Known-good measurements/values for Presonus Eris 3.5 PSU

[Elly D.]

Hello.

I'm looking for known-good measurements or photos of some passives of the primary side PSU of the Presonus Eris 3.5 studio monitors.
Board version is v1.2 (2018-05-04).

I've already looked for typical application circuits in datasheets and application notes and some come quiet close, but do not 100% match:

- https://www.st.com/resource/en/application_note/an246-uc3842-provides-lowcost-current-mode-control-stmicroelectronics.pdf (pg. 19, closest, non-100% match, no optocoupler)
- https://www.ti.com/lit/an/slua143/slua143.pdf (pg. 13) (good, no 100% match)

Specifically I'm interested in:

- R49
- C without designator below R49
- C10 or C19 (hard to read, below R51)
- R54
- R60
- R1, R61 (1210 in parallel)

Diagnosis:

- No char marks elsewhere except around the PSU controller.
- Switching transistor Q3 shorted between drain and source
- Components in the switching transistor path blown
- UC3842B shorted across multiple nodes
- F1 did it's job
- The NTC is OK
- Switched transformer winding ~200mOhm (sounds legit)

Blown or bad measured components are marked with a red "x" in the partial, reverse-engineered schematic:



(No guarantee that the schematic is 100% correct. U3 in the schematic is supposed to be U8.)

Damaged area details:





U8 marking:



PSU bottom overview:




Teardown guidance:

- Remove all back plate screws
- Unplug the cables
- Discharge and check primary side caps (i.e. using Low-Z) or handle very carefully
- Pull the knobs off and remove all screws to the connectors and stand-offs
- Use a spudger to carefully lift the glued foam from the backplate around the connectors


Raw measurements from my notes:

- NTC (undesignated, next to CN1) = 3.2Ohm
- F1 = open
- Q3 = TSF 10N60M
  - open on pins 1 and 2 / 1 and 3 (gate vs. source/drain)
  - shorted in both directions on pins 2 and 3 (source and drain)
- U3 = UC3842B
  - Ground vs. Isense vs. Vin shorted
- R61 = 191k
- R1 = 180k
- R61 || R1
- R54 = 570k [with U3 removed]
- C19 (C10) = 1.9k, capacitance not measurable, 34k [with U3 removed]
- R51 = 9.4k, [also with U3 removed]
- R49 = 17.9M, open with [with U3 removed]
- Cx (undesignated, below R49) = open
- R28 = 20k
- C9 = capacitance not measurable
- C58 = capacitance not measurable
- R32 = 2k
- C59 = capacitance not measurable
- R31 = 14.9k
- R60 = 6.5k
- R36 = 19.8k
- C57 = 26µF
- R102 = 560k [with U3 removed]


Thank you in advance.

[crozac]

r61 and r1 = r500

[mladen82]

Any further help, please post

[Ashtar Sheran]

Hello, can someone told me the parametrs of U8 and Q3 part? I dont know what to order.

[tunk]

Maybe this: https://www.st.com/en/power-management/uc3842b.html

[Elly D.]

Thank you for the replies.

On a later board with version 1.3,  I could see these changes:

- "C19 (C10)" is actually or now C10
- CX (undesignated) is C55 now

The board from my first post was v1.2.

- R54 = 470 Ohm [in-circuit] (label=471) (0603")
- R49 = 50 Ohm [in-circuit] (label=510) (0805")
- R1, R61 = 500mOhm [in-circuit] (label=R500) (1206")
- R60 = 10 Ohm [in-circuit] (label=100) (0603")
- C10 in 1.3, (C19 in 1.2) measured=140nF  (probably 100nF) (0603")
- CX / C55 measured=140nF (probably 100nF) (0603")