RCF HDL-20A Class D Amp - No power.

[beavernuggetz]

Hello everyone,

I am having issues with 2 RCF HDL-20A amplifiers which fail to power on. Unfortunately the manufacturer does not provide any sort of schematics which makes this a very difficult repair for someone such as myself with medium level knowledge working with electronics.

Both of these amps had shorted Mosfets 6R90C6 as well as Driver IC S25131D which have been replaced. Also replaced the 2 big electrolytic capacitors, the fuse, cracked Thermistor on the top side, some burned SMD resistors (all on the power/hot side).

Checked remaining SMD resistors & transistors and the values match with those from a working amp.

Was told (elsewhere) to check voltage at the SMD capacitor (labelled 4 on pic 2) to the right of the 1BW Transistor because it should get 14-15 volts otherwise the amp will not power on. When I measure a working amp, the volts are indeed approximate 15 but for the one that does not power on the voltage is low and the cap is only getting 3.1 volts.

Here are some discoveries made. The 3.1 volts at the SMD cap is the same as what is going across the Zener diode (12 volts capable) on the top left. Both have been replaced to no avail.

Another difference is this. There is a small electrolytic capacitor (100uf 35v; first pic at the bottom) which on the working amp is reading as having 27 volts while on the broken one it has absolutely nothing. The capacitor has been tested and both capacitance and ESR come back okay; same values as the one on the working amp.

Anyone have any ideas? The first picture shows the top/bottom sides of the PCB side by side for comparison.

I labelled the main components on the power side of the main board on the second picture. The third picture zooms into the area with the SMD capacitor in question with the 3.1 volts. The 4th picture is a bit of a mess showing the path of continuity between the 12v Zener diode and the components connected to it including the small electrolytic capacitor (100uf 35v) with no voltage.

1 & 2: Big electrolytic caps. Have been replaced.
3: 1BW Transistor - Replaced
4: SMD capacitor with the low voltage - Replaced
5: S21531D: Driver IC. Has been replaced with IRS21531D.
6: LMC555CM: CMOS Timer - Replaced
7 & 8: 6R190C6 N-Channel Mosfets - Have been replaced with exact match.
9: 4702 SMD resistor = 47kΩ. On working system (still attached to the board) it reads 26.68kΩ vs. 40kΩ on faulty one.
10: U1JB: Ultrafast Rectifier Diode

Any thoughts, questions, suggestions are welcome and highly appreciated.