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Old 7th December 2012, 02:04 AM   #1061
davada is offline davada  Canada
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Default auto set level

This cleaned up nicely with an LT1468. There is a little bit of something just above 3KHz but it very small.
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Old 7th December 2012, 04:15 AM   #1062
davada is offline davada  Canada
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Default With no signal

I realized something tonight about the operation of the 339a. When there is no signal there is nothing for the notch filter's balance modulators to lock onto. Consequently the notch filter goes to high gain. There is leakage from the oscillator and this is picked up and amplified. Once this passes through the distortion amplifier in high sensitivity settings -80dB range the signal is so strong it puts the distortion amplifier into hard clipping. So hard it can produce a square wave form at the output of the distortion amplifier.

Ill see if I can trace the leakage from the oscillator and confirm the effect by stopping the oscillator. I think this leakage maybe driving the meter reading higher.

Replacing the distortion meter op amp cured the problem with the meter pinning when the LP filters were switched out and the ground problem with the scope cleared up as well.
It was parasitic oscillation causing this. However the 339a does not like my PC ground.
It still drives the meter up.

In the 339a manual in the manual changes section at the bottom there are some changes to the filters to clear up a parasitic oscillation at 5MHz. It might be worth having a look at
this section. Depending on the vintage these changes may have been made.

Cheers,
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Old 7th December 2012, 04:26 AM   #1063
RNMarsh is offline RNMarsh  United States
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Default Nice !! results


Boy, you are making me drool with anticipation of recieving my OPAmps.... still waiting.
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Old 7th December 2012, 04:32 AM   #1064
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by davada View Post
This cleaned up nicely with an LT1468. There is a little bit of something just above 3KHz but it very small.
LT1468... where? which IC got replaced? U4?
Thx-RNMarsh

Last edited by RNMarsh; 7th December 2012 at 04:36 AM.
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Old 7th December 2012, 04:48 AM   #1065
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
LT1468... where? which IC got replaced? U4?
Thx-RNMarsh
A2U10. Auto set level op amp.

I had changed this to an LT1122 two years ago and had forgotten.

Bad choice at the time, it was oscillating.

One thing to keep in mind with the LT1468 is it has a dual compensation slope. At unity and up to about gains of 35 it has a GBP of 25MHz.
There there is a transition range to a GBP of 90MHz at gains above 40 or so.
Once compensated properly with a feedback cap it's quite well behaved. It it beneficial to include an isolation resistor right at the output pin of the op amp.
This resistor isolates the output from stray capacitance preventing parasitic oscillations.

See Design Features of the LT1468 and the data sheet for feedback compensation. There is a formula.


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Last edited by davada; 7th December 2012 at 05:05 AM.
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Old 7th December 2012, 05:22 AM   #1066
RNMarsh is offline RNMarsh  United States
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Default Greening of the HP339A - How to:

U10 was on my list to replace. That pretty much cleans up and clears up the old 339A for another lease on life... if it doesnt wear out... cheap-o switches and all. My 400Hz filter IN/OUT switch just quit making contact. Before I replace it, i just jumpered the filter in all the time. May never use this below 400Hz anyway.... but I do have a spare junker 339A for parts....

Parts and IC updates and a few gain shuffling here and there and we have a new THD meter. With a pretty low distortion choice of oscillator frequencies. Thx for working with me on this project. I'll finish it off in the next few days -RNMarsh

Last edited by RNMarsh; 7th December 2012 at 05:30 AM. Reason: variable frequencies - at low THD.
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Old 7th December 2012, 05:44 AM   #1067
RNMarsh is offline RNMarsh  United States
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Anyone contemplating these upgrades -- I recommend a desoldering tool which will remove IC and other parts quick and painlessly. I use a HAKKO model 808. Parts will just fall out after you use it on them. -RNM
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Old 7th December 2012, 01:52 PM   #1068
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
U10 was on my list to replace. That pretty much cleans up and clears up the old 339A for another lease on life... if it doesnt wear out... cheap-o switches and all. My 400Hz filter IN/OUT switch just quit making contact. Before I replace it, i just jumpered the filter in all the time. May never use this below 400Hz anyway.... but I do have a spare junker 339A for parts....

Parts and IC updates and a few gain shuffling here and there and we have a new THD meter. With a pretty low distortion choice of oscillator frequencies. Thx for working with me on this project. I'll finish it off in the next few days -RNMarsh
Hi Rick,

I think there is a lot more that can be done here to improve on things.
Rather than posting on each new find I will just work away at it and then do a final post and summery. Perhaps you can give a summery of what you found so far and post it. What caps were changed and to what value. We need to do a performance check every now and then to be sure we're not just chipping bandwidth away or causing a ripple. That would be cheating.

Cheers,
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Old 7th December 2012, 04:36 PM   #1069
RNMarsh is offline RNMarsh  United States
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Perhaps this is Stage One of the upgrades/mods. I have to do more opamp replacements and see how it goes from there and the results.
One can get 10-12 dB improvement by replacing passives with better electrolytic caps (NP) and trimmer pots.
Meanwhile, we sort of monopolized this forum for a stint --- now I'm (temporarily) giving it back to other's oscillators, etc. Thx-RNMarsh

Last edited by RNMarsh; 7th December 2012 at 04:54 PM.
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Old 8th December 2012, 12:12 AM   #1070
RNMarsh is offline RNMarsh  United States
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Default An Ooops Update:

Quote:
Originally Posted by davada View Post
Hi Rick,

We need to do a performance check every now and then to be sure we're not just chipping bandwidth away or causing a ripple. That would be cheating.

Cheers,
After I repaired the intermittant contacts of the 400Hz filter switch (in series with the sine wave), the THD went back up! Apparently, the increased series R (switch contact) dropped the signal level during thd reading and it looked better than it is. But, it ain't bad, either.

Now the HD components without the most important IC's being replaced yet, are: .0007% H2 (-105dB) and .0001% H3 (-115dB). THD+N as read on the front panel meter: .0008% or just under the .001% mark. And, it's going to get better! Can't wait. That's, again, with all filters IN. Thx-RNMarsh
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