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NAD 7155 FM Tuner cuts out to static after 5 minutes, where to begin...

marcthisdate

marcthisdate

Where’s the confounded bridge?
The saga of the 7155 continues... Quick recap, I bought this knowing there were going to be issues but figured it would be a good project for the summer. Cosmetics were good and the price was a steal. It was sold as having no sound and of course it didn't. Also noted it was missing its jumpers so borrowed a pair from one of my other receivers and.... Great sound on the right channel but nothing on the left. Deoxit the daylights out of all of the switches and knobs but in the end it was a cold solder joint on the volume pot. Reflowed that and I was back in business.

Next up was testing the sources so hooked up a dipole and tried to get in some FM first. FM seemed to come in ok but not in stereo and never more than 2 bars on the meter. I have a 7140, using the same antenna in the same position and it gets 4 to 5 bars and stereo in the same spot so figured it was something with the tuner. This one I solved by adjusting the Stereo Switch Threshold and voila, I now have more bars and Stereo.

So now onto my current issue. Once I tune to a station and let it play for awhile (a few minutes, around 5 to 10), FM will go to full static. No bars on the meter and the Stereo indicator is dark. AM plays fine when FM is out, as does everything else. If I turn it off for awhile and let things cool down, it will come back none the worse for wear but will always go back to static at some point.

I'm thinking heat is probably the cause. I tried to reflow the solder in the FM pack components thinking maybe cold solder point of two but the issue persists. So, I'm looking for ideas on where to look next.

Appreciate any help you good folks can give...
 
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Did a bit of troubleshooting today. I tried to reflow again pretty much everything I could find on the tuner board. One interesting behavior. With the cover off, it will play FM indefinitely. I put the cover back on and it cuts out after a couple of minutes. This has got to be a thermal issue, right? Perhaps a component is thermally unstable? How would I troubleshoot this? Do I buy freeze spray, then wait till it cuts out and cool off the hotter FM components (I have a temperature gun) and observe if it comes back?
 
experienced a similar issue once, and by simply pressing down on a board component, the issue would resolve itself. needed reflowing, possibly. sometimes, the components appear to be soldered on and yet if pulled on, they're not.
 
Since you get static, that means the IF section is ok, and problem is either in the front end or power supply to the front end. Get the Freeze Spray.
 
dr*audio said:
Since you get static, that means the IF section is ok, and problem is either in the front end or power supply to the front end. Get the Freeze Spray.
Click to expand...
I'm using this as a learning experience so apologies in advance if I keep the questions coming. If it's the power supply, would I still be getting static? I was assuming if the FM part of the board was getting no power it would be silent. AM is working fine throughout this (Meter responds and it plays great), it's just FM. When FM cuts out, the meter shows no bars and of course the stereo indicator is off.

Thanks...
 
Still cutting out... I just know I missed some solder joint. I did a few more when I did the transistor @Johnny_Law recommended and now when it gets warm, FM sometime goes in and out, rather than just going to static.

Going to stop for the evening, getting late and I don't want to mess anything up.
 
marcthisdate said:
I'm using this as a learning experience so apologies in advance if I keep the questions coming. If it's the power supply, would I still be getting static? I was assuming if the FM part of the board was getting no power it would be silent. AM is working fine throughout this (Meter responds and it plays great), it's just FM. When FM cuts out, the meter shows no bars and of course the stereo indicator is off.

Thanks...
Click to expand...
Frontend power supply is usually separate from the IF supply. AM and FM supply are not necessarily the same, sometimes they are switched.
 
I had time to think about this a bit overnight. When I reflowed I just reflowed the existing solder, adding only if it was needed. This time I removed the old solder on the 3 transistors and redid with fresh solder. So far it's been playing for 20 minutes without issue.
 
And this time I got to about 45 minutes and then back came the static. :(

I ran a fan over the board and it came back running strong. So what I know so far and thinking out loud as to where to go next..
  • I know I have a thermal condition that is causing FM only to fail
  • Something in the solder joints might be at issue. Maybe in more than one place?
  • Could I have a thermal issue on a component as well as the solder joints?
  • Do I get Freeze spray, wait for a failure and spray each board component till it comes back?
 
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