TK-880 polarity protection diode

[GizmoMcGee]

Hi all -- relative newbie here, please forgive any ignorance.

I got my hands on a beat up TK-880-1. I have another of these radios and I love it for GMRS. This one, though, seems to have had a tough life.

What I assume is the reverse polarity protection diode is burnt to a crisp. But the radio powers on, transmits and receives. It actually sounds pretty crisp.

I would like to replace the diode, which is labeled on the PCB as D403. Problem is I can't find a D403 listed in the TK-880service manuals I find online. What kind of generic diode should I sub in for polarity protection?

The radio also blasts a lot of random static when there's an antenna attached. This doesn't interfere with Rx. There's some electrical tape around the coax leading to the SO239 connector, so I'm assuming it needs a new antenna line, but is there anything else I should suspect given the burnt diode?

 

[kb4mdz]

Hmmm, I opened a copy of TK-880H manual I have, Reverse Polarity Diode is labeled as D37, a DSA3A1.

If it's in rough shape externally, it may be in rough shape electronically too. Open squelch noise might indicate to me the squelch control setting is almost fully on.

It may have already been someone's project radio, and had 20 ga. parts & alignment shotguns pointed at it.

 

[mmckenna]

What I assume is the reverse polarity protection diode is burnt to a crisp. But the radio powers on, transmits and receives. It actually sounds pretty crisp.

I would like to replace the diode, which is labeled on the PCB as D403. Problem is I can't find a D403 listed in the TK-880service manuals I find online. What kind of generic diode should I sub in for polarity protection?

The purpose of the diode is to create a short circuit between + and - when the power is hooked up bassackwards. That will blow the fuse in the power feed. The diode is usually sacrificial and once it does it's job, it needs to be replaced.

But, as you've found, replacing the fuse -and- hooking it up correctly will restore power to the radio. It's just lost that protection.

Good idea to replace it. Just make sure it's something that will handle enough current to pop the fuse. Looks like kb4mdz found the part number for you. If you can't find that specific diode, look at the specs for it and find something close.

The radio also blasts a lot of random static when there's an antenna attached. This doesn't interfere with Rx. There's some electrical tape around the coax leading to the SO239 connector, so I'm assuming it needs a new antenna line, but is there anything else I should suspect given the burnt diode?

Sounds like squelch is loose. You can adjust that through the front panel if you have the buttons programmed for it.
And I agree with the above, if it looks like its been abused, it probably has. Electronic components age and periodic alignment is necessary to keep the radio working to factory specs. Those specs play into both receive performance as well as transmit performance.

Trick would be finding someone who will align it for you. Most shops won't support old dinosaurs like this. Finding someone with a service monitor and a service manual would do the trick.

 

[nokones]

If you can't find someone to align the radio, check with Tim at Radios Across America. He has a Tech that specializes in the repair and refurbishing of the Kenwood TK-780/880s and TK-7180/8180s.

As for the loose squelch, I had one of my TK-780Hs do the same thing on the channels that were set for CSQ on the receiver. It appeared to be something very local, possibly in my own residence, to be that was enough to get pass the squelch setting threshold.

 

[GizmoMcGee]

Thank you all, this is very helpful.

I've ordered a new diode (a few, just in case), and they should arrive next week.

While I was poking around on the circuit board I noticed that the ground on the antenna seemed like a bit of a cold joint. Reflowed the solder, but haven't spent enough time with the radio yet since to see if that had anything to do with the squelch issue. Likely not, but I hate a cold solder joint.

And the rough soldering on the antenna lead just underscores that this was probably someone's project rig. Any easy way with rudimentary tools to tell if it needs a realignment, or is this just one of those things I should assume an old abused box like this needs?