A repair job with unintended consequences - UK Vintage Radio Repair and Restoration Discussion Forum

audiomm · 2nd May 2019, 11:44 pm

I have recently moved to a new town, pop. 800 and have had to build and set up my workbenches again (in the garage). My building activity got noticed and it didn't take long for one of the locals to swing past for a 'nosey' which in turn ended up with me being asked to check over a Kenwood KT-48 FM Tuner and the companion KAX-48 Amplifier/Cassette Deck which had apparently been stored for a quite few years due to one unit or the other not working.
I said yes, and a few days later the units arrived but I didn't get a chance to work on them until the following day. When I fired up the Amp (via my lamp limiter) sure enough – no go. A quick check inside and one replacement fuse sorted it so a few cursory voltage measurements later I switched to full mains and confirmed everything was in order. Got 42+ watts per channel before clipping which seemed reasonable.
So then my attention turned to the Tuner – this is connected to the Amp/Deck by a 4 way ribbon cable ( L,R,Gnd & 15v supply). I hooked up a suitable aerial and it worked! All functions were ok so I figured that I could quit while I was ahead – but I didn't did I. The system was still connected to my 'scope and I took a glance at the dual traces dancing in time to the music BUT hello what's this I see? Damn! The left & right channel displays were 180 deg out of phase!
As it happened, I was tuned into the local radio station run by a volunteer community group.They broadcast FM (stereo) and also simulcast on AM (mono) . There are other (more distant) FM radio stations that I can tune into as well as I have an outdoor FM aerial. So I quickly tuned in another station to see if their Left & Right channel transmissions were also out of phase. They weren't and neither were any of the others I checked. Well at least that ruled out a fault in the Tuner – or did it? Just to be sure, I dragged out a Quad FM4 and substituted that for the Kenwood Tuner. Same results. Local station channels out of phase – all other stations in phase. So that pointed the finger at the local station.
On the hour when the newsfeed is taken (in Mono) the signal disappears -which wasn't that surprising.
I called in to the Radio station yesterday and very diplomatically mentioned what I had found. Actually, they were most appreciative and are going to get the local radio engineer (who installed & set up the gear) to sort it out …....sometime in the next few days. When they told me who he was I discovered that he is my next door neighbour! That'll go down well (sarc).

So what started out as a simple repair job has created consequences one would usually never encounter. I have no idea how long that situation has been going on but I suspect since the installation of the FM transmitter some years ago. Anyone listening to FM on a clock/radio or a similar single speaker transistor radio would've had probably just figured their radio wouldn't pick up the signal and just switched to AM and been blissfully happy.

Mike

Diabolical Artificer · 3rd May 2019, 6:15 am

Wow, that's a story to tell the Grankid's..... maybe not if they're like my daughter who switches off at the mention of anything electron related. Still, Grandkid interest or not it's an amazing story full of synchronicity.

Andy.

wd40addict · 3rd May 2019, 8:11 am

I've experienced the same thing. A friend's daughter's band were once interviewed on a community FM station and I listened in. At my old house I had a single mono speaker in the kitchen which was fed with left + right, it kept falling silent. In the living room with stereo speakers the sound kept playing! My suspicion is that they had a reversed wired XLR cable in the studio somewhere.

2nd May 2019, 11:44 pm	#1
audiomm Pentode Join Date: Sep 2007 Location: Ranfurly, New Zealand Posts: 170	A repair job with unintended consequences I have recently moved to a new town, pop. 800 and have had to build and set up my workbenches again (in the garage). My building activity got noticed and it didn't take long for one of the locals to swing past for a 'nosey' which in turn ended up with me being asked to check over a Kenwood KT-48 FM Tuner and the companion KAX-48 Amplifier/Cassette Deck which had apparently been stored for a quite few years due to one unit or the other not working. I said yes, and a few days later the units arrived but I didn't get a chance to work on them until the following day. When I fired up the Amp (via my lamp limiter) sure enough – no go. A quick check inside and one replacement fuse sorted it so a few cursory voltage measurements later I switched to full mains and confirmed everything was in order. Got 42+ watts per channel before clipping which seemed reasonable. So then my attention turned to the Tuner – this is connected to the Amp/Deck by a 4 way ribbon cable ( L,R,Gnd & 15v supply). I hooked up a suitable aerial and it worked! All functions were ok so I figured that I could quit while I was ahead – but I didn't did I. The system was still connected to my 'scope and I took a glance at the dual traces dancing in time to the music BUT hello what's this I see? Damn! The left & right channel displays were 180 deg out of phase! As it happened, I was tuned into the local radio station run by a volunteer community group.They broadcast FM (stereo) and also simulcast on AM (mono) . There are other (more distant) FM radio stations that I can tune into as well as I have an outdoor FM aerial. So I quickly tuned in another station to see if their Left & Right channel transmissions were also out of phase. They weren't and neither were any of the others I checked. Well at least that ruled out a fault in the Tuner – or did it? Just to be sure, I dragged out a Quad FM4 and substituted that for the Kenwood Tuner. Same results. Local station channels out of phase – all other stations in phase. So that pointed the finger at the local station. On the hour when the newsfeed is taken (in Mono) the signal disappears -which wasn't that surprising. I called in to the Radio station yesterday and very diplomatically mentioned what I had found. Actually, they were most appreciative and are going to get the local radio engineer (who installed & set up the gear) to sort it out …....sometime in the next few days. When they told me who he was I discovered that he is my next door neighbour! That'll go down well (sarc). So what started out as a simple repair job has created consequences one would usually never encounter. I have no idea how long that situation has been going on but I suspect since the installation of the FM transmitter some years ago. Anyone listening to FM on a clock/radio or a similar single speaker transistor radio would've had probably just figured their radio wouldn't pick up the signal and just switched to AM and been blissfully happy. Mike

3rd May 2019, 6:15 am	#2
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,667	Re: A repair job with unintended consequences Wow, that's a story to tell the Grankid's..... maybe not if they're like my daughter who switches off at the mention of anything electron related. Still, Grandkid interest or not it's an amazing story full of synchronicity. Andy. __________________ Curiosity hasn't killed this cat...so far.

3rd May 2019, 8:11 am	#3
wd40addict Octode Join Date: May 2009 Location: Stevenage, Herts. UK. Posts: 1,518	Re: A repair job with unintended consequences I've experienced the same thing. A friend's daughter's band were once interviewed on a community FM station and I listened in. At my old house I had a single mono speaker in the kitchen which was fed with left + right, it kept falling silent. In the living room with stereo speakers the sound kept playing! My suspicion is that they had a reversed wired XLR cable in the studio somewhere.