Classic II No Audio After Recap repair strategy and case studies

[Elemenoh]

No sound after recap of a Classic II is quite common and it's not always the same culprit. I'm going to use this thread to document the root cause of no audio on a couple of my Classic II MLBs and provide a tip for testing audio with an oscilloscope. If you have run into the same problem, please chime in with what the root cause(s) were.

Process​

In order for sound to work, continuity is needed between U1 (amp), U4 (DFAC), and many of the nearby capacitors and resistors both top and bottom side.

After recap, I perform the following steps testing audio between each step:

1. Ultrasonic clean the board.
2. Test with and without headphones to rule out a stuck/corroded jack.
3. Remove U4 and U1. Scrub the pads and look for obvious broken traces.
4. Test the speaker's resistance. It should be about 60 ohms.
5. Continuity test the audio (out) circuit with Bomarc drawings as reference. Top side first, then bottom side.
6. Swap test U1
7. Swap test U4

Case study 1​

After a thorough clean underneath U1 and U4, I eventually found a flaky connection through a via off of C4 (topside) leading to C32 (bottom side). A small bodge with 40 AWG enameled wire did the trick.

Scope Tip​

Sometimes I like to test both with my ears and eyes. The last stop for audio out before reaching J11 or the headphone jack is LP1. Using an ATX extension with the logic board outside of the system, you can probe LP1 while playing back audio to see if any signal is reaching the point out output. I use Snooper to play back tones and samples which is easier to manage compared to alert sounds.

You can also probe pin 7 of J11 for speaker audio out, but it can be difficult to make good contact with a shallow probe.

I like to use the sine wave generator on Snooper which makes for an easy to decipher waveform on the scope.

 

[Elemenoh]

Case study 2​

This one was a lot more difficult to figure out compared to case study 1.
There were broken connections between:

• R29 and C39
• R40 and R46
• U4 (DFAC) pins 1 and 2

None of those broken connections were obvious. There wasn't any visible corrosion, black traces etc. It just took methodically probing to track them down.

After checking continuity of the whole circuit twice I ended up swap testing U1 from a working board and the issue persisted. I then swapped U4 (DFAC) from a working board and it worked. The DFAC from the bugged board also worked fine on the known-working board. I must have had a bridge or open I just hadn't spotted earlier.