The Overseas Only Kenwood Model KR 790

[Mark R]

I am Repairing this unit. It had many problems. One major one was the power transformer being about 4-5 volts low on the main B+ line which called for 66.5 Vols after the Main Filter caps but only showing about 62 Volts. Caps and the 5 amp Diode bridge were replaced which helped slightly.

The other strange problem was the small secondary winding from the PT which supplies 18 Volts AC to the "FM FLUORESCENT DISPLAY" circuit, and the small winding that supplies 8 Volts AC to the push button grain of wheat lamps. Those voltages were down from 18 Volts to 16 volts. Which was just enough for the FM display NOT to light up but enough for the signal strength l.E.D.'s to light up.

The 8 volt section was down to about 6.5 volts but the lamps still lit. Also there was a problem in the driver circuit which would turn on the protection relay. At one point with the Voluum control off the relay would close and full B+ would appear at the speaker outputs. Then, when you turned the volume slightly the protec relay would open. Found a bad component in the driver circuit which caused this.

So now, I had to solve the problem of low voltage at the FM 18 Volt AC area. The fix was a separate 110 Volt AC input to 18 Volt Output transformer to feed the input of that area. GREAT you say......Bam... Another problem pops up. This unit has a "Karaoke" system. It has a LOz Mic input and a REVERB!!!.

So, you can put on your favorite music and balance the music with the front panel controls with the mic input And sing along.

This circuit has proven a tough one. This circuit will pass audio to the outputs when the "Karaoke" switch is in the OFF position. Also the blend for Mic and Music does work in the on position but no source audio. All transistors test good with my B&K transistor tester. All the IC's were replaced. Diodes check good. I have narrowed it down to the master switch which has an "OFF" .."MIC".. "ON..MIC & SOURCE". I am waiting for an eBay small board that has one similar to the one in the unit. the Diff is the shaft..

Anyway this unit has taxed me... L.O.L There is only a two page schematic for this unit world wide. No service manual that I could find with theory of operation or adjustment settings. Take a look at the Reverb section of the schematic. See what you think.

I think it's the switch.. BWTFDIK..L.O.L.

Mark R

[CapLeaker]

The power issue(s) shouldn't be a surprise, since it is a transformer based PSU. Means output voltage increase and decrease in sync with the input voltage. Lets say you got 110VAC input, the output will be lower than if you had 130VAC on the input. The voltage cannot be 66.5VDC to the dot. Same as the 18VAC supply. It all is dependent on the input VAC that you got out your power outlet. Some folks have higher, others have lower voltage. To me, there wasn't much wrong with the amp first place.

As for your other problem with the karaoke / reverb. That is a pain in the rear end, so many things to check. There is the switch (should be easy to verify), there is a muting circuit, there are series DC blocker caps that could be bad... etc. I think the best idea here is to borrow an oscilloscope go through that board and check that muting circuit and follow the audio until it's lost somewhere.