Adding switch and line input to Zenith MK2068 Console

[bbs71364]

Howdy,

I've finshed restoring my grandparent's 1962 Zenith MK2068 Stereo console, it has a 9H20LZ1 preamplifier/tuner and a 5k29 amplifier. The Preamplifier connects to the Amplifier with what looks like an XLR connection, but it's a left and right signal with ground connection.
(Posted it about it earlier on here: https://audiokarma.org/forums/index...in-zenith-5k29-amplifier-from-console.962388/)

I'd like to put a DPDT switch on the rear of the console and switch this connection from the 9H20LZ1 and an unbalanced stereo line-in connection. It seems extremely straightforward, but is there anything I should consider to protect the amplifier and/or the preamplifier?. I'd be connecting an AirPort Express Gen2 to add AirPlay2 functionality. I could switch the DPDT switch to go between the AirPort Express and the Preamplifier.

I've seen this modification from D-Lab on Youtube here:

and since my volume knob is in the 5k29 amplifier, it makes just as much sense to splice this in between the cable that goes between the pre-amp and the input on the amplifier.

Should I be aware of any RIAA EQ curve that is needed? or additional resisters for the load for the AirPort Express? (it's not designed as a headphone jack, so I'm guessing I don't need the resistors)

Thanks!

 

[cademan]

You could tap into either the tuner or turntable outputs.

 

[gadget73]

Does this have a power transformer? I can't open the schematics from here

If no power transformer you really need to be careful with this. Some things from this era had no isolation from the power line and jacking in an external input can be dangerous. There are ways but its something that really needs consideration before you connect stuff.

 

[cademan]

It has a power transformer and uses the 6BQ5 output tubes.

 

[gadget73]

ok, no worries then

 

[cademan]

I have this same amp and tuner. The tuner has its own power transformer so it can be used with any integrated amplifier. The amplifier will automatically turn on when plugged straight into the wall so there is no need to have a wire jumped to power up by itself as the original tuner had the power on/off button on it.

Once you get the amp to power up, you can just add a switch at the hot wire side and then continue to replace capacitors or upgrade whatever needs to be upgraded.

I removed all the controls on mine so it is now effectly a power amplifier in which I can use any preamp of my choice.

 

[cademan]

You could tap into either the tuner or turntable outputs.

This is for when you are using the tuner however when using the phono mode, it will not turn on unless you turn on the original turntable. If you want to use the amp by itself, you will just need to get the amp to power up by itself.

If you want to use the amp with your airport express, you will need to either add a separate plug to the amp to be able to plug it straight into the wall or use it with a power strip.

That so called XLR jack (plug) on the amp is the left and right input with common ground.

 

[cademan]

Plug this wire straight into the wall for the amp to power up by itself. (see yellow arrow at bottom)

Red arrow is the right and left inputs with common ground. Speaker connections are on top.

 

[bbs71364]

Hi there! I know I’m reviving my old thread. Thank you to everyone who’s commented. My grandparent’s Zenith console is doing well! I ended up going with the switch between the Preamp output and 3.5mm connection for an AirPort Express as I spoke about and it works fantastic.

One thing that has bothered me for a few years since, but never got around to looking at it.. I seem to have a 60hz hum - but it appears to be directly tied to the volume pot, and it doesn’t actually move (get louder) as I increase the volume, and when I get to 10 (as high as it can go) the hum goes away. It’s also not there around 4-5. I’ve troubleshooted the entire signal chain thinking I was injecting noise in somewhere (happens when no source is connected) I’ve also connected a transformer isolator from my wall outlet thinking it might be my power, but I can’t seem to correct it externally. I’ve done everything except for wiring a ground to the chassis.

Here’s a video demonstrating the issue- it isn’t consistent through the volume range:

Any thoughts or ideas would be greatly appreciated!