Kenwood Trio KA6004 power board repair

Dandy said:
I did a light cleaning of the pots and switches when I started, but after reporting the DC offset, and reading some other posts in AK on bad channels, I did it again. Including the Normal/Separate switch at the rear. I left it to dry out overnight.
Click to expand...

I just want to be sure that you proceed corectly with the contact cleaner.

You should spray the contact cleaner inside the control (or switch), and move the control from one end to the other several times just after spraying, when the control is still wet of contact cleaner. Repeat a second time.

Some switches are not easy to clean. I often drill a small hole at the rear of the sealed switches to make sure the contact cleaner reaches the contact inside the switches. Again, you must move the switch at all positions several times when the switch is wet of contact cleaner.

Before replacing the relay, compare the sound at pins 20 (L out) and 16 (R out) on the power board.
 
This is really helpful guidance. I think we often just guess that the fluid has reached all the switch contacts. Thanks also for telling me which pin is R out. I'll report back.
 
Ecluser, you are an amazing troubleshooting guide! I replaced the Normal/Separate switch after the original fell apart in my hands. The contacts were in bad shape. (I've learned that most slide switches are better to replace.)

As I was now able to make sure that the Normal setting was working correctly, I connected pins 20 and 16 to the L and R speaker outs and -- hey presto, full audio! What a great feeling to hear the amp working. :D

I guess I need to get a new relay to make a tidy permanent fix? Can you advise on the correct specs? I can't see any markings on it.

I attach a couple of pics showing the temporary connections (one red and one black) which also shows the dirty condition of the top (pic 2).
 

Attachments

  • DSCN2685.jpg
    DSCN2685.jpg
    109 KB · Views: 82
  • DSCN2686.jpg
    DSCN2686.jpg
    79.9 KB · Views: 80
Must I understand that you pulled out the relay?

You must determine the voltage at the coil and measure the voltage drop in Re49, when the relay is closed. From this voltage drop and the value of Re49 (680 Ohms from a previous post) you can compute the current in the coil.

This is an important parameter. Remark that if you can't find the exact replacement you can adjust the value of Re49.

The relay must have 2 poles and should be physically compatible.

I can't say more than this because the schematic doesn't show any original part number for the relay. I have the chance to have a local store with more than 30 different relays on hand, and I usually find the one I need, or I adapt them to the circuit.
 
Hi, no I have not removed the relay. The procedure you mention sounds like I could mess up the amp if I make a mistake. As the offset voltage measures fine, maybe I should just bypass the relay as for the test?

If I find someone with a service manual for this amp, it might have a part number for the relay. I have a KA4004 that I could use for parts. Is it likely that that relay would be the same?

Thanks for your help. I appreciate it.:thmbsp:
 
I'd just like to add that having soldered proper connections to the speaker outlets bypassing the relay, the amp is working fine. I think I'm going to leave the repair at this point and just enjoy it, apart from finding out how to insert a new bulb for the ON switch! Thanks again to ecluser for invaluable assistance.
 
Don't do that. Your amp has no protection at the moment.

Can you post a picture of the speaker relay?

For the bulb. The usual procedure is to remove the knobs and the front plate. At this point you should see the bulbs. There is the power bulb and one bulb for each input.
 
I have one of these units as well, and I have checked the voltage across Re49 (680 ohm) with the relay engaged... around 21.5 volts, and approx 20.5 volts across the relay coil. These will be a little low, as Rk7 is marked 330 ohm, but is high (417 ohms as measured, and that probably explains why my relay click seems a little "weak"). Looks like a 24V relay with coil resistance around 680 ohms should do the job. As you are in the UK, RS components has a suitable replacement 450-0352 (assume that there is stock of these in the UK), same physical size as the original, and 650 ohm coil resistance

http://australia.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=4500352

Below is a photo of my unit, seems it may be an earlier manufacture, it has Ck7 33uf 50V fitted (smoothing capacitor for the relay voltage), and also has an additional 50uf 50V electro fitted on the relay side of Re7 that looks original. The relay (mounting bracket removed) is on the lower right.


KA6004PowerPCB003sn.jpg



Hope this helps

Cheers

John
 
You guys are too kind! Thanks John and ecluser. :thmbsp: I'll investigate the availablity of the relay as noted. With the temp measure, I noticed today that a faint hum remains after the power is switched off at the power switch, so I've been switching off at the plug too.

I tried removing the front plate, but got stuck. I'll get the bulbs done too.

As a matter of interest, given the good performance of this amp in its repaired but unmodified state, would either of you recommend replacing any caps in the preamp or poweramp board for even better performance?

And, the Speaker A switch has just jammed... I'll see if it can be repaired, but I'm wondering about replacement options for this push switch? This amp has worked hard in days gone by!
 
Last edited:
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.

I replaced the power indicator bulb.

I also sorted out the speaker switch (a plastic rim had slipped loose and was obstructing its return). It works fine, though I gave speaker A and B switches more switch lubricant.

But now, there is no signal from the L Channel and the left channel voltage at pin 20 is -16mV (minus). :sigh: There is still preamp output from both channels.

I checked Qe18 but this measures correctly B-E is 0.7. (B4.2V, C10-.2V, E3.5V).

The relay John identified is available in the UK, but I'm holding off till I know that this is the next stage of the repair. What could have caused the loss of output on L? (There is still a faint hum from the L speaker, and with the power off, the offset rises to nearly a volt.)
 
There is nothing bad with negative offset voltage, just another flavour...

Do you mean there is no audio signal at pin 20?
 
Yes. There is no audio signal from the L speaker connection and there is continuity between that pin and the speaker connector.

I guess I'm also puzzled that there was a positive offset voltage before, after the repairs.
 
Could it be that you shorted accidentally the left output pin on the relay with the chassis, and the left output transistors were damaged?

Can you measure again the bias voltage across each emitter resistor (0.47 Ohm) on the power board?
 
Wow, I hope I've not damaged the L output transistors. The amp was unplugged when I took the cover off the relay to look for markings. But, I keep learning that you have to treat old gear with care. I also noticed when replacing the Normal/Separate switch that the wires snapped very easily. So, I think I might have disturbed a connection somewhere.

I'll measure the bias voltage and report back.
 
Bias voltages are: R1 26mV, R3 27.5mV, R2 34.7mV, R4 33.4 mV. Probably should be nearer 15 each, but not too bad?

Please, what can I check next?
 
Last edited:
Since you have an acceptable bias current in the output stage, the output transitors and the driver transistors seem good. Furthermore, you have a low offset voltage, which seems to confirm that the differential input stage is also good.

Can you connect the input signal directly at the base of Qe1? I suspect a bad solder at Re3 or Ce3. Make sure Ce1 is not shorted.
 
I've checked Ce1 (not shorted), replaced Ce3 and checked the soldering of Re3 (it measures correctly).

When I connect the input directly to Qe1, the same right signal comes on, just full volume.

So I thought maybe I'm looking at the wrong channel side of the board? After checking that Ce2 is not shorted, I replaced Ce4 (for balance) and checked Re4 for soldering and correct resistance.

Now, when I insert a signal directly to base of Qe2 the other channel comes on strong too!

So, it sounds like something got screwed up on the front, when I removed the front panel to replace the power light bulb. Could it be the vol or balance pots? I've given a lot of switch cleaner to the A and B switches, but that didn't make a difference. I'll do the same for the vol/bal pots, but would be glad of other guidance.

Would this be a good time to order the new relay?

David
 
I'm a little lost at the moment for the relay. You have strong signal from both channels, with the relay in action or with the relay bypassed? At least, both power amplifiers are good.

Do you have signal on both channels at the pre-out ? The Normal/Separate switch?

There are so many possibilities to lose a channel in this circuit that I strongly recommand that you built an AC probe (just a small cap in series with a shielded wire)to trace the signal with a tape deck. This will be good time investment in your case because you will be able to trace the signal anywhere.
 
Sorry. I should have explained that I removed the relay bypass again, just as another A/B check.

I still have a good signal on both channels from the pre-out and the Normal/Separate switch should be okay. But, I remember that the wires to this switch were very brittle. I'll check to see if they are still making correct contact!
 
Well, if you have a good signal on both channels from the pre-out, and good signal at the output when you feed the bases of the input transistors on the power amplifier, the problem is between the pre-out and the base of the input transistor, for the bad channel.

Even if the Normal/Separate switch is new I recommand that you treat this switch with contact cleaner too.
 
You must log in or register to reply here.
Back
Top Bottom