I am trying to fix a Sony ST-3950 tuner. I have solved tuning noise using Tuner Spray 600 on the variable tuning capacitor, but it is still hard to tune on to a station. For instance, the bandwidth for the stereo indicator to light up is extremely small and very close to the edge (of the middle) of the discriminator indicator.
However first thing I need to fix is the tuning signal meter that always shows near full signal strength (5), even between stations. It seems that the tuner thinks there always is a strong signal. Note this only happens in FM mode, not in AM mode.
I know there is a modulator involved, could it be that for some reason it is the modulator signal that is confusing the meter? FYI this is my first tuner that I am trying to fix, I have done several amps.
I do not have a signal generator as assumed in the service manual, but I have seen a number of Youtube examples where some tuner fixes can be done without, such as this one: YouTube
So I would appreciate some guidance on the way forward.
However first thing I need to fix is the tuning signal meter that always shows near full signal strength (5), even between stations. It seems that the tuner thinks there always is a strong signal. Note this only happens in FM mode, not in AM mode.
I know there is a modulator involved, could it be that for some reason it is the modulator signal that is confusing the meter? FYI this is my first tuner that I am trying to fix, I have done several amps.
I do not have a signal generator as assumed in the service manual, but I have seen a number of Youtube examples where some tuner fixes can be done without, such as this one: YouTube
So I would appreciate some guidance on the way forward.
How much do you know about RF, and superhets, and FM?
The general rule with RF is that you need either knowledge or test equipment, preferably both. The more of one you have the more you can cope with less of the other. With neither there is not much you can do.
The general rule with RF is that you need either knowledge or test equipment, preferably both. The more of one you have the more you can cope with less of the other. With neither there is not much you can do.
Thanks for your feedback. In answer to your question: nothing as you have probably guessed from my intro. It's a 50 Euro tuner so there is not much to lose. What would you think causes the signal strength to indicate full signal with or without being tuned to a station?
Could be local interference, front end oscillating, IF amp oscillating, faulty signal strength circuit.
Your best bet may be to find someone who has had a similar fault with that one or a similar tuner. Checking DC voltages against the service sheet is always a useful fault-tracing technique.
Your best bet may be to find someone who has had a similar fault with that one or a similar tuner. Checking DC voltages against the service sheet is always a useful fault-tracing technique.
Thanks! Taking measurements in accordance with the manual is always good. I'll start with that. Oscillating parts is a good suggestion too. Haven't found a thread on the ST-3950.
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