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Old 7th October 2012, 12:44 AM   #581
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Quote:
Originally Posted by EssB View Post
The schematic in fig. 8.4 is a bit fuzzy but readable enough.
If you can read it then why don't you post it? Obviously, if I could read it then I wouldn't ask him for a clear copy!

Thanks for the link. very useful!
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File Type: jpg HP239 AGC.JPG (96.6 KB, 223 views)

Last edited by dirkwright; 7th October 2012 at 12:47 AM.
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Old 7th October 2012, 12:53 AM   #582
EssB is offline EssB  United Kingdom
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Because you already have it! see post 532 Low-distortion Audio-range Oscillator

Last edited by EssB; 7th October 2012 at 12:54 AM. Reason: add post #
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Old 7th October 2012, 01:01 AM   #583
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Quote:
Originally Posted by EssB View Post
Because you already have it! see post 532 Low-distortion Audio-range Oscillator
I'm sorry, I don't follow you. I have that manual. That's where I got the fuzzy figure. There is no figure 8.4 BTW.

Do you mean the attached schematic instead?
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File Type: jpg HP239 AGC zoom.JPG (137.1 KB, 209 views)
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Old 7th October 2012, 01:08 AM   #584
EssB is offline EssB  United Kingdom
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Quote:
Originally Posted by dirkwright View Post
I'm sorry, I don't follow you. I have that manual. That's where I got the fuzzy figure. There is no figure 8.4 BTW.

Do you mean the attached schematic instead?
Yes!
It says Fig. 8-4 in the bottom right corner
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Old 7th October 2012, 04:48 AM   #585
1audio is offline 1audio  United States
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The oscillator circuit it the same as the HP339a, schematic available here: http://bama.edebris.com/manuals/hp/339a look at pgs 103-106. Much better than the HP239 scans, just chopped into several pages. Much simpler than the KH4025 circuit but slower settling.
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Old 7th October 2012, 07:41 AM   #586
coluke is offline coluke  Italy
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Here is a better quality 239A (simplified) schematics.

Resistor values:
Ra = 10k (X 10 range), 1k (X 100, X 1k, X 10k range)
Rb = 100k (X 10 range), 10k (X 100, X 1k range), 1k (X 10k range)
R49 = 2k
C42 = 220u

Ciao,

L.
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Last edited by coluke; 7th October 2012 at 07:56 AM.
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Old 7th October 2012, 12:26 PM   #587
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Quote:
Originally Posted by coluke View Post
Here is a better quality 239A (simplified) schematics.

Resistor values:
Ra = 10k (X 10 range), 1k (X 100, X 1k, X 10k range)
Rb = 100k (X 10 range), 10k (X 100, X 1k range), 1k (X 10k range)
R49 = 2k
C42 = 220u

Ciao,

L.
Thank you!

So, bear with me a minute about what they are dong here. Are they changing these resistors and capacitors for each range to change some kind of filter for each range? Are they changing the settling time based on which range is selected?

Last edited by dirkwright; 7th October 2012 at 12:49 PM.
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Old 7th October 2012, 12:28 PM   #588
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Quote:
Originally Posted by 1audio View Post
The oscillator circuit it the same as the HP339a, schematic available here: http://bama.edebris.com/manuals/hp/339a look at pgs 103-106. Much better than the HP239 scans, just chopped into several pages. Much simpler than the KH4025 circuit but slower settling.
Thank you!
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Old 7th October 2012, 01:02 PM   #589
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Quote:
Originally Posted by coluke View Post
Here is a better quality 239A (simplified) schematics.

Resistor values:
Ra = 10k (X 10 range), 1k (X 100, X 1k, X 10k range)
Rb = 100k (X 10 range), 10k (X 100, X 1k range), 1k (X 10k range)
R49 = 2k
C42 = 220u

Ciao,

L.
Now that I see this circuit more clearly, what is the input impedance for the node where CR1 connects to the output of the oscillator? Since the other end of CR1 is AC connected to ground via a capacitor Ca, I would think it would be a dead short for either some part of each cycle or for some corner frequency that I can't figure out right now. Should there be a buffer to drive this diode?
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Old 7th October 2012, 03:38 PM   #590
coluke is offline coluke  Italy
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Quote:
Originally Posted by dirkwright View Post
Thank you!

So, bear with me a minute about what they are dong here. Are they changing these resistors and capacitors for each range to change some kind of filter for each range? Are they changing the settling time based on which range is selected?
Generator dynamics changes with oscillation frequency, so you have to change ALC loop gain and response time in order to optimize every range for both low THD and short settling time (and stability, too) - in general a single time constant loop can't work (and actually won't) from 10 Hz to 100 kHz with standard peak detectors. Heavy filtering will guarantee low THD but very long settling time; light filtering will shorten settling time at the expense of a much higher detector injected THD - there's a trade off. In the actual 239A THD @ 1 kHz is lower in the 10 X100 range than in the 1 X1k range, but the generator won't probably work @ 10 kHz in the X100 range...

As far as the schematics is concerned, my 239A clone includes a buffer just before the full wave detector, and it performs actually measurably better than the original one.

L.

Last edited by coluke; 7th October 2012 at 03:46 PM.
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