UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Amateur and Military Radio

Notices

Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

Reply
 
Thread Tools
Old 23rd May 2020, 11:10 am   #61
crackle
Dekatron
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 3,547
Default Re: Superstar 6900N V6 - Fault with SSB mode

Quote:
Originally Posted by SiriusHardware View Post
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.
I cant detect any audio on the scope when I key in SSB mode.

Mike
__________________
BVWS Member, M7MJE
The KB Museum

My collection in the Radio Museum
crackle is offline   Reply With Quote
Old 23rd May 2020, 12:07 pm   #62
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 4,238
Default Re: Superstar 6900N V6 - Fault with SSB mode

Looking at area D1-D2 (sheet 3) now, take your scope first to Q502 collector and if you don't have audio there in SSB TX mode, look for it on pin 7 of U500, NJM4558 IC.
SiriusHardware is offline   Reply With Quote
Old 23rd May 2020, 1:34 pm   #63
crackle
Dekatron
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 3,547
Default Re: Superstar 6900N V6 - Fault with SSB mode

Hi
I cant see Q502 on the layout drawing.
Can you please give me a clue.

thanks
Mike
__________________
BVWS Member, M7MJE
The KB Museum

My collection in the Radio Museum
crackle is offline   Reply With Quote
Old 23rd May 2020, 1:40 pm   #64
Andrew B
Hexode
 
Andrew B's Avatar
 
Join Date: May 2015
Location: Bradford, West Yorkshire, UK.
Posts: 398
Default Re: Superstar 6900N V6 - Fault with SSB mode

Where is the circuit diagram located?
__________________
Concerned regarding the lack of migrating electrons.

Andrew now M0IIQ
Andrew B is offline   Reply With Quote
Old 23rd May 2020, 2:20 pm   #65
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 4,238
Default Re: Superstar 6900N V6 - Fault with SSB mode

Quote:
Originally Posted by crackle View Post
Hi
I cant see Q502 on the layout drawing.
Me neither at the moment! It's a shame that 'find' trick doesn't work on the layout the way it works on the circuit diagram.

Don't worry about it for now, just go to U500 pin 7 and see if you have audio on that pin in:

-SSB TX mode
-AM TX mode
-FM TX mode

Back later.
SiriusHardware is offline   Reply With Quote
Old 23rd May 2020, 3:39 pm   #66
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 14,556
Default Re: Superstar 6900N V6 - Fault with SSB mode

For a filter method SSB generator there are two frequency schemes which can be used.

If you're feeling troublesomely rich, you can have a carrier oscillator, feed it into a balanced modulator and get a DSB signal centred on the carrier frequency (though there is little leakage of the carrier itself through to the output, then you switch it through one or other of two crystal filters offset from the carrier. One filter will pass USB, the other would pass LSB.

This is done in some expensive receivers (Racal RA1772, Marconi H2900 etc) where they sometimes want to receive both sidebands simultaneously and independently. But mostly two crystal filters costs too much.

Individual crystals are cheaper, so more usually a single filter is used and the carrier is switched to an offset on one side or to the other. So the DSB signal from the balanced mod moves so the appropriate sideband falls into the filter bandwidth.

Is that PWM from the processor filtered to make a cheap DAC used to pull the crystal using the varactor diode?

If so it can be set to three offsets, USB, LSB and something that gives a comfortable beat note for A1A morse reception.

And, yes, I know what you mean about inadequate heat sinking and aggrandised power claims.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now   Reply With Quote
Old 23rd May 2020, 3:43 pm   #67
crackle
Dekatron
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 3,547
Default Re: Superstar 6900N V6 - Fault with SSB mode

Hi
Lots of audio on FM & AM but none on USB or LSB.

Mike
__________________
BVWS Member, M7MJE
The KB Museum

My collection in the Radio Museum
crackle is offline   Reply With Quote
Old 23rd May 2020, 4:16 pm   #68
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 4,238
Default Re: Superstar 6900N V6 - Fault with SSB mode

David yes, if that's PWM going into U510D the mark-space ratio of the constant digital stream going in on BFO_FRQ determines the DC voltage which appears on the output of U510D, or it may be that the processor itself outputs variable DC voltage from a DAC and the filter components around U510D are just there to knock out any little bumps and spikes on the output from the DAC. We can ask crackle to scope that in due course, to find out which.

Either way, the processor provides a variable DC voltage via U510D to D507 (varicap) and uses that to pull the frequency of that oscillator up and down, shifting one sideband or other into the pass band of the narrow filter.

Something is muting the microphone audio in USB / LSB mode. What is it, and why? And what does it have to do with the mysterious transmitter fault which only presents on SSB modes?

Some more voltage measurements now, all in SSB TX mode.

Base of Q512
Base of Q513
Base of Q514
Base of Q515

I would expect that you will see either near enough 0V, or about 0.6V to 0.7V on one or more of these points.
SiriusHardware is offline   Reply With Quote
Old 23rd May 2020, 5:18 pm   #69
crackle
Dekatron
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 3,547
Default Re: Superstar 6900N V6 - Fault with SSB mode

Hi
Tests done in USB TX mode.
Base of Q512 = 0v
Base of Q513 = 0.74v
Base of Q514 = 0.69v
Base of Q515 = 0v

Mike
__________________
BVWS Member, M7MJE
The KB Museum

My collection in the Radio Museum
crackle is offline   Reply With Quote
Old 23rd May 2020, 5:28 pm   #70
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 4,238
Default Re: Superstar 6900N V6 - Fault with SSB mode

As you can probably see, that means the SSB ALC transistor Q513 and the 'Mic Mute' transistor Q514 are both turned on for reasons unknown. Let me think about that for a while, anyone else, feel free to pitch in with theories.
SiriusHardware is offline   Reply With Quote
Old 23rd May 2020, 6:08 pm   #71
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 4,238
Default Re: Superstar 6900N V6 - Fault with SSB mode

The Q12 / Q48 chain which generates the SSB ALC signal may be getting upset by the absence of the +V supply to the output stage, as the emitter of Q12 is normally held at a voltage determined by the resistor divider formed by [R66 / R66/ W6] and R67, which normally has +Ve supply on the other end of it. At the moment there is nothing on the other end of it because the +Ve power to the output stage is disconnected.

Can you reinstate Link L10 (power to output MOSFETS), remove links J3 and J4 and for the time being leave L11 (power to RF driver MOSFET) out.

With those changes made, measure the voltage on Q513 and Q514 bases in SSB TX mode again - if you see the same heavy current draw again, then of course desist. Otherwise, see if those transistors are still both turned on.
SiriusHardware is offline   Reply With Quote
Old 23rd May 2020, 6:16 pm   #72
Mike. Watterson
Heptode
 
Join Date: Dec 2014
Location: Limerick, Ireland.
Posts: 897
Default Re: Superstar 6900N V6 - Fault with SSB mode

Quote:
Originally Posted by SiriusHardware View Post
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.
And it should have nothing when there is no audio. DSBSC when there is audio and a filter removes the unwanted sideband. Some Double Balanced Modulator ICs have internal DC offsets, which cause carrier and modulation at a low level. Often can be corrected with an external pot. I've used a Double Balanced Modulator IC as an AM modulator by deliberately unbalancing the input at DC. I forget the part, the one with balanced in, balanced out and optional onboard oscillator or single ended input for the Local Oscillator.

There is always second double balanced mixer mixing the RF VCO with the "IF", then a filter, then the output.

If it's 20W FM max, then it's 5W on AM, max, so taking 10W, I think on the pair of parallel FETs (you indeed can't do that with bipolar, but does work with FETs as the hotter one would then need more bias).
So I'd put an identical FET on a heatsink and 6V supply. Adjust gate voltage for 500mA current from supply, as two would be 1A (6W). Class A AM is 50%, so that would be about 3W AM out. Most CBs using series transistor in supply for AM, put that near full on for FM and SSB, that's 12V, so twice current and x4 power on the FETs as the enhancement MOSFETs in linear mode are like resistors.

If the bias on the gates is much higher than the bench test, then any bias adjustment is suspect.

I've seen faulty SSB ALC cause 200W military amps to fail. The output transistors are expensive!
Mike. Watterson is offline   Reply With Quote
Old 23rd May 2020, 6:18 pm   #73
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 14,556
Default Re: Superstar 6900N V6 - Fault with SSB mode

I agree, Q514 should kill any signal from the microphone, and the line controlling it is called MIC MUTE.

Q513 will also be killing it

MICIN seems to come from pin15 U904 a multiple DAC. THese things are often used as gain controls and there are 8 of them in this beastie.

Pin 15 is the output of multiplying DAC number 6. its input, pin 16 comes from the output of DAC number 5 (pin 14) and the input of DAC5 goes into pin 13.

Now that comes from a BL1551 part, which is a CMOS analogue switch which appears to select either an echoed or a not echoed version of the signal from the microphone itself. OK, this is definitely a CB set!

THe first microphone stage U201A has agc around it by means of D900 D901 Q900 Q901 done in PNP and NPN forms to compress both + and - signal swings.

OK that's the mike signal traced back to the mike itself. Back to the area around Q514 etc.

The microphone signal enters U501A whose gain is switched by Q518 (on = more gain) onwards to U501B with the circuit reading unhelpfully right to left!
U500A and B do some filtering and SSB mod goes out via R508 to the balanced modulator U503. Q502 is there to gate off this signal feed whenever FA8V is high. It's worth checking this.

One possibility is that there is a fault in the mode control and power supplies area. I wonder if the set isn't really going into SSB mode, but is leaving some control lines in the state that kills parts of the SSB signal path.


I would take a little while to have a look at Q522 Q526 Q527 Q528 Q529 Q530 and the supply and control lines they switch

CW8V, FM8V, AM8V, UL8V, FM MOD EN, AM MOD EN, and BFO B+

Build a table of these voltages for each mode the radio can be set to.

Let's check that things really are going into the modes we expect. Sorting the whole lot might save a lot of confusion.

The set's got a fault on it. It's nice to guess what it could be then test that guess but guesses involve a lot of assumptions and assumptions are how you lead yourself astray or round and round in circles. So let's make no assumptions. They are too dangerous when you're looking for the unknown.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now   Reply With Quote
Old 23rd May 2020, 6:27 pm   #74
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 4,238
Default Re: Superstar 6900N V6 - Fault with SSB mode

Quote:
Originally Posted by Radio Wrangler View Post
CW8V, FM8V, AM8V, UL8V, FM MOD EN, AM MOD EN, and BFO B+
Build a table of these voltages for each mode the radio can be set to.
Posts #32 and #35-#37
SiriusHardware is offline   Reply With Quote
Old 23rd May 2020, 7:30 pm   #75
crackle
Dekatron
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 3,547
Default Re: Superstar 6900N V6 - Fault with SSB mode

Sorry
I made a mistake when I wrote up the results here. Q514 & Q515 were transposed.
Does that make any difference.
Shall I still do the test again with L10 refitted and J3 & J4 removed

Results should have been
Tests done in USB TX mode.
Base of Q512 = 0v
Base of Q513 = 0.74v
Base of Q515 = 0.69v
Base of Q514 = 0v

Mike
__________________
BVWS Member, M7MJE
The KB Museum

My collection in the Radio Museum
crackle is offline   Reply With Quote
Old 23rd May 2020, 8:04 pm   #76
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 4,238
Default Re: Superstar 6900N V6 - Fault with SSB mode

That means that MIC MUTE is not asserted by the microprocessor which is a relief as it was difficult to imagine why it would be.

Instead it is the BP MUTE transistor which is enabled and this will not cut off the microphone audio as it is clearly intended to kill the TONE (roger beep? 'BP'?) output from the microprocessor.

So now we can say that it is the ALC MUTE transistor Q513 which is keeping the microphone audio muted in SSB mode, therefore yes please, go ahead as outlined in #71 and see if Q513 is still turned on in SSB TX with those alterations made.

Mike W, just working to restore SSB RF to the input of the transmitter at the moment. We will eventually get back to the transmitter. If you've read the thread through you will know that crackle has owned this from new and has not altered any factory set adjustments.

It is possible that with new output devices fitted there was a need to readjust the bias, but as we do not have the service procedure or manual, knowing what to set it to would be a bit of a guess. I prefer to leave things exactly as factory set / undisturbed until there is absolutely no alternative.
SiriusHardware is offline   Reply With Quote
Old 23rd May 2020, 8:40 pm   #77
crackle
Dekatron
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 3,547
Default Re: Superstar 6900N V6 - Fault with SSB mode

Ok that has made a difference.
Base of Q512 = 0v
Base of Q513 = 0.035v
Base of Q515 = 0.692v
Base of Q514 = 0.002v
I had the meter set to lower volt range.

thanks
Mike
__________________
BVWS Member, M7MJE
The KB Museum

My collection in the Radio Museum
crackle is offline   Reply With Quote
Old 23rd May 2020, 9:13 pm   #78
crackle
Dekatron
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 3,547
Default Re: Superstar 6900N V6 - Fault with SSB mode

Quote:
Originally Posted by SiriusHardware View Post
It is possible that with new output devices fitted there was a need to readjust the bias, but as we do not have the service procedure or manual, knowing what to set it to would be a bit of a guess. I prefer to leave things exactly as factory set / undisturbed until there is absolutely no alternative.
Here is a bit more background to the fault.
I suspect that this issue with SSB running away when keyed even before any audio was present in the mic has been happening before, but cant be certain.

The first time the finals blew I noticed by the fact that the OP power had dropped to just 3 or 4 watts on FM and AM. I only replaced the one mosfet that time, I think it was one of the OP pair.
The second time I had to replace both the OP mosfets, again the symptoms were loss of power.

The last time I changed them was as a result of what appeared to be a short in the radio. The current was so high and L27 (ferrite link) got so hot it melted its solder connections and dropped out.

Following replacing the link and fitting the new Vishay IRF520 devices the radio seemed to be working very well. (in fact slightly better than before on FM, power wise)
It wasn't till I tried some SSB contacts over a week later that I noticed the PSU display was reading in excess of 11amps. This was also causing the radio to alarm DCLO indicating the voltage was lower than the set alarm value. During the alarm conditions the OP is normally cut (the same happens if the radio detects a high SWR) so I dont know if there would have been any RF on the OP or not.
But in this instance the supply current was still showing as 11 amps.
The low voltage was purely internal in the radio as the PSU was still reading 12.29v, which was normal for it as it normally gives 12.3v. It could have been volt drop over the radios supply cable and internal losses in the radio.

This is only guess work, but the original IRF520 devices that were fitted were unbranded and unmarked and may have been less able to survive the heavy current, the same may possibly be said for the replacements which I bought off ebay.
I finally decided to go for some branded types that were sold as Vishay and obtained from Farnell.

Mike
__________________
BVWS Member, M7MJE
The KB Museum

My collection in the Radio Museum
crackle is offline   Reply With Quote
Old 23rd May 2020, 9:20 pm   #79
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 4,238
Default Re: Superstar 6900N V6 - Fault with SSB mode

Thanks for the background info - we can never have too much information about the case history.

Now if you revisit U500 pin 7 (Output of the microphone amplifier strip) and C539 (audio input to the balanced modulator circuit), you should be able to scope audio there even in SSB TX mode.

If you go back to Q520 collector in the balanced modulator circuit and then the collectors of Q51 and Q49 in the early transmitter stages, when you key the microphone and speak you should now be able to see double-sideband, suppressed carrier RF, or 'a mess' as described by Radio Wrangler earlier.

When looking at the signals on Q51 and Q49 and anywhere else in the transmitter I would suggest, as someone said earlier, switching to the X10 position on your scope probe if it has one, and turning up the volts / div to compensate for the loss of signal size / height.

In the X10 position the scope probe places less load / has less effect on the circuit being measured than it does on X1 - the penalty is that the signal reaching the scope is then only a tenth of what it was, so that's why you need to turn the volts / div up two or three clicks.
SiriusHardware is offline   Reply With Quote
Old 23rd May 2020, 10:19 pm   #80
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 4,238
Default Re: Superstar 6900N V6 - Fault with SSB mode

Mike W, regarding your suggestions about bias in #72: If, and only if, we now have a DSB-SC signal barking at the back door of the transmitter, I propose first to reinstate the power link (L11) to the driver MOSFET but leave J3 and J4 removed to keep the output MOSFETs turned off.

If the radio keys up in SSB mode and still does not guzzle current, what I am then considering trying is removing link L10 again - breaking the power feed to the output MOSFETS - and inserting an ammeter. Then, we fit only one bias-enable link at a time, J3 then J4, to see if the excess current being drawn in SSB TX mode is being drawn primarily by only one of the two MOSFETs or whether both are drawing roughly the same amount.

At a rough guess, what sort of current would you suggest each MOSFET should be drawing under no-RF-input signal conditions (ie, no voice input applied) in SSB TX mode? (Vsupply = ~13V).
SiriusHardware is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 10:55 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Copyright ©2002 - 2020, Paul Stenning.