Hi, I have a tr200 on the bench these days with some serious noise issues, due to the behaviour of the noise I strongly suspect it to be a clear error somewhere, I just cannot see it.
If loudness is on everything is fine, except if I hit the Tape/monitor switch, then noise comes back.
If volume pot is all the way to max everything is fine, turn it 1/20 back down, noise is back, no gradual change, On/Off.
Character of noise is low frequency buzz, like ground loop I believe. Level of it is very high, around moderate listening level through the speakers, and the tone controls seems to do their thing with the noise, so it happens “before” that stage.
Probably worth to mention, “crosstalk” between inputs and output stage is very high. With no input selected I can still hear the music relatively loud and clear considering it should be quiet, this follows volume pot and can be turned up and down to some extent.
Amp has been more or less fully recaped, including those ,33uf in loudness loop.
All switches and pots have been given some contact spray without any effect.
Any ideas?
If loudness is on everything is fine, except if I hit the Tape/monitor switch, then noise comes back.
If volume pot is all the way to max everything is fine, turn it 1/20 back down, noise is back, no gradual change, On/Off.
Character of noise is low frequency buzz, like ground loop I believe. Level of it is very high, around moderate listening level through the speakers, and the tone controls seems to do their thing with the noise, so it happens “before” that stage.
Probably worth to mention, “crosstalk” between inputs and output stage is very high. With no input selected I can still hear the music relatively loud and clear considering it should be quiet, this follows volume pot and can be turned up and down to some extent.
Amp has been more or less fully recaped, including those ,33uf in loudness loop.
All switches and pots have been given some contact spray without any effect.
Any ideas?
Attachments
Full service manual with schematics:
https://audio-circuit.dk/downloads/tandberg2/Tandberg-TR200-rec-sm.pdf
https://audio-circuit.dk/downloads/tandberg2/Tandberg-TR200-rec-sm.pdf
Wild guess here: is the frame of the loudness switch not grounded? I can't spot the Loudness switch on the front panel. How about frame of source selection switch? If its frame is floating, that might contribute to crosstalk from other sources.
Good luck!
Good luck!
Thanks!
The button for loudness switch is hidden under a lid on lower part of front panel so not normally visible.
I don’t think any switch or pot on this unit is grounded, but there is some info in the manual suggesting this kind of crosstalk/feed should be much lower than what I’m getting
The button for loudness switch is hidden under a lid on lower part of front panel so not normally visible.
I don’t think any switch or pot on this unit is grounded, but there is some info in the manual suggesting this kind of crosstalk/feed should be much lower than what I’m getting
I think all metal switch frames and pot shafts should be grounded. If not, they act as antennas into the signal paths. Floating selector switches would act as coupling caps between inputs. Try touching switch frame with a metal tool to see if your body couples more hum into the amp.
I agree with you on that one, and to an extent this would set the bar for performance and I think the "crosstalk" with no input selected could very well be explained by this. The noise however is another story and is the real problem here, the level of this is not like a background buzz from the speakers with no music, its more like I'm careful not to blow any speakers with it! 🙂
Osvaldo:
Noise is in both chanels at similar level? No left is a bit louder
What if you remove one chanel supply? you mean audio signal or psu?
Are those opamps is sockets? Tried to remove them? Soldered directly on pcb, its part of the phono/riaa network which seems to work fine.
Was the noise before recaping? Yes it appeared after a couple years on the shelf, figured it probably was those 50 years old elkos causing some fun, but no success.
Osvaldo:
Noise is in both chanels at similar level? No left is a bit louder
What if you remove one chanel supply? you mean audio signal or psu?
Are those opamps is sockets? Tried to remove them? Soldered directly on pcb, its part of the phono/riaa network which seems to work fine.
Was the noise before recaping? Yes it appeared after a couple years on the shelf, figured it probably was those 50 years old elkos causing some fun, but no success.
have you tried to uncouple preamp and amp via C601 and c602 to know if it comes from one or the other?
This manual is very well done, have you checked all the voltages and current at the checkpoints?
This manual is very well done, have you checked all the voltages and current at the checkpoints?
Check the voltages around Q501 and Q502 it sounds like something is oscillating as a result of erroneous gain .
Try to remove supply to one channel. If it continues, it possibly be a PSU defective, it is difficult both channels has the same problem.
Thanks for all suggestions! However I need to stress the fact the unit is working 100% when loudness is on, no noise at all.
The noise is only a problem with loudness off or tape monitor on.
Just tried and output stages are silent without preamp section connected and when I switch L/R between pre/amp (wires between stages) the louder channel moves to the other side.
Tone controls are working on the noise, if I turn the bass up noise gets bassier same with treble.
The balance control however is not working on the noise, that is I can turn the right channel up and down with it, but left stays the same. This is only with the noise, with music in loudness mode balance works fine.
Psu I think works fine, but have not observed it during the noise mode, however can’t see clearly how this could be triggered by loudness switch?
I will try pulling the schematic from manual, it’s the only one I have found.
The noise is only a problem with loudness off or tape monitor on.
Just tried and output stages are silent without preamp section connected and when I switch L/R between pre/amp (wires between stages) the louder channel moves to the other side.
Tone controls are working on the noise, if I turn the bass up noise gets bassier same with treble.
The balance control however is not working on the noise, that is I can turn the right channel up and down with it, but left stays the same. This is only with the noise, with music in loudness mode balance works fine.
Psu I think works fine, but have not observed it during the noise mode, however can’t see clearly how this could be triggered by loudness switch?
I will try pulling the schematic from manual, it’s the only one I have found.
Thank you for clarifying about loudness switch--- I was going to ask that very question.
If you select Tape Monitor, what happens if you short vs. open the monitor input jack? The noise is either present or absent in both channels?
With volume at 0, is noise always absent? Is noise a consistent buzz, or an erratic crackling?
Interesting that the noise follows one preamp input channel. What happens if you disconnect just one channel? Then the opposite channel?
If you select Tape Monitor, what happens if you short vs. open the monitor input jack? The noise is either present or absent in both channels?
With volume at 0, is noise always absent? Is noise a consistent buzz, or an erratic crackling?
Interesting that the noise follows one preamp input channel. What happens if you disconnect just one channel? Then the opposite channel?
Just measure between all legs and ground and get, 12.5V (collector), 3.5V (base) and 3V (emitter) on Q501 roughly the same on Q502
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Most loudness controls create a bass bost to compensate low frequencies at low volume control settings, this is why I suspect over the power supply. Perhaps some of the new caps is faulted and thus a bad decoupling of the supply line at the lower frequencies boosted by the control, making it to motorboat or regenerate.
Also when playback or monitoring the signal, also there is a bass notes reinforcement giving again the oportunity to regenerate trough power lines.
Also when playback or monitoring the signal, also there is a bass notes reinforcement giving again the oportunity to regenerate trough power lines.
Prime candidates for oscillation are emitter followers IE Q503 and Q504 check around these 2 transistors for any dry joints especially C513 and C514 . Maybe check the voltages around Q503 and Q504 also .
Just observed something interesting I think is very relevant: if I touch the solder point of R537 with the multimeter probe or a screwdriver for that matter, the noise goes completely away and music can be played. Tried to re-solder the resistor with no effect.
As an experiment, you might tack a 100pF cap Q503 base to ground. The sensitivity to "touching" seems suggest an oscillation. The cap is suggested as a diagnostic, not necessarily a fix.