I recently received a SignaLink USB unit that that the owner could not get working.
SignaLink USB Schematic courtesy AB8VJ
I connected it to my PC and my FT1000D, that I regularly use via my homebrew interface and it promptly went into Transmit mode without any signals present from the PC. No matter where I set the controls it just went into PTT. Removing the radio connection dropped the PTT for about 1 second but then it went into continuous PTT ON/OFF at a speed that could be set with the delay control. Even shorting the SPKR input to ground made no difference.
After a lot of digging on Google, I eventually came across a schematic for the item and began checking that the correct components were fitted in the correct place.
Lo and behold, after a lot of prodding around with my DMM I found a 27k resistor, associated with U1, that is shown on the drawing as a 1K.
U1 just happens to be the PTT signal amplier and with a 27k feedback resistor it gives 27 times more gain to the PTT system than it should have, enough in fact to trigger the system from internal noise.
I strapped a 1k resistor across the offending item and the PTT stopped pulsing On/OFF. I plugged the FT1000 connector back in and, to my surprise, I immediately saw signals on my HRD waterfall. I was surprised because I had not yet pressed the Power ON button. I believe that because the MIC5205 Vref chip and the USB audio chip are permanently connected to the 5V USB, when the USB connector is in place, there is enough DC floating around the system to make it functional on receive, albeit with slightly lower gain. This doesn't bother me too much so I haven't changed the wiring to the power switch. It simply means that you can watch band activity without powering up the SignaLink.
Also shown on the schematic is a 33uFd cap across the Vref output which does not exist on this unit. That may be an error on the schem, because there doesn't appear to be anywhere to fit it on the board.
My next step is to do the Vbias to Vref mod, shown on the link below to OZ1PIF mods.
Scroll down almost to the bottom of the page to see the simple 1 resistor change and 1 wire link to quieten down the internal noise level.
Maybe if I had done this mod first the problem would have gone away.
My FT1000 gives a fixed audio output of 500mV p-p and this is way too much for the SignaLink and you had to set the RX pot to almost zero, so I replaced the SPKR link with a 10k resistor and now you can change the input level, if necessary.
p.s. The owner is very pleased to know that it now works and it wasn't down to his inability to set it up but he Never want's to see the B&**dy thing again.... :)
OZ1PIF SignaLink USB mods
Update Feb 9th : I have now completed the Vbias/Vref mod and all is working fine.
I have also added two diodes, a 1uFd capacitor and a BFY51 transistor and I now have CW keying directly on the programming link. My rig cable already has the FT1000 key line so all I needed to do was rectify the 'Mon' output via two OA202 diodes with a 1uFd cap across the ouptut and feed the DC level into the base of the BFY51. The MCW now only exists within the SignaLink USB and the radio is keyed via the BFY51.