Inside the audio path
Every restoration entry on this site talks about capacitors, decibels and heads. This page is the other half of the story: what those parts actually do to the music, shown rather than told. Everything below is drawn from this machine's own factory schematics — the boards named here are the boards in the deck.
The journey of a signal↑ TOP
From a whisper off the tape to professional line level — tap each stage.
A coil the size of a fingernail reads the tape's magnetism. The signal here is a whisper — a thousand times quieter than what leaves the machine.
C51/C151 couple this whisper into the first amplifier — the positions the recap protects first.
The numbers
Factory levels at 1 kHz, model 2S: head -50 dBm, EQ amp out -15 dBm, line amp in -30 dBm, line out +4 dBm (600 ohm), phones -24 dB. From the simplified diagram in the parts-and-schematics manual.
The numbers under each stage are the factory's own: the head hands the first amplifier a whisper around a thousandth of the level that leaves the XLR jacks. Every capacitor in the recap lives somewhere along this line.
The gatekeeper: coupling capacitors↑ TOP
Music passes through. DC stops at the wall — unless the part leaks.
Bass roll-off starts below 0.1 Hz — the lowest note on a bass guitar is ~41 Hz, so at 10 µF the music never notices the capacitor.
The numbers
High-pass corner f = 1/(2πRC). Into the EQ amp's ~150 kΩ input, 10 µF puts the corner near 0.1 Hz — nearly nine octaves below the lowest note on a bass. Leakage is the DC that crosses anyway: standard grade ≤ 0.01·C·V µA, low-leakage grade ≤ 0.002·C·V µA.
About a third of the electrolytics on these boards do exactly this job: pass the music, block the DC. The recap chose their replacements by the one number that matters here — how much DC sneaks through anyway.
What the microamps threaten↑ TOP
Why microamps get watched, and why the demagnetizer runs after every session.
A worn capacitor lets a whisper of one-way current through the replay head. Slowly, the head itself becomes a weak permanent magnet.
The numbers
Steady leakage in the microamp range is mostly survivable — practitioner rule of thumb puts real danger orders of magnitude higher. Replay-head magnetization = raised noise floor + progressive short-wavelength (HF) erasure. Distortion and pops belong to record/erase-head magnetization. Fail-short endpoint documented in tantalum reliability literature (KEMET/AVX). Remedy: AC demagnetization before serious playback and after each bench session.
Why 100-volt parts in a 25-volt machine↑ TOP
Run a 100-volt part at 5 volts and its leakage all but disappears.
At 5 V this 100 V part typically leaks 0.01 µA — under the 1 µA printed limit the old low-leakage grade guaranteed. The printed limit is a promise at full voltage; the curve is what the part actually does at yours.
The numbers
Datasheet limits are specified at rated voltage: standard grade ≤ 0.01·C·V µA, low-leakage grade ≤ 0.002·C·V µA. Below rated voltage leakage falls steeply (manufacturer curves: ~3% of the rated-voltage figure at 20% of rated voltage). Curve shown: typical 10 µF/100 V part; dashed line: the 1 µA printed limit of the old low-leakage grade at 10 µF/50 V.
This curve is the quiet engineering argument behind the whole capacitor shopping list: voltage headroom isn't waste, it's free margin — and at these bias voltages a healthy modern part outperforms the printed limits of the parts it replaces.
The recap these graphics explain is documented in the playback-first bench guide.