Salas DCG3 preamp (line & headphone)

[IDM]

I am having an issue with my DCG3 build.

One channel is up and working perfectly the other is measuring +16.9V offset!!

The DN2540 CCs is measuring 120mA and the BF256B CCS is measuring at 5mA (both exactly the same as the working channel).

I can see no obvious poor joints or incorrect resistors or transistor orientations. Any thoughts on the best way to find and rectify the issue?

Thanks

Ian

 

[Salas]

Check the bad channel's Mosfet tabs insulation to sink / chassis is good first. The DMM's continuity buzzer will tell.

 

[IDM]

Hi I did check and indeed just checked again and there is definitely no short through the MOSFET tabs

Ian

 

[Salas]

Then see if M1 M2 are good i.e. showing as much Vgs each as in the good channel.

 

[IDM]

OK,

Big difference here.

The good channel has a VGS of approx 4.51V for both M1 and M2.

The bad channel has 12.6V on M2 and 8.23V on M1.

 

[Salas]

Probably finished parts (static shock?)

But also check voltages around the Q1 Q2 BCs vs the good channel in case they are problematic.

 

[IDM]

Sorry, to be a bit slow, but what voltages should I measue on the BC560's?

 

[Salas]

Let's start with Vbe

 

[IDM]

On the good channel:

Q1 VBE = 644mV, Q2 VBE = 0mV

On the bad channel
Q1 VBE 7.58V, Q2 VBE = 630mV

 

[Salas]

We caught one!

 

[Salas]

Replace Q1 to see what happens

 

[IDM]

Change Q1?

Though I need to check the hFE for both I guess?

 

[Salas]

By the way, good opportunity to install female pins for those two. So to can put and compare BC327-40s at a point which tend to promote a more realistic sound in this preamp according to the builders subjective majority vote.

 

[IDM]

Will do as I have loads of BC327-40's

I will report back

 

[Salas]

Change Q1?

Though I need to check the hFE for both I guess?

Just to know if it's the culprit or not, hfe match doesn't matter.

 

[Salas]

Will do as I have loads of BC327-40's

I will report back

Good

 

[IDM]

Hi I just replaced the BC560's with BC327's and on powerup, I got exactly the same result with huge offset and weird vbE readings on the BC327's.

Would the next logical step be to replace the IRF9610's and replace the BC327's again?

 

[Salas]

Replace M1 & M2 alone first

 

[Salas]

Did you install 0.1 inch pitch female pins for Q1 Q2 quick swap as well?

 

[Salas]

Got a transistor tester to see what happened with semis after removal?