- BUILD/TEST DETAILS
During Round Table events, time tends to be in short supply. For this reason, the transceiver build has been split into separate receive/transmit sessions in the hope that at least the receiver is fully finished and tested by everyone taking part.
Adding further smd components once leaded items have been added can often be difficult, because the pcb assembly usually no longer sits comfortably on the bench. This problem was resolved here by making up some jigs with cut-outs to locate such offending components.
The red solder resist rather obscures the board track, so here is a clearer view:
The receiver build includes the supply regulator which feeds the whole transceiver.
First, fit the smd components. Work from the top of the component card downwards, ie, resistors first, capacitors second, etc. This avoids a couple of situations where otherwise a component might be found difficult to fit because of surrounding components.
Next, add the leaded items - the following order is as good as any:
Fit the 8v regulator, IC3 - the leads need to be bent at right-angle at the point were they thin down. The body (tab) is soldered to the groundplane. Use a relatively large, hot iron for this, ensuring that the body is kept firmly pushed down onto the board during soldering.
Fit D1, the opto-diode. Only one side is soldered down to the pcb - the other is bent upwards at 45 degrees. It should not make contact with the pcb, even a non-conductive part of the board (the impedance at this this point is many tens of Mohm). The side soldered to the pcb can be identified by noting that within the clear moulding it is the thicker of the two structures. Before soldering, cut this side back to 5mm length (but do please make sure you have got the correct leg!). This device must be accurately placed, and this can be done by soldering the short stub centrally above the 1mm wide solder resist relief pattern.
Note the alternative position for D1 as below, if the finished build is going to be receive only. In this case, D1 straddles the centre line (shown dotted on the smd side).
Fit TR1, the input FET. Only two leads are soldered to the pcb, the third is bent upwards and connects to the opto-diode, D1. The two leads that connect to the pcb should be cut short, about 3mm from the body. The third lead, and the floating opto-diode lead should be shaped as the picture, trimmed, then soldered together. This lead should not make any contact with the pcb at any point.
Finally, fit the 15 way D plug. The item supplied will be one of two types, as shown below:
The solder bucket type on the left will be quite a tight fit onto the end of the pcb, however, the solder spill type on the right will be a very slack fit. In this case, solder the non-ground side pins first and then turn over to the ground pins side and fill the gap between the pins and the pcb with solder. Do not bend the pins down onto the pcb. Again, a relatively large, hot iron should be used to do this.
Testing/setting-up the receiver
Ideally, the receiver would be tested via the test/interface pcb - but that may not have been built yet - so interconnections will be referenced to the 15 way D connector instead.
Connect a 12v supply to the board (+ve to pin13 and -ve to pin 8). For the cautious, the PSU could be current limited to 100 mA. The current taken should be around 65 mA.
Measure the voltage at the output of the 7808 voltage regulator. It should be between 7.8 and 8.2 volts.
Now measure the drain voltage of TR1 and adjust pot R4 for a reading of 2.5v.
There should now be audio (noise, with probably lots of hum, both optically and capacitively induced) appearing at both rx outputs - ie, pins 9 and 10. The level should be about 50mV rms.
That concludes Rx alignment. If the required final build is receive only, then fit the two pillars to the board (stud end), and fasten with the two M3 nuts. Tighten from the non-smd side, then slide into the aluminium tube, securing with the two countersunk M3 screws. Capacitive hum pick-up will now be found to have disappeared.
Having built and tested the receiver, the transmitter section can now be added, starting with the smd build. For the Finningley RT, a number of jigs have be made so that pcb can be held firm whilst the chip components are added, but a small vice could also be used - anything to keep the board in a fixed position.
As with the receiver build, work down the Tx smd component card in order:
Note that for C17 (10uf/16v), the pcb silk screen does not show a '+' sign
The correct orientation is for the + side to be fitted towards the middle of the pcb.
Having build the rx section first, placement of R46 becomes slightly obstructed by some of those rx components, but every over component has good access for soldering. Load plenty of solder around the drain (large tag) of TR3 and TR4 - it will not spread beyond the allocated solder resist cleared pad, and good heat transfer to the pcb track is required. Use a relatively large, hot (800F) bit for this.
Having fitted the surface mount components, now add the leaded items - there are just three of them. The following order is as good as any:
Push the centre pin of the tilt switch through the single board mounting hole (the bottom side screen is marked 'Tilt Sw'). With the switch flat to the pcb, use a relatively large, hot (800F) iron to solder the metal case to the pcb at opposing sides onto the two solder resist relief pads provided. Then solder the centre pin to the pcb on the other side of the board.
Fit the two-pin Berg plug on the bottom side of the pcb into the holes marked 'Test'. Use the two shorter, bent leads to enter the holes, and point the other side towards the nearest board edge (ie, away from the 'Test' legend).
Finally, fit the 10mm LED. You will need to trim the leads first, as below:
Testing/setting-up the transmitter
Ideally, the transmitter would be tested via the test/interface pcb - but that may not have been built yet - so interconnections will be referenced to the 15 way D connector instead.
Connect a FLOATING12v supply to the board (+ve to pin13 and -ve to pin 8). The PSU could be current limited to 500 mA. The current taken should be around 70 mA (this includes the rx current).
Take pin 11 (Tx key) of the 15 way D connector to 0v. The transmit LED should light. Turn pot R28 and note the supply current. Adjust for an increase in current of 250 mA from the non-keyed value (ie, about 320 m. Do not set higher than this - you'll only have modulation problems later on if you do this).
Now connect an oscilloscope across 'Test' pins - be sure that the 12v supply ARE floating before doing this. With pin 11 still connected to 0v, set the scope to show the dc standing voltage across the test pins. Take pin 15 (tx tone) also down to 0v - you should now see a 1 kHz superimposed on the dc standing deflection. Adjust pot R23 until the tone 100% modulates the standing dc current - a little bit of distortion of the sine wave may be seen, but don't worry about that. Disconnect the pin 15 connection to ground.
Connect an electret microphone to pins 14 (live) and 7 (ground). Check that full modulation is seen on the scope - expect there to be some clipping also. As you speak, you should notice some slight flicker on the Tx LED.
If you've got this far without problem, you can assume that the pcb is pretty much OK!
This should be quite straightforward in its build. An even simpler version is possible for anyone building a receive only assembly.
Full transceive build
It is sensible to build in the order shown below, working your way down the component list on the left-hand side. As with the main transceiver board, if the 15 way D connector is a pcb pin variant, solder the active pin side (pins 9-15) first - there will then be a gap on the other side (all are 0v connections) between the pins and the pcb. Do not bend the pins, just fill in the gap with solder.
Receive only build
This build is very simple!:
-o---- E n d o f w r i t e - u p ----o-