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Tandberg 2075 FM Tuner Crackling Issue

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dendazen

New Member
Hi, guys.
I am servicing Tandberg 2075 for my friend, it was his grandpa's and was stored at home for 10 years.
Initial test I connected my laptop to TAPE1 and TAPE2 and EPIs M100 on SPEAKER A sound was clear and loud. Volume, Balance switches were scratchy and Treble switch the upper part was not moving and stuck to the lower one, so I removed that Treble knob and cleaned it with warm water and some soap. It fixed it.
After I started testing FM Tuner and here is the problem, MONO R plays perfectly to either speakers or to my Senheiser headphones, MONO L+R same quality of clearance but volume goes down just tiny bit. Stereo -> Right spearker, or right ear of the headphones plays good, the left one is crackling. And finally MONO L both ears are crackling. So I opened the unit up and cleaned all the switches and push buttons with deoxit, I also cleaned and worked all trimpots on FM Decoder board. Cleaning didn't help deoxit. I looked at the schematics in service manual and measured all symetric components on FM Decoder board, all nominals seems up to the service values, no dead caps or transistors. So I am not sure what else to look at, if anyone here has/had experience with these guys, maybe you have a few tips? Otherwise the unit is marvelous, the sound out of TAPE1 is just pristine and clear. EPIs don't have a very good bass, but with this amp adjusting bass makes them sound so nice. I appreciate any suggestions. Thank you.
 
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Hi dendazen,

Since you've already narrowed down the signal path, I’d check two things to pinpoint the issue:
  1. Connections between the FM & Mode/Filter boards: Try cleaning the jumpers and wire harness first. If that doesn't help, check for cold solder joints on the board pins (a gentle tap test with a chopstick will do the trick).
  2. Left channel output transistors: Hit them with some freeze spray while playing FM to see if the crackle changes or stops. I chased a very similar right-channel hiss on my TR-2030 where everything measured perfectly out of circuit. A couple of forum members walked me through the freeze spray and ground test method, and it instantly exposed a noisy transistor breaking down under load. You can see how we tracked it down in my old thread here.
Good luck,
Steve
 
wandererxp said:
Hi dendazen,

Since you've already narrowed down the signal path, I’d check two things to pinpoint the issue:
  1. Connections between the FM & Mode/Filter boards: Try cleaning the jumpers and wire harness first. If that doesn't help, check for cold solder joints on the board pins (a gentle tap test with a chopstick will do the trick).
  2. Left channel output transistors: Hit them with some freeze spray while playing FM to see if the crackle changes or stops. I chased a very similar right-channel hiss on my TR-2030 where everything measured perfectly out of circuit. A couple of forum members walked me through the freeze spray and ground test method, and it instantly exposed a noisy transistor breaking down under load. You can see how we tracked it down in my old thread here.
Good luck,
Steve
Click to expand...
Thank you very much, Steve, will do both and report back.
 
wandererxp said:
Hi dendazen,

Since you've already narrowed down the signal path, I’d check two things to pinpoint the issue:
  1. Connections between the FM & Mode/Filter boards: Try cleaning the jumpers and wire harness first. If that doesn't help, check for cold solder joints on the board pins (a gentle tap test with a chopstick will do the trick).
  2. Left channel output transistors: Hit them with some freeze spray while playing FM to see if the crackle changes or stops. I chased a very similar right-channel hiss on my TR-2030 where everything measured perfectly out of circuit. A couple of forum members walked me through the freeze spray and ground test method, and it instantly exposed a noisy transistor breaking down under load. You can see how we tracked it down in my old thread here.
Good luck,
Steve
Click to expand...
Wow, I think i found the culprit, was tapping on transistors and U301 chip, which i think is Motorolla FM Multiplexer IC, when tapped the crackling disappears, but so does sound as well, when tapped again it appears again, when I switch to Mono R though when i tap it, sound becomes noisy, when tap again, clear. So something is up with it i think, will order a replacement. Thank you very much.
 
Stop tapping and start measuring. Use a signal generator (cheapo MP3 modulator will do just fine) and use a test tone (1 kHz) as modulation. Then scope pin#5 of the MC1310P MPX decoder chip. Now you can start tapping away again and see if the tone from your speakers is consistent with the signal on your scope. If so, U301 has gone phut. If not go down the line Q304, L301, L309 (?), Q308 (?, the numbering is hard to read and seems funny). Maybe there is something wrong with the switching diode CR305. Finally, look for PCB cracks. This is for the mkii BTW, the older version (42598) uses an active T-notch filter for 19 kHz. Same story.
 

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