FAQ
TL;DR: Bosch GML 20 with no sound? In 3 documented cases, “check the PCM1808 analog-to-digital converter” first; it’s a recurrent fault that can mute audio. Some sets also need the TAS5706B amp replaced. [Elektroda, fellowes20, post #17109842]
Why it matters: This FAQ helps DIYers and technicians quickly diagnose no-sound/low-volume issues on Bosch GML 20 and estimate repair parts and steps.
Quick Facts
- Symptom pattern: no speaker output, headphones play very quietly while on-screen volume changes. [Elektroda, Kob82, post #16865258]
- Frequent root cause: PCM1808 ADC failure on the amplifier module; confirmed in multiple units. [Elektroda, fellowes20, post #17109842]
- Secondary damage: A short in the PCM1808 circuit can also take out the TAS5706B class‑D amp. [Elektroda, Kob82, post #17126605]
- Typical parts cost: PCM1808 5 pcs ≈ 30 PLN; TAS5706B 1 pc ≈ 50 PLN. [Elektroda, Kob82, post #17126605]
- Standby transformer guidance: approx. 3–5 W, ~8–9 V secondary for the SB rail. [Elektroda, 398216 Usunięty, post #17126852]
How do I fix a Bosch GML 20 that powers on but has no sound?
Start with the audio path. Replace the PCM1808 ADC on the amp module, then verify/replace the TAS5706B power amp if output is still muted. One report fixed audio after new PCM1808s but later required the TAS5706B as well. Use proper SMD rework and test speakers and headphones after each step. [Elektroda, Kob82, post #17126605]
Is the TAS5706B always to blame for no sound?
No. As one technician notes, “the power amplifier is not always to blame.” Diagnose the front-end first, especially the PCM1808 ADC on the amplifier board. Swap parts only after confirming rails, clocks, and I2S lines. [Elektroda, maniek1603, post #16901425]
What symptoms suggest a front-end (PCM1808) issue on the GML 20?
Typical signs include no speaker output, headphone audio present but extremely low, and the on-screen volume bar moving normally. This points to digital audio path or ADC issues rather than user controls. Verify with known-good headphones and inputs. [Elektroda, Kob82, post #16865258]
Can a failed PCM1808 damage the TAS5706B amplifier?
Yes. A short within the PCM1808 circuit can also take out the TAS5706B, leading to complete speaker silence. Replace the PCM1808 first, then recheck speaker output; if still dead, replace the TAS5706B. [Elektroda, Kob82, post #17126605]
How much do the parts cost, and how many PCM1808s might I need?
One repair used 5 PCM1808 chips (≈30 PLN total) and one TAS5706B (≈50 PLN). The batch approach helps when multiple ADCs test marginal or when stocking spares. That’s a concrete 5-piece statistic from a successful repair. [Elektroda, Kob82, post #17126605]
Quick 3‑step: how do I test/replace the audio path on GML 20?
- Inspect the amplifier module; reflow or replace the PCM1808 ADC.
- Power‑test; if speakers remain silent, replace TAS5706B.
- Verify headphone and speaker outputs, then burn‑in for several days.
“Replace PCM1808 first; TAS5706B may also be needed.” [Elektroda, Kob82, post #17126605]
Do headphones work when the main speaker amp is bad?
They can, but very quietly. Users report headphone audio present at low level while the display shows volume changes. This indicates upstream audio conversion/control works partially, but output stages are compromised. [Elektroda, Kob82, post #16865258]
What is the TAS5706B in the GML 20?
It’s the SMD power amplifier IC (class‑D) driving the speakers. In documented cases, replacing it restored speaker output after front‑end repairs. Handle with proper thermal control during SMD rework. [Elektroda, Kob82, post #16872538]
What is the PCM1808 in the GML 20?
It’s an analog‑to‑digital converter (ADC) on the amplifier module. Multiple technicians advise checking or replacing it first when the unit powers up but audio is missing or low. [Elektroda, fellowes20, post #17109842]
What transformer spec should I seek for the GML 20 standby supply?
Use an approx. 3–5 W transformer with about 8–9 V secondary for standby. This matches observations from a teardown inquiry on the GML 20’s PSU section. Confirm pinout and mounting before substitution. [Elektroda, 398216 Usunięty, post #17126852]
My transformer reads short and blows the fuse—does that track with GML 20 failures?
Yes. A reported unit had a shorted transformer that heated rapidly and blew the fuse once disconnected from the board. Replace the faulty transformer, then re‑evaluate the downstream rectifier and regulators. [Elektroda, Kob82, post #17126605]
I don’t have electronics experience—where should I take my GML 20?
Use a radio/TV or car‑electronics service that handles SMD rework. Costs depend on diagnosis and shop rates. In‑person evaluation is required before quoting. [Elektroda, 398216 Usunięty, post #16866080]
