Kenwood KA-4020 Integrated amp. Would like to bypass amplification stage

[pjug]

@restorer-john great stuff

 

[Ast]

OP has stated he simply wants to use the unit as a preamplifier, my directions are to do just that in a non-invasive, reversible way.



Troubleshooting and repairing the power amplifier (if it actually has a fault) is a whole other story.

No need to ground the lifted R71/72 leads. They are there for him to inject a signal later should he wish to.

The fact that one channel is fine, suggests the power amp final stages are not actually damaged. The Class A front end (uPC1298), may be damaged, or the fault could be upstream. The uPC1237HA protector IC drives a single speaker relay. If there is a DC offset (from a fried output IC) the relay won't connect any speakers at all. So there is no harm leaving the power amp stage powered up, unused and input floating (it's tied by 100K to 0V anyway).

 

[Ast]

Hi John, I really appreciate your support on this. Thanks for the suggestions but concerning the other suggestion I don't have the tools nor the dexterity to accomplish that task, dang those parts are way to small for me! I'll take my chances with the ribbon wire, I don't want to cut the wrong wires so could you help me out one last time by telling me on what side are the desired wires, so A or B? Thanks again.

 

[pjug]

What does the end of the ribbon cable look like when you disconnect it?

 

[Zek]

I don't want to cut the wrong wires

Don't cut any wire, just make some pins with solid hard wire to connect.

 

[pjug]

Hi John, I really appreciate your support on this. Thanks for the suggestions but concerning the other suggestion I don't have the tools nor the dexterity to accomplish that task, dang those parts are way to small for me! I'll take my chances with the ribbon wire, I don't want to cut the wrong wires so could you help me out one last time by telling me on what side are the desired wires, so A or B? Thanks again.

To me it seems like it would be much easier to connect like John described. Are you in the USA or Canada? I can send you some shielded wire with solid 26AWG conductors (~0.016" dia). You should be able to slip the conductors underneath the jumpers and resistor leads and kind of wrap it around to make the soldering pretty easy. There are 2 conductors plus shield wire though, so you won't use one of the insulated wires.

 

[Ast]

To me it seems like it would be much easier to connect like John described. Are you in the USA or Canada? I can send you some shielded wire with solid 26AWG conductors (~0.016" dia). You should be able to slip the conductors underneath the jumpers and resistor leads and kind of wrap it around to make the soldering pretty easy. There are 2 conductors plus shield wire though, so you won't use one of the insulated wires.


View attachment 26442

 

[Ast]

Hi, I'm in Québec Canada, I've got the wires and the soldering kit. But these pasts to cut and solder are sooo very small to me and I don't have the cutters. But I like Johns idea of using a pin, I'll try this and will let you know. Thanks again for your help.

 

[Ast]

What does the end of the ribbon cable look like when you disconnect it?

I pull but cannot get it to budge, I'll figure out a way but need to know on what side the wires are situated, is it A or B ?

 

[pjug]

Maybe the cable is soldered on the power amp end. As for the pin numbering, look at John's post #18. Pin 1 is nearer to the center of the board; Pin 12 is closer to the corner.

 

[Ast]

I think I got it, A is the answer. Thanks again and I'll let you know how it works out.

 

[Ast]

OP has stated he simply wants to use the unit as a preamplifier, my directions are to do just that in a non-invasive, reversible way.



Troubleshooting and repairing the power amplifier (if it actually has a fault) is a whole other story.

No need to ground the lifted R71/72 leads. They are there for him to inject a signal later should he wish to.

The fact that one channel is fine, suggests the power amp final stages are not actually damaged. The Class A front end (uPC1298), may be damaged, or the fault could be upstream. The uPC1237HA protector IC drives a single speaker relay. If there is a DC offset (from a fried output IC) the relay won't connect any speakers at all. So there is no harm leaving the power amp stage powered up, unused and input floating (it's tied by 100K to 0V anyway).

Hi John, I've completed the task and tested it on the AMP. 2 things: number one In order to get volume I must turn the know to at least 3/4 of it's travel and number 2 there's quite a lot of humming and buzzing, is it possible that my cable is picking this up from the transformer? If so how would I go about shielding it?

 

[restorer-john]

Zip tieing the shielded output cable to the power transformer secondaries is a no-no. You also must keep the three wires you've pulled apart as close together as possible, not leave them floating up in the air and route the cable carefully. Have you put RCAs on the rear panel or just run the cable out? If you have put chassis mount RCAs, use isolating washers and the PCB earth, not a chassis earth, or you may get hum from eddy currents in the chassis.

The front end is passive in direct mode, so the level will be relatively low. All you have between the line source and the output in direct mode is a 330 ohm resistor and the 100K pot. There is nothing wrong with using the volume pot up that far, as it is really just an attentuator in this case. Obviously, if you are using the RIAA stage or the tone controls, you switch in some active circuitry.

 

[Ast]

Zip tieing the shielded output cable to the power transformer secondaries is a no-no. You also must keep the three wires you've pulled apart as close together as possible, not leave them floating up in the air and route the cable carefully. Have you put RCAs on the rear panel or just run the cable out? If you have put chassis mount RCAs, use isolating washers and the PCB earth, not a chassis earth, or you may get hum from eddy currents in the chassis.

The front end is passive in direct mode, so the level will be relatively low. All you have between the line source and the output in direct mode is a 330 ohm resistor and the 100K pot. There is nothing wrong with using the volume pot up that far, as it is really just an attentuator in this case. Obviously, if you are using the RIAA stage or the tone controls, you switch in some active circuitry.

I cut an RCA cable and kept the male ends to plug directly into the AMP then passed the cable through a hole I made in the chassis and added a grommet. The wires are spread out only to show you and I'll do as you suggest when I reroute the cable. I'll reroute the cable but still would like to shield it (how can I do that) just in case.

I understand the volume level but I would have liked to be able to have better control on it, now it's low and full blast and hardly anything in between lol !

Again, your support is greatly appreciated, thanks again.

 

[Ast]

Zip tieing the shielded output cable to the power transformer secondaries is a no-no. You also must keep the three wires you've pulled apart as close together as possible, not leave them floating up in the air and route the cable carefully. Have you put RCAs on the rear panel or just run the cable out? If you have put chassis mount RCAs, use isolating washers and the PCB earth, not a chassis earth, or you may get hum from eddy currents in the chassis.

The front end is passive in direct mode, so the level will be relatively low. All you have between the line source and the output in direct mode is a 330 ohm resistor and the 100K pot. There is nothing wrong with using the volume pot up that far, as it is really just an attentuator in this case. Obviously, if you are using the RIAA stage or the tone controls, you switch in some active circuitry.

Hi John, I've passed the cable on the other side away from the power supply as far as I could. I wrapped the cable in aluminum duct tape and put electrical on top of that to make sure that it did not touch any part of the chassis or anything else. Did not change a thing unfortunately. It seems to be internal.

 

[Ast]

What does the end of the ribbon cable look like when you disconnect it?

Hi, I'm not getting answers from John, do you have any suggestions? I've passed the cable on the other side away from the power supply as far as I could. I wrapped the cable in aluminum duct tape and put electrical on top of that to make sure that it did not touch any part of the chassis or anything else. Did not change a thing unfortunately. It seems to be internal.

 

[pjug]

Maybe you did not ground the shield? I don't know if you want to use shielded cable anyway since the ribbon is not shielded. You might just want small wires well routed to the RCAs on the panel. Anyway I'm afraid I'm not going to be able to help much, although I'd still be happy to send the shielded wire (I am in NH so it should get to you quickly).

Be patient with John - he'll probably get back to you.

 

[Ast]

Maybe you did not ground the shield? I don't know if you want to use shielded cable anyway since the ribbon is not shielded. You might just want small wires well routed to the RCAs on the panel. Anyway I'm afraid I'm not going to be able to help much, although I'd still be happy to send the shielded wire (I am in NH so it should get to you quickly).

Be patient with John - he'll probably get back to you.

Thanks my friend but I think that it will not be necessary, your support is very much appreciated. Although I don't understand why it's not working and if this does not work out, then I'll just go ahead and buy a pre amp. This was just a project.

 

[restorer-john]

Hi, I'm not getting answers from John, do you have any suggestions?

The hum you are getting is most likely due to the fact you removed the ribbon cable (L/R/Gnd) instead of following the suggestions I gave you on soldering directly to the components and earth point (J153) on the power amp PCB I identified for you in post #18.

Use J153 (jumper) as your ground point (easy to solder to a top board link) for any shielded cable you are running for a 'pre-amp out'.

What you have effectively done is this (you have cut the 'earth' reference for pin 2):

Cheers.

 

[Ast]

Ok John, no worries I'll redo. The image where you marked in red, could you be more specific and show me on what site of the resistance do I cut and solder my wires. That would help, thanks again.