KENWOOD KA-4004 Recap & Restore

[eizner23]

Doing well! sorry those changes didn’t pan out I should’ve emphasized power supply. If it plays great momentarily on both channels then it doesnt, it could be an issue with something common, such as the power supply.
look for faulty part in the power supply as I mentioned above. If it happens after unit warmup you can try hitting parts in PS with freeze spray.

 

[Zeus12]

Hi @eizner23 !

Glad you´re doing OK. Got it hooked up now as a "Pre" to a Pioneer SA-7500 and it´s sounding SWEEEEEEEEEET!!! That recap of the Tone Board really made a difference to the sound.
Great combination! Oh, the joys of mixing and matching equipment. So many variables!!! Pre, Amp, Speakers, Cables, DAC....the possibilities are endless!!

OK, so we definitely know that the problem has to be SOMEWHERE on the Main Amp Board. I changed ALL THE CAPS AND TRANSISTORS so that leaves us with Diodes and Resistors, correct? What I really don´t want is to take them ALL out and test them. If I could at least know the AREA of the problem. Any voltages measurements I can take with my DMM? Testing points, for example? Another question I have is that if a component is unstable wouldn´t that mess up the Bias? How come I have a rock solid Bias?

By the way, that noise/distortion comes mostly at the beginning. Then it can go for almost an hour or two working PERFECTLY and then again, for a few seconds, the noise/distortion. That´s what´s driving me CRAZY!!! Why is it teasing me?

Cheers!!!

 

[Semanon]

Did you clean the 'separate' switch? That will be in the circuit even when separated. Can't recall from my 6004, but other models require you to open it up to clean it. You could bypass the switch with jumpers to test.

 

[Zeus12]

Hi @Semanon

Thanks for the tip. Will clean the switch. Don´t know how I could bypass the switch with jumpers though. Would love to try first and eliminate variables. I have the Switch on "Separate" no and absolutely no issues.

Cheers!!

 

[Semanon]

Jumper as shown and put the switch in separate. This will connect the preamp to the amp and take the switch out of the circuit. Use alligator clips or mini-grabbers.
.

 

[Zeus12]

Hi @Semanon

Thanks again for helping me out!! Appreciate your time and labeling the part on the schematic. OK, I see it clearly.

If you could please enlighten me a bit more and help me understand that part of the schematic. I am dying to learn how to read schematics. I have dreams at night visualizing it...It is still looking like "Chinese" but I´m slowly beginning to understand it.

So, from what I understand, by doing that, we are essentially taking out the C15 Ceramic Cap (560pf)? Resistors R202 and R203 should be working OK because they "work" on the "Separate" position?

One last thing. What do you think about the Ka-4004´s Pre-Amp section? Would it be better than the Pioneers KA-7500´s preamp? Just by looking at the circuitry the KA-4004 has a simpler Tone board. Doesn´t that mean that by having "less" path to travel, the better the sound? All I know is that the sound is BETTER by combing these 2 than individually. So probably that´s why when you move up the Hi-Fi ladder, more and more people prefer separates.

"For with much wisdom comes much sorrow" --Let me be a wise man in Hi-Fi please!!!!!!!

Cheers!!

 

[Semanon]

Let's see where this goes. In separate mode:

- The green line is the Right signal & yellow is the Left path from the tone board to the amp board input. Note there is a complete pathway from the transistor output to the amplifier input with no other devices in the way (except a 1uF coupling cap). Also note that the 'MAIN IN' jacks go nowhere

Transistor > High-filter switch > Separate switch > Amplifier



When the switch is set to SEPARATE, here is the result.

Yellow and Green only go to 'PRE OUT' jacks. Trace them and notice they no longer reach the amplifier inputs.

The blue and pink go to the amp inputs.


I do not have a photo of the 4004 switch, but:

- Trace the wires from the tone board to the switch for each channel
- Trace the wires from the amp board to the switch for each channel

Use a jumper to connect the tone output to the amp input, thereby bypassing the switch.

 

[Zeus12]

Wow!!! Woke up this morning and saw these beautiful colors. Thanks again buddy!!

OK. Please be patient with a freshmen! I´m still learning to understand these crazy lines with circles, triangles, numbers, dots...

This is what I understand. Please correct me if I´m wrong.

The flow starts top left, goes right and then down.

So, for the KA-4004, 1 (Pre Amp) which means Phono amplification is engaged when in PHONO mode only, 2 then goes through the Muting and Loudness switches and 3 to the Tone Amp. The Signal should STOP there when in Separate Mode. Just using deductive reasoning here. The amp is working perfect when in Separate but NOT when in Main. The Main Amp board is engaged ONLY when in MAIN mode. Oh, forgot that Power Supply Board. Looks to me like it´s to power the upper boards in the diagram. Not exactly sure. The caps should all be OK on it and perhaps that Qk1 transistor is the culprit? Didn´t check that one. All other transistors have been changed (except the Pre Amp board).



So, try this first and then go from there.


The Pre Amp part, like I mentioned before, is WORKING great!! The BASS and TREBLE controls are very subtle and it sounds to me very sweet and neutral. The highs are perhaps a little rolled off and I usually give it one "click" up. So worse comes to worse, I can always use this little gem as a Pre Amp.

Cheers!!!

 

[Semanon]

Correct on the signal flow.

There are two power supplies, one for the tone & phono preamp and one for the main amp (typical for Kenwood and others). The main amp PS is on the Main amp board (blue box). If you are having a PS problem with the amp, this is where it will occur.

The yellow box is the PS for the Phono and tome amps. If the preamp is fine, then QK1 will not be the problem. QK1 is the voltage regulator transistor for the Phono and Tone amps.

 

[Semanon]

Remind me again, please: is the distortion in both channels?

 

[Semanon]

Reading schematics is a science learned by repetition (at least for me). I'm going to go over the power supplies for the amp. Forgive me if this is too basic.

From the plug to the transformer are all the AC line voltage stuff: power switch, outlets, fuse. The hot side of the AC is shown in orange. Green is neutral/ground reference

The transformer has 5 coils: (2) primary; (3) secondary.

• Primaries are (2) 117 vas wired in parallel.
• Secondary voltages are:
  • 18-0-18 (Blue) for a split rail PS to the amp board
  • 7.5 (Yellow) for the pilot lamp
  • 40 (Cyan) for the phono and tone amp

 

[Semanon]

Here is the phono and tone amp PS with main parts labelled. Remember this configuration. You will see it time and time again in schematics

 

[Semanon]

Pilot light circuit with fuse and lamp. Quite simple.

 

[Semanon]

And finally the Main Amp PS. You can google the labelled elements and get an explanation of the function


Hope not being too basic....

 

[Zeus12]

What can I say? Can´t thank you enough for the wonderful explanation and colorful diagrams. I would love to learn to speak "Hi-Fi Schematics"! I know it takes time and practice to learn to speak a new language.

Yes, the noise is happening on both channels. Interesting on the 2 power supplies used in Kenwoods. Can see it very clearly on the diagram. Have to really pay attention to those line and see where they go. So that takes out the PS for the Phono and tone amp out of the equation. That would leave us with either the 2 filter caps or it could be one of those Bridge Rectifier Diodes (De12-De15)? I tested the 2 filter caps and both tested OK (within specs). Or could it be one of those .022uf snubber caps?

The noise/distortion is a kind of lowering of the volume in one channel or then both, lasts for a short while (less then a couple of min.) and then works great again. So maybe it´s running out of power or something related to the PS? It is not a loud pop or harsh pop corn sound--which is more or a transistor going bad sound, if I´m not mistaken. I wish there would be some "audible guidelines" for parts failures. Kind of like a car mechanic. In 2 seconds, just by listening to it, he knows it´s a problem with the carburetor.

Can´t be the electrolytic caps, transistor or D1-D2 diodes since they are all new. Oh, forgot. Could be a bad resistor? I thought that they never go bad --and if they do, you can visually see discoloration. All the resistors look fine. Anyway, I´m rambling on and don´t have a clue, BUT, thanks to nice people like you on this forum I am learning new things each day.

Just like John Gotti said, "I´m gonna be a gangster or I´m gonna die trying", I say, "I´m gonna fix this little baby or I´m gonna die trying".

Have to keep at it and keep practicing. Thanks again buddy!! Will keep you updated on this little soap opera.

 

[Semanon]

Here is where I'd start: when it happens again, wiggle the separate switch. Don't change the switch position, just wiggle it. If that does not work, turn the volume down to a reasonable level, then switch the switch to separate and back again. If neither of these actions fixes it or makes a change, it's probably not the switch.

It could still be a bad trace or flaky component. However, nothing in the PS would do what you describe, except maybe a flaky diode.

If the switch test does not do the trick, open the unit up and stand by with a chopstick or some other non-conductive stick. When it happens again, gently poke around the Main Amp PS to see if you can reproduce or find the source of the issue.

Best of luck.

 

[Zeus12]

Hi @Semanon

OK. Here´s an update on my little soap opera.

1. Cleaned the "Separate" switch with Deoxit and a Q-tip. I did the "wiggle" on it and no changes. The solder joints on the 3 pins look OK.


2. Wanted to take some voltage measurements on the Main Amp board and unfortunately, either I´m doing something wrong or I need a better DMM. I´m connecting the negative lead to "Chassis" and the positive (red) lead on the individual pins. The only pins I was able to get a reading was pins 7 and 8 (3.6V) and 1.46V second time. ????







From another post online I found this reading for the Pins Voltages on the Main Amp board. So obviously, I´m doing something wrong.


So, from what I see, Pins 12 and 13 are VERY IMPORTANT because they would tell us if any of the Diodes (De12-De15) are defective, correct? Isn´t this the way we troubleshoot the "area" of the problem? We check and see the voltages at some key pinouts and we work our way back.

I also used the chopstick trick and "gently poke around the Main Amp PS". NO CHANGE.

Cheers!!!

 

[Semanon]

Let's break it down. Pardon me if I'm too basic.

• Pins 10 and 15 should be measured using VAC, and should be around 18 VAC.
• Pins 12 and 13 are important, and the voltages shown seem sensible (18 VAC * 1.414 (RMS to P-P conversion) = 24.5)
• Pins 1,2,7,8 should be 0. Have you measured your DC offset? Pins 2 and 8 feed the speakers and should be in the low 2 digit millivolt range

Does your relay click?

• Pin 18 feeds power to the protection circuit 45 volts seems OK
• Pin 17 powers the relay. Something around 24 volts is good. If the relay is not clicking, it should have under 12 volts and maybe 0.

Last item to check:

Have you checked the relay? Pitted contacts can make the sound really poor. You may be able to slide the cover off the relay and slip a piece of paper soaked in deoxit between the contacts to clean. No sandpaper or files.

 

[Zeus12]

@Semanon

Thanks again, as always, for enlightening me.

Yes, relay clicks in after 3-4 sec. and sounds like it´s working fine. Will look into "sliding the cover of" and work some Deoxit in there.

The problem with the voltage measurements is that my DMM (cheap amazon buy) shows 1 or -1 instead of 24V or 45V for example. It works great for Bias setting, DC offset, checking fuses, etc, but I can´t get it to work for checking the pins. Have it on 20V setting and it´s this little thing: https://www.amazon.com/Multimeter-M...tance-Continuity/dp/B07J6294H2?language=en_US

Was looking into getting a Fluke. The 117 or 177 for example? The Fluke 87V is way too expensive for the moment.

In the meantime, I will go ahead and change the pending Diodes on the Main amp board and see if I find that "FLAKY" one.

Thanks again for putting up with a total newbie!!!

 

[Semanon]

Thanks again for putting up with a total newbie!!!

One of the things I love about this place so the willingness to share knowledge. My pleasure
to pay it forward.

A Fluke is a lifetime investment. Mine has been reliable and accurate and durable. I have the 115 abd done feel the need to look at another meter. If you like doing this work, a good meter is a great investment.