There have been (since the launch of the kit) a number of complaints relating to FM reception and RDS problems. I have built this kit 5 times now, and I have had 3 units (completed by other people) in for repairs. I have not (yet) had any complaints from the owners of the ones I have built: however, each one of the repair units I have looked at has had the same reported problem - poor FM reception but reasonable AM, plus loss of RDS TA/TP capability and poor tuning and lock.
Having looked at the two most recent repair units the same construction problems were evident - the connection of the coax to the tuner board in the bottom of the unit was poor. When corrected, the units worked perfectly on my test rig, pulling in stations from quite a distance as well as national RDS FM stations.
So, what's the source of this mysterious problem?
On the tuner PCB, the input signal solder pad on the PCB is adjacent to the grounding pad. There's only about 1mm between them: on the version of the kit with the multistrand core conductor, if there are any stray conductors from the core they can short the signal to the grounding pad. On the single strand core version, the cable can twist bringing the shield braid in contact with both earth and even the signal core. In both cases, the input signal is simply shorted out and the tuner board gets no input from the antenna. You can see that during construction, attention to detail when you are connecting the antenna lead to the board is
very important.
On the third repair unit, the single core conductor had snapped off at the PCB due to fatiguing. This was mainly due to the lack of stress relief - the cable ties supplied with the kit were not used and the core broke off as a result of twisting.
On the second repair unit, as well as the cable shorting out at the tuner board, the shield was not connected to ground at the socket connector end of the coax - so even after the repair, the unit still didn't work. This was due to the shield braid not connecting to the earthing olive inside the socket connector, giving no connection to earth. This is a pretty common problem in DIY coax connectors, something I have seen repeatedly over the years I've been doing this stuff (so don't feel guilty)
I've come up with a slight modification to the build method of the tuner to stop this happening and hopefuly ensure good quality signal reception.
Firstly, I would stress that these kits are pretty easy to build and are good value for money. However, you need to know what you are doing, have the right tools, and to be careful with your soldering - especially on the tuner board. Take the care to do it properly. I am inclined to say that the construction order (processor board then tuner) tends to leave the builder impatient to get it finished off quickly as you reach the second board (the tuner) meaning that attention to detail drops along with build quality - and hence the quality of the signal you will eventually receive.
Follow these steps when you are attaching the coax cable to the tuner board.
1) Begin with the coax cable. If you have the cable supplied with moulded on plug, cut the plug cleanly off the cable with clippers. Insert the cable (regardless of which type you have) through the lower drilled hole in the tuner box and feed it through to the inside sufficiently so you have a length of lead to work on easily..
2) Carefully strip the coax back 25mm.
Trimmed coax I use a scalpel to do this (carefully, of course!). Be careful not to cut through the shield braid completely - if you do, you'll have to cut the cable back another 25mm and do it again.
3) Separate the strands of the braid and twist it into two different pigtails on opposite sides of the core
Pigtail braids
4) Cut the centre core back to 10mm length, then remove a further 5mm core insulation to expose the centre conductor.
Trimmed core As before, be careful not to cut completely through the core. If you have the multi-strand core, twist the core into a single pigtail in the same way as the shield braid. Put the resulting cable to one side.
5) Locate the two holes in the Tuner PCB for signal input and signal shield. Fill in the shield hole with solder. Don't use too much, just a little. Wick off excess.
6) Dab a small blob of epoxy or hot glue over the sealed up hole. Don't put on a large amount: the idea is simply to obscure the contact pad. Be careful not to cover the signal input hole next to it.
7) Insert the signal wire from the prepared coax into the solder hole, making sure that the insulator butts up at right angles to the circuit board. Solder the contact making sure that the insulator core remains tight against the board as you solder it.
8) Solder the two pigtails to the can of the tuner, and through the large PCB hole at the corner of the can nearest to the signal hole. Tip: Lay the cable flat against the can as you do this. Use only enough solder to hold the joints properly, and make sure that the joints are good (not "grey"). Use
these two
images as examples.
9) Make continuity checks with a multimeter to ensure that the signal conductor is not shorted to the earth shield, and vice versa. If there is a short, investigate the cause and rectify before proceeding.
10) Using either fast-setting epoxy glue or a hot-glue gun, apply a fair amount of glue to fix the cable against the can and seal the connection.
Glue seal (Note in this picture I have put on too much glue near the screw post hole which caused problems later during mechanical construction - don't overdo it). Ensure that the cable remains perpendicular to the board and flat against the can of the tuner module.
11) Insert the tuner board into the case taking care to slowly feed the cable play back through at the same time and not pinch it betwen the board and case side. This is a bit difficult to do easily since the original kit did not have the moulded on socket connector: you are forced to insert the board at an angle with the cable already through the case hole - it's quite difficult. Take care and do it slowly. I find I have to use reasonable force to push the board in, but be careful - again - that you don't overdo it and damage the case, can or board. Try to end up with a loop of cable a few centimetres long above the board to play with.
12) With the board in place, insert the screwposts and washers, and screw them down finger tight.
13) Feed the loose loop of coax slowly back out of the box until you have about 40mm of cable between the board and the side of the box. Be careful not to disturb the cable-locating glue on the board.
14) Using the supplied Ziplock cable tidy, feed it around the coax and push the loop against the side of the case. Tighten the ziplock as far as you can with your fingers and check that the cable is tightly fixed by pulling on the cable outside the box. If it is secure, trim off the excess length.
Stress relief for the coax
15) If you have the kit with the non-integral socket, strip the coax at the other end per instructions. However, do
not roll back the shield braid.
16) Carefully feed the shield braid
through the supplied connector back screw-on cover, followed by the metal earth contact olive. Proceed to connect up the signal conductor to the insulator block as per the instructions.
Shield through olive
17) Wrap the shield back around the
outside of the olive, taking care not to allow stray shield wires to contact with the centre conductor where it goes into the insulator block.
Shield around olive
18) Insert the insulator block into the outer connector cover, and screw the back cover over the outer cover, pinching the shield braid between the back cover and metal olive.
Cover and back cover
19) Make continuity checks with a multimeter to ensure that the signal conductor is not shorted to the earth shield, and vice versa. If there is a short, investigate the cause and rectify before proceeding.
20) Fit the top (processor) board carefully over the connector header and fit the screws and washers. Feed the coloured control cable bundle through the case and follow the remaining instructions to finish off construction. However, make sure you fit the second supplied ziplock to the bundle in a similar manner to 14).
Stress relief for control cables
21) Unfortunately, there is no simple way to test the unit before getting to this point other than the constructional checks specified in the kit instructions. At this point, without fitting the lid, you should connect up the unit to the antenna and harness and try a functional check. If all is well, the green LED should light and you will get a good signal from the unit.
If it doesn't, then unforch - you will have to simply go back and check the above sequence. This either works, or it doesn't - there isn't really a halfway house.
If you have any questions about this mod or how to go about it, drop me a PM and I'll be glad to help.
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