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After letting the Board run for an hour or so output voltage is 0V but opamp pin 6 is running on -17v again. One of the transistors near the opamp is really hot.
This project is driving me nuts.
After letting the Board run for an hour or so output voltage is 0V but opamp pin 6 is running on -17v again. One of the transistors near the opamp is really hot.
This project is driving me nuts.
This is not a project for beginners, but you can learn a lot and will be rewarded in the end.
"One of the transistors near the opamp is really hot" - this would be one of
Q6 .. Q9, compared your picture in 4485 to that of post 4518, all BC337 ..
Q6 .. Q9, compared your picture in 4485 to that of post 4518, all BC337 ..
Yes.
If opamp pin 6 is running on -17v there seems to be a massive balancing problem again. Thought that I solved that by reducing values of R43a and b. Something gone south here while I was letting the board run? Which side do I have to start looking into if the opamp is running on -17V?
They're 40v parts it's really hard to kill one if it's fed through the shunt. Might be worth scoping the shunt, it could be oscillating at hf. The r2 boards don't have any shunt compensation.
https://live.staticflickr.com/65535/49298518913_f4e729619a_k.jpg
Annotated board above shows all the shunt compensation mods, 33uf on the input is in case you're running long leads between pre reg and main boards.
J113 and 1k resistor are a better choice for the shunt than j107 and 3.3k. Replace all the 16v caps with 30v+, they run at c25v on the input and c18v near the output, thry only last a couple of years if you use the original nichicons. Use Panasonic fc 35v for the 10uf 16v position instead of the blue nichicon, not a low impedance cap.
I've built units that have ten years on them with these cap choices. The first I built needed parts swapping after a few years as they'd start to drift and got crackly.
For the main power rail caps 6.8v and 16v it's a toss up between low esr, super long life, but a bit leaky organic polymer from Wurth or shorter lifespan Panasonic fm, fc, or fk if you want the lowest leakage. These see no real ripple so they last way past rated life. I'd choose the fk caps but they're way hard to find these days.
https://live.staticflickr.com/65535/49298518913_f4e729619a_k.jpg
Annotated board above shows all the shunt compensation mods, 33uf on the input is in case you're running long leads between pre reg and main boards.
J113 and 1k resistor are a better choice for the shunt than j107 and 3.3k. Replace all the 16v caps with 30v+, they run at c25v on the input and c18v near the output, thry only last a couple of years if you use the original nichicons. Use Panasonic fc 35v for the 10uf 16v position instead of the blue nichicon, not a low impedance cap.
I've built units that have ten years on them with these cap choices. The first I built needed parts swapping after a few years as they'd start to drift and got crackly.
For the main power rail caps 6.8v and 16v it's a toss up between low esr, super long life, but a bit leaky organic polymer from Wurth or shorter lifespan Panasonic fm, fc, or fk if you want the lowest leakage. These see no real ripple so they last way past rated life. I'd choose the fk caps but they're way hard to find these days.
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Yesterday evening I powered up the board and took some measures on TP 5,6,9,10. They are all close to the values in the attached pdf.
Opamp PIN 6 was around -3V, output voltage around 0.050v
Then I started to clean the lathe while listening to some music from my old Kuba Tube radio. Every 2 or 3 minutes measuring all voltages again.
Out of a sudden I realized that the output voltage as well as the Opamp PIN 6 was close to 17V again! Looking closer at myself taking these measures I realized that my old multimeter seems to have an issue switching the ranges, because when I turn off the Amp and the Multimeter for a few seconds and then back on again the measurements are back to normal.
Will do some further investigations tonight.
Opamp PIN 6 was around -3V, output voltage around 0.050v
Then I started to clean the lathe while listening to some music from my old Kuba Tube radio. Every 2 or 3 minutes measuring all voltages again.
Out of a sudden I realized that the output voltage as well as the Opamp PIN 6 was close to 17V again! Looking closer at myself taking these measures I realized that my old multimeter seems to have an issue switching the ranges, because when I turn off the Amp and the Multimeter for a few seconds and then back on again the measurements are back to normal.
Will do some further investigations tonight.
That makes sense. If the output offset is in the low mV range it's all good. 50mV is fine, it'll improve as the caps form.
Next test sesseion. Measurements directly taken after the start. Different multimeter.
TP9,10 close to 17V
TP5,6 13,2V and 14,2V
Opamp 17V
Output 12V
Turning the trimpot does not have any influence.
Why do these values jump from one day to another?
So multimeter is out of the mix. Back to the Board.
TP9,10 close to 17V
TP5,6 13,2V and 14,2V
Opamp 17V
Output 12V
Turning the trimpot does not have any influence.
Why do these values jump from one day to another?
So multimeter is out of the mix. Back to the Board.
At 17v that opamp is hard up against the rail. If you've got a scope get it on the side where the transistors were heating up, see if its oscillating.
So here we are again.
On the TP we have -11,4V, -11,5V vs 17,5V, 17,5V. Opamp -18V, Output -7V. Again totally different.
That is the output. Ext. Riaa looks the same.
Test points look like this.
On the TP we have -11,4V, -11,5V vs 17,5V, 17,5V. Opamp -18V, Output -7V. Again totally different.
That is the output. Ext. Riaa looks the same.
Test points look like this.
looking like noise, not oscillation. does it disappear when you shorten the input? (this could be just the white noise from the input resistance, amplified)
Also, if I remember correctly, the input bias should be adjusted without input connection (or maybe 10k to ground), so that even a small current gives a meaningful reading. Then the input is shortened and the servo should do its magic. The DC resistance of a MC cartridge is close enough to zero to do the same.
the gain of a phono stage is highest at lowest frequencies, and the paradise is no exception. so even a small input bias current offset may cause the servo to no longer be able to compensate, as it is designed to be relatively "weak". R43a,b lower values will make it stronger. And, if you have a MC cartridge connected and look at the output DC voltage, then pull the input cable out and see a jump in output voltage, it is an indication that the input bias needs adjusting.
just my two cents....
Also, if I remember correctly, the input bias should be adjusted without input connection (or maybe 10k to ground), so that even a small current gives a meaningful reading. Then the input is shortened and the servo should do its magic. The DC resistance of a MC cartridge is close enough to zero to do the same.
the gain of a phono stage is highest at lowest frequencies, and the paradise is no exception. so even a small input bias current offset may cause the servo to no longer be able to compensate, as it is designed to be relatively "weak". R43a,b lower values will make it stronger. And, if you have a MC cartridge connected and look at the output DC voltage, then pull the input cable out and see a jump in output voltage, it is an indication that the input bias needs adjusting.
just my two cents....