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Old 27th April 2012, 05:38 AM   #11
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Hi Ricardo,

I'll get back to the PSU questions later, after looking very carefully at the schematics.

About the measuring equipment needed for my modifications: you definitely need some sort of DC voltmeter for the offset trimming and a takemeasure for measuring the length of coaxial cable. For everything else you can find solutions if you get creative/pragmatic:

-The audio signal level measurements are relative measurements, so you can do them with an audio recording device having a VU or peak meter or a signal waveform display. A computer with a sound card and Audacity will work fine (as long as you make sure no-one touches the recording level setting between the measurements).

-As a stereo signal generator, you could use one of those very low power FM transmitters that people use nowadays to listen to their MP3 player over their car radio, if these are legal in your country and if you have one or can borrow one. Play your favourite Bach cantate over the left channel of the transmitter and adjust the receiver until you hear the least music in the right channel.

-If you don't have one of those, tune in to your favourite FM station and adjust VR301 until you like the stereo imaging best. This probably won't be the theoretically correct point, but who cares as long as you like the sound?

Best regards,
Marcel
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Old 27th April 2012, 09:17 PM   #12
RCruz is offline RCruz  Switzerland
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Perfect.

I will search for a FM transmitter

Looking forward to your analisys of the psu.
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Old 28th April 2012, 02:47 PM   #13
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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...
It looks to me like the collector of Q706 drives switching transistor Q705 which turns on or off the 13.7 V supply. Regulator transistor Q701 then supplies the reference (zener diode) of the 5.6 V regulator through resistor R703. Hence, switching off the 13.7 V automatically switches off the 5.6 V. On top of that, the line regulation of the 5.6 V supply is much better than it would have been had R703 been connected to an unregulated supply, because there is less ripple current flowing through the zener.

So you can disconnect the emitter of Q701 and connect your alternative regulator to everything that is normally connected to the emitter of Q701. To retain the normal on/standby switching function, your regulator has to have an enable input. You can drive an active-low enable input from the collector of Q706 and add a pull-up resistor. For an active-high enable, connect the emitter of Q705 to an always-on supply carrying the proper high level, connect its collector to the enable input and add a pull-down resistor.
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Old 28th April 2012, 11:04 PM   #14
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Perfect

I am now drawing the layout of the new shunt.... I will let you know the outcome.
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Old 3rd May 2012, 11:07 AM   #15
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Hi, I am Carlos from Denmark and I have a Technics ST-G70 tuner. Could you recommned some modification? I've heard to change some capacitor would develope the sound quality a bit. Here is a link, in Germany they made some modification:
Klangtuning für Technics ST-G 90 - Hi-End Tuner
Could you check the picture? I wrote them a letter, but only they asked the following about the capacitors:
"
Take Nichicon Fine Gold.
For the power supply / Elna smoothing power supply it.
But they didn't tell me about the value of the capacitors. (Voltage, F)
Can you help me?

With kind regards,
Carlos
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Old 15th July 2012, 07:12 PM   #16
RCruz is offline RCruz  Switzerland
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Originally Posted by MarcelvdG View Post
It looks to me like the collector of Q706 drives switching transistor Q705 which turns on or off the 13.7 V supply. Regulator transistor Q701 then supplies the reference (zener diode) of the 5.6 V regulator through resistor R703. Hence, switching off the 13.7 V automatically switches off the 5.6 V. On top of that, the line regulation of the 5.6 V supply is much better than it would have been had R703 been connected to an unregulated supply, because there is less ripple current flowing through the zener.

So you can disconnect the emitter of Q701 and connect your alternative regulator to everything that is normally connected to the emitter of Q701. To retain the normal on/standby switching function, your regulator has to have an enable input. You can drive an active-low enable input from the collector of Q706 and add a pull-up resistor. For an active-high enable, connect the emitter of Q705 to an always-on supply carrying the proper high level, connect its collector to the enable input and add a pull-down resistor.
Hi Marcel
Just finished building the 14v shunt (Salas BiB p2p) and my plan is to power it using an external TX.
To retain on/off capabilities I am thinking using a 12v power relay to switch on the shunt.

I will disconnect the emitter of Q701 and feed there the 14Vdc.

Can I use Q701 emitter output to power the12v relay (that should switch on the secondary of the external tx, just before the rectifier diodes)

I plan to use a PCB Power Relay – G5NB-E (12V coil, 720r).

Regards

Ricardo
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Old 15th July 2012, 08:18 PM   #17
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Hi Carlos

I do not know if the ST-G 90 is similar to the tuner I am upgrading, so solutions might be different.

I believe that a good psu is very important to any audio equipment so that is where I am starting.

In the link you provided, they alk about replacing psu caps, output caps and output opamp... this gave me "food for the mind" and I am already searching for a good replacement for the AN6554 quad opamp... but that will be second phase only.

For now I will be replacing the basic 14vdc psu with an external shunt, hoping to retain on/off capabilities (this is a family tuner and I need everyone to be able t use it).

Let us know if you have a schematic for your tuner so we can study it.

Regards

Ricardo
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Old 16th July 2012, 07:57 PM   #18
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Originally Posted by RCruz View Post
Hi Marcel
Just finished building the 14v shunt (Salas BiB p2p) and my plan is to power it using an external TX.
To retain on/off capabilities I am thinking using a 12v power relay to switch on the shunt.

I will disconnect the emitter of Q701 and feed there the 14Vdc.

Can I use Q701 emitter output to power the12v relay (that should switch on the secondary of the external tx, just before the rectifier diodes)

I plan to use a PCB Power Relay – G5NB-E (12V coil, 720r).

Regards

Ricardo
Yes, that should do the trick (at least I can't think of any reason why it wouldn't). Maybe it is a good idea to disconnect the electrolytic capacitor at the base of Q701 (C707?) to make the relay switch faster and reduce contact wear. You won't need a diode for clamping the voltage at switch-off, as this is already taken care of by the zener (in forward conduction mode) and Q701.
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Old 16th July 2012, 08:08 PM   #19
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Hi Ricardo,

First of all thank you veru much for your help. I understand it. I have the ST-G70 service manual. Where can I post it? Do you have emal to send?

Regards,
CaRlos
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Old 18th July 2012, 08:39 AM   #20
RCruz is offline RCruz  Switzerland
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Hi Carlos... you have PM
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