Hypex Ucd400OEM offset problem

[vuki]

Hello,

I bought 4 UCD400OEM V4 modules second hand (supposed to be NOS), but 3 of them have ca. 2Vdc offset (one module has positive two have negative offset).
As far as I can tell (checking with dmm onboard components) all transistors and diodes are OK.
Offset is independent of input - if I ground the unbuffered inputs offset doesn't change. It does respond to DC Adj pot - I'm able to move offset ca. 500mV up or down
Modules oscillating frequency is ca 360kHz.
Of course, I contacted Hypex, but they don't support OEM and also the version is obsolete.
Does anyone have any suggestion - what could cause dc offset?

PS. wiring is fine - I also have 2 Ucd400OEM V7 modules that in place of defective ones work fine.

 

[vuki]

Fixed! 😀 Input transistors of comparator were half dead - funny thing was that they looked OK on multimeter's diode test?

 

[Electrone]

Well done! I was just writing that what I think could cause the DC offsets are components like transistors being out of tolerance.

 

[namcir]

what is the part number of the comparators input transistor?

 

[vuki]

Mmbt5401

 

[namcir]

can you kindly point where is it?

 

[vuki]

IIRC those were t12 and t13 but I'm not sure anymore.

 

[Antana]

T12 and T13 are both BC846S 🙂

 

[vuki]

As I said- I really don't remember anymore- it was more than year ago and I didn't save the data. The easiest way beside having schematcs 😀 is to trace input signal to the first differential pair. I remember that transistors of the pair were not next to each other.

 

[namcir]

as you have version 4 and version 7 is there a big difference in sound characterisitics?

 

[vuki]

Not a big difference if any. Recently I compared ucd400oem to ncore mp252 - big difference- ncore had better resolution - more details.