Well, all was going well and I was doing RBN signal strength comparisons on 20M, while switching antennas to compare. My newly completed Mag loop was kicking the random wire 1:9's butt, typically by 8dB, sometimes more, all was good until....

Suddenly no more RBN reports??? ... Uh-oh... I'm not certain but my guess is that at some point during all that antenna switching, I probably transmitted my canned RBN VVV CQ message one or more times into a non-load. Took me a little while to figure it out.

The receiver seems fine, The side-tone monitor sounded normal, but nothing was going out. No smoke or dramatics, just no output on a previously working rig.

So here's what I see now in receive mode:

Current draw: ?260mA in receive ?13.8VDC input (All measurements with DMM) I have not got the scope on it yet.

Q1,Q2,Q3: ? All show 12.5V at drains, all the time
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 6.3V at gates ? all the time?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 0 V ?at source

Q6: ? ?13.2V ?at emitter
? ? ? ? ? 12.5V ?at collector
? ? ? ? ? 12.6V ?at base.

IC3 pins: ? 1 +5V ? ? ? ? 14 ?+5V ? ?
? ? ? ? ? ? ? ? ?2 ?+5V ? ? ? ?13 ?+5V
? ? ? ? ? ? ? ? ?3 ?+6.2V???12 ?+5V
? ? ? ? ? ? ? ? ?4 ? ?0V ? ? ? ?11 ? 0V
? ? ? ? ? ? ? ? ?5 ? ?0V ? ? ? ?10 ? 0V
? ? ? ? ? ? ? ? ?6 ?+5V ? ? ? ? ?9 ? 1.1-1.9v varying???
? ? ? ? ? ? ? ? ?7 ? 0V ? ? ? ? ? 8 ?+5V

Q4 normal: ? Drain 12.5V ? Source and gate 0V... on key down, source drops to 0v, gate to +5V

My preliminary read is Q6 seems to be on even in receive mode, the collector is energized, but Q4 seems normal...??? There seems to be a voltage of =5.3V at the output jack at all times, but I'm thinking it has to be a misreading by my DC voltmeter, but I don't see how that could get past C29, so I'm thinking it must be an AC voltage that my meter "sees" as +5.3VDC. Obviously I'm gonna need to get the scope out. It seems as though if it were actually 35-40Vpp my meter would be reading higher than that.

The IC3 pin3 +6.3V sure looks weird. It's the same 6.3V that shows up on the Q123 gates (which makes sense) but where the hell is it coming from? Also the IC3 pin8 at 1 to 1.9V??? WTH? Could that be my meter looking at a 3.3V square wave?

I'll clearly be digging further into this but I'd like to hear what you guys think about this... Is IC3 toasted? My gut says just replace all the transistors, which I can do, but do the symptoms suggest that IC3 is fried also? I'd like to get a clearer picture, or at least a decent theory about what's going on before I start replacing parts.

Thanks,
Gordo?

Is IC3 toasted? My gut says just replace all the transistors, which I can do, but do the symptoms suggest that IC3 is fried also? I'd like to get a clearer picture, or at least a decent theory about what's going on before I start replacing parts.

Gordo,

Maybe.
Remove Q1,2,3,6. When your symptoms are reported these are usually blown as voltages indicate.
Check voltages on IC3 RX and TX although pin 6 seems to indicate it has also gone.

73 Alan G4ZFQ

Had to wait for parts on a slow boat from China.... Replaced Q1,2,3,6 ?Q4, IC3, and.... same reading, with the exception that pin 9 on IC3 reads about 2.6 in receive.

Q1,2,3 hot... ?12.9v at drains, goes up to 13.0 on transmit
? ? ? ? ? ? ? ? ? ? ? ?6.3v at gates, all the time on rev, goes to 7.3v on transmit
? ? ? ? ? ? ? ? ? ? ? ?0v at sources on rev, and xmit.


Still 6.3v at pin 3??? (same on drains of course)

I'm buffaloed here.
Does anyone have any more ideas? If I could just order another original QCX I'd do it in a heartbeat, but not as taken with the new mini, or, the QCX plus. I just want to get my baby back on the air...

Gordo

Some progress... Q6 is shorted from emitter to collector dumping full Vcc (13.2v) onto the Q1-3 drains at all times. and somehow driving 6.3v to the gates, also full time.

With Q6 removed, all the (replaced) IC3 pin readings are spot on with pin 3 now reading 0v in receive mode and a 4Vpp square wave on transmit. Why did Q6 blow? I don't know... Next step will be to replace Q6 (again) and see what happens... If everything works, great. If it blows again, at least I have a clue where to be looking.

More will be revealed?

Q6 just pops occasionally for no understood reason. Replaced mine with a small power package PNP and no problems ever since.?

Still not right... with all new semiconductors, still have a strange reading at pin 3... about 7.5VDC with a small AC component of about 100mV (probably from PS ripple etc). I've pulled all the power amp transistors and Q6, and with these out of play, IC3 signals at all pins are the correct voltages and waveforms in both receive and transmit modes, So at least we're good to that point... ?I'm taking it back to good, known signals, and building outward again from there, one component at a time, rather than the whole lot. This way I can follow the waveform and keying signals outward from known good points, dealing with only one variable at a time.

Keying circuit with Q6 replaced again (Q4 already checked and good)... new transistor... 13.25V Vcc at emitter, 0V at collector in receive...check! ?12.8V at collector and into L4 at keydown... check! ?5V squarewave still good at IC3 on keydown... check!

Making progress... We know the LPF train is good already so only the PA remains. Install 1 BS170 transistor... 5V square wave at pin 3, and YAHOO! 12.6v sine wave across a 50 ohm load at output. ?Install another BS170 Squarewave still intact and about 25v sine wave into dummy load. Install final BS170 ,power up, 5V square wave clean at the drains, and a gorgeous 36.8 Vp-p sine wave into the load. WHEW!!!

I have no idea why it didn't work when I replaced the whole lot of transistors... I really don't, but clearing all the confusing bits out, back to known good signal points, and rebuilding outward one component at a time, and following the signal chain as I went, seems to have done the trick.

I'll tell you one thing. I'm gonna be DAMNED careful going forward, not to go keydown into ANYTHING but a 50 ohm dummy load, or a well tuned antenna. I will say that in a certain way it was time well-spent, and I learned a lot more than I planned.

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