Sansui tu-717 distortion

[spacemen12]

Hi,
I have a Sansui TU-717. This tuner is functional, but I have perceptible distortion on all FM stations. According to the signal meter the strength is not the problem. I always get above 4/5, and more often it looks like 4.9/5. The only other unusual this is that the station are ot on the spot, i.e. I get 95.1FM at 94.9. I don't care about this small deviation, what I care more is about the distortion.

The distortion is not extreme, my girlfriend cannot get what I am talking about, but an audiophile friend picked it up right away.

Everything else is perfect, i.e. stereo light, tunning meter, etc.

I would like to get rid of the distortion, but I don't know where to start. I know we can upgrade those tuner, but my main objective here is to get rid of the distortion.

If you can help me identify the cause or better find a solution I would be please.

[Salas]

Time to send it off to a highly qualified technician. Its vintage gold and it will enjoy an inspection and general service anyway. I don't think that anybody can just guess at what it may be happening. I recommend you to e-mail anatech , he is a moderator here and Canadian too.

[robjak]

Hi
I just got done tweaking TU-717. Its quite astonishing after $20 filter mods and alignment:
WIDE
Mono .015% both channels
L+R .016/.022
L-R .083
SEP 58dB
NARROW
Mono .123% both channels
L+R .129%
L-R .104%
SEP. 28dB (with a dual separation pot relay it would be 50db)

@91.1Mhz,72dbf, 75khz deviation, 1.023khz mod, 9% pilot, no SCA, no RDS

So your tuner is capable of much better performance. The audio circuits have a lot of coupling caps to replace for best bass. You would be hard pressed to better it for the money. A Tu-517 is very slightly better at rejecting MPX noise on weak stations.
If you have any q's feel free to contact me
John

[spacemen12]

Originally posted by robjak

So your tuner is capable of much better performance. The audio circuits have a lot of coupling caps to replace for best bass. You would be hard pressed to better it for the money. A Tu-517 is very slightly better at rejecting MPX noise on weak stations.
If you have any q's feel free to contact me
John

Hi robjak ,
Do you think that by changing those caps, my distortion problem can go away?

What is your list of mods? Is is the same as the one on fmtunerinfo?

What else than black gate can we use?

Thanks

[robjak]

First some q's to help narrow down the problem

1)Can you describe in detail what the distortion sounds like?
2)Have you tried a different tuner to see if this distortion is still present?
3)Are you certain that your antenna is getting a good signal, not just a strong signal but one free of multipath?
4)Does the distortion change with antenna orientation or with a different station?
5)Are you using a directional outdoor antenna?
6)What is the distance to your station?

Increased distortion (THD) in FM tuners is caused by the drift and aging of critical circuits such as the ceramic filters and detector coils. This is more pronounced in vintage tuners suchas the TU-717.

On the Sansui I worked on the caps did not influence THD at all. The owner did not see the need to upgrade the caps as the midrange sound was so exceptional even with the old caps!

To my ears the tuner lacked low end that could be upgraded with bigger coupling caps. I have a more extensive list of cap modds than Jim but its muy $$$. Also there's stuff you can do to flatten the high freq. response and power supply upgrades but those are advanced mods.

There are no exact substitutes for Black Gates IMHO but you can use the richer sounding Elna Silmics or Cerafines in contrast to the thin sounding Nichicon Muse. Sanyo Oscons can sound good in some cases also.
Bigger caps = lower frequency response but do not cheat by using lower than 25V caps 50V is best. On the higher end tuners Sansui used a composite cap made from two back to back polar caps bypassed with a poly to good effect.

Electrolytic caps lifetime is typically 15 years under good conditions BUT they can last longer if they where high quality components to begin with. High temperatures degrade the caps much faster than that. Also if the unit was left unpowered for many years these caps will lose their polarization and will "leak' upon first power up. Old bad caps will sound fuzzy and ill-defined in the bass. When I get an old tuner I usually let it run-in for a few days before aligning it.

First I cleaned the frontend contacts with a pinpoint application of Deoxit.( See Audiokarma "how to clean an analog tuner " sticky).
I changed the wide and narrow ceramic filters with carefully matched ones from Bill Ammons (see FMtuners matched filter page).
The narrow filters came with an amplified filter board to compensate for the filter loss. I exchanged the ceramic caps in the detector circuit for polystyrene ones.

Then I re-aligned the detector coils and IF group delay equalizer for lowest distortion and reset the separation pot for best separation.

Unfortunately to do this crucial step you need some rather specialized test equipment like a FM stereo signal generator in good calibration. You will not get proper results just by swapping in filters and caps. So as Salas said you need to find a GOOD tuner tech WILLING to do this.I don't know any tuner techs in Montreal, but Mike Williams or I could easily tune-up a 717 as long as you can ship it to the 'states

John

[anatech]

Hi Salas,
Thank you for the plug. You're very kind.

Hi spacemen12,
I often find the demodulator (quadrature coil or discriminator coil) is out of alignment. That will greatly affect the distortion. Sometimes coupling caps can age, but this would more likely create a different type of distortion rather than just bass loss. The power supply should also be checked out at this age.

The other dreaded fault is someone who gets in there and just starts adjusting things. That can create lengthly repairs, so can component replacement by people who don't solder well.

I do audio service, including complete tuner alignments. My equipment is pretty much all Hewlett Packard, with one Radiometer Copenhagen stereo generator thrown in for good measure.

Now, if you are not in a rush I might be able to help you. Right now I'm pretty busy with work. Just use the button below to send me a message.

-Chris

[spacemen12]

Originally posted by robjak
First some q's to help narrow down the problem

1)Can you describe in detail what the distortion sounds like?
2)Have you tried a different tuner to see if this distortion is still present?
3)Are you certain that your antenna is getting a good signal, not just a strong signal but one free of multipath?
4)Does the distortion change with antenna orientation or with a different station?
5)Are you using a directional outdoor antenna?
6)What is the distance to your station?


John

1) The distortion is mostly in man voice (I would guess badly around 600Hz, don't trust the number)
2) Distortion is present on the entire FM band
3) I don't know what is a multipath, I would like to know.
4) No
5) No, T antenna. I live on the 2nd floor.
6) less than 3km

[anatech]

Hi spacemen12,

Increased distortion (THD) in FM tuners is caused by the drift and aging of critical circuits such as the ceramic filters

Ceramic filters do not often drift that far. The initial center frequencies are not very tight either. Worrying about this is more of an improvement and you may not notice it.

1) The distortion is mostly in man voice (I would guess badly around 600Hz, don't trust the number)
2) Distortion is present on the entire FM band

Still sounds like your FM detector.

3) I don't know what is a multipath, I would like to know.

That happens when the signal bounces off a building or some other surface. The direct signal arrives, but the reflected signal arrives a touch later and interacts with your direct signal. Since they are on the same frequency, the interaction passes through. The only way to "fix" a multipath situation would be to use a directional antenna aimed to reduce the strength of the reflected signal. This will probably not be the direction that gives you the strongest desired signal.

-Chris

[zoranaudio]

Have you tried to switch to Mono? Is than distortion lower than in stereo mode?
You can match to the right frequency of the broadcasting station with trim. potentiometers on the PCB inside, but that is work for the technician.
Also try to connect just piece of wire about 70 cm long ( to the hot point of the coaxial Ant. connector) , perhaps your signal is coming too strong!!! and see if that makes any difference in distortion.

[spacemen12]

Originally posted by zoranaudio
Have you tried to switch to Mono? Is than distortion lower than in stereo mode?
You can match to the right frequency of the broadcasting station with trim. potentiometers on the PCB inside, but that is work for the technician.
Also try to connect just piece of wire about 70 cm long ( to the hot point of the coaxial Ant. connector) , perhaps your signal is coming too strong!!! and see if that makes any difference in distortion.

Hi,

Mono: no change, same distortion as in stereo.

I've tried the 70cm long coaxial, and no change. The strength of the signal is the same as my T antenna, and the distortion, unfortunately, is still there.

Any other test to pin point the problem?

Thanks