SG505 puzzle

[jackinnj]

my Textronix SG505 had been operating as close to perfectly as one could imagine until this afternoon -- now we have some 120Hz problems -- has anyone ever had the internal +/- 15 V regulation system on this thing go wonky?

 

[anatech]

Hi jackinnj,
Like everything else, yes. How old did you say they were?

The repair is very straight forward and unlikely to affect the calibration, unless the 120 Hz residual was not found at the time of it's last cal.

-Chris

Edit: check your rack / supply also.

 

[jackinnj]

yeah, I will check the pass transistors on the TM5006 before I start screwing around with the SG505 -- I had looked at the residuals back in the early summer and it wasn't a problem at that time so it must be the mainframe. the SG505 was yielding THD of 0.0005% to 0.0008% back at the time I did the article on the Super Oscillator.

the TEK mainframe is on almost all the time.

 

[anatech]

Hi jackinnj,
At least you have a simple problem!

-Chris

 

[jackinnj]

fwiw -- here's the power supply regulator used in the SG505 -- Q2 and Q3 are located in the TM5006 chasis:

An externally hosted image should be here but it was not working when we last tested it.

 

[anatech]

Hi jackinnj,
Did you find the fault?

Wishing you success ......

-Chris

 

[jackinnj]

yes, it was the F29555, F30555 pass transistors -- insulated TO-220 versions of the ubiquitous power supply transistors. I also gave the TM5006 a good airing with the compressor.

 

[anatech]

Both?
Interesting.

 

[jackinnj]

I didn't bother to check -- the SG505 worked well in all the other bays -- for the moment I have replaced the 2955/3055 with another On-Semi device of which I have thousands.

I have an SG502 which is completely dead -- some of the physical construction is identical to the SG505 but the 502 dates from 1971 -- this could be an interesting fixer-upper.

 

[anatech]

I have a couple Tek modules but no rack. Would you sell your 502? I don't mind fixer upper.

-Chris

 

[jackinnj]

the SG502 goes for around $10 on Ebay -- incredible since just the planetary drive potentiometer would cost you $40.

 

[anatech]

Thanks, I'll keep my eyes peeled then.

-Chris

 

[jackinnj]

the SG505 uses 2 stages of amplification -- probably an order of magnitude less distortion than the SG502 -- here's a link to the SG505 schematic:

http://www.tech-diy.com/TestEquipment/SG505/SG505_Oscillator.gif

 

[anatech]

Hi jackinnj,
Very kind. Thank you for the diagram. Now I will have to find one, I have time ....

-Chris

 

[jackinnj]

Let me know when you receive it -- I have one tweak which had a nice effect -- as you might be aware the SG505 has its own regulated power supply -- I placed a 1.25 inch strip of insulated mu-metal around the power supply section and it knocked down some of the 60, 120 and 180 Hz artifacts. These were really out of control when the pass transistors blew.

My guess is that the SG505 would be happiest in a TM501 -- all by itself, or perhaps in a TM503 with a DA4084.

 

[EchoWars]

I know where a SG505 is for sale (not mine)...it isn't cheap though...

 

[anatech]

Hi Glenn,
Thanks. I think I'll take my time on this. It appears as though I'll be off work longer than I coud have dreamed. Not fun.

I have time, little money and less physical ability.

-Chris