Amek Angela - 'frying sounds' when the phantom power is on.

[smilan]

Hi,
One of my A-10 modules makes 'frying sounds' only when the phantom power turned on.
I've replaced C1 and C2 with new capacitors, checked the values of resistors R1, R2, R7, R8, R9 and  transistors  Q1-Q10 (with a transistor tester) and everything seems to be alright.
I've also replaced U3 and U4 with op amps from another channel that works fine and nothing makes a difference, the  'frying sounds' doesn't get away on this channel.
What else can I check?
Attached schematic
https://i.ibb.co/N9KFnk4/angela-a-10.jpg

 

[pucho812]

does it only happen on this one channel and only with phantom  engaged?

Have you moved the channel around to another location to see if it follows the channel? Have you tried a different known "good" channel in it's place to see if it is the slot?

 

[smilan]

does it only happen on this one channel and only with phantom  engaged?

Yes

Have you moved the channel around to another location to see if it follows the channel?

Yes

Have you tried a different known "good" channel in it's place to see if it is the slot?

Yes
I've tried all the above, it's happens only with this specific channel in any slot.

 

[pucho812]

o.k. so we have it isolated to the channel.

From what you tell me, it only happens with phantom engaged.  Check solder joints.  looks alone may or may not tell you they are o.k.. Normally a frying sound indicates something went up. Check for discolored resistors. Check and see if any of the transistors are getting too warm to touch? Same for opamps...

 

[rock soderstrom]

Did you clean the contacts of S16, S17 and S18? Especially S16 is suspected.

 

[smilan]

o.k. so we have it isolated to the channel.

From what you tell me, it only happens with phantom engaged.  Check solder joints.  looks alone may or may not tell you they are o.k.. Normally a frying sound indicates something went up. Check for discolored resistors. Check and see if any of the transistors are getting too warm to touch? Same for opamps...

I've checked all the solder joints and continuity of the PCB lines and all seems to be good.
None of the transistors / opamps are  getting too hot to touch.

Did you clean the contacts of S16, S17 and S18? Especially S16 is suspected.

I've already cleaned S16, S17 and S18 with Deoxit D5 and  replaced S16 but it didn't made any change. Now I've also replaced S17 and finally problem solved, the channel is dead quite with the phantom turned on!
Thank you very much for your support.

 

[rock soderstrom]

I've checked all the solder joints and continuity of the PCB lines and all seems to be good.
None of the transistors / opamps are  getting too hot to touch.
I've already cleaned S16, S17 and S18 with Deoxit D5 and  replaced S16 but it didn't made any change. Now I've also replaced S17 and finally problem solved, the channel is dead quite with the phantom turned on!
Thank you very much for your support.

Good news! 

 

[pucho812]

I've checked all the solder joints and continuity of the PCB lines and all seems to be good.
None of the transistors / opamps are  getting too hot to touch.
I've already cleaned S16, S17 and S18 with Deoxit D5 and  replaced S16 but it didn't made any change. Now I've also replaced S17 and finally problem solved, the channel is dead quite with the phantom turned on!
Thank you very much for your support.

good news to hear. I would say the internals of s17 were dirty or something. but glad it is all working now without the sound of frying.