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Riddle Me This Batman: NAD loses FM reception only when case is attached. SOLVED

Johnny_Law

Johnny_Law

o.O
Subscriber
Edit: Solved, see post #7.

Here's a real humdinger. Yesterday, I turned on my NAD 7155, tuned into a local FM station, and left it on in the background. I stepped outside for a second, and come back in, and all I hear is static. It wouldn't pick up any stations at all ... I hit the FM search button and it would scroll up and down and never lock onto a station. But I switched to AUX input, and that worked fine.

So I hopped online for a little while, did some searching, and surmised it must be a loose component causing the FM to drop. This made sense, since I had to repair a couple loose solder joints in the power supply. I removed the case and turned it back on, intending to wiggle stuff until the FM reception snapped back. But, lo and behold, FM came in loud and clear from the moment I turned it back on.

Today, I left the case off and left it tuned into an FM station all day, listening and waiting for it to lose reception. I planned at that point to check for anything loose or hot, or somehow try to isolate the problem component. However, all day it played great, FM loud and strong! So I say huh, weird, and put the case back on.

But within 5 minutes of putting the case on, it suddenly turns back into static! So I pull it back off, feel around for hot transistors, even freeze spray them all with a duster can to cool them back off. But it won't pick up the FM station again! I turn it off, wait another 30 minutes or so, and turn it back on. Ta-da, FM is back.

I wasn't sure if the case thing was coincidence, so I let FM play for 5 minutes or so, and put the case back on. Soon thereafter, static again. Weird!!!

The question now is, hmm, how to diagnose. I figure it may be an issue with a transistor that is on the verge of failing, and when the case is on, gets just too warm and fails. What's strange is that even if I take the case back off and wait a while, it'll never tune back in unless I turn it off and let it sit for a little while.

I'd love to know if anyone has experienced anything like this before! And if my heating-failing-transistor theory makes sense or not. I'd love to fix it!
 
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Humdinger? Such language... You DO understand that AK is supposed to be a family-oriented site, right? ;-}

While it might be a stress/flex issue caused by attaching the cover, I would suspect that the temperature change is more likely the cause. Perhaps you could test this theory with a heat gun or hair dryer warming up the tuner section, or possibly even warming up the mode switches.
 
It seems to be disconnecting the antenna . My guess is a bad solder joint between the antenna port and the tuner.
 
I forgot to mention, I did poke / move / jiggle every single component on the FM board as well as the antenna connection with a non-conductive rod (a plastic pen) twice - once when the FM was working, once when it was all static. I found nothing to be loose and no amount of physical manhandling altered the reception at all. So I've done that much to investigate the cold solder joint theory both on the board and at the antenna.

I like the heat gun idea, I'll try that today.

I also have not unscrewed the board and looked underneath for any signs of bad solder joints on the board itself, or scortches from heat-failure (both were present in the pre-amp's power supply, which I repaired).
 
Solved. I'll preface with, because of the genius design putting high temp power supply components underneath the FM board (and essentially trapping the heat between the two boards), it's little wonder all NAD 71XX receivers will eventually suffer from solder joint issues.

The 7155 is an awesome unit, tons of power and headroom, sounds great, can drive really hard loads ... BUT ... the design flaw is putting the FM board on top of the power supply / amp board and not adequately ventilating.

So some of the transistors on the FM board are located directly above the pre-amp's power supply components. The same components that are notorious for overheating and going bad (this is why many 7155, 7140, etc have dead pre-amps).

Here's a pic of mine - check out the blackened board. The resistors had totally fried their solder pads off the board.

y1aR3TH - Imgur.jpg

Notice the black areas on the bottom of the FM board.

WirlUTK - Imgur.jpg

So I used the heat gun method (good call @Watthour ), and found that heat applied to Q102-Q104 here resulted in fading to static. This time the FM came back after I turned the heat gun off and waited a minute for it to cool down.

SoDgR7c - Imgur.jpg

Here's what was happening. Q104's solder joints had totally broken underneath, but it was being held in contact by a thin layer of glue on the solder side. When that glue heated up, it stretched, and Q104 sagged down, disconnecting the solder joints! Then when the glue cooled back down, it pulled the solder joints back together and viola, FM reception again.

Here's Q104's solder joints from below (notice the clearly lifted joint).

KFTLNdT - Imgur.jpg

After re-flowing the solder (with some new solder added), we're back in business. Thanks all for the input and hopefully this thread can help someone in the future who is stumped by their NAD 71XX receiver acting up like this.
 
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Johnny_Law said:
Solved. I'll preface with, because of the genius design putting high temp power supply components underneath the FM board (and essentially trapping the heat between the two boards), it's little wonder all NAD 71XX receivers will eventually suffer from solder joint issues.

The 7155 is an awesome unit, tons of power and headroom, sounds great, can drive really hard loads ... BUT ... the design flaw is putting the FM board on top of the power supply / amp board and not adequately ventilating.

So some of the transistors on the FM board are located directly above the pre-amp's power supply components. The same components that are notorious for overheating and going bad (this is why many 7155, 7140, etc have dead pre-amps).
Click to expand...

Good find!

Lee.
 
Hey there, total newbie here. I've been using this forum for quite some time to learn about, well, everything related gear I'm researching and I've been amazed at the scope and detail found in the threads - you guys cover everything!

Today, however, I was totally surprised to find this thread - I'm experiencing the exact same issue. It was crazy to read the set-up to the solve as it was exactly what I'd experienced.

I've got the unit torn half way down, waiting for my "work day" to end before diving in to check the solder pads, I'll let you know how it goes, but I'm now optimistic about being able to fix this, today, based on and thanks to this thread. Thank you @Johnny_Law!
 
Well, dang it, flipped the board over and did not have the same dark areas as Johnny_Law, and the solder points he fixed look fine on mine. In fact, the entire thing looks super clean. For me, the one other point was how sensitive the grey antenna lead was to the touch, the signal would get much stronger when I'd put my finger on it and move it some. That said, there was a time, with the cover off, when the FM came in strong and nowhere near the level of static I got when I put the cover back on. Hmmm, I'll keep poking around. I'm not a pro with soldering so I don't want to just start roaming around trying to refresh connections.

IMG_5321.jpg

Any ideas? Remember, I'm pretty much a novice, but, happy to try just about anything.
 
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Thanks, and yes, that's definitely the plan. For reference, the picture you posted clearly showed some lifting and separating (sorry, sounds like a bra commercial) but are there any other telltale signs of a bad solder connection? As I look at the board I'm seeing some that are slightly discolored, kind of brown around the base, but none look like they've come off the board or appear loose.

Thanks again for the support, I know I can fix it. And man, when it was fired up it really does have an great sound, I just want to be able to put the case on it!!
 
In my limited experience bad solder joints show up in a few ways. Most characteristic is the little ring in the solder about half way up the joint indicating the solder has cracked. May or may not cause your issue.

Another one is where the solder cracks around the lead, visible as a rough line around the top of the lead. Harder to see and the component may indeed not feel loose.

Lifted traces or the solder cracking off the trace complete are harder to see, and occasionally the trace will fracture at the edge of the solder from a lifted pad flexing.

All of them can manifest as failure for something to function with changes in heat or position. Quite often touching components will temporarily restore the connection, and bad joints tend to heat up from high resistance through the bad solder.

Cold joints where the solder wasn't completely melted and properly adhered initially can show up as grainy looking joints if eutectic solder was used. Non-eutectic solder always looks matte (although it's not very common)

The ones I had on my KA-80 were much more obvious -- serious overheating lifted the traces, melted the solder, and enlarged the holes in the board! Nothing sneaky about those. Not the first time either, one of the solder pads was jumpered when I had it repaired years ago.

Failed components are harder to find by looking, unless they are physically damaged.
 
Thanks, @HPMguy, very helpful!

As I poked around I was able to find a few that looked a bit brown, oxidized (?), so even though it know it's likely better to pull the old solder first before applying more solder (I don't have a solder puller yet...) I still went ahead and warmed up the joint along with the two leads where the antenna attaches to the board. I then did an A/B test to see how long the signal would stay consistent with the case off, then with the case on - there was no difference and thankfully none of the horrendous static I'd heard after putting the case back on last time. Feels like I still need to do some testing around actually hooking up a physical antenna, so I'll do that soon as well. When I had the case off and would touch the antenna lead coming from the back of the unit then terminating at the board I noticed a considerable improvement - later learned I basically became the antenna, so that makes sense.

Oh, and the Dynamic Separation feature is also very interesting in the way it colors and almost distorts the sound.

Bottom line, I'm making progress, which is good. More testing to come, thanks again, team!
 
Johnny_Law said:
Solved. I'll preface with, because of the genius design putting high temp power supply components underneath the FM board (and essentially trapping the heat between the two boards), it's little wonder all NAD 71XX receivers will eventually suffer from solder joint issues.

The 7155 is an awesome unit, tons of power and headroom, sounds great, can drive really hard loads ... BUT ... the design flaw is putting the FM board on top of the power supply / amp board and not adequately ventilating.

So some of the transistors on the FM board are located directly above the pre-amp's power supply components. The same components that are notorious for overheating and going bad (this is why many 7155, 7140, etc have dead pre-amps).
Click to expand...

Just picking up on this thread again. There are plenty of other amplifiers and receivers that fall woefully short of adequate ventilation. The Pioneer SA-9800 I have, the regulated power supply board is mounted upside down underneath two huge power transformers. There is nowhere for this heat to go, in fact the transistor regulators will almost burn you because of how hot they run. I've worked on a Yamaha preamp with the closed case, where again the regulator transistors have gotten so hot, it has caused a solder joint to go bad, and the thing dies.

Lee.
 
Closing the loop here. My retouching of a few solder points seems to have helped - no more insane static with the case on, happy that's behind me! Also picked up one of those cheapy RCA FM antenna's and geez, this unit sounds amazing now that the signal strength has been resolved and the static is gone.

Thanks again for the advice, much appreciated!
 
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