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HP339A distortion analyser
HP339A distortion analyser
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Old 9th July 2019, 04:36 AM   #851
slbender is offline slbender  United States
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Join Date: Aug 2004
Location: Queens, New York
Quote:
Originally Posted by 1audio View Post
The 1:3 ratio suggests the range knob is on incorrectly. Check that it goes to the correct end stops first.

Hi guys,

After reading through the mods section abridged by Sync, I noted that for the 1468 at U1 it did state removal of CR7 and CR8 as well as C45 and C48, the mica caps, for the best overall performance. I left the diodes on the board, while not connected to pin 8, or really anything else, except +15 Volts, they are essentially disconnected, except for stray capacitance coupling. I doubt that they still may affect the circuit in a meaningful way.

Last night I made a few more adjustments here and there, tightened up the setscrews on the switch couplings, and made up a shielded 1k ohm load plug, that I wrapped in clear plastic tape, then copper tape. Then covered it with duct tape. I then made one final adjust of the null pots on A4, and installed the A1 shields, and the top and bottom covers.

I was also thinking about replacing R51 on the Q2 Jfet, in each set, with a 1k resistor in series with a 2k 25 turn trimpot, and also replacing the 10k Output Level potentiometers, with a modern 10 turn wirewound pot, but decided I could always do those two things in the future if the results are not so hot. While I have more 2k 25 turn trimpots on order, I have yet to order any 10k 10 turn Output Level type pots.

At this time, both of the HP339A AC Voltmeters agree with my Fluke's ACV readings within about 1/2 %.

However, things are looking quite good.

Unit #2, formerly 12% THD some ten days back, on the Residual Noise test reads -103.0 dB. That is below the scale starting point and over 10dB better than the spec. The 1 kHz. distortion, with the 80 kHz. filter on, at a signal level of 3 Volts came in at -98.0 dB, or around 0.0013% again way better than the spec. The unselected combo of an LT1468 at U1 followed by an LME49710 at U3, apparently gains a couple of dB over the selected HA2625 parts of 35 years ago, but realistically it is an improvement of about -2.5 dB to -3.0 dB.

Unit #3 my old set, with all the original 1983 IC's still installed, clocked -102.5 dB on Residual Noise, and THD down at -96.5 dB or 0.0015%. There is some jitter, and the meter does move down to -97.0 dB back and forth continually, but -96.5 is still a pretty respectful reading. This set was just slightly worse than spec some five years back, when it would come in at 0.0020% to 0.0022%, but I had never adjusted the null trimpots on A4 previously, that I remember.

At TP1 both sets show about 6.68 Volts, +/- just a shade under 20 V. p-p.

So both are quite in the green.

Good enough!

My thanks to all who helped, commented, and made suggestions.


Steven

The photo is Unit #2 Residual Noise at -103 dB.
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File Type: jpg IMG_20190708_222854.jpg (706.1 KB, 93 views)
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Last edited by slbender; 9th July 2019 at 04:44 AM.
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Old 9th July 2019, 02:36 PM   #852
davada is offline davada  Canada
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Location: Fort St John, BC Canada
Excellent.
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Old 9th July 2019, 05:07 PM   #853
anatech is offline anatech  Canada
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Location: Georgetown, On
HP339A distortion analyser
Encouraging. I might actually make time to get into my sets.

-Chris
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Old 16th July 2019, 01:05 AM   #854
RNMarsh is offline RNMarsh  United States
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Join Date: May 2012
Location: 2457 Cascade Trail; Cool, CA. 95614
Quote:
Originally Posted by slbender View Post
Hi guys,

After reading through the mods section abridged by Sync, I noted that for the 1468 at U1 it did state removal of CR7 and CR8 as well as C45 and C48, the mica caps, for the best overall performance. I left the diodes on the board, while not connected to pin 8, or really anything else, except +15 Volts, they are essentially disconnected, except for stray capacitance coupling. I doubt that they still may affect the circuit in a meaningful way.

Last night I made a few more adjustments here and there, tightened up the setscrews on the switch couplings, and made up a shielded 1k ohm load plug, that I wrapped in clear plastic tape, then copper tape. Then covered it with duct tape. I then made one final adjust of the null pots on A4, and installed the A1 shields, and the top and bottom covers.

I was also thinking about replacing R51 on the Q2 Jfet, in each set, with a 1k resistor in series with a 2k 25 turn trimpot, and also replacing the 10k Output Level potentiometers, with a modern 10 turn wirewound pot, but decided I could always do those two things in the future if the results are not so hot. While I have more 2k 25 turn trimpots on order, I have yet to order any 10k 10 turn Output Level type pots.

At this time, both of the HP339A AC Voltmeters agree with my Fluke's ACV readings within about 1/2 %.

However, things are looking quite good.

Unit #2, formerly 12% THD some ten days back, on the Residual Noise test reads -103.0 dB. That is below the scale starting point and over 10dB better than the spec. The 1 kHz. distortion, with the 80 kHz. filter on, at a signal level of 3 Volts came in at -98.0 dB, or around 0.0013% again way better than the spec. The unselected combo of an LT1468 at U1 followed by an LME49710 at U3, apparently gains a couple of dB over the selected HA2625 parts of 35 years ago, but realistically it is an improvement of about -2.5 dB to -3.0 dB.

Unit #3 my old set, with all the original 1983 IC's still installed, clocked -102.5 dB on Residual Noise, and THD down at -96.5 dB or 0.0015%. There is some jitter, and the meter does move down to -97.0 dB back and forth continually, but -96.5 is still a pretty respectful reading. This set was just slightly worse than spec some five years back, when it would come in at 0.0020% to 0.0022%, but I had never adjusted the null trimpots on A4 previously, that I remember.

At TP1 both sets show about 6.68 Volts, +/- just a shade under 20 V. p-p.

So both are quite in the green.

Good enough!

My thanks to all who helped, commented, and made suggestions.


Steven

The photo is Unit #2 Residual Noise at -103 dB.







THx -RNMarsh
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