My SE/30 has developped distorted sound in 2026

[Mu0n]

This SE/30 has been logic board recapped around 2019, it was my 3rd or 4th ever soldering job and I did a passable job of it with tantalum caps.
Its analog board has yet to be done as well as the power supply. It's been a stable workhorse and I've even left it running for days uninterrupted for #MARCHintosh.

Here's a recording from Super Studio Session in 22 kHz, but running a Studio Session song of 6 channels:
Pay attention to how every instrument has this rough distorted feel.

Here's a recording from Studio Session (so, 11 kHz) running the same song, but from my Mac Plus:
Every instrument is immediately clearer.


Before you think this is a System 7.1 on the SE/30 vs a System 6.0.8 on the Plus issue, consider the following: on the SE/30, the sound becomes PRISTINE again if I use the headphone jack instead of relying on the internal speakers.

I know that if both the speaker and heaphone sound output are problematic, it points to a sound chip problem, but what does it mean when only the speaker quality is bad?

 

[techknight]

Missing one-half of the waveform. Either the upper or lower transistor failed in the audio output stage, or you are missing the -12V.

 

[Mu0n]

Missing one-half of the waveform. Either the upper or lower transistor failed in the audio output stage, or you are missing the -12V.

ok, those are good avenues to test. -12V seems like the low hanging fruit, I'll check my external floppy drive port, I think?

How do I test these transistors? The sound chip is UE10, correct? This is what I have that I think is the most relevant schematic page for the SE/30

 

[YMK]

Q1 and Q2 form an emitter-follower output stage.

Check for the the +12/-12 supplies at each collector.

If the upper half of the output is distorted, replace Q1.

If the lower half is distorted, replace Q2.

Be aware, they are different parts.

 

[Mu0n]

Q1 and Q2 form an emitter-follower output stage.

Check for the the +12/-12 supplies at each collector.

If the upper half of the output is distorted, replace Q1.

If the lower half is distorted, replace Q2.

Be aware, they are different parts.

How do you propose to check for distortion? Send a pure sustained sine wave through the Sound Driver's 4-tone synth and check the output at each transitor on a scope, and look for sinus impurities?

 

[YMK]

A sine wave test would work.

Q1 and Q2 have their emitters and bases tied together, so the output is shared.

A failed transistor will be obvious; You'll be missing the upper or lower half of the output.

I'd replace both as a matter of course. They're cheap and used throughout many logic boards.