Salas DCG3 preamp (line & headphone)

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#6,581
Wasn't that a Homer Simpson thing? He looks at something and it blows? Uiuiui! :rofl: (I feel similar)
I'm not sure if I observed this correctly: Had the impression the fuse didn't blow right away, it was more like it was gone (the led-bar, that is) after power-on again...

Haven't yet examinated the situation...
What is the procedure to examinate the state of jfets and the likes?
 
#6,582
BJTs simplest check is with DMM in diode mode. You look for one diode drop. Say its NPN. You put red probe on base and you read about 0.5V when black probe is touching each other pin. For PNP you put the black probe on the transistor's base.
Just shows if junctions are broken or not. Crucial. No need to remove it from board.
 
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#6,584
Just measured J1, J2, Q2, Q4 both sides (the other pair to complete and confirm it)
They all look similar:
J1 left:
source-drain: 94R
source-gate: 1.1R

J1 right:
source-drain: 70R
source-gate: 1R

J2 left:
source-drain: 90R
source-gate: 1R

J2 right:
source-drain: 94R
source-gate: 1R

Q2 left:
source-drain: 0.26V
source-gate: 0.64V

Q2 right:
source-drain: 0.112V
source-gate: 0.54V

Q4 left:
source-drain: 0.26V
source-gate: 0.66V

Q4 right:
source-drain: 0.27V
source-gate: 0.67V

I'm not quite sure what it means besides that no part is really off of each other.
Beginning to hope there's just another silly wiring-glitch or soething?
What do you conclude of this, or where should I put my fingers on next?

Thank you so much!
 
#6,585
Went one tiny step further:
Confirmed transformers, they're good and connected the right way.
Swapped transformer (connected one channel only)

DCSTB
channel 1 is +-17.4VDC,
channel 2 is still outputting weird numbers, +24VCD -42VDC (bright LED-bar on the +side)
 
#6,592
What would happen is inconclusive readings regarding the internal diodes of the transistors conducting or not.

Meantime pull the suspect out to probe it again in isolation. Do you have a cheap component tester for seeing more?
 
#6,597
I am struggling to understand the mechanism by which the DC servo in the DCG3 can affect the sound of the amplifier. All the authors I have read say that servos can affect the sound of an amp, but I'm just not seeing it. As a load on the output, the 1meg resistor is insignificant. And the output of the servo, a very slow moving DC signal of a few tenths of millivolts that brings the offset back to zero: how can that be heard? I would love an explaination of why a properly designed DC servo can affect the sound of an amplifier.
 
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#6,598
Cyberpup said:
some stock of all the transistors

Best way to insure you won't need them!
Click to expand...
I may reluctantly do so…
Keeping a stock of parts is a bit a part of the rabbit-hole, isn’t it? And, as I have built several other things and more often than not ordered some spares, but still haven’t managed to develop sort of a muscle-memory on electronics (I still don’t have that comfy feeling of understanding), it’s basically a big, growing bunch of loose parts…

OTOH: „I can see clearly now the partisoutofstock…“ (
)
😀
 
#6,599
Cyberpup said:
I am struggling to understand the mechanism by which the DC servo in the DCG3 can affect the sound of the amplifier. All the authors I have read say that servos can affect the sound of an amp, but I'm just not seeing it. As a load on the output, the 1meg resistor is insignificant. And the output of the servo, a very slow moving DC signal of a few tenths of millivolts that brings the offset back to zero: how can that be heard? I would love an explaination of why a properly designed DC servo can affect the sound of an amplifier.
Click to expand...
Another way to see it is as a dynamic high pass filter on the feedback node.
 
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