13.8 volts through the KEY PTT jack. Oops

[David Dill]

Dear friends,

I inadvertently sent 13.8 volts through the front panel KEY jack on my Hermes lite.

I reset databases and tried the ring/tip reset.

I also looked over the boards and there is no visible smoking or damage to components.

On reboot, I have all four status lights but

while connecting to Thetis, the two ADC lights D4 and D5 go off.

The main relay connects on Thetis power up but there is no RF signal, 

relays engage with band switching, but only a 600hz tone is heard on some bands, and nothing on others.

While tuning on Thetis, a small noise "wake" is created in the waveform, regardless of band.

Steve helpfully guided me to troubleshoot the power supply short points.

All were >500 ohms except one.  

1V2 to GND is 392 ohms

All readings are:

3v3 to GND  673 OHMS

2V5 to GND 2.95k

1V2 to GND 392 Ohms

Vbias to GND 161.8K

Vsupply to GND 29Meg

VOP to GND 1.63k

VPA to GND 17.2K

+3V3 and +2V5 1.42k

+3V3 and +1V2  1K

+2V5 and +1V2 1.14K

Thanks for any thoughts or clues for troubleshooting this.  

73 all

David Dill

W0NP

[Don W9BHI]

This unfortunately happens quite too often. There is no protection on the board to prevent this. I mentioned this issue on the Facebook forum and the designer of the Square SDR decided it would be a good idea to put a Schottky blocking diode on the PTT line on their next production run. Maybe Steve will implement this sometime in the future.

[Rajiv Dewan]

Indeed so.  I just put a popular high-end European 2m transverter into service and its PTT voltage was 13.6V.  It is more common than we realize.  A Schottky diode with small forward voltage drop is a good solution.  An optoisolator would be even better to protect from any other transients.

Raj, N2RD

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[Steve Haynal]

Hi David,

Is the 1V2 power supply at 1.2V DC when the unit is on? Perhaps the power regulator was damaged.

73,

Steve

kf7o

[David Dill]

Hi Steve

Yes, there is 1.2VDC at 1V2 .

Other voltages to check?

thanks

David

W0NP

[Steve Haynal]

Hi David,

You can check voltages at the points identified on this page:

https://github.com/softerhardware/Hermes-Lite2/wiki/Hardware

It is likely at least bank6 is damaged on the FPGA.This is the bank which connects to PTT. You might find someone willing to replace the FPGA for you.

73,

Steve

kf7o

[Rajiv Dewan]

A good solution is to use an optoisolator like PC817.  You do not even have to roll your own.  They are cheap.  Two 4 channel boards for $8 at Amazon https://a.co/d/bt5kBhe.  A schematic for modifying them for PTT isolation is attached.

Raj, N2RD

On Jan 22, 2025, at 10:15 AM, Don W9BHI <donand...@gmail.com> wrote:

[Ezio Insinna]

Hello Rajiv and all,

Thanks for your suggestion to use an optocoupler but I've two remarks about the schematics

1. The 30K (R1) resistor is way too high to ensure a clean operation.

2. The bridge to connect the grounds is NOT a good solution and should be removed. In fact we want a total isolation and connecting 

to a common ground seems to go against it.

Otherwise it's a good suggestion

73

Ezio - F5MWA / K3EK

To view this discussion visit https://groups.google.com/d/msgid/hermes-lite/83647B06-722E-4AD3-9096-D81EA4F94084%40gmail.com.

[Rajiv Dewan]

Both are good suggestions.  

Rajiv

— Sent from my mobile

Rajiv Dewan

rmd...@gmail.com

On Feb 10, 2025, at 10:34 AM, Ezio Insinna <eziomi...@gmail.com> wrote:



Hello Rajiv and all,

Thanks for your suggestion to use an optocoupler but I've two remarks about the schematics

1. The 30K (R1) resistor is way too high to ensure a clean operation.

2. The bridge to connect the grounds is NOT a good solution and should be removed. In fact we want a total isolation and connecting 

to a common ground seems to go against it.

Otherwise it's a good suggestion

73

Ezio - F5MWA / K3EK

On Mon, Feb 10, 2025 at 2:45 PM Rajiv Dewan <rmd...@gmail.com> wrote:

A good solution is to use an optoisolator like PC817.  You do not even have to roll your own.  They are cheap.  Two 4 channel boards for $8 at Amazon https://a.co/d/bt5kBhe.  A schematic for modifying them for PTT isolation is attached.

Raj, N2RD