There had been a long gap after the completion of the mcHF UI board to start with the RF board soldering. Mainly it was for the time required to source all the components needed for the RF board.So to cut it short, I started with the RF board soldering and completed soldering about 90% of the components using hot air soldering. I left the smd inductors, relays to be soldered in the end using conventional soldering iron since hot air would melt their plastic bodies. And the hand wound inductors were the last parts to be soldered into the RF board. All done in about 2 weeks. Now, the real challenge :) Testing it and making it work.
Connected the board to a limited current power supply. And pressed the on/off button. But nothing happened. It didn't power on. :((Before soldering the RF board components, it was working..see the photo and video in my last post.)Found that the SMD fuse in the power supply section was blown out. And the worst part is, I didn't have much fuse left for experimenting. There was a Schotky diode for reverse polarity protection. It became my central point of suspect along with few others. After taking the suspects out of the RF board, it was found that they all were totally innocent in this issue.
The next suspect was the transmit side with the PA consisting of 2 nos of RD16HHF1 MOSFETs rated for 16W 3-30MHz, Torroids, LPF, BPF stages, Antenna switching etc. So the best thing to isolate the stage was to cut the power to that stage. The 12V supply comes to this PA stage through an RF choke RFC8 which is a single turn 24SWG copper wire through a small binocular core(BN43-2402). I wanted to make the receive part working before going for the transmit. I removed RFC8 cutting the power to the PA stage. Replaced the blown out fuse with a new one.And there it is, the Rig powered up showing the spectrum display. So the fuse blowing issue was isolated to the PA stage. Thought of checking it later, once the receiving is found to be working.
Connected my speaker...And guess what...there was no audio at all.I increased the AF Gain to max, RF Gain to max. But still no audio.
I traced through the DC voltages for the ICs in the receive stages and everything seemed to be fine. Checked for any soldering bridges in the processor for the digital signal input/out put pins. But that too seemed fine. Checked CODEC, Switching IC, Speaker amp. Again here my suspect became the SMD Audio amp, the LM386M-1.
The DC voltages seemed to be fine. To check if the Audio amp was working, I isolated it from rest of the circuits by removing the coupling capacitors. Connected audio from my Samsung Tab to the input of the LM386M-1 and played a song. There came crystal clear audio from the speaker. And there goes LM386M-1 out of the suspects list.
Next was the Codec IC, WM8731. After removing the isolation made in the LM386M-1, I did the same thing with the codec IC. Isolated the input to the codec and connected the audio from my tab playing an mp3 at low volume. But this time there is no audio from the speaker. With this I became doubtful if the codec's working.The analysis continued for two evening sessions. On the third evening,I noticed a strange thing which went un-noticed in the initial analysis.
There was a short between pin 15 and 16 which should not be the case according to the schematic. Across these pins are connected, C22 and C23. C22 being an SMD Tantalum, I thought it must have been blown out. I desoldered it and the short was still there between pin 15 and 16. And the pulled out tantalum was also not shorted. Removed C23 SMD capacitor and the short between the pin 15 and 16 is there no more. :)
Even though the pulled out capacitor didn't give any short while continuity check, I replaced it with a new one after cleaning up the board. May be there was some un-melted BGA soldering paste(mixture of lead and flux in the form of paste) under C23 which lead to this short. Anyways, I didn't go for further testing it. I went ahead with connecting the rig to power supply and then switched it on.
:):):)
The rig came up with audio in speaker :). Connected my 20m antenna to it and searched around the band and rest you can see it in the below video :).
I have built many conventional radios in the past and I feel that the happiest moment is when,you hear some one talking from some where in this world, through the radio that you newly built. And this time also it was no different with this cute little QRP SDR.
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