Some pictures of the mods

BEFORE...

 

... AFTER

      

 

 

Voltage checks

Before and after the modifications I had noticed strange voltage values in some test points of the circuit : 15,5V instead of 20V at the rectifiers output.  I measured 15V on R71 and 12V on R72. These were well below the specifications (20V, 18V and 16V).

My processor is wired for Europe (230VAC)  with a 10Kohms resistor in series with the transformer primary winding, as per the Service Manual instructions. I was astonished to measure only 97V-AC instead of the expected 115VAC  in the transformer primary winding, so I replaced the 10Kohms with a 8,2Kohms resistor to allow more voltage to flow. Nothing changed, same low values in the EQ circuits. So I suspected the original transformer to be the weak link : I removed it from the PCB and measured voltage output from secondary windings without load (22 Volts) and under load, which showed a dramatic voltage drop. Conclusion : one crappy transformer here !

To correct this situation  I quickly  replaced the original trafo with a 44000 SERIES from Myrra,  10VA 2x15V (www.myrra.fr) which allowed me to get rid of the 10K resistor - primary winding is designed for 230VAC - and made internal connections even more simple and clean. I only had to drill two new holes in the PCB for the screws holes of the new trafo (see diagram). For the sake of signal to noise ratio the transformer was entirely shielded with copper foil. Secondaries were connected in a 15V-0V-15V configuration (center taps soldered together).

After installation of the new transformer the voltages were back to specs, although a bit high (22V, 19V and 16V) but nothing to worry about : the only active components in here are BC239C transistors and they will withstand easily that values. They are now powered exactly as intended by the designer of the EQ, which is crucial to obtain correct gain and distorsion figures. Voltages were checked at test points of all transistors and all are within specs. 

Setup

Some pictures of the setup here .

Figures and performance

 Now it's time to measure performances, let's see what this device is doing. Performances were verified through Righ Mark Audio Analyzer 6.0 with an external sound card from M_Audio, the Transist USB.

Click on picture to open a new window.

 

 

 

Equalizer's controls efficiency, equalization curve

 After the modifications  the equalizer shows equalization results  matching the Service Manual specifications as shown below

 

Reference : all controls set to dot marks

 

 

Below 40 set to "CONTOUR" (green curve)

 

High frequency set to "DECREASE" (green curve)

 

Treble control position 1 (decrease, green curve)

 

Treble control position 2 (decrease, green curve)

 

Treble control position 4 (increase, green curve)

 

Treble control position 5 (increase, green curve)

Download

Bose 901 series I/II Service Manual

 

Related links

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