8 & 3.3 co-ordinates, Ill do these checks tomorrow AM

thanks

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Old 22nd May 2020, 11:07 pm   #43
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Ugh, sorry. Must be near my bedtime. I meant C1 to C4, sorry.
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Old 22nd May 2020, 11:46 pm   #44
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Just catching up with this thread.

There's not much that I can really add.

The big brute which is Q55 from memory is something like a 150 volt 25 amp (or thereabouts) transistor, also used in audio amplifier applications and was the part that had failed on the radio that I had to repair last year. The circuit around this area, including that actual transistor type seems to have undergone considerable changes during production, and it makes you wonder what actually causes these failures and whether there's some oscillation or stray RF somewhere that's getting in where it shouldn't, but only sometimes and under certain conditions. Mike said something about the power control not turning the power all the way down like he thought it should - I can't remember what sort of range this control had on the set that I had here, other than it worked and I remember having it set to an output power that a 5-7 amp test power supply would happily run it at. I remember talking to the radio's owner on it before taking it back to him and we tried it on both FM and SSB - AM was not checked, but I think I possibly did a quick 'monitor' type check with another radio in the same room just to prove it was also working.

As for the Q17/18 transistors - I also can't seem to see them on the diagram, so perhaps another more recent change during production. I can't see that shorted turns on L27 will have any effect in this instance, but I well remember the inductor with just a few turns that used to overheat and develop shorted turns on the old '059' chassis, killing the RF output with lots of heat still - I'm actually working on two home base models of that chassis when I get time, both having had multiple issues, as you can imagine - perhaps a story for another thread.
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Old 23rd May 2020, 12:01 am   #45
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Default Re: Superstar 6900N V6 - Fault with SSB mode

I think mike was talking about there being a limited range of supply voltage control via the power output control in AM TX and I can see why that would be - whatever the unmodulated supply voltage for the transmitter is in AM mode, it has to be able to swing equally plus and minus from that point.

The unmodulated transmitter supply voltage can't be allowed to get too close to zero otherwise there will be nowhere for the supply voltage to swing down to when modulated.

The nice thing about electronic (PDF) circuit diagrams is that you can search the text on them with 'find' just as you can with any other document. If you are viewing in Adobe reader, go to edit, find, and a little text search box pops up, upper right. If you use that to search on Q17 or Q18, you will find them in the left half of square D3 on sheet one - in the receiver (first mixer, it looks to me).

Last edited by SiriusHardware; 23rd May 2020 at 12:11 am.
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Old 23rd May 2020, 12:28 am   #46
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Ah yes, looking back he'd stated 5 watts, but had also taken voltage readings on the different modes. The Americans that some of us may remember down the bottom end of the old AM band with their 2 watt radios driven with maximum 'swing' into distortion for maximum volume driving into large amplifiers for maximum effect. It may sound like a strange thing to say, but with AM coming back into use, I've noticed that it's not as nice, or easy to listen to as I seemed to remember it being in the old days, due to all the 'noise' on the band (27Mhz) these days, and I wish some of them would just 'up' their audio so that I could stand a chance of hearing them at a distance. I happen to know that all the 'operators' that are now using AM local to me are using straight rig power - amplifiers don't work well on AM with standard carrier power. I was listening to a group on AM (mid band as they call it now) tonight (actually three of them are still talking on there now as I type this) and at least one of them was using a 6900 - there's a lot of them about. One of them a while ago switched a small amplifier on and they all wondered why his audio sounded clipped and distorted, they'd all forgotten the old days. Anyway, he soon switched it off again.

It's possible that Mike's radio has two separate faults, although we may all end up being surprised at the final outcome - so long as there is one. We're certainly all learning something about these particular radios.
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Old 23rd May 2020, 12:45 am   #47
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Default Re: Superstar 6900N V6 - Fault with SSB mode

I've actually always preferred the hard, crisp sound of FM over AM.

Discounting the effect of receiver AGC for a moment, the 'loudness' of an FM signal is determined by how far it is being deviated from the centre frequency, not by the strength of the signal. The deviation is the same no matter how close or how far away the signal is coming from, so the volume is constant. When the signal is very weak, noise starts to encroach, eventually making the audio hard to pick out, but the audio from the sending station doesn't really get any quieter.

With AM, the 'volume' of the signal recovered on detection is directly proportional to the strength of the signal being received, so stations further away will sound quieter and stations which are very close will sound very loud and distorted. This is why A.M. radios really did need an RF gain control, for those times when you might be travelling in a close convoy for example.

In theory, the AGC on AM receivers should ensure that weak signals are amplified (along with any noise which happens to be around, unfortunately) and strong signals are attenuated, but the effect of signal strength on the volume of a received AM signal is always quite noticeable to me.
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Old 23rd May 2020, 12:58 am   #48
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Yes, actually one of the chaps on there tonight who I happen to know is over the age of 70, happened to mention to the chap that wondered why his linear amplifier made the sound distorted and clipped on AM, that did he remember when everyone had AM rigs in their cars back in the old days and you pulled up behind the person you'd been talking to for an 'eye-ball' (meet up, in English), that the sound would go really distorted and clipped. What was really funny was that the other chap said that he'd never noticed this happen, and what the older chap hadn't taken into account was that the other (younger) chap didn't remember the early days of AM and was thinking of just FM. I resisted the temptation to jump in and say that it just applied to AM. A couple of them are still on there now talking about motorcycles - a lot of them that are on round here are bikers and ex-bikers.
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Old 23rd May 2020, 1:32 am   #49
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Did we find out what Q17 and Q18 did, or is it still a mystery?

Or perhaps I need to have a closer look at the diagram/s.
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Old 23rd May 2020, 7:39 am   #50
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Q17 & Q18 Ah yes there they are in D3, a couple of FET's.
It is odd how they look like a bit of an afterthought or replacements.

It is slightly annoying why the circuit was drawn with some transistors represented as just lines and no circle around them. It makes them difficult to spot unless you know and understand the circuit and where to find them.
As they are in the receive circuit, which is working extremely well, I will not worry about them. It would be very difficult for me to attempt to re-flow the connection anyway.

Off upstairs to carry out the last set of tests that have been set for me.

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Old 23rd May 2020, 8:22 am   #51
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Default Re: Superstar 6900N V6 - Fault with SSB mode

The TP301 shows a nice wave form of about 100mV albeit very condensed, (tight)

The test on Q520 was not so good, can I confirm the collector is the single pin on the left in this instance.
All I get there when I key on USB is a slightly fuzzy trace, but so sign of modulation with my voice. see trace here

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Old 23rd May 2020, 8:37 am   #52
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Default Re: Superstar 6900N V6 - Fault with SSB mode

OK, backtrack a little bit to C539 in circuit area C2, sheet 3 - set your scope as you would to look at audio (as a rough guideline, adjust the horizontal timebase until the scanning line only just becomes a solid line, that will put it in the right ball park area).

On C539, the input to which you will see is labelled 'SSB MOD', you should see audio when you key the microphone and speak into it.
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Old 23rd May 2020, 9:13 am   #53
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Quote:
Originally Posted by Techman View Post
Did we find out what Q17 and Q18 did, or is it still a mystery?

Or perhaps I need to have a closer look at the diagram/s.
No mystery, not an afterthought, they are the mixer for the receiver. Two J-FETs in a single-balanced configuration. The wanted signal comes in through an RF amplifier with varactor-tuned frequency as a preselector, then operates the two JFETs as a balanced amplifier. But the gate voltage of the JFETs is fed via the centre tap of the input transformer secondary and is the receiver local oscillator from the synth.

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Old 23rd May 2020, 9:17 am   #54
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Default Re: Superstar 6900N V6 - Fault with SSB mode

I think crackle was just referring to the slapdash way in which they've been put on, slightly twisted and not too well soldered. (Photo in #29).
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Old 23rd May 2020, 9:32 am   #55
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Quote:
Originally Posted by crackle View Post
The TP301 shows a nice wave form of about 100mV albeit very condensed, (tight)

The test on Q520 was not so good, can I confirm the collector is the single pin on the left in this instance.
All I get there when I key on USB is a slightly fuzzy trace, but so sign of modulation with my voice. see trace here

Mike
U503 appears to be the balanced modulator. Q523 manufactures the carrier signal using the crystal Y500 and a varactor diode D507 to pull the frequency.

This is the first step in making SSB.

Q520 should be amplifying/buffering the output of the balanced modulator. At this point there should ideally be NO carrier. U503's balanced acton should null it by cancellation. Talk into the mike and the waveform on Q520 should look to be a right mess. Put in a pure tone to the microphone input and look with a spectrum analyser and you should see two tones, 1kHz above and 1kHz below the carrier frequency, and a little bit of carrier leaking through.

This is a DSB-SC signal.... Double sideband, suppressed carrier.

It now needs to go to a narrow crystal filter to remove the unwanted sideband to turn it into a single sideband signal, then it needs to pass through stages of linear amplification on the way to the antenna.

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Old 23rd May 2020, 9:42 am   #56
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Default Re: Superstar 6900N V6 - Fault with SSB mode

RW / David, would appreciate you bringing your powers to bear on how the balanced modulator circuit works. Audio in is to 'Signal In' (Pin 5), Carrier in is to 'Car In' (Pin 7), balanced mod out is to OUT 2 (Pin 3) and on to Q520. Looking across to the right, there is a signal BFO_FRQ coming in from the right and I think that is a PWM signal coming from the processor.

That PWM waveform is converted to its DC voltage equivalent by filter / averager U510D and the DC voltage is then used to shift the frequency of the 10.6975MHz 'BFO' oscillator by applying it to the varicap D507.

In SSB TX mode I think the purpose of this will be to shift the carrier frequency slightly up or down so that only one chosen sideband (the upper or the lower) slides through the SSB IF filter.

Q505 / Q516 both look as though they are designed to intentionally offset the carrier null adjustment set by W502, so that the modulator does produce a carrier output in CW mode for example.

Datasheet for the balanced modulator IC:

https://www.mouser.com/datasheet/2/2...94_E-62933.pdf

I've just noticed it is also a demodulator, or at least can be used for that purpose.

Edit: (cross posted with above) ...David is obviously a mind reader....

The issue at the moment is that crackle doesn't seem to have his DSB signal on Q520 collector, hence asking him now to look for the audio input going to SIG IN.

Last edited by SiriusHardware; 23rd May 2020 at 9:50 am.
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Old 23rd May 2020, 9:57 am   #57
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Chances are if it's drawing current with no audio on SSB (or AM) try the 2SB688 voltage regulator hasn't gone dead short supplying 13.8v directly to the MOSFETs.

Common fault is 20W is too much for these radios due to their poor design, MOSFET finals and lack of cooling and heat dissipation for the regulator, sooner or later something gives.
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Old 23rd May 2020, 10:11 am   #58
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Default Re: Superstar 6900N V6 - Fault with SSB mode

crackle, sorry but I don't think I made this clear: When looking at Q520 collector for the signal-out from the balanced modulator you should (of course) be in either USB or LSB TX mode.
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Old 23rd May 2020, 10:42 am   #59
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Default Re: Superstar 6900N V6 - Fault with SSB mode

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Chances are if it's drawing current with no audio on SSB (or AM) try the 2SB688 voltage regulator hasn't gone dead short supplying 13.8v directly to the MOSFETs.
This is a V6 (not V3) chassis so the power regulator / modulator transistor is a TIP36 rather than a 2SB. Although they certainly are a common source of problems, the radio in this case is still working in AM and FM TX modes, in which that transistor also controls the supply voltage to the transmitter. See all earlier posts in the thread.
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Old 23rd May 2020, 10:45 am   #60
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Default Re: Superstar 6900N V6 - Fault with SSB mode

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crackle, sorry but I don't think I made this clear: When looking at Q520 collector for the signal-out from the balanced modulator you should (of course) be in either USB or LSB TX mode.
Yes I assumed that.

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