[meesterzee]

I am having trouble with the headphone outputs on my MC624. Signal from headphones is non-existent unless pot is cranked all the way and then I get pops/clicking distortion. This happens on both the front and rear headphone inputs with low or high gain set in the controller. All other ins/outs appear to be functioning without issue. There is no visible heat damage and all solder joints appear fine.

Testing the headphone output showed the following:

1. Signal for left/right we correct at the high side of the headphone pot P5
2. The TAP of P5 into U6b.5 was at RAIL (not sure if it was HI or LOW rail)
3. U8b.7 was at HI rail


I'm wondering if I have a bad pair of op amps?

[JPK]

Do you have this issue in both ears?
Please report the DC voltages on every pins of U6 and U8 against 0V.

[meesterzee]

The crackling/popping is only in the left ear. I measured the straight DC voltage (no audio being passed) on U6 and U8.


U6

• 0 vdc
• 0 vdc
• 0 vdc
• -16 vdc
• -12 vdc
• -12 vdc
• +8 vdc
• +16 vdc

U8

• 0 vdc
• 0 vdc
• 0 vdc
• -16 vdc
• 0 vdc
• +14.5 vdc
• +14.5 vdc
• +16 vdc

[JPK]

There is something wrong in both channels.
The pins 6 & 7 should be at 0VDC.
Does it change something in the voltages if you remove the ribbon cables going to CN24 and CN25.
What are the DC voltages on TP9 and TP10, against 0V? Should be 0VDC.
Same question after removing U6 and U8 but with a wire connected between pin 7 and 0V.
Look for bad solder joints or solder bridges in the area U6 to U9.

[meesterzee]

I checked everything above and finally with U6/U8 pin 7 wired to GND I was able to read 0VDC at both test points. Replaced both op-amps and everything seems to be working perfectly! Thank you so much for your help!