Rising DC Offset

[pmchoong]

that right one

Thanks, ZM. I'll do it tomorrow.

 

[pmchoong]

Hi ZM, I have 18.22 mA and 18.24 mA on the board with the mosfets removed.

 

[Zen Mod]

that's good

now put one pair of (checked!) mosfets back (one in positive rail , one in negative rail ) and test

be sure that small bjts are good - put new ones if you can't be sure

 

[AndrewT]

Hi ZM, I have 18.22 mA and 18.24 mA on the board with the mosfets removed.

Those measurements confirm that 0.02mA are exiting the circuit by another route.
That other route is probably the Power Ground to PSU Zero Volts.

Whereas post99 (left pic) shows 0.28mA exiting by another route. That big ground current probably indicates a fault.

 

[Zen Mod]

no,at least as I understood - in post #99 , he put Ammeter in parallel to existing PSU rails , which is obviously wrong way of current measurement

 

[pmchoong]

I measured it again and now it is at 18.20 mA and 18.30 mA. But the decimals numbers fluctuate, ie. it goes from 18.00 up to 18.20. It goes up and down between 18.10 to 18.25 mA. Does this matters?

 

[Zen Mod]

no

proceed , as I wrote in #105

 

[pmchoong]

i have connected a pair of mosfets (Q5 and Q7 but in Peter Daniel's board, it is Q8) and the ampere reading is attached. The bjts are good as they are new ones.

 

[Zen Mod]

gnd connected between PSU and aleph pcb ?

 

[pmchoong]

Yes, except the board's SG in. The inputs are connected to the Gnd as well.

 

[Zen Mod]

something's bloody wrong

give me pictures

do you see any difference vs. functional channel ?

 

[pmchoong]

I can't see any difference between the two except the grounding which is different as I have changed this grounding according to Andrew's comment. i did both the amp at the same time together but somehow this channel gives a problem.

 

[pmchoong]

Please let me know which part do you want me to enlarge.

 

[Zen Mod]

well , if mosfets are not having mixed pin wires , and if they're good ...... I can't see anything wrong

though - I'm not having capture of top and bottom copper of PD boards , so I can't say which mosfet position(s) are alive , meaning - of which mosfet position is source resistor monitored in Aleph CCS

if you're sure that all 4 mosfets are well and alive - connect them all

connect that pcb exactly in same manner as functional pcb , for now

 

[pmchoong]

I have removed inputs off the gnd and connect and it is now the same as the functional unit. Attached is the measurement. I do not know why is the positive rail differs so much from the negative rail.

Thanks, ZM. I appreciate your help.

 

[Zen Mod]

sorry - we are running in circles .......

- according to measurements in #106 - input LTP is functional ;

- recheck all resistor values in Aleph CCS area ( positive side of output stage)

-in fact - recheck all resistors (easier to say that , than to list them by function )

- while doing that , re-check all soldering joints

- if you're sure that mosfets are OK (consult mosfet testing article from FW site, how to properly test them ) , connect them all

- connect all wires in exactly same fashion as on functional channel

- fire it up ; check output offset and Iq ; best place to check Iq is measuring voltage across one of source resistors in negative output stage side (those connected to negative PSU rail)

 

[pmchoong]

Thanks ZM, I'll remove the board and check everything including PSU. I thought the positive rail is giving the problem.

 

[AndrewT]

I have removed inputs off the gnd and connect and it is now the same as the functional unit. Attached is the measurement. I do not know why is the positive rail differs so much from the negative rail.

Thanks, ZM. I appreciate your help.

that difference of over 6mA must be exiting via another route, probably to the PSU Zero Volts.
Measure it and confirm you know where the current is going.

 

[Zen Mod]

that measurement is completely illogical , so best to ignore ;

what counts , so far , is what's measured in #106

 

[AndrewT]

Why is determining where the missing current is going to, so "completely illogical"?