Hojo's Ham Blog: 2018

Monday, March 5, 2018

PCB Etching notes

PCB Etching Notes

I did a little research, and found it still took some trial and error to come up with a PCB etching procedure that worked reliably for me. I also noted that, since I don't etch boards all that often, I kept forgetting how I do it. So, with that in mind, here are my notes on PCB Etching for my "Note To Self" category.

Materials

Circuit design

I use Diptrace to design my PCBs
Set the trace width and spacing to 16 mil, if possible. Thicker traces like this work better for hand etching. I've succeeded with 10 mil, but the traces were very thin and uneven. Thicker is better.
To put letters on the board (copper characters), use "Objects / Place Text". Hit Enter to complete text entry. Then right click on the text. Under "Properties" set the "Type" to "Signal". That will leave the letters as copper on the board, and leave some space around it.

Saving Gerbers

Nothing special. Save the Top layer for use in etching.

Print using Gerbv

Note, I use Gerbv in Windows. I don't know how these instructions might work on Linux or Mac systems.
By default, gerbv will show the layer in color, rather than in a strong black monochrome. I set the following properties before printing:

Under "Layer / Change Color"

Set Opacity slider to 255.
Set Color name to "#09090e"
The image will turn black on the screen.

Under "Layer / Modify Orientation"

Set "Mirroring about Y axis"
This adjusts for the fact that we're putting the paper face down, reversing the image on the copper.

Do test prints, check that it's very dark, and that the size is right.
Laser print on the Gloss paper and cut it out.

Copper Clad preparation

Scrub the copper clad with steel wool to get it shiny.
Clean it with Isopropyl alcohol.
Keep fingers away from the surface.

Toner transfer

Use 1 or 2 pieces of scotch tape to adhere the gloss paper, face down, onto the copper clad.
Warm up the Laminator on the 5 mil (hotter) setting.
Run the copper clad, with paper on upper face, through the laminator about a dozen times. I rotate the copper clad end to end as I do it. After about 8 passes, as the copper is getting really hot, I rotate the copper clad 90 degrees and run it through sideways a few times. When I think it's nearly done, I may do a pass or two with the paper side down.
The copper clad should be unpleasantly hot to handle by the time you're done.
After about a dozen passes, drop the copper clad into a pan of water. Keep it submerged for about 5-7 minutes.
Remove the copper clad from the water, and peel off the paper. If all goes well, the paper should pull away leaving the toner on the board.
The toner should be well secured to the board. Go ahead and rub it with your fingertip to make sure that any glue or paper pulp is removed from the bare copper. Swish it in the water and make sure the copper looks clean.
As the toner dries, it's not uncommon for it to turn a bit white. That's OK. When it does that, make sure the bare copper spots (to be etched) do NOT have white on them. If they do, the acid won't etch the copper away. I usually just keep rubbing with my fingertip until the copper areas remain copper colored when the board is dry.

Etching

Dry the board off, and then drop it into a pan of the etchant.
It takes about 10 minutes or so for the copper to come off. Note, the copper turns pink during one stage of the etching. That's NOT the fiberglass PCB. The pink copper is still conductive, and needs to be etched away. Let is soak longer.
When it's done, you should see yellow PCB where the etching occurred.
Remove from the etchant, and rinse with water.
Note, you can save the etchant and reuse it.
After etching is done, remove the toner from the board with acetone. I put some on a few napkins and scrub the board.
Rinse the board off again, until it's nice and clean.
Inspect the copper and traces to make sure it all came out right.

Liquid Tin

I put liquid tin on the board after etching. It protects the copper from tarnishing so that soldering will be more effective later.
Put the PCB in a dish and pour liquid tin on it. The copper will turn silver immediately.
Rinse with water. Save the unused liquid tin.

Monday, January 8, 2018

Hojo and the case of the deaf Tentec Omni V

A friend from the local ham club handed me his Tentec Omni V. He hadn't used it in a while, and was preparing to sell it to another ham when he noticed it appeared to be very weak on receive. He asked me to take a look.

Power supply

Right off the bat, I checked the power supply. It was reading low. The trimmer pot was right next to the activity LED, so I couldn't resist dialing it up to a proper voltage.

Initial problems

I powered up the rig and found the LED display blank. I opened the covers and wiggled a few things directly behind the LED display. It flashed back on. The board was not seated very well, apparently.

At the same time, one of two incandescent bulbs behind the S-meter was intermittent. I snugged up the fixture with a pair of pliers, and the bulb became more reliable.

Having taken care of those little physical problems, I could finally try injecting a signal into the radio. Sure enough, it was quite hard of hearing. I needed a signal of about -20 dbm to be discernible. I confirmed the problem existed across multiple bands.

Diagnostics

I opened up the radio and started tracing the signal. I injected a tone on 14 Mhz (1) and followed the path. It got through the antenna selection circuits and initial attenuator just fine. It reached the front-end mixer board (2), and exited that board mixed to 9 Mhz (3). I also verified that the signal disappeared when the injected signal was stopped. So far so good. The signal passed through the 9 MHZ IF board (3) -> (4) unmolested and was amplified slightly. From there, it went into the Pass Band Tuning board (4), and finally into the AF/IF board (5). There's where the trouble began.

I measured a good signal coming into the board (5 above). It passes through a few amplifier stages first thing. I checked at the end of that stage and found no signal. Tracing back, I found the signal disappeared after passing through a J310 Fet.

I next confirmed that there was power on the "+REG" rail. It was reading 8.78 volts. Unfortunately, I had no idea what the voltage was supposed to be. After about 40 minutes of tracing the +REG rail back to a power board, I found a knockoff sentence in the service manual that +REG should be 8.5 volts. So, it was a touch high, but shouldn't have been a problem for this situation.

Having narrowed it down to this Fet, I checked my parts bin, and voila, I had a few in stock!

The repair

The board had approximately a jillion connectors on it. I carefully photographed everything before removing it.

Now that I had a better view of the FET in question, I confirmed the model, and that my parts-bin FET matched.

I pulled the FET and replaced it.

Testing

After putting things back together, the radio was hearing loud and clear. My Service Monitor can generate signals as low as -130 DBM. The signal was clearly discernible all the way down.

Talking with the owner, he was satisfied that the repair was sufficient, so I stopped there, rather than attempting to find alignment instructions.

Success!

Monday, January 1, 2018

Zeta FX-79 Buffalo Build - Port planning, Smartport and RSSI

Serial Port Planning

The Omnibus F4 Pro V2 offers 3 UARTs.

UART1 - Used for SBUS
UART6 - Available
UART3 / I2C - Available if I2C is not needed

My Ublox Neo-M8N GPS with Compass module will need UART6 for the GPS, and the I2C pins for the Compass functionality. Unfortunately, I want to do Smartport as well, but don't have a UART. So, instead, I'll use softserial for the Smartport connection.

My configuration will be as follows:

UART1 - Used for SBUS
UART6 - GPS
UART3 / I2C - Compass
Softserial - Smartport

Smartport

The Softserial ports are available on the Omnibus F4 Pro V2 via two pads on the front of the board. I followed the FrSky SmartPort using SoftwareSerial procedure at the bottom of the Omnibus F4 page. They call for a 1k ohm resistor between the RX and TX pins. I had one in 0603 in my parts bin, so I put it between the pads, and then attached a small wire-wrap wire to the RX pin. It ended up working fine.

I love surface mount soldering! Here's a 1K 0603 resistor from my parts bin.

There's the resistor, temporarily sitting on top of the processor. I circled the two pads it's going to be crossing below.

Here's the resistor installed, with a bit of Wire Wrap wire connecting to the RX pin.

After confirming that all worked well, I went ahead and secured this wire with a dab of glue to the top of the processor chip, for strain relief.

In order to use the Softserial port, it needs to be enabled in the iNav Configurator.

I enabled Softserial, and while I was at it, also enabled telemetry.

After a reboot, I went into the ports page, found the Softserial port, and configured it for Smartport at 57,600 bps, as suggested.

The final step was to go into the CLI and apply two settings:

set smartport_uart_unidir=OFF
set telemetry_inversion=on

Having done that, I was able to go onto my transmitter, do a "discover", and telemetry values populated from the flight controller.

One additional note. The GPS telemetry does not appear for a "discover" on the radio until you have a GPS lock. This caused me quite a bit of frustration. Once the telemetry is discovered, it will be on the list on the radio, thereafter. You do not have to rediscover it after it is in the list.

RSSI

The Omnibus F4 V2 uses a tiny little pad in the middle of the board to receive RSSI data from the receiver. Many people are soldering a small wire to the pad, and just hanging it off the Flight Controller, with a bunch of glue and hope holding it in place. I didn't really like that solution. Instead, I opted to hijack the RAM pins to make the RSSI externally available.

The RAM pins on the Omnibus F4 pro V2 are wired together, but otherwise isolated, if you do not solder a jumper elsewhere on the board, linking them either to VCC or the 5v rail. Since I left the jumper undone, the RAM pins are just tied to each other. Since I'm powering my Camera and VTX via an external wiring harness, the RAM pins are unused.

I soldered a small jumper from the RSSI pad to one of the RAM pins. I will then solder a heavier gauge wire into the other RAM pin hole, and connect that to the wiring harness going off to the receiver. This should provide greater strain relieve for the RSSI jumper. I think this is a much more resilient solution. Here's the jumper wire going to one of the RAM pins. This picture does not illustrate the wire harness leading off the board.

One thing to note. I was concerned that the RSSI pin might have problems. The MCU on the F4 board is running at 3.3 volts. The receiver runs at 5 volts. I was worried that the RSSI pin might be overdriven by the receiver. I wondered whether a voltage divider would be needed to scale the voltage down. Fortunately, FrSky had considered this. The RSSI value is documented to be a PWM value between 0 - 3.3 volts. So, even though the receiver is running at 5 volts, it is safe to run the wire to the Flight Controller. So, the result was, I didn't have to do anything special. I felt better having researched it, however.