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Grundig 5195 bass distortion on FM
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Autor:  kristof.vdm [ Mo Mai 31, 2021 10:28 ]
Betreff des Beitrags:  Grundig 5195 bass distortion on FM
Hello everyone,

I'm new on this forum, so I hope I'm not violating any rules by writing in English.
I do understand German, so feel free to respond in German if that's better for you.
However, I do not feel super comfortable writing in German myself. I hope that's OK. :)

I'm currently restoring a Grundig 5195 and I'm a bit stuck. I think there are some people here with much more knowledge on tube radios, hopefully some of you can put me on the right track again.

I'm experiencing extremely distorted bass sound while using FM. I made a video of how the radio sounds right now: https://www.youtube.com/watch?v=lYjLyLfGBMs
As you see in the video, when I reduce the bass potentiometer, the sound is fine.

The work I already did on this radio:
  • Replaced all paper caps & elcos
  • Replaced the filter capacitors
  • Replaced one broken EL84
  • Aligned both AM & FM

I tested with different speakers, no effect.
Measured resistances of the primary windings of the output transformers:
  • left channel transformer: 99.6 ohm
  • balance transformer: 289 ohm & 289 ohm (total 578 ohm)
  • right channel transformer: 94.7 ohm

Voltages on the EL84s:
  • pin 2: 4mV
  • pin 3: 5.99V
  • pin 7: 247V
  • pin 9: 221V

I tried to do the following to isolate the issue (unsuccesfully):
  • Connected an external tuner (and also a bluetooth module) to the pickup input (TB pin 3) --> sound OK, no distortion
  • Connected the radio to an external amplifier via the tape output (TB pin 1) --> sound OK, no distortion
  • AM also seems to be OK

However, in all 3 of these cases, the volume is significantly lower than when using the full FM chain (internal tuner + internal amplifier).

So, as I said... I'm stuck now. I don't really know where to look any further for this issue. I hope some of you can give me some direction.

:danke:
Autor:  Erwin Goossens [ Di Jun 01, 2021 4:53 ]
Betreff des Beitrags:  Re: Grundig 5195 bass distortion on FM
did you also check these 3 components? cathode voltage is slightly lower but this shouldn't be a problem.
C48 kathode ECC 83
C98 kathode EL 84 1/2
R103 kathode resistor EL 84 1/2
Autor:  Peter2K [ Di Jun 01, 2021 8:45 ]
Betreff des Beitrags:  Re: Grundig 5195 bass distortion on FM
Hi Kristof,

do you have a FM Transmitter? Then you could also check the FM by playing with volume of your signal source.

BR,
Peter
Autor:  kristof.vdm [ Do Jun 24, 2021 9:04 ]
Betreff des Beitrags:  Re: Grundig 5195 bass distortion on FM
I think I found the problem! IF transformer (b) was completely misaligned. I think this is the AM discriminator in the FM chain, right?
It was basically tuned for a maximum instead of a minimum. As a result the amplitude of the FM signal was way too high.
I now quickly tuned this transformer for a minimum and the distortion is completely gone. The volume on FM is now quite a bit lower than the volume of an audio signal on the pickup input though. So, the next thing I'll do is to again align the FM thoroughly, because I think it can still get a bit better.

For completeness, the answers to your questions:

Erwin Goossens hat geschrieben:
did you also check these 3 components? cathode voltage is slightly lower but this shouldn't be a problem.
C48 kathode ECC 83
C98 kathode EL 84 1/2
R103 kathode resistor EL 84 1/2


These are all fine (I had already replaced C48 & C98)

Peter2K hat geschrieben:
Hi Kristof,

do you have a FM Transmitter? Then you could also check the FM by playing with volume of your signal source.

BR,
Peter


No I don't have an FM transmitter.
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