Bogen MO-100 P.A. Amplifier
The Bogen MO-100 was originally designed for constant-voltage audio distribution, rated at 100 watts, and very configurable to various installations. The customer has two of these, and this is the second one in for restoration. After some basic checks, the power supply filters and all internal coupling capacitors were replaced, a new power cord installed, and a new volume control & knob installed. Everything checked out at DC, so it was time to try it on an audio generator and the oscilloscope.
I FORGOT TO CHECK THE OUTPUT TRANSFORMER IMPEDANCE!
The original output transformer had a shorted turn in it. Only 3 watts of the rated output made it to the load. After that, when the primary impedance was measured, the shorted turn was discovered. 1000 ohms was measured with no load. This should measure very high (a theoretically perfect transformer would measure OPEN). Oh, what to do!
Edcor transformer (http://www.edcorusa.com/) saved the day. A 5000 ohm plate-to-plate 100 watt transformer was ordered. This will change the output terminal configuration, but is otherwise a superior higher-fidelity transformer. It has a standard 0-4-8-16 ohm output winding.
After a bit of drilling and soldering, the new transformer was on. This is a trial for the customer, so the output terminals are not relabeled. If he likes it, the other amplifier will be likewise modified, both transformers fitted with black end bells, and output terminals relabeled appropriately.
Recalculating the feedback loop: The original circuit used a winding of 49 ohms to apply inverse feedback to the input. The new transformer has a maximum of 16 ohms. The original feedback series resistor was 51K. Run it all through this formula:
( Old Rfb / (√Old Zo) ) • (√New Zo) = New Rfb
( 51000 / (√49) ) • ( √16) = 29000
The new target value is 29K or as close as possible. A test with the audio generator and oscilloscope showed full power from 20 Hz to 24000 Hz. Nice!
View from the other side:
The (Original) Schematic:
Note the error: the feedback loop ties to the “49 OHM” terminal, NOT the “COMMON” terminal! I wonder how many techs this has confused!