DIY rebuild of the Tek SG505 V3

This Blog post will detail the third stage of my project to rebuild the Tektronix SG505 instrument.

Here is the link to the post describing the second version of the design: 

https://www.paulvdiyblogs.net/2025/08/diy-rebuild-of-tek-sg505-instrument.html

Here is the first post about this project:

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

The reason for the third revision

After the investigations of the second version, I wanted to separate the Power Supply from the Generator in the same enclosure. I also wanted to put the Generator circuits into a full metal enclosure to make the output of the generator as clean as possible and as a minimum, remove the mains hum.

After a very long search, this is the only enclosure I could find that has the required height and width for the front panel layout. It is a ProMa 130 0044 and is also available from Amazon. The outside dimensions are 165x110x80. I would have liked a black enclosure, but alas I couldn't find one. There is one available from ProMa though, with part number 130 0045. I can always spray paint it black myself if I develop the urge. The current Front Panel design will need some modifications and will replace the aluminum panel. The generator PCB will slide in a slot close to the bottom. Unfortunately, the dimensions are a little different so I can't use the current PCB in this enclosure, not even to try it out. 

This all means a new PCB for the Generator, for the Power Supply and for the Front Panel.

Splitting the Power Supply

There will be a separation of the noisy mains related parts of the circuit, that need to go outside of the enclosure for the generator. In essence, it means that the transformer, the bridge rectifier, the main capacitor reservoirs and the 40V regulator need to be on a separate PCB that will be housed outside of the generator.

This part of the project will be described in a dedicated post here:
https://www.paulvdiyblogs.net/2025/11/the-diy-sg505-mains-power-supply.html

The shunt supplies for the +/-16V rails can move to the main generator, and also the 12V supply can move to the main board. They are quiet and will have no negative effect on the generator. I hope. It also makes the interface from the Power Supply enclosure simple, because I will only need to use two wires for the 40V supply that feeds the other three rails.

The circuit after the transformer and the bridge will get some more filtering to avoid mains related noise getting into the generator.

The Power Rails on the Generator PCB

These three rails are very quiet by themselves and can now move to the generator PCB.

No major changes from the previous design, I just added a few extra capacitors and ferrite beads to the power input lines. This may still change a little based on the new layout.

Just when I was about to order the PCB from PCBWay, I decided to skip the idea of using a heat sink for the LM317, because it will still get too hot and raise the temperature in the box. It will now be mounted isolated to the side of the aluminum enclosure.

The Generator circuits

The other circuits stay the same, will just get a revised layout and incorporate the three power rails and needs to fit in the new enclosure.

I finished the new version of the generator PCB, now with the power rails on it.

This is what is ordered. On the bottom part you can see that the LM317 is now flipped around and moved closer to the edge of the board. It will be mounted isolated on the side of the enclosure to remove a considerable heat source.

The Front Panel

This is the updated front panel fitting the new enclosure.

The golden rings around the holes connect the front ground fill to the back ground fill to add an EMI shield to the inside circuits. 

The rings on the back are larger so will connect to the metal parts of the switches, connecting them to the shield. The 4 mounting holes in the corners also have exposed holes on the back and will connect the shield to the aluminum enclosure. 

The enclosure itself is not connected to earth ground but floating. I have created the possibility to connect the GND of the PCB circuits close to the output BNC to an exposed pad on the front panel. In that case, the circuit GND will be connected to earth GND when the BNC is connected to a DSO.

Building up the boards

I received the shipment with the three PCB's and I have built up the power supply by transferring most of the parts from the old board, added the additional parts and tested it. No problems.

The next step was to add the solder paste droplets to the main generator board and transfer the parts one by one from the old board. I used my heat gun to remove them and put them on the new board. When that was done, I reflow soldered the board. Because I used smaller solder paste droplets this time, the reflow process went a lot better, with only a few tiny solder ball bearings and a lot less of the flux gue. I did not clean the board just yet, I wanted to test the functionality first.

Discovering issues

Bad Solder joint

At first I wanted to check the voltage levels of the three power rails. The +16 was only about 9V and then dropped to 3V, the negative 16V was about 30V, the 12V was OK. Although nothing got warm, I quickly shut it off. After connecting the switches and potmeters to the connectors such that the generator could function, I applied power again and saw a welcoming sinewave, albeit with some distortion at the top half. Hmmm, partial good news. When I checked the 16V rails, I still noticed a large unbalance, and that explained the distortion. The good news is that the most complicated circuit seemed to work OK, but the most simple circuit did not, but why?

The hunt for the shunt supply imbalance turned out to become more and more strange. To a point where I started to remove the parts from the shunt supplies one by one, but without any improvement. Using my Lab Supply instead to first power the +16.5V and the -16.5V everything worked, I then supplied the 40V supply, further up-stream and that also showed the correct currents and the system worked fine. Now really puzzled, I used another one of the spare generator boards and started to add the minimum amount of the same parts I just took from the board for the shunt regulator to make it function, which it did flawlessly. Even powering the generator from the second board showed the correct balanced voltages. I was flabbergasted. After thoroughly cleaning the board and resoldering the components back to the board in pairs, everything worked. Bad solder joint!

Wimpy transformer

I also discovered that the mains power supply had a wimpy transformer, so that also needed attention. Details are in the other post: 
https://www.paulvdiyblogs.net/2025/11/the-diy-sg505-mains-power-supply.html

Mistake on the front panel

When putting everything together, I first mounted the construction for the main potmeter and the reduction unit. It fitted perfectly, unlike with the first front panel. So, happy with that result, I added all the other switches and potmeters and proceeded to slide the board into position, when I hit a barrier.

Turns out that I made a serious measurement mistake with the position of the rotary switch for the multiplier. It was bumping to the board and also bumping to the reduction unit. Moving the hole up and left solved that issue, but it will mean another turn of the front panel.

This is how it looks now, so close...

This is the inside view of the now fully working instrument:

Mains related hum

When I did the first FFT tests, I still saw some 50Hz hum and some harmonics. When touching the metal parts of the front panel, it got sometimes worse, sometimes better. I did not have the main potmeter knob mounted, and when I touched the metal axel, the hum got worse. Connecting the Earth GND from the output BNC to the metal parts of the main potmeter and reduction unit with a wire did not do anything, but connecting it to the common GND of the main board reduced the hum dramatically.

It turned out that the mounting holes for the main potmeter support and the reduction unit did not connect the metal parts to the common ground of the PCB. I used star washers on each support to improve that. I also added a blank ring to the layout around one more hole of the contraption to improve that going forward.

Connecting earth GND to common GND?

I intentionally connected the front panel shielding to the metal enclosure to create a Faraday cage, but I separated it from the common GND of the board.  

In my current setup, with the USB connected EMU0202, however, that produced too much unwanted mains related hum. 

I already added a solder tab on the back of the front panel as an option to make the connection possible. When I soldered a wire to it, and connected the other end to a solder lug I added to one of the supports for the main potmeter to connect the two GND's together, it solved the hum issue completely. But now the instrument is earth grounded through the EMU to the laptop, which by the USB-C cable to the power supply is connected to earth GND.

The other possible connection for the instrument to get earth GND connected is through a BNC cable to a CRT or DSO and that will connect it to earth GND as well.

The original SG505 has a switch to connect the common GND to earth GND. If you also want to have the option to separate or connect the enclosure from earth GND, you could add a toggle switch to the back of the unit, or a sliding switch on the side. I'm undecided at this moment, but it's easy to add afterwards.

Result after the fixes

After all these mishaps and corrections, I wanted to share the first FFT from the generator, hot of the press. Note that I was able to quickly trim the second Harmonic visually into oblivion (0.00002%).

Result, no hum, no noise. 

Unfortunately, with H2 visually gone, H3 is now sticking out, but the rest of the harmonics are virtually invisible. 

During my testing with the updated power supply and mounting everything on the front panel a few times while fixing things, I noticed that I could no longer adjust the H2 harmonic as low as it was above. There is now also some hum visible, so when the new supply arrives, I will look at it again in more detail. 

Just for reference, my DIY version of the generator seems a bit better than the original one. Albeit using different measurement tools.

I'm almost there...

Happy with all these results, I uploaded the updated power supply V4 and the updated front panel to PCBWay for production. They gracefully continue to sponsor my activities, despite my screw-ups.😇

It will usually take about a week for the shipment to arrive.

Stay tuned for more information while I go about making the final changes...

A Github repository is available here Note that 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!