Support QRP CoA Everywhere

Friday, 7 May 2010

ARCI 72 part challange - realised error

Morning after... over the shreddies and visual check I realised as I had swapped FET from BS170 to 2N7000 having run out of one TO92 version (see for the two TO92 pin configurations) obvious error... I had connected one of the FET switches for mute backwards at the LPF input (shared with RX as forms half the BPF), anyway on TX instead of the FET passing RF to the LPF instead I was feeding the RF directly into the RX chain (abt 4W). Oops! I hit the key for a second and seeing no RF out and a brief squeak/squeal from AF stopped and heard no RX afterwards. Damage done but no magic smoke!. The IRF510 PA still seems to work so I have a TX :-) anyway might explain why it doesn't work ho hum! The 8 pin DIL PIC 16F629 (like old 509) was hosed as a result as the switch before the AF stage sent the RF into the PIC pin instead!

Normally a sensible builder would have reverse diodes etc in line and plenty of decoupling caps, voltage regulator, used npn transistor to switch the FET's and added an extra stage on the LPF as only 30dB down but omitted all this stuff to manage the part count! I realised I could have saved 3 as didn't need the xtal filter input transformer and couple caps used to decouple DC I had doubled up in two places (BFO and VFO into mixers).

Had this been a DC RX then 72 parts easy.... still learnt I could build a half decent <600Hz IF filter with two crystals not 4 or 5 :-)

Going to be interesting to see how the others actually managed it.

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