Help. FISHER GOLDBUG g2

[Skippy]

"Why are capacitors c2 and c57 placed in parallel and in different places? Why in different places?" Two capacitors are used to obtain the correct resonant frequency. The coil was originally designed for 7.8kHz detectors, like the Fisher F5. Making it work at 19kHz obviously means new capacitor values.
Having two capacitors allows you to fine tune the total C . For example, if the design needs 550 nF, you use 470nF, then 68 / 82 / 100nF to obtain 538 / 552 / 570nF, whichever works best.
The two capacitors can be different dielectric types, with different temperature behaviour. For example putting a small value polyester capacitor in parallel with a larger polypropylene one can give you nearly zero variation with temperature.
Why the position? Well the earliest "revision 1" had two small capacitors ... sometimes with a large cap fitted to small pads. Then later revisions had the big cap fitted properly. Then it was turned around, probably for mechanical reasons, so it fits the PCB better. But ... capacitors do talk to each other if they are close, because there is capacitance between them. Moving them further apart, or putting them at 90 degrees can reduce this effect.

[Arcoman]

"Why are capacitors c2 and c57 placed in parallel and in different places? Why in different places?" Two capacitors are used to obtain the correct resonant frequency. The coil was originally designed for 7.8kHz detectors, like the Fisher F5. Making it work at 19kHz obviously means new capacitor values.
Having two capacitors allows you to fine tune the total C . For example, if the design needs 550 nF, you use 470nF, then 68 / 82 / 100nF to obtain 538 / 552 / 570nF, whichever works best.
The two capacitors can be different dielectric types, with different temperature behaviour. For example putting a small value polyester capacitor in parallel with a larger polypropylene one can give you nearly zero variation with temperature.
Why the position? Well the earliest "revision 1" had two small capacitors ... sometimes with a large cap fitted to small pads. Then later revisions had the big cap fitted properly. Then it was turned around, probably for mechanical reasons, so it fits the PCB better. But ... capacitors do talk to each other if they are close, because there is capacitance between them. Moving them further apart, or putting them at 90 degrees can reduce this effect.

you explain very clearly. I understood that it was a tuning one. but that different type of dielectric = thermal stability, I didn’t guess something. And now it’s clear why there is such a strange inductance tx and px

[Arcoman]

This is required during Tx modification, when this is done after checking amplitude on C57, if is less than 5V resistor can be connected back if it is higher you need protection 4V zener diode.

C57 it main capacitor, C2 is fine tuning capacitor.

without this resistor, it works well too. may not install it. Why the additional voltage on px? I heard that it is being removed. and everyone is happy.

[Arcoman]

This is required during Tx modification, when this is done after checking amplitude on C57, if is less than 5V resistor can be connected back if it is higher you need protection 4V zener diode.

C57 it main capacitor, C2 is fine tuning capacitor.

I understood or rather found what r71 is needed for. it corrects the phase. well, to hell with it. There is another one that also turns the phase. therefore it is clear why the phase left. It was 75 on the mount, it became 68 when I removed it. I was thinking about p 14.

[Arcoman]

Why have you quoted MY post, when ( nearly ) then add 150 nF ( or 100nF plus 68nF ) across C16, to make it about 630 nF.
.

which one is better to put? so that this side = is heat stable. c16 at 400 volts costs. and I haven’t installed u7)) hasn’t arrived yet. I wonder how it will be with her. It all started because of her.

[pito]

which one is better to put? so that this side = is heat stable..

cbb capacitor

[Arcoman]

cbb capacitor

that is, the same. Ок

[Arcoman]

What is the quenching capacitor used for? or at a frequency of 19 kHz it does not work as a damping one. With an increase in its capacity, the current will increase. this device is very interesting.

[Skippy]

"which one is better to put?"
It's not that important in this case, a polyester cap will be OK.
The 27R and 470nF make a high pass filter, with a corner frequency of 12500 Hz. If you reduce resistance to 20 R, the C needs increasing to keep the filter frequency the same.

[Arcoman]

"which one is better to put?"
It's not that important in this case, a polyester cap will be OK.
The 27R and 470nF make a high pass filter, with a corner frequency of 12500 Hz. If you reduce resistance to 20 R, the C needs increasing to keep the filter frequency the same.

and what's in the middle

[Arcoman]

"which one is better to put?"
It's not that important in this case, a polyester cap will be OK.
The 27R and 470nF make a high pass filter, with a corner frequency of 12500 Hz. If you reduce resistance to 20 R, the C needs increasing to keep the filter frequency the same.

since you know everything in such detail. why 12500Hz and not 19? The controller handles these processes. she is carrier. or auxiliary? very interesting. with every post of yours. I understand that I’m not dropping anything.

[Skippy]

The amplifier only has to amplify 19 kHz, and a tiny bit above and below. So it is normal to design the amp so that above 19kHz, the gain gradually reduces; and below 19kHz, the gain also is reduced. This reduces the noise levels from unwanted sources, like 50 / 60 Hz power supplies, radio interference, thermal noise from resistors , and more.
These R-C filters are simple first order types, so their response is gentle. Choosing f_c = 12.5kHz allows the 19kHz signal to pass with little loss of gain, and little phase shift. If the designer had chosen f_c = 16 kHz, the amp would also reduce the wanted 19k, and also change its phase slightly.

[Arcoman]

The amplifier only has to amplify 19 kHz, and a tiny bit above and below. So it is normal to design the amp so that above 19kHz, the gain gradually reduces; and below 19kHz, the gain also is reduced. This reduces the noise levels from unwanted sources, like 50 / 60 Hz power supplies, radio interference, thermal noise from resistors , and more.
These R-C filters are simple first order types, so their response is gentle. Choosing f_c = 12.5kHz allows the 19kHz signal to pass with little loss of gain, and little phase shift. If the designer had chosen f_c = 16 kHz, the amp would also reduce the wanted 19k, and also change its phase slightly.

OK. here a man drew it for me. and states that: “The LC circuit creates a shelf in the frequency response in which the operating point should be at a frequency of 19.2 kHz. In the standard circuit, the operating point is at a frequency of 25 kHz.” and attached the following diagram.

[Arcoman]

OK. here a man drew it for me. and states that: “The LC circuit creates a shelf in the frequency response in which the operating point should be at a frequency of 19.2 kHz. In the standard circuit, the operating point is at a frequency of 25 kHz.” and attached the following diagram.

When the operating point is on the shelf there and the phase depends little on frequency, which increases temperature stability

[Arcoman]

it is necessary to change the choke 220 to 330 and increase from с15 to 470nf