Sunday, 11 June 2017

Transverter Setback

More work this evening on the transverter, after that is, building a clothes prop for Julie's washing line!

I spent some time taking some readings of the various stages using an RF probe and DVM. With the 28MHz drive set at 5dBm (the most I can give it) the input to the SBL-1 mixer measured 147mV (this is the 5dBm IF signal), the oscillator input 114mV, and the mixer output around 2mV. The input to the Tx buffer stage 4mV, and the output 30mV. All well and good. I tuned my MVT-7100 scanner to 70MHz USB and even without defeating the squelch could hear the thing from a good 5m away!

After a lot of searching and enquiring after equivalents, I decided first to use a 2N3866 in place of the specified ZTX327 transistor for the driver amp. I had just two of these. Well, I managed to snap the emitter leg off of the first one! So, I built the stage using the second, but then decided to try and find an equivalent to the MRF237 specified for the PA.

Putting down another pad
By an odd coincidence, something on the net linked me to the circuit of the Marconi RC-690 PA strip, where I discovered there is an MRF237 - and I have one of these boards! Trouble was, I couldnt find it! So, after tearing the workshop apart I eventually located this PCB - and discovered that there are some ZTX327's on it as well!

Its from here on that it all started to go wrong!

Getting the ZTX327s off of the board was awkward, and when it finally came, a bloody great blob of molten solder fell on my finger! Thats coming up in a nice blister. I removed the 2N3866, which by this time included all the surrounding parts of the driver amp, and rebuilt it using the ZTX327 i'd just removed. On powering up and measuring the output using the RF probe, all looked great, well over 100mV, but now the input measured only a few mV, not the previous 30mV - it seemed to be badly loading the previous stage... and then I started to smell it!

Oh my gawd it was running hot! Incredibly hot! The PSU's meters showed this stage dragging around 250mA. I powered down and had a think. Back to the books and internet for a bit of research, and decided to check the bias was sensible. Disconnecting the drive by desoldering the leg of the trimmer, I measured the DC voltage on the base - 0.68V, hmmm, seems about right to me? But, it kept dropping, quickly to 0.46V then more slowly. And it still got very hot. I tried a blast of freezer spray and it made no difference, and by this time the clean peaks on the spectrum analyser had degenerated into a mass of sproggies!

I suspect ive killed this transistor! But why? Was the bias at fault? Is the 1uH choke too low in value? Or was the transistor already faulty, or damaged in being removed from the PCB? Because im lazy, and im regretting this now, I didnt bother to test it before hand.

Tomorrow, i'll start this stage again. The old PCB has one more ZTX327 to salvage. This time, however, i'll test it before soldering it in!

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