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Old 30th November 2012, 01:04 PM   #931
davada is offline davada  Canada
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Well I was tired when I wrote the above. Maybe I will pursue it more later.
I forgot to re-solder the cap canceler back in and this will effect stability.
Any notch filter will be more stable if it is slightly off tuned from deepest notch but this just brings us back to where we started.

I'm wondering if we really need all the range HP put into the auto tune circuits. Maybe placing a fixed resistor across the LDRs would work and reduce the V across the LDR.

It's difficult to measure the V across the LDR because placing the probe across upsets the auto tune and circuit impedance. I can try using the TEK 7a22 diff amp PI and retuning for the
difference. This may put us in the ball park.

Cheers,

David.
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Old 30th November 2012, 01:58 PM   #932
1audio is offline 1audio  United States
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Quote:
Originally Posted by RNMarsh View Post
Well since it is the LDR... where do I get some to use in a ser/par fashion? Who makes them? make and model to buy??

Thx - Richard
Silonex Silonex Inc.: Products: Audiohm Optocouplers Allied seems to be the US source. You will need to reverse engineer the parts in the box to match to Silonex part numbers. Please share when you figure it out. The Silonex parts should be much lower distortion than the original.
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Old 30th November 2012, 02:07 PM   #933
1audio is offline 1audio  United States
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David-
You would add the resistor in series to reduce the voltage on the LDR, but you might need an additional resistor in parallel if the LDR doesn't get low enough in value. It would reduce the tuning range, but maybe still enough.
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Old 30th November 2012, 06:03 PM   #934
RNMarsh is offline RNMarsh  United States
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Default semi-auto tune

I am Ok with a partial auto tune and partial manual tune for deepest null and lowest distortion.

Last edited by RNMarsh; 30th November 2012 at 06:13 PM.
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Old 30th November 2012, 06:52 PM   #935
RNMarsh is offline RNMarsh  United States
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Off the physical part --> The brand/model of the LDR (Photomodule A3E1 and A3E2 of the HP339A): VACTEK, VACTROL. Mfr P/N 1990-0644 (date code: 28480).
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Old 30th November 2012, 07:48 PM   #936
1audio is offline 1audio  United States
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Unfortunately that number doesn't cross to a Vactrol part number. I found a good PDF on the parts from Vactrol here: VTL5C3/2 Datasheet pdf - Photoconductive Cells and Analog Optoisolators (Vactrols) - PerkinElmer Optoelectronics I don't know who if anyone still owns Vactrol. . . We need at least a dark resistance to figure out what parts are in it. Their charts also show the distortion descending to very low with less than .2 V RMS across the LDR. They also talk about aging.

Reducing the range of the autotune becomes an issue if the manual tune capability is limited. How much resolution dores the switching bring to the nulling? In the ST1700 I believe they left off the last digit or two to save money since it could tune that far.
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Old 30th November 2012, 08:55 PM   #937
RNMarsh is offline RNMarsh  United States
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I saw those pdf also. The web site should have a phone number somewhere to ask what the equiv p/n is now. Or a cross-reference. A clue to the (LD)R value is the fact that the LDR is across a resistor in the op-amp feedback.... (output to neg input) the value limits the range the LDR is effecting the opamp. That value is - 3.48K.
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Old 30th November 2012, 11:26 PM   #938
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
Unfortunately that number doesn't cross to a Vactrol part number. I found a good PDF on the parts from Vactrol here: VTL5C3/2 Datasheet pdf - Photoconductive Cells and Analog Optoisolators (Vactrols) - PerkinElmer Optoelectronics I don't know who if anyone still owns Vactrol. . . We need at least a dark resistance to figure out what parts are in it. Their charts also show the distortion descending to very low with less than .2 V RMS across the LDR. They also talk about aging.

Reducing the range of the autotune becomes an issue if the manual tune capability is limited. How much resolution dores the switching bring to the nulling? In the ST1700 I believe they left off the last digit or two to save money since it could tune that far.
The PerkinElmer LDRs are way up in distortion unless they are operated in the low 10%.
They do better if LP filtered. That is in series with a shunt cap to ground. But that was measured on the 339a. I have yet to re-measure using a spectrum analyzer and I might find it was mostly noise.

I will have the E1 and E2 LDR data ready tonight. They share the same part number.

Cheers,

David.
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Old 30th November 2012, 11:30 PM   #939
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
I saw those pdf also. The web site should have a phone number somewhere to ask what the equiv p/n is now. Or a cross-reference. A clue to the (LD)R value is the fact that the LDR is across a resistor in the op-amp feedback.... (output to neg input) the value limits the range the LDR is effecting the opamp. That value is - 3.48K.
The 3.48k resistor is worked in a very narrow R range. So the LDR can be 400K and up.

I measured 402K with the max drive current into the LDR for the phase detector circuit on the 339a when the LDRs were lifted. The drive circuit was saturated under this condition.

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Old 1st December 2012, 12:03 AM   #940
RNMarsh is offline RNMarsh  United States
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400K across 3.48K wouldnt change the amp gain at all. With full LED current, the LDR value will be lowest.
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