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Old 31st Mar 2017, 10:04 am   #21
Chaparal
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Default Re: How does this circuit work?

>>I did wonder about having a variable resistor in the feedback loop

I think they choose it to avoid extra shielding - because the Poti is at the front panel and the Op-Amp somewhere at the motherboard

Martin - if you find red elko's (ROE) - change them all

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Old 31st Mar 2017, 10:11 am   #22
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Default Re: How does this circuit work?

Quote:
Originally Posted by Chaparal View Post
This 'Monitor Amp' has a Main Amplifier with filters and attenuators in front
(Input to D3)
Peter
Hi Peter,

you are right. The "main amplifier" is just on top of the "audio amplifier", built with 2 of the same OpAmps.

I attach a bigger picture of the circuit, hope its readable.

cheers
Martin
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Old 31st Mar 2017, 10:23 am   #23
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Default Re: How does this circuit work?

Martin

do you have a datasheet from 'µAF 772 TC' Op-Amp ?
I can't find it.
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Old 31st Mar 2017, 10:50 am   #24
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Default Re: How does this circuit work?

Quote:
What do you think of C23 (on pin5 of the OpAmp) being leaky? Its the same type of black electrolyte miniature of which I had two failed (see my other thread).
You would definitely hear some issues with the audio if that cap is failing.

However, it depends what you mean by a crackling sound. I'd expect a C23 failure to cause fast tripping or muting on audio peaks on certain volume settings. Would this sound like crackling though?

Does the mute comparator opamp section toggle the comparator output on a scope when you hear the 'crackle' sound? Look at opamp pin7 or the circled TP10 on a scope to see if the system is rapidly gating the mute system on speech peaks? The job of C23 is to damp this out and provide a graceful entry in to mute rather than a staccato effect on speech peaks?
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Old 31st Mar 2017, 11:00 am   #25
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Default Re: How does this circuit work?

Also, measure the DC voltage at pin 6 of the opamp and keep a note of it. The setting of the 4.7k trimmer resistor next to pin 6 of the opamp sets the threshold of the muting/squelch at a low negative DC voltage. This setting is unlikely to have been tweaked by anyone but you may wish to adjust the muting characteristic even after you replace C23.

Quote:
Its the same type of black electrolyte miniature of which I had two failed (see my other thread).
How many more caps are there that are the same type as C23 in the test set I wonder?

If C23 turns out to be OK then maybe the system is going unstable at certain volume settings. I'd expect the three ceramic caps around the audio amp (68p, 330pF values) to be reliable. They are probably there to help with system stability up at very high frequencies. The 27.4R damping/load resistor across the speaker is probably going to help with stability and the output 100uF cap here needs the be healthy with a low ESR. It might go unstable if someone has fitted different transistor types in the audio amplifier? If it goes unstable it will probably do it at a much higher frequency. It could go unstable at several hundred kHz and chop up the audio?

But definitely replace C23 if this type of cap has a history of failing in your unit.
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Old 1st Apr 2017, 1:43 am   #26
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Default Re: How does this circuit work?

It does sound like there could be two separate issues here.

The crackling at the low audio position is possibly the mute circuit kicking in. Maybe a fresh C23 will clean up how this happens and you could experiment with tweaking the 4k7 pot R51 next to pin 6 of the opamp/comparator to see if you can shift the mute to allow a lower audio level before it mutes.

Without this comparator based mute feature I doubt it would be possible to achieve a satisfactory level of quietness with just the 47k volume pot on its own when set to minimum. It's bound to have some residual resistance so someone wearing headphones may still hear enough leakage to be annoyed when the volume pot is turned fully down.

So maybe the mute is there to totally blank like a squelch. The 33.2k resistor is probably there to adapt the DC level of the amp as the volume is turned up to steer things away from the ballpark -300mV mute threshold set by R51. This will stop the comparator falsely muting at high volume settings.

Quote:
When I put up the volume, at one point it gets distorted and asymmetrical. When that happens, R37 and R40 get very hot, with R40 significantly hotter than R37.
Not sure why this happens yet. Could be instability and/or a faulty part somewhere.

At the moment the only thing I can add is that the reason that the negative transistor has a 27R collector resistor R40 and the positive transistor has R37 up at 68R is because of the approx -5V DC shift at the AF amp output at full volume control settings. The -5V shift means the negative side has less headroom for the AF waveform wrt -15V so it needs a lower value resistor here (27R rather than 68R) to be able to deliver enough current and keep the system in closed loop at highish volume settings without any clipping.

But at some point it will clip and the output will distort. However. if it stays in closed loop I'd expect this amp to deliver quite low distortion despite its clunky looking circuit. TP9 should then behave as a low distortion voltage source with a very low source impedance.
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Old 1st Apr 2017, 6:51 am   #27
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Default Re: How does this circuit work?

G0HZU_JMR

Attention to message #16

250 not 300 mV

Peter
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Old 1st Apr 2017, 6:41 pm   #28
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Default Re: How does this circuit work?

Thanks Peter, I missed that. If the official setting here is -250mV then I guess we can predict the setting of the volume pot where it should mute. The DC input current via the 33.2k resistor into the node at pin2 is 5.5/33200 = 0.166mA. So to be at the -0.25V mute threshold at the amplifier output at TP9 the feedback resistance for the volume pot would be 0.250V/0.000166A = 1505 ohms.

Presumably it will be a logarithmic 47k pot? So 1505 ohm would be quite a few degrees of movement? Also, I'd expect that the level of sound from the speaker would be quite audible at this threshold? So the sudden drop into mute could be quite aggressive? Lowering the voltage from -0.25V to -0.1V with the 4k7 trimpot R51 would mean that the drop into mute would happen at a lower volume with the volume pot at 600 ohms? But this may bring some side effects of its own.
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Old 4th Apr 2017, 8:37 pm   #29
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Default Re: How does this circuit work?

Hi all,

thanks for your precious input. Meanwhile I found the time to make some few experiments and measurements, here are the results:

The circuit seems to do some sort of audio-volume-autoranging. Meaning that, when I raise the volume, either with the integrated generators or by an external generator, the volume in the loudspeaker at first raises too, at some point settles back to a lower level, starts to raise again and so forth.

I noted the voltages around the OpAmp with volume 0:
pin1: -0,979V
TP9: -1,083V
pin5: -1,049V
pin6: -0,381V
pin7: -12,80V

At one point I noticed the OpAmp getting hot and not working anymore as usual (no audio). After power-cycling the unit it went back to normal. I didnt have the scope on, and remember hearing a faint high pitched noise. Maybe there was oscillation, possibly induced by accidentally short-circuiting a pair of pins of the OpAmp during voltage measurements. Perhaps some of the voltage measurements are invalid due to that.

So far for today, cheers
Martin
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Old 4th Apr 2017, 8:48 pm   #30
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Default Re: How does this circuit work?

Quote:
Originally Posted by Chaparal View Post
Martin - if you find red elko's (ROE) - change them all
There are indeed 3 red Roedersteins on the board (apart from a hundred of the black electrolytics). They are C42, 43 and 44, all used to decouple the audio from the various inputs. They are located close to the switch D10 which is above R137.

Since the audio passes the main amplifier and C3 and looks well I do not suspects these Roedersteins for the moment.

cheers
Martin
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Old 4th Apr 2017, 9:37 pm   #31
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Default Re: How does this circuit work?

My first thought is that the voltage at pin1 should only be slightly below 0V at minimum volume. I was expecting maybe -0.02V and not -0.979V. This assumes that the volume pot resistance is low at 0 volume. eg less than a hundred ohms.

Maybe something is different about your circuit compared to the official circuit but your voltages suggest to me that the opamp isn't functioning as expected at 0 volume. What is the voltage at pin 4 and pin 8 of the opamp when it is like this? Also the voltage at pin 2?

The only thing I can think of is that the opamp is drawing more current than usual on the negative side and this is turning on TR4 slightly. Hence the small reverse current back into the opamp via the 100R resistor. But the opamp should be trying to do something about all this and it doesn't seem to be fighting this seemingly illegal condition.

I would try another opamp and maybe also replace transistor T4 in case it is leaky from C-E but maybe just measure those voltages at pins 4 and 8 and pin 2 first?

The only other thing I can think of is that the analogue switch D3 may have a problem and it is directly summing in a fairly strong current to the opamp node at pin 2 of the opamp. i.e. the switch has an illegal positive dc offset that is summing current into the opamp as an error signal from its pins 4 and 9. This could be due to an internal leakage fault in the analogue switch. This might cause the opamp to behave oddly in a way that isn't obvious from a quick look at the circuit.
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Old 4th Apr 2017, 11:34 pm   #32
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Default Re: How does this circuit work?

If the switch was faulty it might be leaking current into the opamp node at pin2 of the opamp as per the image below. This might fit your symptoms?

A slightly brave way to test for this would be to deliberately short pins 2 and 3 of the opamp together with a 100R resistor at a high volume pot setting.

Don't do this test at a very low volume pot setting or you could overheat the volume pot!

This would then force the opamp to (almost) ignore all summing inputs from the AF or the 33.2k resistor (or the leaky switch?) and the output voltage at the output of the opamp at pin 1 should go very close to zero volts. TP9 should also go very close to 0V if all is healthy in the opamp and transistors?

If it does go to 0V at pin1 with this test then maybe the switch is leaking current into the AF stage?

It shouldn't harm anything to do this test as long as you don't connect the 100R resistor across the wrong pins on the opamp? Make sure you connect pins 2 and 3 together with the resistor and not something else!

Ideally you could short pin 2 directly to pin 3 to do this test but there is a slight risk of passing lots of current through the volume pot if you do this. Only try directly shorting pin 2 and 3 together with the volume pot set to mid or high. Probably not wise to short pin 2 and 3 at low volume pot settings in case one of the transistors tries to dump current through the volume pot via the 27R or 68R resistor. The volume pot could get hot like this and maybe even degrade the wiper contact.
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Old 4th Apr 2017, 11:52 pm   #33
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Default Re: How does this circuit work?

A safer (wiser but slower?) way to test for switch leakage would be to remove the opamp and the 33.2k resistor and disconnect to volume pot and see if there is a DC voltage at the switch at pins 4 and 9? See how much current it can deliver?
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Old 5th Apr 2017, 12:27 am   #34
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Default Re: How does this circuit work?

Whoops! My suggestion that connects pin 2 and 3 together might not give predictable results at pin1 of the opamp. The output pin1 would become uncontrolled if pin 2 and 3 were shorted so it wouldn't prove anything. Sorry, it was a poor suggestion in post #32.

I think you would have to do it by taking out the opamp and the 33.2k resistor and disconnecting the volume pot. Or are these switches also on sockets? Could you swap the D3 switch easily?
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Old 8th Apr 2017, 10:05 pm   #35
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Default Re: How does this circuit work?

Hi all,

First: I swapped the OpAmp against another one somewhere else on the board - no change.

Then I made again voltage measurements, for three distinct conditions:
1. volume at zero
2. volume at max, no singal
3. volume at max, with signal (600Hz sinewave)

I noticed that the amp starts having distortion when the volume pot is at around 30%. Voltages are very different around the positive and negative transistor, suggesting (to me) that T3 maybe faulty.

I also noticed that the amp regularly starts oscillating when I switch on the generator while the volume is at max.

Here are the detailed results in the form "condition 1/cond.2/cond.3". A scope capture is attached. Some reading come directly from the scope, the mV reading are from a voltmeter.

a) voltage at TP9: -2,3mV / -8,08V / na
b) voltage at pin 2: -2.0mV / -1,9mV / na
c) voltage at pin 4: -14,6V / -14,3V / -13,2V + ripple 2,3V
d) voltage at pin 8: +14,5V / +14,2V / +14,1V + ripple 500mV
e) voltage at R21 (at Emittter of T3): +14,5V, stable in all conditions
f) voltage of R22 (at Emitter of T4): -14,9V, stable in all conditions
g) voltage at pin 5: +11,1mV / -8,18V / -6,6V + ripple 200mV
h) voltage at pin 6: -400mV / -400mV / -400mV (adjustable with R51)
i) voltage at R37 (at Collector of T3): -1,4mV / -8,07V / -5,60V + 7,6Vpp
k) voltage at R48 (at Collector of T4): -2,3mV / -8,08V / -6,34V + 2,7Vpp

Attached is a picture of the voltages at R137 (red) and at i) and k) (green and yellow). This does not look good to me. In particuar I wonder why all values are negative...

I could make a thorough measurement of voltages around each transistor to verify if they are working as they should. Changing them means separating two boards (secured by a dozen screws and whatnot) to gain access to the solder side, so I would like to be sure.

cheers
Martin
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Old 9th Apr 2017, 2:11 am   #36
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Default Re: How does this circuit work?

Hi Chaparal,
PHP Code:
do you have a datasheet from 'µAF 772 TC' Op-Amp 
Martin, I think TL082 can be a god equivalent nowadays and NTE-858M should be same/similar too.
Regards, Karl
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File Type: pdf uA772 equivalent NTE858M.pdf (28.2 KB, 40 views)
File Type: pdf Fairchild-uA772.pdf (178.4 KB, 45 views)
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Old 9th Apr 2017, 8:37 am   #37
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Quote:
Originally Posted by karesz* View Post
Martin, I think TL082 can be a god equivalent nowadays and NTE-858M should be same/similar too.
Hello Karl,

thanks for this info. For now I do not suspect the OpAmp, since the symptoms do not change when swapping it against another one.

I will make some more measurements to see where the oscillation comes from, and look at voltages around the transistors T3 and T4.

The mute circuit seems to work as it should. I can modify with R51 to obtain a lower volume level where it starts to mute.

The "auto-ranging" of volume I mentioned earlier is controlled via the "Main-Amp" circuit through attenuation piloted by D11. The commands to D11 are given by the microprocessor itself. I recall a programming option where you can stop the unit to do auto-ranging on the meters. Perhaps this applies to the volume-control, too - the manual is mute on this topic.

cheers
Martin
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Old 9th Apr 2017, 11:00 am   #38
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Default Re: How does this circuit work?

Quote:
Originally Posted by Apfelmus View Post
First: I swapped the OpAmp against another one somewhere else on the board - no change.
There does look to be a change...

The DC offset at TP9 now looks about right at both Min Volume and Max Volume

Min volume should be a few mV below 0V. You got -2.3mV.

Max Volume should be -(5.5/33200) * 47000 = -7.8V DC.

You get -8.08V but there could be a tolerance issue here wrt the 33200 ohm resistor and the 47k volume pot in the real hardware.

Before, you had different voltages here. I would change the transistors now.

Could you put up a photo of the real circuit components? Someone on the forum might be able to offer advice on dismantling or desoldering etc.
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Old 9th Apr 2017, 11:32 am   #39
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Default Re: How does this circuit work?

-13.2V at the base (i.e. pin 4 of the opamp) looks suspicious to me. That looks bad for Vbe of one of the transistors? I would just change both of the transistors now.
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Old 9th Apr 2017, 4:04 pm   #40
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Default Re: How does this circuit work?

Upon special request, here are some pictures of the circuit:

1. the unit from behind. I had to remove the rightmost card to make the measurements. Nevertheless the selftest still runs through without notice.

2. the AF-detector board taken out (2 cards)

3. the card I am working on

4. detail with the audio amp: the OpAmp, two transistors, the trimmer for the mute level and (to the left of the trimmer) the D3 switch. Although switches in DIP-packages are all socketed, the round packages are soldered as well as the transistors.

5. the other side of the board

cheers
Martin
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