SG505 Op1 Restoration and Distortion Measurements

[@PrimeHiFi]

Awesome video and work. I recently restored my AA501 and SG505 test set and had them calibrated. They’re super helpful pieces of equipment in my HiFi repair lab.

[@leonardpeters3266]

That QA403 is a slick outfit. I would like to afford myself a Tek distortion setup. But I have a hard time justifying the expense.

[@ZenwizardStudios]

Other than an input voltage limitation the QA403 can replace 2x SG505's and 2X aa501's it is very will positioned for testing 2 channel amps. But a dummy load it needed to soak the amp current. AA501's have a 200Vac input rating where if i remember correctly the QA403 is 32dBv.

[@leonardpeters3266]

I guess I didn't look close enough.I didn't realize it had its own generator. That really is slick. Looks like they won't have more manufactured for a little bit. In demand for obvious reasons.

[@ZenwizardStudios]

Even though it is sampled the DAC's in the qa403 are really impressive. @TheSignalPath does a really deep dive in to the circuits as well as shows some loop back of the generator Video can be found here on his channel : kzbin.info/www/bejne/eJ26q6ODoL1oh7M

[@tomaskey6844]

Nice. Can’t wait to see these units put to use.

[@dominicokelly9160]

The SG505 is an amazing piece of equipment… if you see one- buy it!

[@willthecat3861]

No don't. 1) it's older than you. 2) 99.9% of you don't need it. 3)many of you don't know what harmonic distortion is, how to measure it, and what the lower bound on it should be. That's my experience. That's my opinion, and I am sticking to it.

[@dominicokelly9160]

@@willthecat3861 I’m sure most people watching this video are doing so because they have an interest in an ultra low distortion analyser and may very well have extensive experience in the field- but good on you for assuming that you know more than the other viewers. But aside from that the fact that it’s old is in many cases a bonus because it uses discrete components. Plus they’re generally not that expensive… in my opinion it’s excellent value for money.

[@FireandFrostHVAC]

Good video. Why did you upgrade the capacitance to 1500uf from 470? Does adding that much extra filtering help the THD or is there another reason? Thanks Zen.

[@ZenwizardStudios]

I am not sure if I did it or if tek did it in later models. I could tell from the boards that this was a very early production unit. My later serial number has less tantalum caps and more electrolytic ones. With some higher values. The TLDR : is quieter power rails will do nothing but help the performance of the plugin. In this case it was measurable. Increasing the capacitance on the power supply rails will have an impact on the THD but it will also help the active circuits find equilibrium easier and not have to work as hard. Given that there is less noise on the power rails. The more stable the rails the less the regulators need to work to maintain their regulation set points. Capacitance on power rails (NOT RF circuits) can be increased with caution in 2 places. Inrush current when the capacitor first charges at power on. The component stressed here is the rectifying diodes. For larger capacitors they will need to handle the inrush current which can be several AMPS momentarily. This is why in the boards that I am designing you find a 10 AMP diode in a board for 300ma. At switch on that rectifier needs to charge a 10,000 uF cap. So for about ½ a second it pulls a lot of juice. On tube circuits filter caps are much more fragile this is because of the rectifier tubes. If the bulk capacitors are to large the bonding wires in the tubes can turn in to fuses and blow off this will unfortunately destroy the tube. In this case my SG505 has been running with this cap value for years so I was relatively sure an also triple value increase was ok. Also modern manufacturing has increased the capacitance per volume calculation so I was able to achieve 3x the capacitance with almost the same physical space (caps were a little bigger but not much.

[@jenniferwhitewolf3784]

This is the nuts and bolts analog lab stuff I really enjoy. Nice to see you getting the bench ready to work on our AA501s and SG505s. Would the smaller caps that were exchanged also benefit from a uF value increase? Sorry I don’t have a schematic handy to follow along with as I should. Extra filtering is usually of some benefit up until haversine peak charging current becomes an issue. Jumping from 47 to 100 won’t usually be a charge current problem, and with 2x the time constant, and holding capacity.. I am assuming those are DC rail filters. Other functions, may not want to change value.

[@ZenwizardStudios]

The two big caps that were changed were the main rail filters right after the rectifiers. one for the + rail and one for the - rail.

[@johnglass]

Really enjoying your continued focus on the TM 500 series and especially the AA501/SG505 videos. Thanks for offering this unusual content.