Hi,
I swapped for a Sansui Eight receiver some four years ago and now, finally, have had time to rebuild it. This has been a learning adventure. Some of the updates have been straightforward but others have taken far more time than I thought they would.
The main issues that I still don't quite understand are as follows:
1. AM reception: After rebuilding the AM board and aligning it the sensitivity of the board is good but it seems overly sensitive to RF noise (when compared to another receiver) just using the built in antenna. If I look at the output on a scope (which itself is a bit problematic because the scope, a digital Rigol, puts out its own noise signature in regions of the AM band...) I can see that the noise is from sharp spikes at 120 Hz. The level is very sensitive to position of the receiver so presumably it is somewhere local and very directional. However the impact of this noise on another Sansui (a 2000x) is much less noticeable.
I am just trying to understand the reason for the difference (I don't know if filters could/should be used to help).
One idea is that it could be a function of the JFETs in front end and the width of the IF response. The Eight appears to pass much higher frequencies than the other tuner and so that tuner's lower frequency spectrum may minimize the more annoying components of the high frequency 120 Hz overtones. I don't have a spectrum analyzer but the spike should be dominated by odd order overtones. I suppose I should look at both outputs on my scope....
Is there something else I should consider?
2. Signal ground of the tape RCA jacks: I was chasing what I thought was a ground loop issue with the phono preamp board. According to the schematic the signal ground from the tape RCA jacks passes through the phono board and is then tied to the chassis. I measured the resistance between the RCA jack ground and that of the chasis and measured just over six ohms. For something that is tied to ground this seems a bit high.Other points that I have tested are easily under one ohm.
Is there something odd here?
3. Phono preamp board: Here is where things get really peculiar. I noticed that with the phono inputs shorted and at high volume there was excessive hum (120 Hz...I believe). I don't know if it was there before the recap (all the ECs, two BPs and two tantalums) but it annoyed me. I couldn't easily trace it down because when I connected the scope probe to the board it went away completely. So as a temporizing measure I simply shunted a pair of 100 pf ceramics caps across the L/R signal output and ground. This eliminates nearly all the hum (unless someone usually plays their stereo at full volume with zero audio input).
Can someone suggest what is going on here?
4. Phono board output (monitored at the tape REC RCA jack across a 3K load resistor): There is a very high frequency sinusoidal "hum" of about 150 mV peak to peak. For the left channel it is at about 8.7 MHz (yes megahertz) and for the right channel it is at about 9.1 MHz. Something is setting up a high frequency oscillator. Certainly there are no audio amplifiers that will need to handle this (I suspect a cap will shunt this this to ground) so it probably much ado about nothing but it seems quite odd. There is so much RF noise these days in the environment I suppose it may originate from something that is external.
Another mystery I have no answer for.
Thanks, in advance, to any Sansui gurus at AK.
Michael
I swapped for a Sansui Eight receiver some four years ago and now, finally, have had time to rebuild it. This has been a learning adventure. Some of the updates have been straightforward but others have taken far more time than I thought they would.
The main issues that I still don't quite understand are as follows:
1. AM reception: After rebuilding the AM board and aligning it the sensitivity of the board is good but it seems overly sensitive to RF noise (when compared to another receiver) just using the built in antenna. If I look at the output on a scope (which itself is a bit problematic because the scope, a digital Rigol, puts out its own noise signature in regions of the AM band...) I can see that the noise is from sharp spikes at 120 Hz. The level is very sensitive to position of the receiver so presumably it is somewhere local and very directional. However the impact of this noise on another Sansui (a 2000x) is much less noticeable.
I am just trying to understand the reason for the difference (I don't know if filters could/should be used to help).
One idea is that it could be a function of the JFETs in front end and the width of the IF response. The Eight appears to pass much higher frequencies than the other tuner and so that tuner's lower frequency spectrum may minimize the more annoying components of the high frequency 120 Hz overtones. I don't have a spectrum analyzer but the spike should be dominated by odd order overtones. I suppose I should look at both outputs on my scope....
Is there something else I should consider?
2. Signal ground of the tape RCA jacks: I was chasing what I thought was a ground loop issue with the phono preamp board. According to the schematic the signal ground from the tape RCA jacks passes through the phono board and is then tied to the chassis. I measured the resistance between the RCA jack ground and that of the chasis and measured just over six ohms. For something that is tied to ground this seems a bit high.Other points that I have tested are easily under one ohm.
Is there something odd here?
3. Phono preamp board: Here is where things get really peculiar. I noticed that with the phono inputs shorted and at high volume there was excessive hum (120 Hz...I believe). I don't know if it was there before the recap (all the ECs, two BPs and two tantalums) but it annoyed me. I couldn't easily trace it down because when I connected the scope probe to the board it went away completely. So as a temporizing measure I simply shunted a pair of 100 pf ceramics caps across the L/R signal output and ground. This eliminates nearly all the hum (unless someone usually plays their stereo at full volume with zero audio input).
Can someone suggest what is going on here?
4. Phono board output (monitored at the tape REC RCA jack across a 3K load resistor): There is a very high frequency sinusoidal "hum" of about 150 mV peak to peak. For the left channel it is at about 8.7 MHz (yes megahertz) and for the right channel it is at about 9.1 MHz. Something is setting up a high frequency oscillator. Certainly there are no audio amplifiers that will need to handle this (I suspect a cap will shunt this this to ground) so it probably much ado about nothing but it seems quite odd. There is so much RF noise these days in the environment I suppose it may originate from something that is external.
Another mystery I have no answer for.
Thanks, in advance, to any Sansui gurus at AK.
Michael