KENWOOD KA-4004 Recap & Restore

[Semanon]

Regarding the meter, inexpensive meters should be fine for general use. They will lack accuracy, over-voltage protection, longevity, and durability. But when they work they should be fine. Try it on a few batteries and other known devices (like wall power supplies) to see if you get expected readings.

 

[Zeus12]

Greetings fellow audiophiles,

Thanks @Semanon for enlightening me along the way!!! I will get a Fluke (used one). Saw a 179 for a fairly good price. The 87V, which is what many say would be the best for electronics use, is way too expensive for my budget.

Changed the Diodes De12-15 (S-1.5-02) with a 1N4003. I checked them and they all measured more or less around this value:


Also checked the two oil filled caps (Ce22,23) and they checkout within specs so I left them in there. I lifted one leg for the measurement. Original value 0.022uF:


I rechecked the Bias, connected it and for a moment I thought it was fixed. I was getting ready to start jumping up and down, but, again, the same fricking symptoms............... I still have the De5-De10 pending to change. Could it be one of those little "thyristors" close to the Bias pots? Or one of the "ceramic" (Ce1,2) or "mylar" (Ce15,16) caps? The symptoms normally take place at start-up and I have been listening to it since yesterday and it´s working GREAT! Maybe those De3,4 Diodes (STV-3). They are screwed to the heatsink so maybe it has something to do with temperature?

So far, this is what I have changed:


Oh, forgot to check the relay that you mentioned before. Gonna keep trying!!

Cheers buddy!!!

 

[Semanon]

Avoid touching the STV-3 unless absolutely necessary. The leads are very fragile and they are expensive or difficult to replace (if you can find them).

If your bias is good, the STV-3 are probably fine.

If the issue is the same in both channels, it is most likely the power supply.

 

[Zeus12]

Hi @Semanon

OK, so the STV-3´s will stay in. I have had it ON now for a more than a couple of days and I haven´t heard ANYTHING wrong with it yet, apart from the issues I have been describing happening mostly at start-up.

Was reading another thread regarding the Harman Kardon 730 - which is my next little project -- and I came across this:

A couple times it has cut out, and been quiet and distorted. I had to crank the volume on a crappy speaker to get it back
I had to crank the volume on a crappy speaker to get it back
Typical symptom of dirty relay contacts.
https://www.audiokarma.org/forums/index.php?threads/left-channel-issues-with-hk-730.372163/

Could it be the relay then? I thought the relay is just responsible for the DC NOT going to your speakers. So, if it´s working OK and clicking then that shouldn´t be the problem? The relay on this is S51-2017-06, according to the manual. You did mention the relay a couple of posts ago.

Learning more and more everyday!!! As always, big thanks for your help buddy!!!

 

[kwayde]

I just finished working on my friend's KA-8004 which had a problem with noise, drop-outs. When I replaced the relay the problem was gone. I was able to see that the relay contacts were slightly out of alignment causing bad connections.

 

[Zeus12]

Hi @kwayde

Big thanks for sharing your experience and helping me out!! Will check out that flaky relay then. Love this little amp and plan on keeping it. For only 18 watts it sounds big, authoritative and musical. Would love to listen to the 8004. Must be amazing!

“Ready to Lead, Ready to Follow, Never Quit.” -U.S. Navy Seals Official Motto.

 

[kwayde]

Hi @kwayde

Big thanks for sharing your experience and helping me out!! Will check out that flaky relay then. Love this little amp and plan on keeping it. For only 18 watts it sounds big, authoritative and musical. Would love to listen to the 8004. Must be amazing!

“Ready to Lead, Ready to Follow, Never Quit.” -U.S. Navy Seals Official Motto.

If you're able to, tap lightly on the relay with a stick or something non-conductive when the problem arises to see if the tapping will make a difference.

 

[Semanon]

Could it be the relay then? I thought the relay is just responsible for the DC NOT going to your speakers. So, if it´s working OK and clicking then that shouldn´t be the problem? The relay on this is S51-2017-06, according to the manual. You did mention the relay a couple of posts ago.

It could absolutely be the relay. Dirty contacts, pitted contacts, misaligned contacts, weak coil all can create dropouts and distorted sound.

These are very early protection circuits, the X004 series were the first for Kenwood. My 6004 also had a bad relay. Should be replaced. Get a real one from Mouser or Digikey. Lots of fakes at auction.

If there is no DC offset, you can TEMPORARILY bypass the relay. Just be aware that you put your speakers at risk. Just bypass one channel. That way you will hear the distortion in only one channel.


Regarding the protection circuit: There is a circuit on the main amp board that senses if the AC audio signal contains more DC than the predetermined threshold. If it does not, the circuit activates the relay coil. It there is DC, the coil is not engaged. The contacts carry the speaker-level signal to the speaker and headphone outputs.

 

[Semanon]

This is the protection circuit. The terminal circled in red provides a path to ground for the 24 volts the relay coil (via Qe20).


The relay is circled in blue. The part to the top is the coil and diode. The bits to the bottom are the contacts. Trace the signal lines from the speaker terminals or headphone jack to the relay and amp board.

 

[Zeus12]

Thanks @kwayde and @Semanon --taking your time to prepare those diagrams!! I am dying to learn to "read" those schematics. I stare at those lines and symbols everyday!!! So, from what I understand, everything goes "through" the relay (signal) and it can effect the sound.

I have never changed a relay before so this will be a first for me.
I would like to order it from mouser but I don´t know which one would be a good replacement. Found these 2 options online:
https://www.ebay.es/itm/11512971619...GyvnBvOlSXEc3JjJ5g3w2M66sQV3hDrBoCQz4QAvD_BwE

https://www.ebay.es/itm/370537058089

I´m so excited!!!!!!!! I´ll be dreaming about relays tonight. Every night I dream about "circuits", transistors, capacitors, diodes, resistors...

Cheers!!

 

[kwayde]

I can't remember which relay I used to replace the faulty one in my friend's amp. See if yours is marked to find the correct one needed.

Are you in the Europe area?

 

[Zeus12]

Hi kwayde,

Yes, I´m in Spain. From another AK post saw this about the KA-4004 relay:
7A HP125 250VAC, 3A 30VDC.

https://audiokarma.org/forums/index.php?threads/kenwood-trio-ka6004-power-board-repair.330127/page-2

Cheers!

 

[kwayde]

Hi kwayde,

Yes, I´m in Spain. From another AK post saw this about the KA-4004 relay:
7A HP125 250VAC, 3A 30VDC.

https://audiokarma.org/forums/index.php?threads/kenwood-trio-ka6004-power-board-repair.330127/page-2

Cheers!

That link is to a KA-6004. I've been thinking about the relay I put in my friend's unit and I seem to think I put a 24v relay in it. I've been looking at the schematics of the
KA-4004 and the KA-8004 as to the voltages the relays need to operate right. The KA-4004 shows the coil connects to a 40v tap.



On the KA-8004, the relay connects to a 31v tap, but I'm fairly sure I pulled a 24v relay and replaced it with a 24v relay. I'm going to my friend's house tomorrow and can check the part I pulled to be sure. My memory isn't the best.

 

[Zeus12]

Hi again,

Yes that thread is from a KA-6004 but I think the user Dandy is mentioning his KA-4004.

Help needed again please. I removed and inspected the relay without disconnecting it to see if it had any markings. There are none, unlike my out- of-service KA4004 which has 7A HP125 250VAC, 3A 30VDC.

Another thing I don´t understand though is why is it working GREAT now and only acts up when just turned on? Could it be just dirty contacts? Maybe a little bit of DEOXIT will fix it?

Anyways, no rush! It´s working GREAT NOW!!! As long as it stays on there is nothing flaky in the sound.

Cheers!!!

 

[Semanon]

By the time it goes through the 470 ohm resistor, drops .6 volts for the transistor, and hits the last resistor, it will likely be 24 volts (being lazy...i dont want to do the math).

Measure the voltage across the coil when activated.

My 6004 was 24 volts. It used solder lugs, not PCB pins. I used an Omron LY2 24DC. Check your voltage and connection type before ordering.

https://www.ia.omron.com/product/item/6302/

 

[Semanon]

Another thing I don´t understand though is why is it working GREAT now and only acts up when just turned on? Could it be just dirty contacts? Maybe a little bit of DEOXIT will fix it?

If it is still open, post a photo of the rekay

It could be acting poorly for any number of reasons:

• The coil could be weak when it is cold
• The driver transistors for the relay could be failing when cold
• The contacts could make better contact when hot

Cleaning will make some difference. Soak a strip of paper in deoxit, then drag it between the contacts.

Look at the contacts with a magnifier to look for pitting, scratches, loss of the silver coating, black residue. The relay is 50 years old. Since it is an electro-mechanical part, it takes more wear than most electronic components. If you can replace it you should. Even if it has not failed yet, it will do so sooner than later. It may not solve the problem, but it will remove a suspect, and provide a longer life between repairs.

 

[Zeus12]

Hi there @Semanon

Thanks for "adopting" me!! You have been enlightening me all along. What an amazing invention the Internet is. Shame that so many people --especially the younger generation -- use it for stupid things.

You mention that some possible reasons for this could be that the driver transistors for the realy could be failing when they are cold. I have changed ALL transistors on the Main amp board and Pre Amp board. I left the 4 original transistors in the Phono board --that shouldn´t effect anything since I´m not using a turntable, correct?

As I mentioned before, it has been playing GREAT, and I didn´t take any photos of the relay since it didn´t even cross my mind that it could be defective. I have this pic: There is also a little diode (I think) connected to it..
It looks like it´s encapsulated.


I will open it up and take some more close-ups and do the Soak a strip of paper in deoxit, then drag it between the contacts. I still don´t know exactly which model to order from Mouser but I will have to put that on hold since I JUST ORDERED a batch of caps and transistors for my next project -- Harman Kardon 730. Oh, what a beauty that one is.

Take care buddy!!

 

[kwayde]

Hi there @Semanon

Thanks for "adopting" me!! You have been enlightening me all along. What an amazing invention the Internet is. Shame that so many people --especially the younger generation -- use it for stupid things.

You mention that some possible reasons for this could be that the driver transistors for the realy could be failing when they are cold. I have changed ALL transistors on the Main amp board and Pre Amp board. I left the 4 original transistors in the Phono board --that shouldn´t effect anything since I´m not using a turntable, correct?

As I mentioned before, it has been playing GREAT, and I didn´t take any photos of the relay since it didn´t even cross my mind that it could be defective. I have this pic: There is also a little diode (I think) connected to it..
It looks like it´s encapsulated.
View attachment 2544275

I will open it up and take some more close-ups and do the Soak a strip of paper in deoxit, then drag it between the contacts. I still don´t know exactly which model to order from Mouser but I will have to put that on hold since I JUST ORDERED a batch of caps and transistors for my next project -- Harman Kardon 730. Oh, what a beauty that one is.

Take care buddy!!

If you're going to attempt to clean the contacts of the relay, be careful. The relay I changed out of my buddy's KA-8004 had contacts with small holes on one side, the other side had the raised contacts that raise up into the small holes. I found the contacts are easily shifted sideways, so be sure they are aligned correctly when finished.

 

[Semanon]

@kwayde, looks like exactly the same relay as the KA-6004. Here was mine (disassembled). Looks like someone went at it with a pickaxe.


Here is the link to the overall Omron LY page.

https://www.ia.omron.com/products/family/949/lineup.html

Lots of others copy them. Omron LY2 DC24 is what I used. You can also Google "24 VDC, DPDT, plug-in ice cube relay." In my opinion, it's not worth saving $3 to get a poor quality relay.

If you really want to be sure:

- Measure the case
- Measure the voltage drop across the terminal.
- Match from the vendor's stock

The diode is there to control the outflow of current when the relay coil is disconnected from damaging other components. Any reasonable diode will do, or you can reuse the one that's there. It sees very little use.

https://electronicsarea.com/why-is-there-a-diode-connected-in-parallel-to-a-relay-coil/

 

[kwayde]

@kwayde, looks like exactly the same relay as the KA-6004. Here was mine. Looks like someone went at it with a pickaxe.
View attachment 2544622

I thought they were sloppy, contact arms were easily moved laterally out of alignment.