Troubleshooting Panasonic RC-7589 radio

[pretro]

Hello everyone,

I’ve managed to clean up a Panasonic RC-7589 that was really dirty inside — clock is up and running and working like a charm (thanks to the motor oiling and repair videos ) and the light bulbs have all been replaced. So things are improving… Unfortunately, the radio is a different story. AM reception seems poor compared to other similar era clocks and I can tune in a few stations. FM reception is pretty much nonexistent. I get static but can’t find anything up and down the dial but static. I’ve carefully looked for loose connections, leaking or fishy looking caps, and used contact cleaner and all switches and pots. I have zero experience troubleshooting the radio circuit beyond these simple steps.

It appears that the unit uses the AC power cable as an antenna but I haven’t chased down the exact wiring yet — a broken connection to the power cable would be my next check as I could potentially see the symptoms matching that of a missing antenna.

Does anyone have any suggestions on troubleshooting the radio?

Thanks for any suggestions you might have.

[pretro]

Thanks . I started looking more closely at the electrolytic caps and pulled and tested the majority — most were actually out of spec and I’ve replaced them. There are still two more that I don’t have suitable replacements for and they are now on order. Unfortunately, no luck improving AM and solving the FM signal issue (still nothing at all across the bands).

I think I spotted the antenna connection as I was replacing the caps — goes from AC input, through a high voltage ceramic cap (seems likely that you would do this for an antenna using the power cord!), and then over to what I would guess is part of the FM section (e.g., coils and such in the neighborhood). No obvious issues on that line unless the ceramic cap has failed (doing its job due to a surge, bad wiring, etc.) and now cuts the antenna short.

Clearly some more troubleshooting ahead!

[pretro]

Thank you ! This will be very, very helpful!

I just tried disconnecting the AC antenna line and replacing it with a stand-alone wire. No joy...

Grounds look OK. I cleaned up some solder joints but still need to give it a careful inspection. Thanks for the suggestions.

I just noticed a disconnected/cut wire lurking over near the ground that was in a glob of glue hidden from view. In the service manual I think you can see it in the photos (e.g., page 8). It looks like the black wire that is between the ground and band selection switch that is cut off as it leaves the bottom of the photo. So far I can't spot any obvious places where it might have been attached. Picture attached for reference. Feels like a bit like a red herring but it is not long enough to match the photo from the service manual.

Update: I managed to find the other end of the wire buried in some of the insulating goop on the circuit board. It was an adventure getting in there to replace it. It turns out it routes through the board and then connects to the chassis via the tone potentiometer. Given a missing ground connection, I was hopeful... but once again, no luck...

[pretro]

Thanks once again . I'm clearly learning something new every day!

I would assume the transistors are the most likely failure points in the amp sections? It looks like some many (all?) are no longer manufactured (e.g., the 2SC920). Beyond another radio, is there a reasonable replacement/substitution strategy for these?

[pretro]

Got it -- I've worked more on repairing and restoring 70's and 80's computers vs. flip clock radios so this is a bit new to me. Your help is much appreciated.

Yes. I followed your advice and applied some pressure across the circuit board but didn't pick up any noticeable differences in behavior as far as signal reception goes -- a few "pops" over the speaker but nothing else significant. I then carefully looked over the board and found a few suspicious solder points and reworked them but no luck in helping to resolve the issue. I'll do another pass with a magnifier and then start working my way through the transistors.