23 May 2011

Comb generator, full diagram

It was back in 2007 when I designed and built a simple comb generator to test the wideband response of RF filters.

An interested builder asked me for further details about my circuit, but it was so simple and flexible that the only help I could provide was to complete the circuit diagram.

The circuit shown above generates an uncalibrated and non-constant "RF comb" from the XTAL frequency well up to the 500 MHz region. The comb is good for relative measurements, such as peaking a filter.

Comb peaks were observed to be spaced as much as the base XTAL frequency: the smaller the rock, the more dense the spectrum will be. Note that you might need to increase C's across the XTAL for lower frequencies.

An alternative for much more dense comb in HF is the "VE3DNL Marker Generator", do a web search for references and diagram. I think both of these circuits should be in every builders' shack.

20 May 2011

Low Vcc voltage on ACC port (FT817)

Twice in the same week, that's the occurrence of a fault occurred at the ACC port of two unlucky FT817 transceivers.

The symptom is a low voltage on the "13.8V" line of the ACC port when an external keypad or FR or IFR is connected to it (or another ACC powered device, of course). The cause has been a short between the "13.8V" line and ground, that has damaged the tiny R1235 10 ohm 1/8W resistor. In both reported cases the resistor has increased its resistance, effectively protecting the remaining circuitry.

On the circuit diagram it is obvious that R1235 is the only protection on the unfused Vcc line (internal battery or external supply) for the 13.8V ACC output. There is a resettable fuse, but that one goes towards the FINALS board and has nothing to do with the ACC port.

How to fix it? I can think of three ways:
  • replace the resistor with an identical one (it is tiny, you need proper equipment, SMD reworking ability and a firm hand);
  • replace the resistor with a physically larger one, and a fuse if there's room for that [but be aware that a 1/4W resistor won't blow up the same way!]
  • parallel the existing resistor with another 10-15 ohm one, but still consider adding a 500mA fuse
Do not simply short R1235. Old R1235 can be gently cut in half instead of being desoldered.

Summing it up. Be very careful with the Vcc line on the ACC port. If you are reading this because the damage is already there, refer to Yaesu assistance or a skilled electronic technician (yourself?) to trace and fix the damage.