Saturday, March 28, 2020

GPIB - USB GPIB controler arduino based

This will be part of a series for test instrument remote control, I started this with my own "instrument" design and code a USB temperature measurement on this blog post and bellow hardware:

 This time and because I have several branded instruments with GPIB bus control, decided to try this nice project  that consist of an Arduino connected to a GPIB plug and acting as a poor man GPIB controller.

The idea of the original project was to create an affordable alternative to more expensive solution of USB to GPIB, PCMCIA or even PCI GPIB cards that still a bit on the expensive side for the common amateur.

Basically only had to follow the instructions on the project page, I used an GPIB cable cut in half since it was cheaper than just buying the plugs and can still make a second unit with the half left:

... I chose to label all the cables to pin numbers to keep it easy, otherwise I could had messed up. The cabling stayed a bit squeezed because I under estimated the box size needed.

Used a 1.5m GPIB instrument cable with plugs in both ends.

Another view of the "mess":

For the usage of the system, bellow an example with the Agilent 6621A power supply on GPIB address 1:

Basically, you set the GPIB instrument address (you want to comunicate with) with "++addr" in my case the power supply is set to 1, then you set output 1 to 5 volts (VSET1,5) and then "ask" the instrument to read the output 1 value (VOUT?1 and ++read). The 9.99v on the image was the output of a previous testing setting to 10V.

For the configuration of the USB port and output I get on a Linux system it's like this:

115200 baud 8N1, in my case the device is loaded under /dev/ttyACM0

 I had some testing with other Instruments:

R & S NRVS power meter:

Marconi 6960A power meter:

Besides the number of pins to connect from the GPIB plug to the Arduino the hardest part is going trough the instruments GPIB programing manual and trying to check which of the commands work. Interesting enough the one that should be standard; get instrument id (ID?) I never got and answer using this board when testing.

Right now this project is a backup option/solution because I manage to get a very cheap PCI32 GPIB card and am using it at the moment. I will provide some more info on that on the future. 

Have a nice day and stay safe!

Saturday, March 14, 2020

HP/Agilent 6621A removing fan noise.

This "mod" not only takes all the fan noise from the Agilent/HP 6621A as it goes a step forward, disables it completely...

I got this power supply to equip the lab with something professional grade, it is in fact a really nice power supply with lots of features but has a fan that no one can stand being near by. Imagine a vacuum cleaner to have an idea.

In the past I did a box to bring the two output from the back to the front along with a voltage meter (converted one of 250v to 25) for visual reference of the output, it's easy than looking at the LCD, below indicating 5V on output 2:

Back to the fan noise, initially was thinking in making a thermostat control to actuate only when needed but I'm not going to use the equipment that often so just disconnected the fan, now I can only hear the hum from the massive transformer, that is still noticeable and something I didn't eared/noticed before. To have an idea the power supply weights around 17Kg

The supply has an internal thermal shutdown circuit so hopefully that will be enough for day to day and low current supply. There is also a small gap on the back panel that latter on I can put a switch to enable the fan without the need to open it.

Here's the fan connection:

and without the plug... end of the fan noise:

Interesting enough the fan is a 115V version, here's the diagram:

as you can see the fan will connect to mains transformer terminals 4 and 5 (120V), clever design.

The massive transformer inside:

and one of the two power modules, amazing engineering:

I might revisit this in the future if I decide on thermostat control.

Have a great day!

Sunday, March 08, 2020

FM deviator meter

Not much, here, found it interesting to have a deviation meter so on those rare occasions I'm working on FM will have something to base measures on.

This one does not comes with incorporated receiver as "traditional" ones do but relies on the discriminator out from a radio receiver or scanner. It will work in tandem with my scanner from previous post.

The circuit is based on the work by VK5DJ and VK5VI
Schematic is this one:

For 1mA panel meter I found 9K1 resistor in series from pin 7 (R15) on IC2 to be enough.

If you download the file from VK5DJ site there will be some mod's and better resolution diagrams, including "veroboard" design.

The end result, here on top of the COM205 scanner:

another view:


...still need to calibrate and "fill" the table on the front panel because It's easier and faster than making an overlay for the 1mA meter

Have a nice day!

Sunday, March 01, 2020

Commtel COM 205 / Realistic PRO 2006 EL backlight circuit and discriminator output

Recently I acquired a Commtel COM205 scanner, for Europe, that is the same as a Realistic PRO 2006 in other parts of the world.

The idea is to use the discriminator out to make a simple deviation meter.

While I bough it second hand I made sure it had the EL backlight was still working and strong just because they have a tendency to fail or dim and I didn't want to install a new one.

Upon receiving, the light only worked for a brief second, immediately I imagine it had failed somehow during transit.
Started to troubleshoot and at some point was convinced it was the high voltage powering circuit that filed because it looked like a very low voltage to power an EL panel (55V) when I was expected thinking that would be in the undred.

After some pocking and even replacing one of the capacitors in the oscillator circuit that generates the high voltage that I realized that was a bad contact on the EL terminals. Good, no more troubleshooting. Anyhow used the occasion to clean some soldering job from previous owner and now it's working as it should.

The back light circuit along with my measurement:

I suspect that if ever fails it will be wither C506 or Q508

The EL panel voltage can be measured at the two terminal above T501 (upper right corner):

If you need to change the EL panel, there's a nice description of the process here. I suspect that replacing with LED will be valid also but there's not much space to place them, maybe a led strip would do the job. You only need the back light for visibility in low light conditions (during the night), during the day it's readable without it.

I used the fact that the box was open from the previous job to include the discriminator out plug and the connection to the test point.
The operation is simple and better detailed from this external post.

My job:

The jack fitted above the DC input:

The inside of the jack:
Connection to the test point TP2 (a vertical resistor):
then a series 12K resistor and a parallel to ground 2.2nF capacitor (I had to parallel 2*1nF).

Another view:
On the output of the jack these are the values obtained (DC and AC): I need to finish the deviation meter.

Have a nice day!