Crown Micro-Tech 1200 with an interesting fault. This amplifier was given to me to fix as someone in the past replaced the output transistors and drivers from another Micro-Tech. All done fine with matching sets but this had a strange problem where there was DC offset bouncing around between 1 and 4 volts on channel 2. When the amp was switched off the DC offset continued but at lower voltage around 1-2V for several minutes! Strange bouncing up and down . The owner complained about the noise it made through his speakers even when switched off. I checked out the voltage translator area and the control/gate transistors for DC protection and ODEP. All seemed fine. Then I powered up again and then switched off but this time removed the ribbon cables to output boards to isolate the offset at swittch off state. The DC offset still bounced around with the main board disconnected. I focused my attention to the main output board and visual check revealed sticky flux all over the driver areas. This flux was conductive. Cleaned thoroughly and then the amp was totally fine. Just goes to show that flux can cause all sorts of problems.
I also found if I re-routed the orange feedback wire (Im guessing this is for connection between high and low side bridge) from original position to going parallel to the case this dropped THD a fair bit and SNR went up. This wire seems to have quite an influence on distortion and snr. In the original position running along transformer side hum was detected through speaker. In new position running along case side it was silent even with headphones.
Anyway, after all that was done I checked/adjusted bias, ODEP voltages and adjust for lowest CMR.
Picture shows the board cleaned, cap mods and re rotored feedback wire.
Also does anyone know what these two small capacitor mods are for?
I also found if I re-routed the orange feedback wire (Im guessing this is for connection between high and low side bridge) from original position to going parallel to the case this dropped THD a fair bit and SNR went up. This wire seems to have quite an influence on distortion and snr. In the original position running along transformer side hum was detected through speaker. In new position running along case side it was silent even with headphones.
Anyway, after all that was done I checked/adjusted bias, ODEP voltages and adjust for lowest CMR.
Picture shows the board cleaned, cap mods and re rotored feedback wire.
Also does anyone know what these two small capacitor mods are for?
