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KENWOOD KA-4004 Recap & Restore

Zeus12

Zeus12

Active Member
Greetings Audiophiles,:)

Got this VINTAGE GEM and I would like to do a complete recap and restoration on it.
KENWOOD KA-4004.jpg

It was 100% original and working great when I got it, with the usual scratchiness of the dirty pots. The only thing I noticed, was ONCE IN A BLUE MOON, the left channel would have a little of static/noise---sounding a bit like boiling water--but it would go away after a few seconds.
KENWOOD KA-4004 Pic2.jpg

I did the usual preliminaries on it: Visual inspection inside, cleaning of the the faceplate, knobs/buttons and cleaning of the pots with DEOXIT.

I started recapping the PREAMP Board (X08-1080-12). Would like to know if I should change the "trannies" Qd1,2 (2SA620WL4) and QD3,4 (2SC1416BL). I changed all the Electrolytics and the 2 Tantalums Cd1,2 (1uf 25WV) with WIMA 1uf 50VDC. I am listening to it know and it sounds perfect. I haven´t heard the static/noise yet, but then again, it was not a regular/constant issue. Perhaps the "aftermath" of the DEOXIT cleaning of the pot?

Here´s a pic of the Preamp Board. Used Nichicon UKT and UFG series caps, by the way:
KENWOOD Preamp Board Recap.jpg

Any other suggestions and advice on this SWEET SOUNDING amp?

Cheers!!:beerchug::beerchug:
 
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@eizner23

Thanks buddy!! :)

Yes, I´m amazed at how SWEEEEET it sounds and how smooth it is. Very balanced sound and has all the controls that I use, no more, no less.

Cheers!!!
 
I have the 6004. These do sound sweet. There are some of the 10 worst transistors in the unit’s preamp. You could change these. See the sticky thread in the DIY forum on 10 Worst Transistors.

I’d probably separate the unit to see if the noise is in the amp or pre amp. I suspect the pre. I’d also check the DC offset. Replacing the differential pairs dropped my offset from 170 & 225 mv to about 3 on each channel.

It’s never my strategy to do a shotgun replacement of transistors. Replace them only if needed or known troublemakers.
 
Last edited:
@Semanon

Big thanks for your help!! Unfortunately I ran into some problems. When I went to check the bias I noticed that the VR1 PC trimmer for the BIAS is missing a part. I can´t move anything on it. The left channel checked out OK, at 20 mV more or less but the right channel is showing 1 (-1). Used the "Voltage Drop" method from the manual. (Took me a while to figure it out since never measured the BIAS this way).

Just to make sure. Am I checking it OK? I´m connecting the leads to those pins of the output transistors.
BIAS FOR RIGHT CHANNEL.jpg

Here´s the messed up trim pot:
Broken PC VR1 Trimmer (BIAS).jpg

I would have to change the trimpot. In the manual it says: PC trimmer (BIAS) 500 Ohm (B)
I only have this at hand: TRIMMER 1K OHM 0.5W PC PIN TOP https://www.digikey.es/products/en?keywords=3386P-1-102LF I would imagine it won´t do, correct? Any suggestions of a suitable trimpot?

Not a good idea to continue with the recap until I get the BIAS of the right channel correct?

Thanks for your help!!!

Cheers!!
 
Greetings again!!

Had this little baby on the "to do" list for a while. I am still getting some uneven sound which sounds like a panning effect, static sound and not a strong stereo image. The right channel shows it perhaps a little more than the left but both have it. I have separated the Main and Pre and found out that the "Pre" sections works great (still uncapped) so it must be in the Main Amp sections.

I have finished recapping the Main Amp Board (X07-1030-01), replaced both Bias trimpots, changed most of the transistors on it. Used the following replacements:
Q1-Q4 KSC1845FTA (all with 430 beta)
Q5,Q6 KSA1013Y (227 beta matched)
Q7,Q8 KSA122AYS (207 beta matched)
Q9,Q10 KSA290AYS (291 beta matched)
Q15-Q17 KSC2383YTA (all with 290 beta)
Q18 KSA1013Y (227 beta)
Q19,20 KSC2383YTA (290 beta)
Keneood KA 4004_53.jpg


Also, changed the 2 caps on the PSU board (C2, C4) and the 470uf axial was within spec so I left it in there.

The relay clicks in normally, and the Bias is set more or less on target (20mV). I also measured DC Offset and got 35mV on Right channel and almost 22 mV on the left channel. So there is something not right with the DC offset. From what I have read, the "differential" pair of transistors are what keep the DC offset at around 0, correct? Are those on the pre-amp board?
Keneood KA 4004_86.jpg
Keneood KA 4004_87.jpg

Checked the big filter caps as well and they look to be more or less within specs. Could it be a leaky diode? What is the sound characteristic of leaky diode, for example? Any suggestions on what I should check for next would be great!

Cheers!!!
 
Hi @eizner23

Big thanks buddy!! :):)

OK. I will replace those with matched 1845´s. Got plenty of them on hand. While I´m in there I´ll replace those small value caps with films. First do the transistors, listen, and then do the caps, for example?

Now the thing I don´t understand is why is it working OK when I had it separated? If the "Pre" Out was working OK, doesn´t that mean that the issue should be somewhere on the Main Amp or PSU board? Another thing I would like to know is, does the signal pass through it no matter what? For example, if I have Tone Defeat ON, would the Audio signal path avoid the tone amp board all together?

From what I heard, this little gem sounds GREAT!! Nice tight low end. Plenty of punch and love the layout of the knobs --especially that big volume nob. Oh, that clicking sound of the Bass and Treble knobs is a treat. Can´t get enough of that sweet and authoritative CLICKING sound! :rflmao::rflmao: The tactile feel probably has to do a lot with how much you like one piece of gear over another --given equal sound more or less. I´m finding out that Kenwoods and Harmans are very underrated.

Can´t wait to change those differentials and see --hear -- the full potential of this little baby!!

Thanks again for enlightening me!!!:beerchug:
 
Hi there again,

Just realized that I did replace those. Sorry, I didn´t look at it carefully. I think I replaced all transistors on the Main amp board. What about that lonely transistor 2SC495 on the PSU board? I didn´t check that one.

What else could it be then?
 
If I have Tone Defeat ON, would the Audio signal path avoid the tone amp board all together?
Looking at the schematic, Q11,Q12 still gets signal passed thru with tone defeat engaged. Those devices may be suspect.

I also measured DC Offset and got 35mV on Right channel and almost 22 mV on the left channel. So there is something not right with the DC offset. From what I have read, the "differential" pair of transistors are what keep the DC offset at around 0, correct?
DC offset in these units with 20mV or lower is acceptable for negligible distortion, but 0 is the target. Some resistors down stream may have drifted, or when you handled transistors while measuring beta the heat from your fingers changed the measurement, potentially causing uneven diff. inputs. High DC offset causes distortion, it what your hearing distortion?

Did you use another external preamplifier unit to test the amplifier board?
How long did you let the unit warm up before measuring offset?

Couple steps i might take...
Maybe try replacing De1,De2 sitting on collector of the diff inputs with 1N4148.
Consider pulling the outputs and reapply thermo paste, check or replace mica insulator.

You can peel thru this thread on more ideas on how to lower the DC offset
https://www.audiokarma.org/forums/index.php?threads/amplifier-distortion-dc-offset-and-you.5634/
 
@eizner23

Big thanks for helping out a "freshmen". Gives me hope to keep at it! Thanks buddy!! :beerchug:


So to recap (no pun intended):

High DC offset causes distortion, it what your hearing distortion? Yes, distortion and unstable stereo image. Comes and goes. Kind of sounds like someone is playing with the balance. Like a panning effect from side to side. No loud popping (pop-corn) sounds though.

Did you use another external preamplifier unit to test the amplifier board? Yes, used the Kenwood as a "Pre" Amp and a Pioneer SA-7500 as "Main" Amp. Worked flawlessly. When I reversed it, same symptoms as I´m describing.

How long did you let the unit warm up before measuring offset? I left it on for about 30 minutes. Ambient temperature around 20-22 degrees (C). The Bias is rock stable on both channels, by the way.

So if all the caps, and transistors have been changed on the Main Amp board the issue should be with either a Diode, Resistor or Varistor? If a diode goes leaky what are the sound symptoms? I was under the impression that Diodes rarely go bad and resistors almost never. Oh, the joys of electricity!!

I have replacements for most diodes on this board. Gonna start with De1,2 (1s1555) and replace it with an 1N4148.

Cheers!!!:beerchug:
 
No problem, im just a sophomore myself!

High DC offset causes distortion, it what your hearing distortion?
Yes, distortion and unstable stereo image. Comes and goes.
Comes and goes = its plays like it should momentarily? Does the issue arise on both channels? And with headphones?
What voltage do you measure on positive side of Ce24?
 
1. Yes, it plays as it should for a bit (momentarily) and it sounds SWEEEEET!!! What a lovely sound! Then it gets those symptoms I described, on both channels. It might start more in one channel, right for example, but it effects both channels. It´s like the sound is drifting from side to side and some distortion.

2. Headphones --- Sorry, haven´t tried. How stupid of me. Will do first thing tomorrow morning.

3. What voltage on positive side of Ce24? Haven´t checked. I´m still learning how to get the voltages. I clip the black lead tp the chassis and then very carefully touch the positive lead of cap C24? Every time I have to probe things with the amp on I GET THE SWEATS!! Don´t want to get any sparks. Still remember when I shorted an undischarged big filter cap and saw this big spark....:rflmao::rflmao:Almost **** my pants.

Cheers!!
 
If it plays great momentarily on both channels then it doesnt, it could be an issue with something common, such as the power supply.
When it acts up take a wooden chop stick and put light pressure on the foil traces around the power supply- there could be a cold solder joint.
If that doesnt give any positive results 2SC495 could be faulty. Its only rated at a half a watt which seems low. You can try subbing it with a beefier C2690A since it looks like you have them on hand. They are both ECB pinout. You may want to replace those 3 e-caps and the 1.5A rectifier diode (anything in the 1N539x series) on the supply board why your in there.

2sc495.jpg
 
Greetings audiophiles,

How are you @eizner23 ? Thanks for your help buddy! Had 1001 things come up since our last exchange.

OK. This is what I have done so far:

Changed the 2 diodes (De1,De2) with a 1N4148TR. From my little "StartoGoo" Multi-funcion Tester TC1 I got these readings for both of them more or less:
Keneood KA 4004_95.jpg

Tried to see if that changed anything and nothing changed.

Then I thought maybe I should recap and change the transistors on the Filter and Tone board (X11-1070-10).

I changed all the caps-- all of them were out of spec. The ones that were most off spec were the 2 "tantantalums" (C1,C2) which showed this (looks like very high ESR?):
Keneood KA 4004_125.jpg

Changed the 4 transistors (2SC1000) with KSC1845FTA--all of them beta matched hfe 430. For the caps, used WIMAs and an UPW Nichicon cap for C21 (47uf 35) --looked like a non audio path cap so that´s why I used it. Still learning to "see" the AUDIO SIGNAL PATH.
Keneood KA 4004_149.jpg

Hooked it back up and it´s playing great, but still has the same issues. Thank God that I didn´t mess it up any more though. I knew the "Pre" was working OK before because I had it separated. At least I know that I have a fully recapped tone board now.

Also did the "take a wooden chop stick and put light pressure on the foil traces around the power supply- there could be a cold solder joint." Didn´t notice anthing. Those leads, traces and solder joints on this little gem are ALL OK. Checked with a magnifying glass. I have to say that I like these boards to work on more than the "green" ones. Just my preference.

Oh, had a more careful listen to it and WHAT AN AMAZING little amp this little beauty is. It has a STRONG bass and a PINPOINT center image. Very sweet, pleasant and non-fatiguing to listen to. Ergonomically is also JUST WHAT I NEED. No more, no less. Love that big Volume knob on the right hand side.

So don´t know what to do? Bias is rock solid and this thing DOES WORK GREAT!!! Just that sometimes it gets those little issues which I have been mentioning all along. Should I keep changing diodes on the Main amp Board? For example: De5-De10 with 1N4148TR Diodes De12-15: S1.5-02 with 1N4003? I have them on hand so all it will cost me is some "will-power".

Cheers!!!:beerchug:
 
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