Not played with electronics for many years but with the Amateur Radio studying I have re-sparked an interest, though I am thumbling around not really knowing a lot of electronic theory.

One of the transceivers I have is a kit built QRP unit (CW only) for the 80 metre band made by HOWES whom have long ceased trading. Purchased second hand and already assembled so it has had a life of use before me. The radio is modular so consists of a 80metre receiver, an 80metre CW(morse) transmitter and a standalone Variable Frequency Oscillator (VFO).  I also have the HOWES SWR and ATU but that's another story.

The VFO is a nice build that seems to behave itself. Have checked it on a frequency counter and it was found to be slightly out of band so has been best aligned to give a range from 3.500Mhz to 3.785Mhz so not quite the full 80metre band but useful none the less.

The transmitter is another nice build. Has a 3 crystal oscillator, so can be used for those three frequencies without use of the external VFO unit, but use with the VFO gives coverage just shy of the full 80metre band (not that all of it is needed for CW use). Have not fully checked out the transmit power yet, building an RF probe and dummy load for that (in progress). I have guesstimated it is a bit low of the 5watt is supposed to be pushing, more like 3 but we'll see.

The receiver is a 80metre CW/SSB unit (HOWES DcRx) with an internal VFO (somewhat narrow) which can be switched to use the external VFO.  Now this unit has had issues. Came with a disconnected wire on the internal VFO so that was disabled by accident or intentionally. The PCB board mount fixings where missing as if someone had cause to be looking at the tracks. The modification to switch select between the external and internal VFO was not to a great standard compared to rest of build. With long wires which made the internal VFO quite unstable and poor quality soldering. The internal VFO had been aligned way off mark, sitting around 2Mhz somewhere (ferrite core on coil wound all way down). Most annoying of all was the AF output would intermittently drop and sometimes pop back in again, by this I mean it would suddenly go quiet for no obvious reason.

I tapped around the components with my finger, wiggled this and that, did all the usual visuals but nothing made the fault occur on demand, so not an obvious no brainer issue.

It has now been plaguing me to the point where I have to dig out and blow the dust off my old oscilloscope that hasn't seen the light of day for well over 10 years. Couple of the knobs have been bashed, only seems to be working only one channel these days but hey it fired up into life and shows me at least one pretty wavy wave form so hopefully good enough to use for fault finding.

Starting at the loudspeaker I traced the audio signal backward through the circuit. Having to sit and wait at each point for the fault to occur before I could proceed. Traced back the signal until I hit the first microchip. So I Googled the datasheet for the two IC's present in the radio so I could see what was what. My probe was sitting on the output of the audio amplifier chip (LM380), the effects of the fault can still be seen there, meaning everything downstream to the loudspeaker is okay. The input side of the audio amp showed the fault present too. So that's the audio amp itself and circuit up to and including the loudspeaker all appearing to be working.

Further upstream the signal goes through the volume control pot (AF GAIN) then a capacitor and resistor in series which I guess is an RC filter? Then next is the output of the Pre-Amp op-amp (TA75071P), the second chip in the radio. The signal is pretty small here so having to adjust the scope so I can see it better.

The volume control checks out. Bit of a drop in the normal signal across the resistor in the RC filter arrangement which I guess is normal "filtering" ? *shrug*... and there is the output present on the pre-amp chip. Soon as the fault occurs I see that the output from the preamp is unchanged! Bingo! Fault lies between the preamp and the volume control. All that is there is the filter (cap/res in series) and feedback to the op amp. The filter's capacitor was tight to the board so could not check the signal without lifting the PCB to get my probe onto cap joints. Fault condition present on the downstream side of capacitor and good signal on the other side.... HI!

The Howes circuit diagram and part list tell me that this capacitor item C12 is a

.1uF (2A104K) polyester capacitor. Does not seem to be a dry joint present causing the fault so I desolder, and remove the offending item from the PCB. My capacitor reading function on my multimeter is dead so I can not test the component. In my mixed bag of capacitors I got from Maplin stores I find a 2A104M capacitor. Same type and value in a slightly bigger package, but the "M" tells me it's a 20% tolerance rather than the "K" which is a 10%. No matter, it's all I got so will have to do. And 'do' it does. Soldered in and radio audio bursts back into life without a reoccurrence of the fault. Chuffed.

UPDATE 6TH MARCH 2009:

Since making the above post a few Howes kit owners have contacted me enquirying about the instruction manuals.I have information regarding the official manuals. When the Howes company stopped trading all the Howes kit manual/instructions where donanted for the care of the G-QRP Club. Electronic (pdf) copies of the manuals are available to members of the club. Membership is £6.00 a year and I am assured is money very well spent. The club's website is located at:

www.gqrp.com