Main Amp Board 8215 continued
Completed amp board
Center Voltage and Bias Adjustment
Once the 8215 Main Amp board of the KA-2000 is rebuilt six new trimmers will have been installed. Prior to beginning any restoration work I made all the necessary amplifier adjustments in stock form to provide a starting point for the work ahead. While working on the amp board each original trimmer setting was measured and the values duplicated for each of its replacements. Even still, the first power up should be done on a dim bulb tester (DBT) with a bulb about twice the value of the W/per channel. Here we have a 13W/Ch amp into 8 ohms so I used a 28W bulb.
Once powered up with the DBT you can check the center voltage for each channel. First, use a DMM to measure DC voltage across the collector (just place your red probe to one of the transistor mounting screws for the collector side) and chassis ground of output transistor Q2. Record the value and then repeat for Q3. Using the DBT will limit the current some so your voltages will be a little under 42V in the 39-40V range. Likewise measure and record the collector voltage of Q1 and Q2. The reading for each of these should be a little under 21V and about half that of the corresponding channel in the 19-20V range. Verify that Q1 is close to half the value of Q2, and that Q4 is close to half the value of Q3. If adjustment is required use VR1 for setting Q1 (left channel), and VR2 for setting Q4 (right channel).
The output transistors as seen top side. face forward, from above. The "C" is for collector.
Still using the DBT, we set a preliminary and safe bias setting for each channel. We do this by measuring the DC voltage at the emitter of Q1 and ground for the left channel. Repeat by measuring the emitter of Q4 and ground of the right channel. At full power the bias should be 14.1mV so while using the DBT set it to about 70% of that value to be on the safe side thus around 9.9mV. Trimmer VR5 adjusts the left channel and VR6 adjusts the right.
Now that the preliminary settings have been made it should be safe to turn on under full power. Allow 20 minutes for the amplifier to get up to full operating temperature. Using the procedures previously described, set the center voltage. Again, the voltage of Q1 should be set to half the voltage of Q2 using VR1 and the voltage of Q4 should be set to half the voltage of Q3 using VR2.
Once the center voltage is set you can move on to the bias adjustment. With the amp at full operating temperature set the bias to 14.1mV for each channel. Red probe to the emitter of Q1, black probe to ground adjust trimmer VR5 for the left channel. Red probe to the emitter of Q4, black probe to ground adjust VR6 for the right channel.
Since we have yet to work on the power supply (filter caps, rectifier etc.) section at this point in the restoration it would be a good idea to revisit the adjustments when that has been completed.
Note for the geeks: The bias current for the KA-2000 is 30mA. This being a quasi complimentary design we measure across only one emitter resistor (R11 L, R12 R) to find the bias. The proper voltage can be calculated by the following: 30mA x 0.47uf = 14.1mV. Quasi complimentary designs were used in the day because the performance of complementary NPN and PNP transistors wasn't what it is today and the reason you rarely see it anymore.