2019-10-07 Zenith Royal 500H Troubleshooting

I got this from eBay, it doesn’t make any sound at all. It has a small crack in one corner but the case is otherwise okay. The screw on the bottom is corroded but the battery holder is in halfway decent shape. It needs a bit of cleaning, the metal is a bit tarnished but it’s all there – pretty good for more than 57 years old. Made in ’62, it’s S/N 811910.

The first thing I found was the earphone jack switch contacts were not closed so the audio output couldn’t get to the speaker. So I scraped the contacts and bent them so they make contact as long as the earphone plug is not plugged in. But I think that after a plug is plugged in, the contacts may no longer make contact. But for now it’s okay.

The next problem was there was still no sound from the speaker. I stripped it down to get to the PC board. I tested the speaker with the DMM and it measured a dozen or so ohms so it seemed okay. I pulled the speaker out of the cabinet to give it a visual and it’s really weird. The voice coil is not in the center, it’s off to one end of the oval cone. I found that there were iron filings all over the cone and voice coil, so I removed them and some dust and put the speaker back.

I still wasn’t hearing anything from the speaker. The electrolytic capacitors had the black plastic cases and those are the ones that usually go bad, so I replaced some and the radio was now making a bit of noise in the speaker when turned on but still no audio. But when I held the PC board in a certain place I heard audio. So I probed around on the board for an intermittent connection and resoldered a few joints, and I think I got rid of the bad connection.

The volume control was noisy so I gave it a shot of Deoxit and it quieted down. It’s now acting halfway like it should, but it needs an alignment to make it more sensitive. The schematic I downloaded has alignment instructions so I’ll try that soon. I’ll have to replace the earphone jack to get it to be reliable.

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