Yes Yes Yes Another Kenwood KA-4002 Retoration

[Butler]

Hi Kenwood/Trio lovers,

Yes another KA-4002 restoration. Picked this up recently, ran it up first on a lamp limiter for a bit and seemed to be working fine for a couple of days and then, nooooooo, that dreaded distortion in one channel, I don't think it had been used in a long time and I was hoping I was going to be able to enjoy this for a while before I would have to get the lid off. Has had a bit of a dust. There is one thing that I haven't seen on any other pictures is this added PCB, with I presume power from pin 38 and then wires into the tape monitor switch and tape dubbing socket?? Anyway just some pictures to be getting on with.

Wooden case, needs a bit of attention.

Has had a bit of a dust and a cap count. Also noticed it has a cable with an earth fitted

You can see that some of the caps are a bit swollen on top so they will be my first port of call, full recap.

Wiring from additional PCB.


Have used @dlucy thread and BOM to from a parts list for the restore, most probably ordered more than I need but when there is a minimum spend to get free delivery, may as well, will always come in handed anyway.

 

[Butler]

It Was a bit daunting at first looking at the PCB from underneath but with the "assemblies-phantom view" for the underside (even has the capacitor “+” labelled on the PCB as well) and my own sketch for the top for capacitor polarity and lots of photo's, was not so bad, time consuming though. Took the additional RS PCB out.

SO, full recap has gone well replacing the main power caps and caps on the PCB. I did notice that a lot of the caps had scorch marks that were quite close to some of the transistors. Tested a few and some were good but some were not but sure they were ready for a change.
Let the amp soak for 15 mins on the Lamp limiter (LL), all good , no shorts, then checked a few readings.

Voltage across the main cap initially was well over 50v so bridge diode replaced and now is to 49.4v (is this right?)

Voltages at the couplers (chassis to collector) Q1/4 were 49.4/49.3v and Q2/3 24.9/23.9v, wasn't sure what these should be, even with the LL on they were about the above the 42v and 21v's.
Come to checking the bias and another missing pin 41 (it was there) but labelled up as 44, so I had 2 pin 44’s. Bias, reading from pins 41(44)/45 39mv trimmed down to 9.3mv, then checked pin 47/49 down to 9.2mv. All connected up, small listening test and sounds good on both channels.

Back to the bench, new trim pots, emitter resistors and a bunch of smaller caps and @dlucy “snubbers” on the main caps, tidied up the loose cables from the addition board (not going back in, but will keep).
Check out pin 44
Took the thermistors (5T31L) out, they read about 1.6k (room temp-18 degrees) and reduced when some heat applied more or less identical.
All done, LL for another 15 mins, then full power, checked bias and tweaked back to 9.3mv.

Now the real test begins, “it’s all about the music”.
Running in setup, PC running VLC, Fubar Dac, Royd Minstral Speakers. The KA-4002 sat on an Armstrong 521.

The amp has been easy to sort as just about every query was already covered on the forum and there were no transistor issues , The lamp limiter is such a valuable bit of kit, saved wrecking the Armstrong 521 numerous times. Lets see how it gets on after an hour or more. Only thing to do is to tidy the case up if all goes ok.

1 hour in and all good 2 Hours later and still good,

Small issues noticed:
-Speaker A+B selector doesn't work
-yet to test the Phono inputs but all others are working
-oh and 2 of the knobs keep falling off

I've been lucky I think

 

[Butler]

Spoke toooooo soon, but some of it was self induced…. Listening was going ok and then I got a little too critical and decided that the sound wasn’t balanced from the left to right speakers (volume), adjusted the balance control a little and just right. The itch grew.

Checked the bais as per service manual instructions, around 9.5mv up and down a little on both resistors, decided to swap speaker cables over on the terminals, very low sound on one channel but both sets of terminals are feed from the same outputs. Decided to investigate the speaker selection control, A+B already didn’t work, in my examination disaster, cracked the rotary selector at the back, definitely no sound now.

Set about trying to understand the schematic. Eventually came the DMM on "continuity" and came up with this.

Which ended up as this.

Sound from Both A, B and A+B speaker terminals, dispensed with the headphone jack and the mono RCA output, would never use them. Cables still there but tucked away in case a selector switch sourced at a later date.

While I was at it changed the speaker terminals and put new transistors (MJE15034G’s) in Qe1/2/3/4, these have quite a high hfe compared to the 2SD155’s I pulled out that all had around 30 hfe the MJE’s are around 150hfe, this was the only considerable difference. Is there any issues with this??

the little thing runs quite warm, the heat sink is quite hot to touch as well, is this normal??, it was the same before I swapped out the 2SD155’s.
Also there is that initial DC offset at turn on causing the bass drivers to flex. Its got a 3300uf main cap and two 1500uf couplers Getting into the nitty gritty now.

Is the running hot normal?? It's been on again for another few hours and no issues, That's how long it takes me to type this out

Anyway, It's had its sideburns (as @hopjohn calls them) all sanded up and wondering whether to have some wax, oiled or varnish on for the weekend. they look great.

 

[Butler]

Been having fun with my KA-4002 the more I seemed to use it the more things started to fail, ended up with over 140mv bias on the right channel, Qe11 was the culprit, read ok when it was cold, when warmed up it read as 2 diodes? Anyway Qe11 replaced and the bias down to where it should be.......I think I initially had a problem measuring the bias in the first place. Anyway that's sorted. New speaker selector rotary switch in place but with only "off", "A", "A+B", still got to wire up the headphone socket.

Next issue, distortion on the right channel spent almost a day measuring, pulling and testing transistors, caps and resistors. Re-flowed just about every track as well. Ends up being the Tape monitor switch. I squirt of servisol switch cleaner and a few in/outs all fine......... at the moment, whats next?

Question 1, So, Can I bridge the tape monitor switch across the pins with the grey and violet (is that the right colour?) cables? to take this out of the equation?



Question 2, there is +2.2v DC offset (starts at 15v then reduces to the 2.2v on both channels after less than a minute), is there a way to get this down to a more acceptable level?

Any help or suggestions would be great please

 

[Butler]

Update on the Tape Selector switch, was looking in the wrong place on the schematic? Found the switch in the right place and out came the DMM continuity probes and on to the switch, the result is the two jumper bars on the switch. No more distortion on the left channel (I did say right earlier) when the board is tapped or the wire moved. Originally it had a additional PCB that must have been used for tape recording/monitoring, this is now gone so having these contacts fixed is not a problem.



Still no thoughts on the DC offset issue?

 

[Butler]

Ok, my initial worry is unfounded, read this thread about measuring DC offset on cap coupled amps

https://audiokarma.org/forums/index.php?threads/measuring-dc-offset-of-cap-coupled-amps.661182/

Was initially seeing the 15vdc coming down to 2vdc with no load after a minute or so, with a load it is 1mv.

So DC offset is good.

Sooo the next question if anyone can help, how do you stop that initial surge of 15v at turn on?

 

[Butler]

Looks like there is no easy solution

https://audiokarma.org/forums/index.php?threads/help-with-cap-coupled-amp-power-on-thump.856738/

 

[Butler]

Well that's that then, Speaker selector switch all done and with the headphone socket as "A+B". All back together, and running in, hope there's not more hiccups. Spoke too soon last time



For turn on, I leave the speaker selector at "off" give it a minute and the turn to "A", hardly any thud

Looking for the next one

 

[Butler]

The Kenny finally earned its HiFi Rack status only to let itself down, back came the low channel volume hisses, crackles on the L channel, all coming from the switch selectors any movement on the switched made it worse. Out it came, you could see some of those very faint hairlines in the tracks, so re-flowed all the tracks and replaced a few resistors and measured some of the capacitors (read a bit high so on the shopping list). I just wonder if this has been part of the problem all along.

Back on the running in rig, so far so good.

 

[taineasy]

Hi Kenwood/Trio lovers,

Yes another KA-4002 restoration. Picked this up recently, ran it up first on a lamp limiter for a bit and seemed to be working fine for a couple of days and then, nooooooo, that dreaded distortion in one channel, I don't think it had been used in a long time and I was hoping I was going to be able to enjoy this for a while before I would have to get the lid off. Has had a bit of a dust. There is one thing that I haven't seen on any other pictures is this added PCB, with I presume power from pin 38 and then wires into the tape monitor switch and tape dubbing socket?? Anyway just some pictures to be getting on with.

Wooden case, needs a bit of attention.
View attachment 2549384
Has had a bit of a dust and a cap count. Also noticed it has a cable with an earth fitted
View attachment 2549400
You can see that some of the caps are a bit swollen on top so they will be my first port of call, full recap.
View attachment 2549409
Wiring from additional PCB.
View attachment 2549420

Have used @dlucy thread and BOM to from a parts list for the restore, most probably ordered more than I need but when there is a minimum spend to get free delivery, may as well, will always come in handed anyway.

The Kenwood Trio KA-4002 is one of the nicest sounding small integrated amplifiers I have ever heard. Designed by the same people who left Kenwood to form Accuphase. Hard to provide enough praise on how good this amplifier sounds. Simply amazing.