Use your mind !

Publié: 27 décembre 2008 dans SDR

18 hours lost after the most stupid mistake I made since I use a soldering iron. And I’m pretty sure I won’t be the only one to make the same error in the future 😦

I must confess… from time to time, I forget to read manuals and schematic diagrams. Particularly when I feel confident about a software, a computer design or… a well known piece of electronic.

That’s what I did with Alex’s MoBo. And it didn’t worked in transmission mode. The symptoms were obvious:

-the signal was correctly coming from the SR board,

-was normally switched via the different FST3253,

-was arriving in good shape at Q2’s (2N2222) base

-was correctly amplified by this very same Q2…

– was correctly routed to Q4 and Q5 gates without troubles (each signal of the same amplitude, in phase opposition compared to the other one, proving that the transformer T5 was correctly wound)

…. And NOTHING was going out of the drains !

I plugged back the “traditional” P.A. which was known in working condition, delivering it’s 20V peak-to-peak HF signal, and I started to measure AC and DC voltages. And I switched back to Alex’s MoBo.

Everything was OK : 12Vcc was present on the drain. I even rewound T6/T202 (just in case… I strongly believe in shamanic actions when I don’t understand what’s happening). Exciter’s signal from the 2N2222 was 4 volts peak to peak… quite enough. Bias voltage on the 2 gates was ok too (2.6 volts.

– Just for info : the base of Q3, 2N3904, is around 0.7 V, the gate of Q6, BS170, must be around 3.2 volts. After the diode, the bias voltage injected on Q4 and Q5 must be close from 2.6 volts (3.2 minus the 0.6 volts of D1’s drop down voltage)

It was obvious that my 2 final transistors where absolutely dead. Was it during a measurement ? did I provoke a short circuit without paying attention ? I dismounted the radiator and…. Saw THIS :

 

THIS IS THE WRONG WAY TO MOUNT THE BS170 !!!!

DO NOT FOLLOW THIS PICTURE

All the FETs have been mounted upside down… exactly like Tony used to do when he designed his board. But Alex’s implementation is different: the flat surface of Q4, Q5 and Q6 must lay on the surface of the PCB.

So, guess what ? I changed the transistors with brand new ones (mounted in the good position), put back the radiator, plugged the board and the different connections, fired the 13.8 Volts, put my probe on pin 5 of J2/P9 and…. NOTHING Again !

… well… I’ll do it short. One hour later, after checking and checking again voltages and signals (without nothing the slightest difference with the working configuratiin) I decided to run a “passive” test, benching every capacitor, every resistor, and every transformer from a pure resistive point of view. And I found the culprit: R9 and R10 were totally burned, toasted !(one having a value of 49 Ohms, the other going up to 220 MOhms). It was the consequence of the first mistake… but 0805 do not make a lot of smoke… that’s the reason why the first test didn’t show anything, didn’t smell anything.

Murphy’s law stroke again… no 2.2 ohms in my junkbox. Only 1.2 Ohms

 

Putting 2 0805 in series need good nerves. Anyway, after this small operation, everything came to life, and I made my first “full power” QSO with my dummy load 🙂

Morality : NEVER READ schematics or manuals before soldering anything. You’ll probably lose a LOT of time, but it’s the best way to learn and use your brain. If you don’t give up before…

Now… let’s build a cleaner audio isolation pcb and think of a 3 elt elliptic filter design

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