1969 Zenith 25ZT120 - AM Drop-Out, Sticking Power Button Issues

[videoroot]

I have a '69 solid-state Zenith 25ZT120 that's part of a console unit that I'm working on restoring. I've ran into a couple issues, and I'm hoping someone might be able to point me in the right direction, as I'm new-ish to vintage radios/electronics.

To start with the most simple issue: the power on button gets stuck in the "on" position. I believe this is called a "latching switch"--you push it to turn it on, and it stays pressed in, and then you press it again to turn it off. My unit has an issue turning off. I've tried cleaning it with electrical contact cleaner, but that doesn't seem to make a difference. If I pull the radio out, I can look at just the right angle to see it failing. There's this little 'V' that gets engaged via a lever when you press the switch on, and when you press it again, that lever is supposed to pull that 'V' back to the off position, but this doesn't always happen. I used to have better luck by removing the switch cover and pushing the rod underneath directly, but even that is giving me issues now.

I can't seem to find any info online about repairing this. It's very likely that my search terminology is wrong here, giving me bad results when googling. I would include pics, but I'm having issues getting a clear one with where it is on the unit. As an added bonus: It's possible that my switch is an aftermarket one, or at least someone messed with it in the past--it's covered in bunch of hot glue for whatever reason.

And while I'm here, might as well ask about another issue that I'm in the process of dealing with as well: I'm getting nothing, not even static, while tuned to AM channels 55 to 115. After 115, there's an audible pop noise, and then everything works as expected from there up.

I've already replaced all the electrolytic capacitors, and I'm in the process of slowly replacing the non-electrolytic molded-plastic/mylar capacitors that are throughout the unit. All the capacitors surrounding the AM IF, RF, Mixer, and OSC are of the disc variety though, so I don't currently have any replacements on hand. Any tips that would save me from simply brute-forcing a fix would be greatly appreciated!

Here you'll find a copy of the schematic. Apologies in advance for the quality; I plan to do a better, higher resolution scan in the future. Hopefully this'll suffice for the time being though.

Thanks in advance!

[damen]

Your AM problem is most likely shorting plates on the tuning capacitor. Sometimes with a good eye and a bright light you can find where they are touching together.

[videoroot]

Originally Posted by damen

Your AM problem is most likely shorting plates on the tuning capacitor. Sometimes with a good eye and a bright light you can find where they are touching together.

I'll give this another look later today or tomorrow and let you know how I make out. Thanks!

I somehow neglected to mention this in my original post, but this issue does not occur when the input selection is set for FM (or AFC), and I believe the same tuning capacitor is used for both FM and AM bands, hence my worry that it might be something else entirely.

[damen]

The tuning cap in your chassis is only for AM, the FM section uses coils.

[videoroot]

Originally Posted by damen

The tuning cap in your chassis is only for AM, the FM section uses coils.

Ah, that makes definitely sense then. Thanks again!

I'm still experiencing the same issue, but I think this is at least pointing me in the right direction. I pulled everything back out and cleaned it a couple more times. To my naked eye at least, it looks just about as clean as it's probably gonna get. At the very least, I can't visibly tell where it might be shorting now.

Now I'm wondering if the issue might be from some of the fins being bent? Some of the fins certainly appear to be touching the back plates as I turn it back and forth, although it sure is hard to tell just from sight alone.

Feel free to let me know what you think. I'll attach a few pics here. Tried my best to get them as clear as I could, but I'm sure you know how it is. Let me know if you have any issues viewing them.

Obviously, I'm gonna need to do more research before I jump straight to trying to bend them back into shape by hand.

[damen]

It's been my experience that most shorting plate issues happen on the very outside ones. Your gang has three sections. It may be necessary to disconnect any wires and use an ohm meter to determine which section the short is occurring. Disconnect and check each section one at a time. Also, if cleaning the power switch mechanism does not correct the sticking problem, I noticed the Sams diagram gives the same replacement part number for the stereo-mono switch as for the power. Maybe swap them till a replacement can be found ? When using the phonograph the power switch is not supposed to be pushed in anyway so that the changer will turn off the amp power at the same time.

[videoroot]

Originally Posted by damen

It's been my experience that most shorting plate issues happen on the very outside ones. Your gang has three sections. It may be necessary to disconnect any wires and use an ohm meter to determine which section the short is occurring. Disconnect and check each section one at a time. Also, if cleaning the power switch mechanism does not correct the sticking problem, I noticed the Sams diagram gives the same replacement part number for the stereo-mono switch as for the power. Maybe swap them till a replacement can be found ? When using the phonograph the power switch is not supposed to be pushed in anyway so that the changer will turn off the amp power at the same time.

I'll have to give that at try next time I'm working on it. Thanks!

Unfortunately, my power and stereo-mono switches are two different kinds of switches. It's entirely possible the one in my machine is an aftermarket replacement. That being said, I might explore swapping them in the future nonetheless.

Today, I bit the bullet and pulled all the old hot glue off of the power switch. As I expected, it's in pretty rough shape. Genuinely looks like someone took a hammer to it at some point (or chewed on it). After cleaning it and putting it back together, and it seems to be working much better now. I'm probably gonna end up using either hot glue or epoxy on it at some point in the future as it'll likely break apart again at some point in the future if I don't.