The Tektronix SG505 instrument is an ultra-low distortion and low-nose oscillator intended for testing of audio grade electronic products. This one came to my possession as a non-working unit and I set-out to fix it.

Front view. Tektronix SG505
Front view

Quick specifications:

Frequency range : 10Hz to 100kHz

Output level : +10dBm to -60dBm into 600 Ohm load

Distortion : less than 0.0008% (-102dB) from 20Hz to 20kHz with a typical figure of 0.0003%

Full specification in the service manual.

As you can see this is an outstanding piece of gear for testing out precious audio gear and DIY projects.

The SG505 is an old piece of an instrument. The exact manufacturing date of my unit is unclear. Close inspection reveals that some IC’s have date codes from year 1979. Being an old tek this SG505 suffers some of the usual problems.

I was happy to discover that my unit had only some old electrolytic capacitor to blame. I do recommend changing the old caps of any old Tektronix unit like this one. I took just 30 minutes and some new parts and this beautiful SG505 was singing again well within specs. The service manual does a great job in explaining the calibration procedure as well.

The calibration is easily preformed and one only needs a good true RMS multimeter. In this case I use my trusty Brymen BM257s. I recommend a good 5 1/2 digit multimeter in case you’ve got one around. Here is the patient waiting for a new capacitor transplantation.

Tektronix SG505 repair and capacitor replacement.
Tektronix SG505 Repair

The service manual is available online here Tektronix SG505 Service Manual

Here are some more photos

Tektronix SG505 inside - transparent PCB solder mask.
Inside showing transparent PCB
Tektronix SG505 inside - top side
Tektronix SG505 inside – top side
Tektronix SG505 inside - back side
Tektronix SG505 inside – back side