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.

[Mark R]

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.

Thanks for the reply "Capleaker" The unit at 120 volt from my Sencore PR570 only gives a reading of what is stated around 62 volts on the rails. If dial up the Voltage to 126 Volts AC, the FM will come on and the rail voltage is at spec. The power transformer is weak and has rust and goo on it. I think it got wet at some point as it was stored in a Basement for years. Sort of like the TV show "Roadkill" where they find junk and make it run.

It you plug the unit into a normal AC outlet it's weak. At full audio out @ 1Khz I can get 123 watts @ 8 dummy load before clipping. The unit is rated at 130 Watts @8 ohms. So I'm sure my Bud can't hear the 7 watt per channel drop without his ears bleeding. So to recap.. normal power form the AC here is around 119 to 121 it varies. But at that level the FM will not work....It needs that extra 2-3 volts AC and the unit needs that extra AC input. My Bud doesn't have a Variac ..L.O.L. and knows nothing about electronics. I need to make it work like they do on roadkill. But All shined up and pretty looking and functional.

Anyway the unit is working except for that Reverb section. I received the new switch in today which I'll install tomorrow. All those caps in the Kenwood are "ELNA" good caps ..In fact the ones I checked in other circuits were all at spec. But I replaced them anyway.

I have all the equipment in my shop to scope out the flow.But the hard part is that Dam switch....Can you follow what is on when and where that switch is moved. Some sections turn off while others turn on. It's downright confusing as there is no Theory of Operation of signal flow description and no service manual available.

All Transistors in the muting circuit and the rest of the board tested good. It may be as you say I cap open somewhere. There are plenty if you look at that Schematic. Good thing I have a Pace MBT 250 de-soldering station.

Anyway I'll keep you posted on the switch change ..but I have bad feeling its a Cheapy Cap.

Thanks Again: Mark R

[CapLeaker]

First probe around and see where the audio doesn’t go but supposed to be and what state the muting circuit is first

[Mark R]

First probe around and see where the audio doesn’t go but supposed to be and what state the muting circuit is first

Good Advice..Now an Update:

Received the Flea Bay switch. The circuit board it was on is laid out differently then the KR 790 Kenwood board as far as Circuit traces go. Even though it was from a Kenwood unit. Installed it and ...no go. So I'm assuming the original switch is good. So....I put the original switch back in. Now I tied my tried and true method. Turn the old duster can upside down...and what do you have??? Freeze Spray.

So I set the unit in the Source & Mic position where there should be audio and started with a few short burst of - 10,000 degree spray..L.O.L As soon as I hit this hidden 1/2 watt resistor I got some crackling and a hit of audio. So it looks like i have an open resistor.

At that point I stopped as I had been going all day with cleaning two of my .45ACP throwers. I smelled bad from Hoppes #9. And I was burnt out from yesterday at the range. So tomorrow I will move some caps that are hiding this resistor and ohm out a few plus that one in the hidden grotto of components.

I didn't look to see what part of the circuit it's in.... I'll do that tomorrow and get back to you. I'm beat.

Anyway..Thanks for hanging in there with me. I appreciate it. It's been my experience on some forums when you ask for help and you get replies with helpful info, it's nice when the person your trying to help lets you know how you made out with there assistance.

So in that spirit, I will get back and let you know what's going on.

Again, Thank You.

Mark R

[CapLeaker]

Seen this a few times, but quite rare. A resistor measuring normal when cold, but as soon as power goes through it, it went open within 10 seconds and the circuit died. Removing power or heat and immediately measuring the resistor with a DMM I could watch the resistance go back to normal. You could be able to verify this by using the can of air upside down aka poor man’s freeze spray and wifeys hair drier or hot air station.
could be also that something is short after that resistor.

Advice: if you do find some dark brown goo over components, remove it. It is not only corrosive, it is also conductive.

[Mark R]

You could be able to verify this by using the can of air upside down aka poor man’s freeze spray and wifeys hair drier or hot air station.

If you read the post above.... I love upside down freeze spray and used it to locate that resistor. I'm about to go at it again now. Will see what else took a dump. Also I have no wifey....L.O.L. And I Do have a hot air gun. Be back ........

[Mark R]

To Day Is Sunday...Day Of Rest... But not for me.... I'm going to check the rest of the resistors and caps....Long Day ahead. No Dark Goo on he components. All good there. I always remove that stuff when I see it

First probe around and see where the audio doesn’t go but supposed to be and what state the muting circuit is first

That is almost impossible. If you don't know what position the switch is in from the schematic you have no idea what's supposed to be on or off. The schematic I posted looks like it's in the off position??? From there you can get some idea of what's what. So in the off position, regular audio passes thru...which it does as in normal operation. And I get Audio

But then as I challenged you .... Next position is Mic & Reverb only on..No Reverb but when I tap the 1/4 " Phono plug in the Mic Socket. I get the Mic, but no reverb and the Blue reverb light is on.


But!!! The third position ..Is Mic and Source with Reverb... You only get Mic no source or reverb. The Blue Reverb Light comes on but no audio or reverb.

So I think it's what your name sake says....Capleaker. L.O.L. If it aint a resistor (Which I eliminated last night) if has to be our old buddy a Cap open somewhere. Since they are used to couple DC circuits..all it takes is for one somewhere to open.

Just finished my coffee ..and might smoke a Cigar before I hit the SENCORE LC103 Cap Checker. My Best Piece of test equipment...My Friend.

Talk To you soon.. Capleker Master....LMAO.

Regards: Mark

[Mark R]

Ok Capleaker ..here's the latest.
Tested every cap and replace a few that the LC103 said were just below -20%. 90 % of those ELNA caps were dead on after all these years. The circuit Power is only + - 24 Volts DC. So not much strane on the caps.

Every resistor was in spec hot and cold. Previously I had tested all 7 of the Transistors in that circuit. I used my B&K 520 Testor out of circuit and they showed no leakage. At that point i had checked everything in the circuit.

So, Next step "SHOTGUN". I had picked up from Flea Bay a Bag 0f 100 pc's of the same transistor for a wapping $8.00!! So I replace all the transistors even though they checked ok.

I took apart the Function switch which is in sections and cleaned the contact surfaces and put it back together.

So now I have.... Mic only..Good. And Mic and Source.... Good... But no reverb either source or Mic. The reverb chips are new and were handled with anti static equipment.

So Now it's time to walk away and ponder what I have and let my neurons come up with an answer. Unless you have something to add.

So, I'll stop here unless you have something else for me to ponder.

Thanks for your Help.

Regards: Mark R.

[CapLeaker]

Do you have an oscilloscope?New chips can be bad. Doesn’t matter if you spent $$$ or $.

[CapLeaker]

So you replaced IC2 and IC3?