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Old 22nd May 2020, 12:05 am   #21
Mike. Watterson
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Normally the Series transistor only varies the supply for AM, it's the AM Modulator and the FETs should be biased slightly on.
FM uses Class C. Normal to have the series transistor fully on and no bias or almost no bias.
The SSB usually needs the Series transistor also fully on like FM, but needs some bias on the FETs, to have them in linear mode. That's about how all these kind of rigs have always worked, except the oldest used NPN output transistors.

So
1) Make user the SSB modulator has no carrier with no audio.
2) Use an RF Decoupled probe to see bias voltage on the output transistors when their supply is disconnected on AM, SSB and FM modes.

At least it's not likely to use expensive RF transistors like a Yaesu, Kenwood, Icom etc multimode 50W rig. Probably cheaper than FT817ND finals, I'm glad I don't have an original FT817 or early FT817ND.
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Old 22nd May 2020, 12:52 am   #22
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Mike, as far as I can see from the diagram there is no change at all to the bias between AM, FM and SSB modes, the amp is operated in 'fully linear' mode for all three modes by the look of it.

In some more historic designs the modulator transistor supplies (unmodulated) half-supply voltage to the finals in FM mode as well, so the AM and FM output power is roughly the same.

However in this design the situation looks to be as you say, with the finals running on near full supply voltage in both FM and SSB modes.

The driver and twin PA output devices are all IRF520 MOSFETs, still readily available from mainstream suppliers.
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Old 22nd May 2020, 6:16 am   #23
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Default Re: Superstar 6900N V6 - Fault with SSB mode

I got the scope out of hibernation and connected it up.
I dont pretend to understand the results, neither do I understand how the different modes are obtained or controlled in this circuit, but here goes.
For these tests the scope ground was on the chassis and the probe was on the collector (centre pin) of Q51
I switched the scope from AC to DC during the tests to see if there was any RF and or DC bias, if that makes any sense. I took a short video of the trace when keying in FM and SSB modes, click link.
FM - (ac) 1 volt, (dc) 8v trace
AM - (ac) 1 volt, (dc) 8v
SSB - (ac) 0 volt, (dc) 8v trace
There was no perceivable difference to the trace when I spoke into the mic, (not done in the video but tested later.)

If I have done these wrong then please let me know or if you need more tests done, please give me clear instruction as I am not familiar with this part of the radio circuitry.

Mike

Last edited by crackle; 22nd May 2020 at 6:22 am.
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Old 22nd May 2020, 8:04 am   #24
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Default Re: Superstar 6900N V6 - Fault with SSB mode

I forgot to mention that with the power to the OP and driver mosfets removed (L10 & L11) there was no appreciable increase in current when the radio was keyed in any of the modes.

Something I have just thought of, when the last lot of driver and finals blew and the L27 link melted its solder and dropped out, could it have been damaged. It looks like a simple link through a ferrite tube, could it be more.
I suspect there was an intermittent problem/fault in the circuitry already which may have accounted for the frequent times the finals were damaged.

I would send this radio back to the dealer if it had not already been repaired by myself the first time the finals blew. (I was told the transistors were not covered, and the likely cost of postage back and forth & repair if not done under warrantee would have amounted to nearly the cost of the radio.) The radio did work on all modes OK after replacing the finals the first time, and also after the second time. But I suspect on the last (3rd time on the 5th May) I changed all 3 of the finals, I may not have used the radio in SSB mode, hence only realising the current problem after 2 weeks.

Mike
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Old 22nd May 2020, 9:07 am   #25
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Try those scope meaurements again, but keep the scope input on 'AC' and set the volts/division to 0.5V/div.

You don't need to post video again, but what you are looking for is:

On FM TX, a steady-height 'band' of signal, much as you saw the first time, but bigger. (Higher).

On SSB TX, a signal which only appears when you speak into the microphone.

If the radio has a roger-beep, turn it on. In SSB mode, you should see a burst of steady carrier the same length as the roger-beep when you unkey.
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Old 22nd May 2020, 12:57 pm   #26
Mike. Watterson
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Usually there are 2 "finals" in parallel and those are driven by a driver, as the gate capacitance is high.

You trying to look at 28MHz with a 15MHz scope? I guess though because it's an analogue scope it will show the RF, but just at a much lower level than reality. I could "see" 144MHz on my 20MHz ancient Tektronix scope, though not really on my current Hameg 20 MHz scope. However I do have an HP141T analyser with a 100MHz, 1200MHz and 20 GHz plugins. Setting the input attenuator is important!
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Old 22nd May 2020, 1:16 pm   #27
crackle
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Hi
I have done that test again.
On both FM & AM mode there is a strong RF signal of 0.37v which is offset from 0 (+.23v and -.14v)
On USB mode there is no apparent signal, the line just goes slightly out of focus.
Both tests done with audio in mic and roger beep on.

Mike
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Old 22nd May 2020, 3:15 pm   #28
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Hmm, that's strange. I'll take another look this evening.

Mike W, yes we're using the only scope Mike (crackle) has, to look at the relative size of the RF envelope in various modes, rather than the absolute size in volts P-P. Sometimes you just have to work with what's available.
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Old 22nd May 2020, 5:35 pm   #29
crackle
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Default Re: Superstar 6900N V6 - Fault with SSB mode

I have been going over the circuit board again with a fine tooth comb and a magnifying glass.
Q17 and Q18 look different to all the other SMD transistors, and the method of soldering also looks different, more hand made if you see what I mean.
Does that bottom left pin on Q17 look like a dry joint?
Click image for larger version

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I cant see them on the schematic, what are they for?
Mike
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Old 22nd May 2020, 5:57 pm   #30
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Default Re: Superstar 6900N V6 - Fault with SSB mode

I'm afraid that looks like what you get when you make something with lead-free solder.

If you give the suspect leg a gentle sideways shove with the end of a toothpick and it moves, there's your answer. If you have fine enough solder tools for SMD working it will do no harm to reflow that joint anyway.
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Old 22nd May 2020, 6:11 pm   #31
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Default Re: Superstar 6900N V6 - Fault with SSB mode

I don't have schematics so I can't be very specific, but some thoughts:

For AM and FM, running the outputs in class-C would give the most bang per buck so full supply voltage for FM and audio modulated (via transformer) full supply for AM might be expected.

SSB needs linear amplification, so the bias arrangements for the output devices would have to be switched to put them in class B. Else distortion would be truly horrible.

If less bang per buck was chosen to get things cheaper, the PA stage could be kept in linear operation and low-level AM modulation done, OR the amplitude modulation could be done by modulating the gate bias of the PA devices this would also involve switching bias arrangements with mode.

So there is the possibility of something wrong in the bias arrangements or the switching thereof. For SSB, expect there also to be thermal compensation issues. The main PA devices are likely the only things which can pull much current without an instant smoke column marking the funeral pyre. 11A is a lot.

Another thing which could eat walloping amounts of current would be something bursting into spurious oscillation.

I don't suppose many people have had to design a real AM transmitter in the 21st century, and put it into a full production type-approved radio, but there were two of mine in the RAF BBMF spit and hurricane that did the flypast for Captain Tom's birthday a couple of weeks ago.

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Old 22nd May 2020, 6:39 pm   #32
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Since you don't appear to have an SSB RF TX signal, maybe it's time to do some more fundamental checks further back in the mode switching area. This will take us away from the transmitter for a while but it may uncover something which turns out to be the cause of all the problems.

If you look at any 'sheet' of the circuit diagram, you'll see that there are alphabetic co-ordinates down the sides and numeric co-ordinates running across the top and bottom edges of the sheet. These provide a handy way to refer to a particular area of the circuit.

Look at areas B1, B2, B3 of the third sheet of five. Here you have the 8V regulator U506 and transistors Q526 through to Q530. These transistors control the supply from the 8V regulator to various parts of the circuit at various times.

The first job is to check that the 8V regulator U506 is actually producing 8V and not some other voltage. Probably the regulator itself will be the easiest place to make that measurement. the output should be 8V all the time regardless of mode or TX or RX state.

Then, Following the rails out from the collectors of these transistors we can see that they have the following functions:

Q530 - '8T' - supplies 8V in any transmit mode. For each mode in turn, check that the '8T' line (Q530 collector) is at 8V when the transmit key is held in.

Q529 - '8R' - supplies 8V in any receive mode. For each mode in turn, check that the '8R' line (Q529 collector) is at 8V when in receive mode.

Q528 - 'FM8V' - supplies 8V only in FM mode. Check that this line (Q528 collector) only has 8V on it when in FM RX or TX mode, not in any other mode.

Q527 - 'AM8V' - supplies 8V only in AM mode. check that this line (Q527 collector) only has 8V on it when in AM RX or TX mode, not in any other mode.

Q526 - 'UL8V' - supplies 8V only in USB and LSB mode. Check that this line (Q526 collector) only has 8V on it when in USB or LSB mode, RX or TX, not in any other mode.

I'll leave you with those checks for now, we can follow the supplies further as they spilt up again into SSB TX only 8V supply / FM TX only 8V supply and so on, but we'll take it just a few steps at a time.
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Old 22nd May 2020, 6:54 pm   #33
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Default Re: Superstar 6900N V6 - Fault with SSB mode

OK will print out that list of checks and note the results.
Now I have got to locate those on the board

Thanks
Mike
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Old 22nd May 2020, 7:44 pm   #34
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Hi
All those tests check out OK.
The 7808 regulator is giving 7.81v in all modes,
Q530 RX=0v, TX= 7.7v all modes
Q529 TX=0.12v, RX-7.7v all modes.
Q528 FM only = 7.7v RX & TX, all other modes 0.05v
Q527 AM only = 7.6v RX & TX, all other modes 0.02v
Q526 SSB only = 7.6v RX & TX all other modes 0.02v

Mike
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Old 22nd May 2020, 8:04 pm   #35
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Default Re: Superstar 6900N V6 - Fault with SSB mode

OK, going a bit further, all in the same area:

Q522 collector = CW8V - should be 8V only in CW mode (RX and TX), no other mode.

D508 common pin = ULA8V - should be 8V only in USB, LSB, and AM modes (RX or TX)

D509 common pin = ULF8V - should be 8V only in USB, LSB, and FM modes (RX or TX)

D510 common pin = FA8V - should be 8V only in FM and AM modes, (RX or TX)

D513 common pin = BFO B+, should be +8V in any mode TX and in USB and LSB RX only.
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Old 22nd May 2020, 8:23 pm   #36
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Two more to wrap up that area.

D511 common pin = AM MOD ENABLE should be 8V in all modes and states except for AM TX mode, where it should be 0V or very low voltage.

D512 common pin = FM MOD ENABLE should be 8V in all modes and states except for FM TX mode, where it should be 0V or very low voltage.
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Old 22nd May 2020, 9:24 pm   #37
crackle
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Results from last tests

Q522 cw=7.7v am=0.05v fm=0.7v ssb=0.7v

D508 fm=0v other modes=7.1v

D509 ssb&fm cw=7v am=0v

D510 fm&am=7v ssb&cw=0v

D513 7v any mode TX, usb&lsb&CW RX=7v

D511 all correct (0.75v less in TX)

D512 all correct (0.6v less in TX)

Did you see my post #29 about Q17, I just wondered what you thought and what Q17 did.

thanks
Mike
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Old 22nd May 2020, 10:02 pm   #38
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Default Re: Superstar 6900N V6 - Fault with SSB mode

OK, having a look at it,

Q55 is a big bruiser of a PNP bipolar, in the main power feed to the output transistors and their driver in the transmitter. Q54 is used to make a Darlington pair with Q55

It looks like a basic linear series regulator.

Q43 is interesting it gets inputs from various pots and the power rails which have been switched for the different modes. Ah, this sets the power supply voltage to the PA (and driver) to set the carrier power for the different modes. Then look at Q53, then Q41 and it takes us to the AM mod line. Aha! Q41, Q53 Q54 Q55 are the amplitude modulator. It modulates the linear power supply feeding the power amp.

For SSB, the power amp will likely be given a good supply voltage and modulation will be at a very low level to make DSB which is then filtered to SSB, then amplified to drive the transmitter strip.

Now one bit I don't like: Q45 and Q46 are just connected across each other in parallel. THey are going to need to be very well matched else one will hog all the current, get most of the heat and not live long.

I wonder what feeds into J3 and J4 they seem to be in series with the bias feed to the gates of the PA transistors and the driver transistor respectively, upstream of the bias setting pots.

Job 1 is to find out which way that terrible current is going.

Fortunately we have ferrite beads we can lift temporarily to break the supply feeds in the PA area.

L27 if you lift that one (and save it carefully) should shut all power off to the PA. There should be nothing else in the radio which could take 11A other than a short.

Does it now take big current?

If so, have a look for a short of the tab of Q55 to ground. Insulation washers get damaged or forgotten.

Ceramic capacitors can fail as a pretty hard short.

David
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Old 22nd May 2020, 10:15 pm   #39
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Hi David
All of the last series of tests have been made with the links L10 & L11 so the driver and finals have no power supply.
In this condition the excessive current condition in SSB mode did not happen.
Nothing feeds into J3 & J4 they are just jumper links like you get in a PC to disconnect or enable something, in this case I believe the bias.
Mike
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Old 22nd May 2020, 10:27 pm   #40
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Taking a few of those points:

J3 and J4 carry '8T' (8V in all TX modes) to the top end of the bias networks for the output FETs. I think they are there so that the bias to each FET in turn can be disconnected, I presume this is so you can shut off one output FET while adjusting the bias for the other.

With respect to disabling the output and drivers, I think that is currently already done via the removal of L10 / L11, although it's possible the same effect could be achieved just by removing J3 and J4.

Q55 tab is unlikely to be short-circuit to GND because the radio works normally in AM and FM TX modes where Q55 is also used to control the supply voltage to the output stage. The radio has a power output control which, if used, varies the supply voltage to the output stage. This is also one of the functions of Q55, and that aspect does appear to be working (see earlier posts).

Edit: As crackle said.
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