Troubleshooting an interface

After we arrived home on Saturday (29th) and unpacked everything from the trip, I set out to determine the cause of my inability to operate digital while mobile. My first suspicion was an open in the cable, so I tested the continuity between the perf. board and the 6-DIN connector – no problems there! The reason I suspected an open was because of a bad kink in the cable where it appeared to have been pinched – in my usual line of work, this would be an instant cause for additional investigation.

My next task was to confirm that the sound card audio was in fact being received at the transformer. This was made challenging by a couple of factors: 1. I was too lazy to haul out my temperamental oscilloscope and 2. the connector pins are very close to each other which makes probing them while trying to multitask a real challenge.

The arrows in the picture indicate the pins which I was attempting to probe which are the data in and ground pins.

Notice that the pins are swapped on a horizontal axis – when compared to the pinout (PDF page 35) of the FT-857D manual. This can be a source of confusion when making a sound card interface, so I’ve added a numbered pinout of the manual drawing to assist with the understanding.

After finding that there was no audio making it to the 6-DIN connector, I had to confirm that the audio was being lost at the output transformer stage. Prior to testing the secondary side, I verified that I had the audio between the two “pins” of the primary winding. I then probed the secondary and it was at this stage that I found the fault in the circuit. There was no audio on the secondary side! There are only two causes which could explain this phenomenon, the first being an open circuit (ie. burnt out winding) which is easy to test – just look for infinite resistance between the two outer-most “pins”. The second cause would be a short-circuit between the pins!

In my sound card interface design (Sound-card interfaces .. home brew one!), I use a potentiometer to vary the output signal level (ie. peak-to-peak AC voltage seen by the radio). At some point, I must have adjusted the potentiometer to such a level as to present what was effectively a short-circuit to the secondary windings, preventing any useable amplitude in the audio signal.

The picture shows a blue box which is the potentiometer which caused all of the grief. After some fine-tuning while setting the computer’s audio output level to 50% and the digital gain (Menu #37) to 50% I adjusted the potentiometer so that only seven “rows” of ALC indication on the display were present.

The manual vaguely states (PDF page 66) to adjust your source audio level until a “few” dots of ALC are present but as high as “16 dots of ALC” on the meter.

Annoyingly, the manual displays a “full scale” display which happens to be “16 dots” tall! My take on this conundrum is to err on the side of caution which suggests operating so that only a total of “16 dots” are indicated which actually means a display which is only “7 rows” high.

My plan is to confirm my suspicion by monitoring the RF output of the radio using my oscilloscope, watching for the first signs of “flat topping” or “bottoming out” of the RF envelope. To do this, I will be making a resistive divider RF sampler – I just need to buy an aluminum housing for the project.

So in the interim, I’ve got my station back up and running which is fantastic! I just wish that I had remembered fiddling with the potentiometer – I could have saved a lot of grief and could have actually “played radio” while on vacation. Doh!