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Old 23rd April 2012, 09:04 PM   #1
RCruz is offline RCruz  Switzerland
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Default TECHNICS ST GT650 TUNER Mod

Just for fun, I will now start to mod an inexpensive Technics tuner hoping to bring itīs sound to a higher degree.

My plan includes powering the output analog opamp with a Salas BiB.

As I would like to make it as user friendly as possible (It is a family tuner), I need all the help I can get to retain all it.s functionalities.

First step would be to power IC 302 with the BiB, but I would like to have the BiB controlled by the main controller ship (power on/off/standby).

The 13.7V are produced by the Q701 and Q705 arrangement... can I somehow feed the 13.7volts from the BiB here without compromising functionality ?
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File Type: png sch4.png (108.1 KB, 161 views)
File Type: png sch5.png (100.6 KB, 155 views)
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Old 23rd April 2012, 09:32 PM   #2
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I haven't a clue what a BiB is, so I can't answer the question, but as the sound quality of an FM tuner is largely determined by its IF filter response and its FM demodulator, have you considered tweaking those?

For example, using a length of terminated coaxial cable for frequency to phase conversion in a quadrature detector will work much more linearly than the typical LC-filter-based or ceramic-resonator-based circuits. Still, it is almost never used (except in Studer tuners like the A726) because it is more expensive and heavier.

For the IF filter, you could replace the ceramic filters with low group delay ripple types, unless those are already used of course.
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Old 24th April 2012, 08:05 PM   #3
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Quote:
Originally Posted by MarcelvdG View Post
I haven't a clue what a BiB is, so I can't answer the question, but as the sound quality of an FM tuner is largely determined by its IF filter response and its FM demodulator, have you considered tweaking those?

For example, using a length of terminated coaxial cable for frequency to phase conversion in a quadrature detector will work much more linearly than the typical LC-filter-based or ceramic-resonator-based circuits. Still, it is almost never used (except in Studer tuners like the A726) because it is more expensive and heavier.

For the IF filter, you could replace the ceramic filters with low group delay ripple types, unless those are already used of course.
Hi Marcel

Thank you for the input. The BiB is a special shunt psu that usually produces great results when powering analog circuits.

As for the coax mod you are refering to, I would really like to implement it but my knowledge about tuners is scarce.... can you point that out in the circuits from the schematics I posted ? (If you need a pdf I can send it to you by mail)

About the IF filter replacements... are you talking about ceramic caps ?

What is a low group delay ripple type ?

Regards

Ricardo
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Old 24th April 2012, 09:49 PM   #4
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Hi Ricardo,

No, I mean ceramic bandpass filters. I saw in the schematic that your tuner uses ceramic filters, but I couldn't read the type number.

There are many types of those filters. Some have a relatively narrow passband, steep roll-off and a poor group delay characteristic. Others have a wide passband, less steep roll-off and good group delay characteristics.

Narrow filters with a steep roll-off and poor group delay characteristic give more distortion, especially on stereo signals, but also more selectivity. So they give the best overall reception when you want to receive a weak signal while there is a strong undesired station with a frequency close to the weak wanted signal.

When you want to receive a strong desired signal and there is no interferer right next to it, then you can improve the sound quality by going for a wide passband and good group delay.

Could you send me the pdf if it has better resolution than the pictures you posted?

Best regards,
Marcel
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Old 25th April 2012, 03:44 AM   #5
tvi is offline tvi  Australia
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The Technics ST-G70/90 uses the same MPX and output IC but simpler filtering, still uses that Class AA config.
6.2V offset into the 6554 quad opamp and 5.7 to polarise the electrolytic blocking caps?
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Old 25th April 2012, 06:57 PM   #6
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Old 25th April 2012, 09:57 PM   #7
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Regarding the IF filters:
you could replace CF101 and CF102 with filters with a wide bandwidth and a good group delay, for example Murata type SFELF10M7FA0G-B0, see

http://www.murata.com/products/catal...p50e.pdf#SFELF

I don't know if this will improve anything though: I haven't found any information about the Panasonic filters RLFFETNG*01L and 02L that are presently used, so I don't know if the Murata parts are better or worse. As the stereo distortion specifications of your tuner are rather good, it could be that it already has wide filters with good group delay characteristics.

On top of that, when you replace the filters, you have to hope that the Panasonic filters are designed for 330 ohm terminating resistance, like the Murata ones. That is rather likely, as 330 ohm is by far the most common terminating resistance for ceramic IF filters.

There is no reason to replace CF103 and CF104, they are only switched in the circuit in the narrow IF mode, which you would normally only switch on when you have bad reception and want a narrow filter. As the filters come in different centre frequency classes, make sure the new CF101 and CF102 and the old CF103 and CF104 have the same centre frequency.

About the demodulator: IC102, AN7274NS, is your demodulator IC and C112, R110, T101 and T102 are the frequency to phase converting circuit. You would have to replace this circuit with a buffer amplifier, for example an emitter follower, driven by pin 16 of the AN7274NS, a DC blocking capacitor, a thin coaxial cable and a terminating resistor to pin 26, the 5.6 V supply. The end of the cable would be connected to the resistor and to pin 16 of the AN7274NS. I can draw up a proposal for a schematic if you seriously want to try this.

You would have to measure the audio output level of the tuner before doing any modifications. Then make the changes and make the coaxial cable one quarter of a wavelength of 10.7 MHz long. For a normal polyethylene filled cable that boils down to
(1/4)*0.67*(299792458 m/s)/(10.7E6 MHz)~=4.69 m.

Then measure the output signal level and if it is too low, which is likely, replace the coaxial cable with a longer one, but make sure it is still an odd number of quarter wavelengths. For example, if the audio level would be 10 times too small, you could replace the cable with one that is nine times longer.

Actually ideally the length should be 3/4+n wavelenghts, as 1/4+n wavelengths would change the polarity of the audio signal. Of course that can be corrected by adding an inverting stage somewhere in the audio path, and I'm not sure if the orginal tuner has the correct polarity to begin with.

Anyway, you probably would have to make the cable a bit longer than calculated, measure the DC offset between testpoints TP101 and TP102, make the cable a little bit shorter, measure the DC offset again and as such trim the cable length until the DC offset is within spec. You would have to do this adjustment with the tuner properly tuned to an RF input signal (either from a signal generator or a strong radio station with no adjacent interferers).

Last edited by MarcelvdG; 25th April 2012 at 10:00 PM.
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Old 26th April 2012, 07:24 PM   #8
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Thank you for your input.

I will look into the Murata filters. As for the phase converting circuit, it looks like a worthile mod but unfortunately I do not have any measurement equipment now so it would be very difficult to implement.

What I believe I can do is replacing Q701 regulator for something much better. (A shunt reg )

But how can I implement it retaining the tuner on/off capabilities ? And where can I feed the 13.7V ? (There is a connection between Q701 emitter and Q702 (5.6v reg) that I do not fully understand)

Maybe I should leave these two regs intact and only feed a clean +13.7v DC into the output quad opamp...maybe replace output caps and see...
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Old 26th April 2012, 07:30 PM   #9
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By the way, if you should change the IF filters, readjust the channel separation if you have access to the required equipment. The required equipment is a good FM stereo signal generator (or a very low-power FM transmitter) and something to measure the output level of the channel that should be silent.

You can even measure the output level with an amplifier, loudspeaker and your ears, as you just need to find out at what setting the silent channel is at its most silent. The channel separation can be trimmed with VR301, as indicated on page 14 of the pdf you sent.
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Old 26th April 2012, 08:20 PM   #10
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That can only be done with the help of a FM stereo signal generator..... but thank you for the tip.... I will keep it for future reference.

Returning to the PSU... I think that the 20V on Q701 collector are obtained from a voltage multiplier (C705 706)... Am I correct ?
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