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Old 23rd May 2020, 10:53 pm   #81
SiriusHardware
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Crackle, is there any chance that you can make yourself up a dummy microphone plug with just PTT strapped 'on' (or with a switch connected from 0V to PTT) and the MIC audio input shorted to GND?

We'll eventually need to do some tests with almost everything fully enabled, but with no SSB RF going into the transmitter.
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Old 23rd May 2020, 11:14 pm   #82
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Hello Newcastle this is Basildon, here are the results for the SS 6900n.
U500 pin 7 a nice audio type signal of around 150mV
C539 we saw about 20mV of audio.
Q520 looked "a mess" but in time with the audio at about 55mV
Q51 also looked "a mess" also in time with audio at about 200mV
Q49 another "mess" but looking like a nice and clean mess at about 250mV

I was tending to take the reading from the roger beep signal, but my voice was a little higher in volts.

I should add that the trace went to a straight line when there was no audio.

There is a 1k pot on the output from the desk mic which can be turned down to zero in effect shorting the input to the radio, and the mike can be latched on.

Thanks for your help guys we seem to be getting better results.
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Old 23rd May 2020, 11:41 pm   #83
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Default Re: Superstar 6900N V6 - Fault with SSB mode

OK that will be just about perfect actually - an easy way to lock the PTT on and to reduce input audio to zero - if it really is zero though. If it really is, that's fine. The R-beep will also have to be off, of course.

If we make progress it's usually because the OP (you, in this case) is patient and methodical enough to make lots of sometimes seemingly irrational or irrelevant measurements and report them accurately. Hopefully, if we continue in this way we will eventually get to the bottom of this, although you will appreciate that it can sometimes be difficult, occasionally even impossible to 'fix' something from 200 miles away.

As demonstrated last night, I don't think in straight lines at this time of night so I'd better continue tomorrow if that's OK - the night owls are welcome to forge ahead with you if they wish. I've outlined what I think will be the way forward from here in #80, but I welcome any other ideas.
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Old 23rd May 2020, 11:48 pm   #84
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Default Re: Superstar 6900N V6 - Fault with SSB mode

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Originally Posted by SiriusHardware View Post
Mike W,
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).
My supply is 12.3v at about 30 amps if that helps.

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Old 24th May 2020, 11:16 am   #85
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Default Re: Superstar 6900N V6 - Fault with SSB mode

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Originally Posted by SiriusHardware View Post
Quote:
Originally Posted by Bigpimp347 View Post
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.
Do you have audio on FM, no audio on AM or SSB?

Check the TIP still. An old trick on Uniden boards (what the Anytone is roughly ripped off from) was to send 13V direct to the final bias board, this gave full power on FM but no audio on AM or SSB. Also there is another smaller transistor which is a companion part to the TIP device sat in front of it, this can also fail.

Sorry for not reading all the replies, but way too many part numbers and big fancy words to try to plough through for a CB radio. If i had it sat here in the shack I'd know what to look at to test, harder trying to put it in text.
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Old 24th May 2020, 12:15 pm   #86
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Default Re: Superstar 6900N V6 - Fault with SSB mode

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Originally Posted by SiriusHardware View Post
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).
No more than about 1.2 A each for output pair at full power setting, or 500mA to 600mA each on AM full power.
The Enhancement MOSFETs or VFETS are resistive when biased linear.
AM is 50% efficiency and supply should be about 6V then from 12V PSU. I'd measure FET currents and FET bias voltages (via resistor & decoupling cap to remove RF) in AM mode. I'd lift supply end of the series supply ferrite, put 1nF ceramic cap to nearby 0V and jumper links so as to easily change from measuring driver FET and output pair.
I'm sure I have a linear RF amp built dead bug with 0V copper 2.5mm buss on a Pentium II heatsink using IRF5xx, maybe IRF520. It's popular as the input gate capacitance is only high compared to a real RF device rather than insanely high. Also very cheap compared to real RF devices. I designed on paper a PA for 138kHz. The gate capacitance isn't so much a problem.
My prototype is push pull with o/p TX binocular core transformer copied from a Racal 200W amp. The input binocular core needs a full 5W as it's a step-down phase splitter to give a 50 Ohm load and cope with the high gate capacitance. There are low value resistors to flatten the response. Getting 7MHz to 29MHz was easy, matching at 3.5MHz was poor.
I did have to use a regulated bias supply and found that changing the FETs sometimes needed bias adjusted.
I abandoned it, deciding that for good stable wide band design you'd need to sample the load currents and have a feedback loop to bias the FETs reliably, very narrow adjustment for linear operation. Also the wideband transformers at input and output are not trivial to design. The very high gate capacitance is a problem. Also I think supply voltage dependant capacitances elsewhere. I concluded there was a good reason only CBs and budget PAs use regular TO220 package power FETs. These FETs are actually multiple parallel FETs on one chip, hence high gate capacitance. Also the TO220 lead inductance, esp on the RF common is a problem. Real RF devices have massively broad leadouts to minimise inductance and two of them for common.
The other problem with TO220 is o/p to 0V capacitance of the tab to heatsink, via insulator. The real RF parts use an internal beryllium oxide wafer as I think only a diamond wafer has better heat transfer (x5 copper!) while being an insulator.
Quote:
Diamond is a good electrical insulator, having a resistivity of 100 GΩm to 1 EΩm (1011 to 1018 Ωm). Most natural blue diamonds are an exception and are semiconductors due to substitutional boron impurities replacing carbon atoms.
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Old 24th May 2020, 1:32 pm   #87
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Default Re: Superstar 6900N V6 - Fault with SSB mode

I don't have any experience of this type of CB PA using IRF MOSFETS but I can offer some general advice...

First of all, if you work on this radio with the PA MOSFETs powered up then I would strongly advise you to wear some decent wraparound safety glasses. When these FETs fail they can make quite a bang and sometimes they can spit out tiny fragments their casing at high speed.

Also, looking at the main PCB it uses a lot of 1206 package SMD parts and the PCB is very large and presumably it will be prone to a degree of flexing. So my other advice is to always make sure the lower side of the PCB is supported when you press on the top (SMD) side to do any rework. Otherwise, even a tiny amount of PCB flexing can crack 1206 package SMD capacitors. This might not cause an obvious failure in the short term but it might cause unreliability over time. It looks like there are a couple of block supports under the PCB and maybe it will be enough to simply make sure the lower cover is in place.

The other thing is that many MOSFETs fail with some degree of gate to channel leakage. This might be enough to turn the device on at high current and this effect will get worse with increasing drain voltage and this kind of fits your symptoms. However, I don't know if these IRF parts fail like this.
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Old 24th May 2020, 1:39 pm   #88
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Note to self:
Do not test bais power FETs with long leads and no decoupling and RF chokes at the pins! Even on a giant heatsink.

I have only IRF510 not IRF520, but principle is the same. These are designed for switching stuff on/off, or low voltage SMPSUs. Not simple to use linear.

I set PSU to 6V and current limit to 1A

Practically off to 2.5V
0.1A @ 2.5
0.1A @ 4.75
Between 5V and 5.1V, oscillates, harmonics interfere with nearby VHF radio
>3A @ 5.3V. First PSU current limiter dies.

PSU voltage regulator now dead, +32V out.
The FET is now short on all pins.

I'd a memory before that the bias on these was super critical for linear operation and needed changed for different FETs. The FET will be most unstable with no carrier (SSB, no Audio). With FM and AM, it's being driven off on every cycle.

I'd suspect you MUST realign the bias pots at the driver and output for whatever the quiescent current is supposed to be on AM, which can be calculated given TIP36 o/p voltages for low, med and high RF power, assuming a 50% efficiency and the stated AM power that those settings is supposed to have, must be about 4W to 5W AM RF at 6V approx (12V supply). Thus current per o/p FET must be about 750mA max (maybe 500mA to 1A is OK?). The driver FET can't be much less as it has to be in the linear region and the gain per FET stage must be poor due to the gate capacitance.

I'd imagine the service instructions for bias for replacing the IRF520 FETs (single or dual FET output) on ANY CB rig with AM will be the same. Basic maths of the standing current needed at 6V for AM for about 2W to 2.5W per device.

Well, I learnt something about ill advised use of 20cm test leads on a FET without decoupling at the pins and series chokes on supplies near the pins.

Now I need to repair my PSU's regulator. I may add an RF filter between the o/p +connector and the PCB. I already added reverse voltage protection diodes on all three outputs when I had to replace the +12V and +5V fixed regulator outputs. I've an IC regulator that does up to 40V in 32V out and 5A, so if I can't fix it, I'll fit that. I think there is a relay to use a lower input voltage at lower outputs to reduce cooling issues.
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Old 24th May 2020, 3:06 pm   #89
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Default Re: Superstar 6900N V6 - Fault with SSB mode

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There is a 1k pot on the output from the desk mic which can be turned down to zero in effect shorting the input to the radio, and the mike can be latched on.
I can't really add anything to the excellent and logical fault finding advice that's already being given. However, it's interesting that the circuit area being investigated has moved to the audio input and microphone area of the radio and it's always been said that the these particular radios should only ever be used with their original stock microphone, otherwise serious damage can be caused. I've never been able to see the logic in this statement or been able to see how connecting a non-standard mic can literally "blow the rig up", (as the common statement you hear goes). As it happens, I was on 'air' with a small group late last night and this model radio was discussed. One of the chaps who I've actually known for probably 40 odd years and who has recently retired, has spent some of his pension money on two of these radios. I think one is the Anytone version which he was using last night and the other is a 9900, which he says he hasn't yet used and is still in the box it came in. He has reasonably good electronic knowledge and has always been a bit of a 'homebrewer' building up stuff etc. He was insistent that using a non standard microphone would be the reason for the rigs failure, but couldn't offer a logical reason other than a voltage being fed back to somewhere it shouldn't. I said that this tended to apply to some of the older sets with an extra 'live' pin in the mic socket and I have actually come across radios where a section of print has been blown O/C by the miss-connection of a microphone, but I can't see how this could apply to the apparent damage to the particular radio in question on this thread.

Those that remember the Anytone 5555 that I repaired sometime last year, will remember that the owner of the radio told me that a friend of his had borrowed it and that he was talking to him on it when the chap said that he was just going to swap the original mic over for a power mic that he had to hand. The radio's owner said whatever you do don't plug that non-original mic into the radio or you'll blow it up, but he was too late and the chap had already plugged the other mic into the set and that was the last heard of his friend on air that night. The next day a phone call between him and his friend confirmed that the radio had failed when the power mic was connected, it was then that I was asked to look at the radio for a possible repair. Regardless of what I tried to explain to the owner of that radio, he was absolutely insistent that the cause of the radios failure was the fitting of the aftermarket microphone by his pal. When I had repaired the radio, I actually found that the stock microphone supplied with the radio was completely unusable, so had to dismantle it to clean up the switch contacts within to get it to work properly, so I can now understand perhaps why the chap wanted to try another mic on the radio. The original mics supplied with these sets do seem to suffer with bad PTT contacts, as I've heard a few on air that have been cutting in and out - probably why people want to replace the original mic with a supposed better one. But universally the advice is "don't do it - you'll blow it up", but with no real reason given.

Perhaps there really is some quirk of these radios and some obscure reason why connecting a non-original mic apparently causes damage, or is it just a myth, as I have to confess I've always thought it was? Stranger things have happened, as they say!
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Old 24th May 2020, 4:27 pm   #90
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Default Re: Superstar 6900N V6 - Fault with SSB mode

My comment about the mic being able to be latched on was not for the standard mic that comes with this radio. It is a base mic that I have adapted.

You are warned in the instruction book which comes with the radio, if you are using another mic, other than the original, to NOT have any connection to pin 4.

I believe the mic wiring for the BR9000 may be the same see diagram below for the circuit for the fist mic.

But I dont wish to hijack this thread about other issues with the 6900n v6

Mike
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Old 24th May 2020, 6:21 pm   #91
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Thanks for all the useful contributions since I last looked in. Just spent a little bit of time reading through. Thanks especially to Mike W for those detailed thoughts on the likely standing current we should expect through the output MOSFETs.

So where were we: Power to finals connected, power to driver not connected, bias supply links J3 and J4 out?

I'm assuming crackle's super PSU must have an ammeter built in, so we'll make the initial observations just using that.

Crackle, leave the power link (L11) for the driver MOSFET out for now.

If, during the following observations, the radio draws anything like 11A, unkey immediately and if it keeps on drawing heavy current after you unkey, slam the power straight off at the PSU.

Can you reinstate bias supply link J3 ONLY, key the radio in SSB TX with zero audio input, roger beep off, and observe and record the current drawn in SSB TX mode?

Then, remove J3, fit J4, and repeat the above measurement.
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Old 24th May 2020, 6:23 pm   #92
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Default Re: Superstar 6900N V6 - Fault with SSB mode

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But I dont wish to hijack this thread about other issues with the 6900n v6
You're completely missing the point.

I'm suggesting that there could possibly be a remote and obscure connection with using the non-stock mic and the fault....Yes, I agree that it sounds absolutely ridiculous, but I'll repeat again 'stranger things have happened' (and all that).
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Old 24th May 2020, 6:37 pm   #93
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Anyway, ignore anything to do with the mic for the time being and carry on with the advice/instructions given in post #91 and let's see where it goes from there. I knew nothing about these radios until I had my hands on one for the first time last year. I'm now learning a little bit more!
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Old 24th May 2020, 6:46 pm   #94
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Repaired the PSU, that's a separate story.

From spending some time trying to make a cheap wideband HF power amp, when I realised CB stuff was using IRF510, about 12+ years ago and looking at my notes, I can't see how you can change the FETs without readjusting the gate bias. It's also a fact that MOSFET PAs with incorrect bias are more unstable with SSB, or CW via drive on/off rather than Supply keying. FM is the most forgiving of bias (because you don't really need much, or even any in Class C). AM bias affects distortion, but the always on carrier means the FETs are less likely to self oscillate.

It's unlikely to be a Mic problem. Since all the switched 8V supplies and TIP36 modulator / PA power regulator via supply volts are OK, then instability due to bias is a suspect, assuming the SSB modulator and final modulators are OK, and it sounds like they are.

The PSU uses 2x 2N3055 parallel as the series regulator. The self oscillating IRF510 with 6V supply and 1A currrent limit destroyed them and I think the dual op-amp. It may have been the op-amp that died first.
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Old 24th May 2020, 6:54 pm   #95
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Default Re: Superstar 6900N V6 - Fault with SSB mode

I think any and all observations regarding this chassis are interesting and will eventually add up to a useful repository of information about the set.

Pin 4 on the microphone socket goes to an ADC input (AN7, pin 50 of the microprocessor) which measures any resistance which appears between pin 4 and ground.

If it sees 47K to ground, that's the 'Up' button pressed, if it sees 100K, that's 'Dn' and if it sees 150K that's 'SQ'. Between pin 4 and the microprocessor ADC input there is a 1K resistor (R608) which should limit the current into or out of pin 4 if it is inadvertently connected to something it should not be.

Anyway, at the moment we are still chasing that transmitter problem.
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Old 24th May 2020, 7:04 pm   #96
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Default Re: Superstar 6900N V6 - Fault with SSB mode

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instability due to bias is a suspect, assuming the SSB modulator and final modulators are OK, and it sounds like they are.
I'm also thinking along this line as to being a likely cause, but we'll see.
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Old 24th May 2020, 7:04 pm   #97
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Default Re: Superstar 6900N V6 - Fault with SSB mode

Something I forgot to mention, because I keep forgetting this set has variable power output - when making the measurements suggested in #91, please ensure the front panel power output control is set to maximum.
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Old 24th May 2020, 7:18 pm   #98
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Default Re: Superstar 6900N V6 - Fault with SSB mode

How to blow up an RF Power amp, even a very highly protected one.

If we leave the world of CB sets for a while and look at amateur radio and commercial practice.

It is common to have a directional coupler on the output and to have detection of both forwards and reflected power. A microprocessor sees these through an ADC and when the PTT is pressed turns on the transmitter and tries to drive it to a low level of power as a test. It looks at the ratio of reflected to forward power to determine the VSWR of whatever the output is connected to. If it likes the result, it brings up the power level to full and enables modulation.

If it doesn't like the result it politely declines to go any further, or it may set a level at which the reflection will not create excessive voltages or currents in the power devices.

All well and good. You can connect shorts and opens and all sorts in-between and it should survive to transmit another day.

Bullet proof?

Er, No. There is one teensy loophole.

Start transmitting into a good load and the protection systems bring on full power. All is well. But if there is an intermittent connection somewhere, maybe going short, maybe going open, maybe arcing, there can be a very fast transient reflected back to the transmitter. Long before the directional coupler and stuff can turn down the wick, there has been a nasty transient hitting the output devices. The energy was alread in the system and bouncing back at the time the directional coupler spotted it. It was already too late to shut things down before the transient got to the set.

Voltage transients destroy power MOSFETs faster and more efficiently than anything else.

Even the big boys and their really expensive radios have this loophole. Sometimes a manufacturer makes it worse.

I was at an openday, back when we had an amateur radio shop in Scotland. Sitting on the counter was an ICOM IC7700 running on demonstration. Nearly new, second hand and a very tempting price. These things were around 7k at the time (a version of the IC7800 with only a single receiver and some improvements) I'd been fancying one for some time. I succombed. It went in its huge shipping container with all its accessories and doodads, and I got loaned a sack cart to drag it up the road (not steep, but definitely up!) to my car (with a big enough tailgate for the box to go in).

From leaving the shop to getting to the car, THREE different people stopped me and said "You know the PAs blow up in those things, don't you?"

They were right, I did know, and I also knew why.

After doing the immensely expensive 7800 flagship transceiver, they did a cut down version in the same size cabinet with a single receiver. They also fitted three different widths of roofing filter and fixed a couple of oversights. Now this radio got slagged off like there's no tomorrow. You've gotta have two receivers to work DX. Who sez? I've got two lug 'oles but I've only got one brain and I can only really concentrate on one receiver. One is enough for me, and if I don't tell the people in DX land, they won't know they shouldn't work me. If I need two receivers at once, I've got half a dozen spare ones, and the IC-9000 looks loke the 7700 and goes to 2GHz.

I was happy with my purchase, wished them well and dragged on.

While they did the IC7700, Icom also brought out a new model of linear amplifier. It had all solid-state T/R switch ing and was fast enough for break-in operation. They designed the 7700 around it. They didn't put in timing to suit the old relay switched linear amps a lot of people already had.

So with an old linear, you press to transmit the power comes up and is happy, then the relay gets round to moving in the linear amp, and breaks the trnasmitter to antenna connection. Big transient!

It might not be enough to do it immediately, but every time you press PTT, it'll catch it eventually, or just do cumulative damage until things run away.

So blown PA devices happen in even much pricier gear. Sruff for governments have the same vulnerability.

ICOM really goofed. They compounded it by changing the PA design to the IC7800 style with ST MOSFETs. My 7700 is the earlier version with Motorola/Freescale devices. I know Freescale very well (now called NXP!) and use a lot of their devices. I can probably scrounge a set of samples if needed.

It's funny that ICOM thought of fitting a whole different PA but didn't seem aware of the root cause.

So I don't think there is necessarily any fault in that 6900 that caused the failures directly. Indirectly, I think having two devices in parallel and expecting them to be closely matched is a stupid decision, making it more vulnerable. Note that J3 isolates the bias to BOTH output MOSFETs together at once, J4 isolates the bias to the driver MOSFET

You can't use them to look at the partitioning of current between Q45 and Q46

They really are paralleled, source-to-source, gate-to-gate and drain-to-drain. Asking for trouble, unless both parts are identical.

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Old 24th May 2020, 7:28 pm   #99
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Default Re: Superstar 6900N V6 - Fault with SSB mode

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Originally Posted by Radio Wrangler)
Note that J3 isolates the bias to BOTH output MOSFETs together at once, J4 isolates the bias to the driver MOSFET. You can't use them to look at the partitioning of current between Q45 and Q46
Ouch, that's right. That's a major difference between this and the V3 chassis. Or is it? Just going to have a look now. Thanks for pointing that out, RW. (That's why we need you).

Edit: ...Yes, the V3 version of this chassis has separate links for disabling the bias to the output MOSFETs and also separate bias presets for each output MOSFET, as well as a third bias setting preset for the driver. Seems like a backward step to have combined the adjustment for both output MOSFETs in this version. We'll let crackle go ahead just in case it is actually the driver which is drawing all the current (unlikely, but...).

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Old 24th May 2020, 8:14 pm   #100
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Default Re: Superstar 6900N V6 - Fault with SSB mode

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Can you reinstate bias supply link J3 ONLY, key the radio in SSB TX with zero audio input, roger beep off, and observe and record the current drawn in SSB TX mode?
Hmm that didnt go too well, with J3 fitted and all the other conditions met, it looked like 18 amps on the PSU and the alarm went off DC lo, it all happened so quick.

I think I may have to do a bit more checking around the TIP36 and take the final mosfets out because I did the same test on FM and the same conditions applied, about 12 amps.

So does it look now like the original fault was bias issues causing self oscillation in the OP pair.

I think I will take out the link L10 to the OP pair and try and check some of the voltages from the TIP36.
And desolder the legs on the IRF520 finals ready to test.

What is the best way to test them with out removing from the heat sink, can a DMM be used?

Mike

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Last edited by crackle; 24th May 2020 at 8:23 pm.
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