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Old 31st March 2011, 10:36 PM   #11
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Default Some headway!

The pot checks out just fine. The there doesn't appear to be any problems with with the resistance of the pot

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Hi, Measure voltage drop across R11,15 (499 ohm) of good left channel
and see if the respective resistors at the right channel are the same ,if
not adjust R10,14 respectively.This should be your starting point reference
before you go on to the rest of the circuit.Also measure voltage across D1,2
led that it is 1.6V as stated.( If the voltage across R11,15 does not change
then measure with respect to common/ground or at the emitter of transistor,
I'm getting rusty at this).
The LED's on both boards match out to be 1.65V on all four, same with R11 and R15 I measure exactly 1V drop across them. Apparently my local electronics shop doesn't have any 100k multiturn pots, they could get them in, but they were about $1.25 a turn, which doesn't make any sense. I'll see any anyone else local can get me some.

It almost looks like the DC offset of the other board moves with the pot too, which means it might be the change of impedance... It's just a lot more noticeable on the right board because the initial offset was much higher then the left. Meaning my only way of fixing this is finding a part that should be quite common, but for some reason is not so. AMB outlined this guide here to eliminate the DC offset using R10/R14:
AMB Laboratories DIY Audio • View topic - large DC offset in left channel
When he says turn it to minimum, the pot should be at it's maximum resistance. AMB in his guide states to use a 10K pot, but the schematic shows a 100K pot, I almost think it should be safer to use a 100K pot. I don't quite understand what he means by "Turn one of the two trimpots to change the offset toward 0mV, a little at a time, and simultaneously turn the other trimpot in the opposite direction.". If the pots already at it's maximum resistance, then what's the point?

I'll also attempt to find some moderate sized TO-220 heatsink that i can use for the output transistors. Something like this:
Digi-Key - 294-1042-ND (Manufacturer - PB1-36CB)
or this:
Digi-Key - 294-1080-ND (Manufacturer - 7-340-2PP-BA)

I think i can partially blame it on thermal run away, since the offset will start at something like ~-1.0mV and as the amp warms up, it'll end up at something like ~-25.0mV and the transistors will be about ~85c.

I'll do some more voltage measurements later dealing with the Jfets, I need a break for now.

Thanks a lot!
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Old 31st March 2011, 11:38 PM   #12
artu is offline artu  Chile
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Yes the DC SERVO has a very limited 'lock' range, that is, it is able to correct little deviations of DC OFFSET because the LEDs are quite 'insensible' to current changes in the bias network of input CSS. So you must achieve a close to 0 V DC OFFSET before re-connecting both servo integrators. To obtain a more 'wider' lock range, you should place a small resistor (22 - 68 OHM) in series between the LEDs and +/-Vs, but as AMB explained first an initial bias close to 0 DC OFFSET must be done.

Cheers
Arturo
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Old 1st April 2011, 05:33 AM   #13
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Quote:
Originally Posted by artu View Post
Yes the DC SERVO has a very limited 'lock' range, that is, it is able to correct little deviations of DC OFFSET because the LEDs are quite 'insensible' to current changes in the bias network of input CSS. So you must achieve a close to 0 V DC OFFSET before re-connecting both servo integrators. To obtain a more 'wider' lock range, you should place a small resistor (22 - 68 OHM) in series between the LEDs and +/-Vs, but as AMB explained first an initial bias close to 0 DC OFFSET must be done.

Cheers
Arturo
Ok, So let my get this straight having more voltage forward on a LED means there's more current flowing through it? If that's the case I should put a 22-68ohm resistor to limit the current a little bit more so i can have a bit more freedom when I adjust R10 and R14.

Now for a proposal, since i have some 5K multiturn trim pots available, I think, I could replace R11 and R15 with ~1K resistors. Would this be a liable option?

I'll post measurements of the voltage drop across the output resistors tomorrow.
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Old 1st April 2011, 07:32 AM   #14
singa is offline singa  Singapore
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Hi, I think you are complicating matters.Sinple voltage checking will
lead you to the bad part.OK measure Q1,2 emiters with respect to
ground and the collectors too and compare with good channel.Same
goes with driver transistors Q3,4.If you can use the lab scope,pump
in a 1 volt sinewave and look at the top and bottom half output of the
jfet,that should give you the condition of the jfets.

PS the above is assuming you have zeroed the offset to minimum.
singa
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Old 1st April 2011, 07:56 AM   #15
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Quote:
Originally Posted by singa View Post
Hi, I think you are complicating matters.Sinple voltage checking will
lead you to the bad part.OK measure Q1,2 emiters with respect to
ground and the collectors too and compare with good channel.Same
goes with driver transistors Q3,4.If you can use the lab scope,pump
in a 1 volt sinewave and look at the top and bottom half output of the
jfet,that should give you the condition of the jfets.

PS the above is assuming you have zeroed the offset to minimum.
singa
My post above is dealing with a possible way of removing the offset since I dont have any easy to sourced 100k trimpots. I'll see what I can find tomorrow.
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Old 2nd April 2011, 01:43 AM   #16
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OK, So any of the horrible previous problems have seemed to go away. Which just leaves me with DCservos that cause a elevated DC offset ~+/-10.0mV. So far both channels DC offsets have been trimmed to always stay below 1.0mV. I achieved this by using two 5K trim pots and two 680ohm resistors. I've done this only with the right board

I took a couple of measurements:
Offset influenced by pot (Measurements taken without DC servo)
Right Max: 3.6mV
Right min: 1.5mV
Left Max: 1.7 (~5 minutes of warm up)
Left min: ~1.1mV

Voltage drops across 24ohm output resistors
Average left positive: 0.971V
Average left negative: 0.972V
Average right positive: 0.957V
Average right negative: 0.955V

Votages at the input of the amp (keep in mind there's a ~600ohm resistor at the input):
Left: ~0.4mV
Right: ~0.1mV

I don't have definite measurements with DCservo, but the left channel was somewhere ~8.0mV and the right channel was ~3.0mV

Anything fishy? I'll try some functions with the scope when i have access to it.
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Old 2nd April 2011, 03:50 PM   #17
artu is offline artu  Chile
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if you got stable measures (3.6mV, 1.5mV), starting the servos should reduce these values, otherwise if one or both increase(s), the failure is in the servo circuit (chips + the integrator cap. resistors ...??)

Cheers
Arturo
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Old 2nd April 2011, 07:01 PM   #18
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Quote:
Originally Posted by artu View Post
if you got stable measures (3.6mV, 1.5mV), starting the servos should reduce these values, otherwise if one or both increase(s), the failure is in the servo circuit (chips + the integrator cap. resistors ...??)

Cheers
Arturo
This is true, I found the DCservo to be quite random at some times. It can make the offset worse if the offset is initially anything above 1.0mV, which is exactly what's happening in the left channel, it's about 13.0mV on average. when i have a chance, I'll defiantly do what i did to the right channel. The right channel's Max offset is ~1.5mV, nice and safe!

I don't know why the voltage drop across the output resistors is so large when the guide says that it should only be about ~38.0mV.

I had a chance to listen to the Dynalo with my grados, I first noticed the soundstage, simply massive! It really brings out the best in the grados, I might up the gain to 5x or 8x... the 33pF compensation capasitor might overcompensate, is this bad?
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Old 2nd April 2011, 07:49 PM   #19
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I am assuming that without servo the max. offset are stable within the limits reported by you.

First note that measuring a 'disconnected' servo is not meaningful, most probably it will swing randomly +/-V depending on output offset. To verify that the servo(s) are working correctly the measurements must be done with the servos 'connected'. Another issue is that when the RC time constant is too little (it falls inside the low end of audio BW) it may induce 'motorboating' instability (a very low freq. oscillation). Resolder the servo circuit to prevent from a 'cold solder' and if that doesn't work, I suggest the integrator cap. replacement, and then the servo chip.

Cheers
Arturo
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Old 2nd April 2011, 07:51 PM   #20
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Quote:
Originally Posted by KnightofAwesome View Post
I don't know why the voltage drop across the output resistors is so large when the guide says that it should only be about ~38.0mV.
You have over-biased the output stage with R19=215ohm...bypass it with a piece of wire for a moment and measure output bias again.
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