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Old 30th December 2012, 11:16 PM   #1391
davada is offline davada  Canada
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Quote:
Originally Posted by scott wurcer View Post
Surprised you can't null the seconds, do you have exactly what you did?
Hi Scott,

I used the exact circuit you posted.
Instead of using 100k resistors I used a 200k multi turn post wire the same.
A 1 meg resistor parallel the PV and a 1K LED bias resistor.

The whole works connected to the jfet as you indicated. The LED was connected directly to the output of the AGC fet driver op amp and the AGC output was disconnected from the jfet.

Nothing else was changed or different from the original connections.

The only thing missing from this configuration is the coupling capacitor in series with the jfet feedback resistor which adds a bit of filtering.

C30, R51, R50 disconnected and floating. PV LED connected to pin 14 through 1K res.

This may have been the difference.
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Old 30th December 2012, 11:18 PM   #1392
dimitri is offline dimitri  United States
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David, are you playing with optocoupler with photovoltaic output and 339a JFET A1Q2 or
with photo FET optocoupler?
http://pewa.panasonic.com/assets/pcs...er-catalog.pdf
http://www.fairchildsemi.com/ds/H1/H11F1M.pdf

Oh, I see... Correct?
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Last edited by dimitri; 30th December 2012 at 11:34 PM.
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Old 30th December 2012, 11:26 PM   #1393
davada is offline davada  Canada
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Quote:
Originally Posted by dimitri View Post
David, are you playing with optocoupler with photovoltaic output and 339a JFET A1Q2 or
with photo FET optocoupler?
http://pewa.panasonic.com/assets/pcs...er-catalog.pdf
http://www.fairchildsemi.com/ds/H1/H11F1M.pdf

Oh, I see...
Hi Dimitri.

The Panasonic APV1122A.

Those other things are terrible.
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Old 30th December 2012, 11:35 PM   #1394
dimitri is offline dimitri  United States
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Quote:
Those other things are terrible.
photo FETs? slow?
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Old 30th December 2012, 11:43 PM   #1395
davada is offline davada  Canada
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photo FETs? slow?
A lot of distortion too for a part that says distortion free in the data sheet.
I was very disappointed.
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Old 31st December 2012, 12:46 AM   #1396
davada is offline davada  Canada
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Default Update on 339a mods

I've done a few more mods to the 339a oscillator and to the balanced modulator board.

I replaced the level pot in the oscillator with a Bourns 10k cermet. This squashed a large 3rd H I just couldn't seem to get rid of. This was a repair.

I replaced the VCR2N with PN4392 Jfet which drove both 2nd and 3rd H down further.

I bypassed the capacitor C30 in series with R51 and added a 100uF electrolytic cap in series with R52. I replace trim pot R30 with a cermet single turn 2K pot.
With some tweaking of the amplitude level trim and distortion trim pots got the following results from the oscillator.

The balance modulator board has R5, R6, R32 and R33 changed to 10k. R7, R8, R34 and R35 changed to 1K. I added a balance 10k trim to A3U4 wired same as the balance trim for A3U2. CR1 and CR7 are removed and R10 and R39 are bypassed with a jumper.
A3U1 was replaced with a 1468 in a earlier mod and R1 is set at 200 ohms.
All trim pots are changed to multi turn cermet pots.

This reduced the noise and distortion when setting the balance trims.
The balance trims are set by probing the test points ATP1 and ATP2 with FFT at 1KHz and tweaking the trims for minimal level of signal and harmonics on the the test points.

Here are the results.

I will confirm this measurement once my Twin T is assembled.
Still waiting on some parts.

Cheers,
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Old 31st December 2012, 04:19 AM   #1397
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Quote:
Originally Posted by davada View Post
Hi Scott,

I used the exact circuit you posted.
Instead of using 100k resistors I used a 200k multi turn post wire the same.
A 1 meg resistor parallel the PV and a 1K LED bias resistor.
Ok, I'll check it out myself, I seem to have a whole week off and nothing but dreary weather ahead. What I want is to check just three or so spot frequencies and I'm happy with fixed oscillators and twin tee filters.
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Old 31st December 2012, 02:27 PM   #1398
vicnic is offline vicnic  Latvia
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Quote:
Originally Posted by davada View Post
Hi Scott,

I used the exact circuit you posted.
Instead of using 100k resistors I used a 200k multi turn post wire the same.
A 1 meg resistor parallel the PV and a 1K LED bias resistor.

The whole works connected to the jfet as you indicated. The LED was connected directly to the output of the AGC fet driver op amp and the AGC output was disconnected from the jfet.

Nothing else was changed or different from the original connections.

The only thing missing from this configuration is the coupling capacitor in series with the jfet feedback resistor which adds a bit of filtering.

C30, R51, R50 disconnected and floating. PV LED connected to pin 14 through 1K res.

This may have been the difference.
I don't like this very big and nonlinear (PV) resistance at FET's gate. In my opinion, PV must be shunted with capacitor (but this make time constant) and so big pot for trimming not needed - you can change this pot to some kohms without problems.
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Old 31st December 2012, 03:07 PM   #1399
davada is offline davada  Canada
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Quote:
Originally Posted by vicnic View Post
I don't like this very big and nonlinear (PV) resistance at FET's gate. In my opinion, PV must be shunted with capacitor (but this make time constant) and so big pot for trimming not needed - you can change this pot to some kohms without problems.

Hi Victor,

I changed the resistors to 1K with no change of behavior. The 2nd H was up about 8dB
from norm. How much nonlinear are we taking here, more than a jfet?

Other than that I found them to work quite well.

Cheers,
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Old 31st December 2012, 03:48 PM   #1400
vicnic is offline vicnic  Latvia
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Quote:
Originally Posted by davada View Post
Hi Victor,

I changed the resistors to 1K with no change of behavior. The 2nd H was up about 8dB
from norm. How much nonlinear are we taking here, more than a jfet?

Other than that I found them to work quite well.

Cheers,
Hi David,
I don't see other reasons for distortions increasing, as circuit at FET's gate. PV also has very big and nonlinear output resistance. I don't know, how big effect it makes - needs calculating or measuring. PV output resistance in series with FET's gate capacitance (also nonlinear).
Maybe you can shunt PV with capacitor (about 1000pF) for experiment...
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