Kenwood/Trio Amplifier Repairs
Kenwood KA-9800/8300 Stereo Integrated Amplifier
This would shoot the LHS speaker across the room intermittently, annoying for the listener and the speaker I guess, also the volume control was as wobbly as a Thunderbird's puppet! The volume control was attacked first, the last 4 sections were coming away from the front ones, tried bending the clips in to the Bakelite but would not hold it together so soldered a couple of wire links across them. The shooting speaker syndrome was fixed by replacing the 2SC1885 transistors which was emulating a diode array every so often, noticed burn marks on the PSU board as some resistors had been (and probably still will be) getting hot, along with the usual dry joints, replaced the capacitors on the board as well.
Kenwood KA-9100 DC Stereo Integrated Amplifier
This one had a faulty transformer and output module on opposite channels? as well as a blown 2SA913 power transitor. I replaced the output modules with discrete Darlingtons; MJE240\350, 2SD1047\2SB807 and mounted them on the heatsinks the module used to occupy. It sounded excellent after.
One channel was distorting. The STK350-030 amplifier front end needed replacing, wrong bias to the output transistor. When replaced the speaker relay needed contacts cleaning.
This is a mono block amplifier from 1979 along with the L-07T turntable a legend in its own? This had been stored in a garage or similar by the corrosion but hey it's an L07M. The amp wasn't working (obviously) a look at the driver board - a plug in board, showed there were several transistors and a 680ohm resistor blown, the driver board is the other picture, both the T0220 and the T092 transistors were changed as were the 2SA810's and 2SC1222, the pot to the left is for the quiescent, the other 2 are for the DC output zero one being for course\fine.
The main board can be checked for problems by powering the amp with the driver board unplugged there should be +\- 62 V at the T0220's on the heat sinks and the o\p relay should 'click' in! If it doesn't then one or more output transistors may be s\c. The DC sensing PCB is on the left of the chassis, there is also a remote relay to connect the mains proper as it can be switched on remotely using front panel DC plug\sockets. There were several dry joints in the on the driver PCB and main board.
This was a 'big' integrated amplifier and weighed over 25 Kg, the fault was simple. One of the fuses from the mains transformer for one channel had blown. This also fed the system monitor IC which senses the DC at the output and switch on delay etc. and as there was no a\c, the IC sulked and refused to power the speaker relay.