Tandberg 2075 FM Tuner Crackling Issue

[dendazen]

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.

 

[wandererxp]

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

 

[dendazen]

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

Thank you very much, Steve, will do both and report back.

 

[dendazen]

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

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.

 

[PE9ZZ]

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.

 

[dendazen]

Thank you, I will measure tonight.

 

[dendazen]

Sorry, guys, I was on vacation, but came back to this Tandberg, I have found the issue, the FM board was actually healthy. I measured decoder chip and it was fine.
I then Unplugged P305 (FM decoder to program selector) and it killed the crackle. Also the crackle would scale with volume, so I was convinced it originated on the A2 decoder board.
I ran an isolation test down the left decoder chain: a 10 µF electrolytic as an AC short to chassis (positive to the node so the DC bias wasn't disturbed) at U301 output, Q305 base, Q307 base, Q309 base, Q309 emitter and D303 anode. The crackle survived all six. I also bridged D303 with a clip lead and that had no change. That read as "noise enters after D303," which is just the connector, and that contradicted the P305 result.
Two things resolved that. First, the last two test points weren't valid shorts: Q309 is an emitter follower driving D303's anode through C332 (2.2 µF), so a 10 µF shunt there only takes the signal down maybe 15 dB. The earlier four points, driven through 1k or more, were real shorts. So the honest reading was "noise enters after Q309's base.", so I started comparing DC between channels.
The DC told the story. J305‑3 (left, D303 cathode) read 10.58 V; J305‑5 (right, D302 cathode) read 9.58 V. Since the anodes are fed from the ~10 V node through R346/R347, the cathodes can't sit above the anodes unless something else is pushing the node up. Measured across C331 and C332 to check for a leaky coupling cap on the decoder board - 0.83 V vs 1.35 V, so no, D303 was actually being held reverse-biased by the cathode side. Something downstream was pulling that node up.

Downstream is Q601's base on the A7 program select board (42425), the common left-channel node for every source, each arriving through its own switching diode. Q601 base read 10.93 V, Q602 (right) 9.6 V. With P305 unplugged, the FM board out of the picture, Q601's base stayed at 10.86 V, it should have collapsed to ~0 through R601 (270k). About 40 µA was leaking into that node. Q602 read 4.1 V unplugged, so the right was leaking too, just less. So I replaced Q601 and Q602 (BC149B → BC549C) on the theory of collector-base leakage. No change. Not the transistors.
So I unplugged the other sources feeding that bus one at a time while watching R601. AM (P604/P404): no change. Phono (P601/P501, from the A6 RIAA/input board 42449): dropped straight to 0 V. Plugged FM back in and both channels were clean.
On A6, each phono output stage (Q503/Q504/Q507/Q508) feeds its switching diode through a 10 µF electrolytic (C515/C516/C529/C530), with a 15k from the bias network holding the anode at 0 V when that input isn't selected. With FM selected, the Phono 1 diode anodes (D501/D502) read 0 V as they should. Phono 2: D503 anode 11.2 V, D504 anode 4.6 V. Selecting Phono 2, D503 went to 12.4~12.6 V (essentially Q507's emitter voltage, design says 10 V). C529 is nearly a dead short; C530 is on its way.
The leaky cap passes Q507's emitter DC to D503's anode, D503 turns on even though Phono 2 isn't selected, and it drags the shared left bus up to ~10.8 V. That forces the FM board's D303 into reverse bias, so FM left audio only gets through on signal peaks - weak, distorted, and with the flickering leakage current riding on top as crackle. Phono 2 itself sounds fine because its diode is supposed to be on in that mode. Every FM board test came back clean because the FM board was clean; its reference point was being hijacked from another board.
Replaced C529 and C530, and C515/C516 at the same time since they're the same part and age. The A6 board was a bit of a pain to get to, but it wasn't too bad.
Lesson for me though the tapping and isolation tests were chasing the symptom's location. Comparing DC between a good channel and a bad one found the fault's location, and the "unplug things until the voltage collapses" test identified the board directly instead of by inference.
Thank you for your help.

 

[PE9ZZ]

Huh. Somewhere else completely. And the bad caps not even electrolytics. Did not see this coming.

 

[gadget73]

Thats odd for sure, but fantastic job chasing that down.

 

[dendazen]

Huh. Somewhere else completely. And the bad caps not even electrolytics. Did not see this coming.

Hi guys, sorry for the late response those caps were actually electrolytics, attached the pic here of the A6, it was a pain in the butt to get that board out and put back in, but restored Tandberg looks and sounds pretty awesome, my friend is very happy about the unit.

 

[dendazen]

Thats odd for sure, but fantastic job chasing that down.

Thank you.