Simple problems can be easy to miss. - UK Vintage Radio Repair and Restoration Discussion Forum

Ancient Geek · 5th May 2018, 9:40 pm

I've been looking for a new project to challenge the old troubleshooting skills for a while, and Mrs Geek wants a small radio for her hobby room, so I got a Panasonic GX30 RF-1630L AM/FM radio for very little dosh. It was described as working on LW and MW, but not on FM, and so it turned out.

Made in about 1985, it has a Panasonic AN7205 9-pin SIL FM front end that comprises RF amp, LO and double-balanced mixer., followed by a single transistor IF amp and a Panasonic AN7220 that does the AM stuff plus FM IF and detector.

Internal examination showed that it had been lightly got at, the 2SC829 looking like it had been replaced. There was evidence of scratchy probing having been done. Squirting in a bit of modulated 10.7 MHz showed that the IF, including the 2SC829 and everything downstream was working fine. The LO did not seem to be working at all, and I was all set to order a replacement AN7205, but my Yorkshire birthright got the better of me. I injected an unmodulated 110.7 MHz signal into the LO output pin of the chip while coupling in a modulated 100 MHz signal to the antenna. Lo and behold, the RF amp and mixer in the AN7205 were fine. So why should the LO not work?

Out of curiosity I did a sweep on the LO tank and did not see anything resembling resonance. On close inspection, the joint at the earthy end of the LO coil looked original, but a bit blobby. A touch of iron and solder wick later and the fault was revealed -- a well-hidden crack in the track. I'm quite pleased to track that one down. I wonder how long it had been like that.

There's still lots of work to be done on this set. It badly needs some retrobrighting before it will be allowed into Mrs Geek's domain,and it will be my first foray into retrobrighting. If it works, I'll write it up in 'Success stories'.

5th May 2018, 9:40 pm	#1
Ancient Geek Retired Dormant Member Join Date: Aug 2017 Location: Bristol, UK. Posts: 136	Simple problems can be easy to miss. I've been looking for a new project to challenge the old troubleshooting skills for a while, and Mrs Geek wants a small radio for her hobby room, so I got a Panasonic GX30 RF-1630L AM/FM radio for very little dosh. It was described as working on LW and MW, but not on FM, and so it turned out. Made in about 1985, it has a Panasonic AN7205 9-pin SIL FM front end that comprises RF amp, LO and double-balanced mixer., followed by a single transistor IF amp and a Panasonic AN7220 that does the AM stuff plus FM IF and detector. Internal examination showed that it had been lightly got at, the 2SC829 looking like it had been replaced. There was evidence of scratchy probing having been done. Squirting in a bit of modulated 10.7 MHz showed that the IF, including the 2SC829 and everything downstream was working fine. The LO did not seem to be working at all, and I was all set to order a replacement AN7205, but my Yorkshire birthright got the better of me. I injected an unmodulated 110.7 MHz signal into the LO output pin of the chip while coupling in a modulated 100 MHz signal to the antenna. Lo and behold, the RF amp and mixer in the AN7205 were fine. So why should the LO not work? Out of curiosity I did a sweep on the LO tank and did not see anything resembling resonance. On close inspection, the joint at the earthy end of the LO coil looked original, but a bit blobby. A touch of iron and solder wick later and the fault was revealed -- a well-hidden crack in the track. I'm quite pleased to track that one down. I wonder how long it had been like that. There's still lots of work to be done on this set. It badly needs some retrobrighting before it will be allowed into Mrs Geek's domain,and it will be my first foray into retrobrighting. If it works, I'll write it up in 'Success stories'.