Kenwood KR-4070 Power Switch Mod, am I on the right track?

[MisterFishey]

For you Kenwood fans, it is decently well known that Kenwood receiver power switches from this era are not built to handle mains voltage with frequent use. I have seen many a KR-9600 whose power switch has fused in the on (or off) position. My friend owns a KR-4070 that was his father's, that I am completely recapping. He told me to draw up a schematic of a possible mod. The theory behind this is that although the switch will fail is used too many times switching mains voltage, at a lower voltage (in this case, 12 volts) it will work fine, and will not need replacement in the foreseeable future.

The mod will consist of a transformer and a relay. The transformer will step down 120 VAC to 12 VAC. The signal will then go to the switch. When the switch is turned to the on position, the 12 VAC will go from the switch to the relay, activating it, which will allow mains power to go to the existing transformer and into the rest of the device.

As I review the schematic, I should really have the other side of the relay activation power source go back to the transformer, not to ground.

This is the schematic of the power supply and on/off circuit as provided in the service manual:

This is the schematic as I propose to modify it:

Does this look like it will work to you? Thanks!

 

[avionic]

Looks like it should work as anticipated..


Simpler , cheaper route...
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=125-152

 

[Jon_Logan]

is there some reason you don't want to use a 120 VAC coiled relay?

 

[MisterFishey]

Looks like it should work as anticipated..


Simpler , cheaper route...
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=125-152

Good to hear it will probably work as anticipated. And the switched power strip is what is trying to be avoided. IMO that would work fine, but I'm not doing this for myself. Besides, this is a good easy intro into DIY.

is there some reason you don't want to use a 120 VAC coiled relay?

The intent of this is to reduce the voltage going through the power switch, since is prone to failure over time. In many units, the power switch has permanently fused itself in the on or off position, and since that has not happened in the unit in question, I hope to avoid that.

 

[Jon_Logan]

The intent of this is to reduce the voltage going through the power switch, since is prone to failure over time. In many units, the power switch has permanently fused itself in the on or off position, and since that has not happened in the unit in question, I hope to avoid that.

I think a 120 volt relay coil would reduce chance of failure. The current through the switch contacts would be greatly lowered.

 

[avionic]

Its probably not the current going through the switch that wears it out but.. the the making and breaking of contacts with current established that arcs and fouls the switch contacts.

 

[MisterFishey]

I think a 120 volt relay coil would reduce chance of failure. The current through the switch contacts would be greatly lowered.

Its probably not the current going through the switch that wears it out but.. the the making and breaking of contacts with current established that arcs and fouls the switch contacts.

So you're saying a simple 120VAC relay (which obviously draws much less current to operate than a receiver) would reduce/eliminate wear on the switch? If so, that would make this super easy to implement, and it would be cost effective.

 

[Sam Cogley]

Its probably not the current going through the switch that wears it out but.. the the making and breaking of contacts with current established that arcs and fouls the switch contacts.

And then, when the resistance gets high enough..."bzzt...*poof*" - no more contacts.

 

[Jon_Logan]

IMO it's the current through the contacts that's going to kill them. The contacts are rated for 120 volts already. So, there's not a danger of arc over to the shaft, etc. Yes, the current would be reduced on the rotary switch. I believe it would solve the problem. But that's my opinion.

To make it super sweet, use a SSR (Solid State Relay). They have very low current (LED) triggers. They offer SSR's with 120 VAC trigger inputs.

 

[avionic]

when the resistance gets high enough

Due to carboned up,pitted contacts...yep eventually they get welded either opened or closed. Or the switch itself starts cooking,burning due to the carbon causing current leakage whether the switch is opened or closed.

 

[Jon_Logan]

I don't disagree that the switch is destroyed by the transformer being switched by an under designed switch.

I don't disagree that arcing causing pitting and carbon buildup on the switch.


I do disagree that the failure of the switch is caused by the switch switching 120 VAC, however.

I believe that if this switch's current load is reduced, it could made to last through as many mechanical cycles as it was designed for. (maybe 1 million?)

I believe that it is a function of the inrush currents and the inductive (transformer) load that causes the damage to power switch.

Bypassing the under designed switch is a good idea.

Personally, I think using a SSR is elegant, yet over kill (but I'd do it). A 120 volt (AC) mechanical relay of modest size (5A rated contacts) should have coil currents that aren't going to destroy the Kenwood's power switch contacts.

Whatever method is chosen, I'd recommend putting a "spark-Killer" across the high power switch contacts. ...it could be just a cap (0.01~0.1uF) or a cap in series with a resistor (~120 ohms).

 

[MisterFishey]

Bypassing the under designed switch is a good idea.

Personally, I think using a SSR is elegant, yet over kill (but I'd do it). A 120 volt (AC) mechanical relay of modest size (5A rated contacts) should have coil currents that aren't going to destroy the Kenwood's power switch contacts.

Whatever method is chosen, I'd recommend putting a "spark-Killer" across the high power switch contacts. ...it could be just a cap (0.01~0.1uF) or a cap in series with a resistor (~120 ohms).

I think an SSR is overkill, and very expensive, especially when compared to normal electromechanical relays. I don't think the small decrease in operating current merits spending 3 times as much. After reviewing all the specs, I think that this relay would work just fine for what I need. The speak killer is a good idea, I've seen it on other equipment as well. Would the cap be a normal ceramic disc one, or a different kind?

 

[Jon_Logan]

....think that relay is a good choice. Yes, Ceramic is a good type of cap to use. Ceramic caps rated 1kV are pretty easy to come by.

Yeah, SSR's are expensive .......but they are sweet for this application

 

[MisterFishey]

....think that relay is a good choice. Yes, Ceramic is a good type of cap to use. Ceramic caps rated 1kV are pretty easy to come by.

Yeah, SSR's are expensive .......but they are sweet for this application

Excellent! But SSR's don't have that satisfying "ping" sound when they activate...

I don't know when I will place the order for these parts, but I'll certainly report back when I get it in... thanks a lot! :yes: