DIY rebuild of the Tek SG505 instrument

This post will describe the building of a complete instrument based on the earlier investigation of the Tektronix SG505 Signal Generator described here: 

https://www.paulvdiyblogs.net/2025/03/diy-build-of-tek-sg505.html

Enclosure and Front Panel

I'm going to use the same enclosure as I used for the SG502, which is a TEKLA KL-22. This is a black plastic enclosure with a metal front panel and back panel. I would have liked to use a full metal enclosure but the ones I found where either too small, too low, or too large and mostly all of them had the wrong dimensions to use a front panel.

To get a feel for the proper handling of the knobs and the overall layout, I typically create a mock-up using a paper front panel with a card board backing functioning as the PCB.

I made a number of changes to the initial layout to make sure I can turn and switch everything without cumbersome finger acrobatics. I also want to group controls together as much as possible, and make the positions logical. Also the position of the BNC output and the main frequency control contraption needs to be positioned correctly.

Here is that mock-up in the final version, after spending a few days relocating and printing and mounting the controls again. (this is not the final version (see the PCB below), some text got moved a bit here and there)

Here is the view from the back:

And here the detail for the mounting of the main potmeter contraption.

This is an earlier version, the mounting holes for the bracket line up in the final one. (;-))

After finishing this, I could finelize the design of the Front panel PCB for The KL-22 , so it has a nice looking feel for the instrument. Here is the 3D viewer result of the final version:

The rectangle in the top is the cut-out for the OLED display that will show the frequency. There are no mounting holes on the OLED display board itself, you have to find a way to somehow glue it to the back of the front panel.

All the holes are isolated from the front and back ground poor to add some more strength and EMI blocking. I will order the black solder mask with white silkscreen lettering as I usually do for all of my front panels.

New PCB Layout

I have also started to create a new layout for the main board, and added the relay's for the range switching, as well as the output amplifier. 

Here are the circuits for the frequency range selection, using a simple rotary switch located on the front panel.

This is the frequency setting for the phase shift amps.

Here are the dampening circuits for the AGC and the Peak Detector.

And here is the circuit for the frequency multiplier rotary switch:

A new Power Supply?

I'm very happy with the Jung SuperReg, but it is more than a bit of an overkill for this instrument. I'm now looking into testing a dual shunt supply that I also very recently used for my Twin-T notch filter and the Victor Mickevics oscillator. I'm using Victor's design to start with. I've used LTSpice to learn a bit more about the principle and I think I now know enough to be dangerous and feel brave enough to give it a try. I also need to add the 12V rail for the relay's and feed the Arduino and LCD display.

This will decide whether I'm going to add the components to the main PCB, or create a separate one, and then add the Arduino counter circuit to it.

Well, after some fooling around with prototypes, I finally got it going on two protoboards.

Here is the schematic of the supply:

The prototype below does not have the transformer snubber and the bridge capacitors implemented.

The power LED is optional. If you're going to use the OLED display, it will show that there is power. During testing, I used an LED to tell me that power is on, otherwise I can't see it visually, and before I added it, to my dismay, I left the unit powered on for a whole night. Not good.

The Shunt Supply principle is quite simple, almost literally taken from my Twin-T notch filter design and I only added trimmers for the key voltages and tweaked the current distribution. I'm now also using a much smaller transformer that I can mount on the PCB.

Here is the first result:

For some reason that I do not understand yet, there are more harmonics of the principle visible and you can also see a bit more of the mains frequency. I'm hoping that a real PCB and complete mains and transformer filtering will reduce that.

The top protoboard on the left is the raw supply circuit fed with the ac from the 2x 28VAC transformer. It has the rectification and filtering plus the LM317HVT that feeds the 44V to the dual shunt supply, located on the lower board. The two boards are connected together using the two 91 Ohm resistors that provide the shunt supply current headroom.

I'll do some more long term testing and look at the temperatures, but it looks like this supply is adequate for this application.

I finished the layout of the new power supply. It will be mounted up-side down from the top of the enclosure. There is a normal transformer located on the bottom for the +/- 17V rails, and the top rectangular in black is the 12V DC power module. That circuit is completely isolated from the +/-17V rails.

And finally, here is a 3D picture of the main board:

The 12V supply for the relays, the Arduino and the OLED display is completely separate from the generator circuits itself.

A reconsideration for the front panel

While working on the final layout for the main board and the front panel, I became more and more agitated with the two screws that mount the potmeter reduction unit that are visible on the font panel.

After some hemming and hawing, I bit the bullet and redesigned the hardware construction such that there are no more screws visible on the front panel. It meant that I had to move some parts on the main board out of the way, and add another bracket and holes to the main board. I also used this moment to slightly change some positions and changed some of the silkscreen text.

This is how the front panel looks now:

And here is the new construct for the potmeter and the reduction unit:

The reduction unit is now no longer mounted on the front panel, but has it's own bracket mounted on the main PCB. The two brackets are made of an L shape aluminum 1mm thick and is 20x30mm. Commonly available in DIY stores.

Here is the main PCB:

On 22-aug-2025 I uploaded the three PCB fabrication files to PCBWay and asked them to produce them under the sponsoring agreement. They already passed their review successfully. I hope to get their approval for the sponsoring soon in which case they can be here in 7-10 days.

Fabrication Details

A Github repository is available here it will be updated with information during the project when I have verified the correct operation. I'm still working on it, so there is very little information there at the moment so please be patient.

If you like what you see, please support me by buying me some Java: https://www.buymeacoffee.com/M9ouLVXBdw

For those that already did, thank you!