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Sansui G-7700

Pure Power DC Stereo Receiver (1979-80)

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Sansui G-7700

Specifications

Tuning range: FM, MW

Power output: 120 watts per channel into 8Ω (stereo)

Frequency response: 5Hz to 75kHz

Total harmonic distortion: 0.025%

Damping factor: 50

Input sensitivity: 6mV (mic), 2.5mV (MM), 150mV (line)

Signal to noise ratio: 78dB (MM), 95dB (line)

Channel separation: 60dB (MM), 65dB (line)

Output: 150mV (line)

Speaker load impedance: 8Ω (minimum)

Dimensions: 505 x 182 x 422mm

Weight: 18kg

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Reviews

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rating
Reviewed Jun 25th, 2018 by g7700

Brilliant reciver, amazing SQ packs a fantastic punch and it looks sublime

rating
Reviewed Apr 16th, 2017 by Bellavita

Excellent Reciever

rating
Reviewed Nov 04th, 2016 by jawbone

Amazing...a bit temperamental with low ohm speakers

 

Comments

re: G-7700 help

Hello,
sansui g7700 sound level problem. When the volume is around 3/1, the output transistors fail. Can you help me with this?
Fatih

re: G-7700 help

If this a recurring problem, make sure the bias is correct first. If the bias is correct, it could be going into oscillation. Oscillation might be due to an open compensation capacitor.
Check things like the 4.7 ohm resistors in series with the output transistors' bases. If you got replacement transistors from eBay, there's a good possibility of counterfeit devices and this could cause major problems. The current Sanken 2SA1695 will work for the 2SA1106 and the 2SC4468 is the replacement for the 2SC2581. The driver transistors will likely be damaged if the outputs are blown. Use Sanken 2SA1859 for the 2SA968 and 2SC4883 for the 2SC2238. I have seen odd things like reduced breakdown voltage on transistors that have been over stressed, which could cause this behavior. Also, check the bias adjustment control and the STV3 stabistor. Always start with the simple stuff first!!! Check voltages, then components. This amp won't drive speakers rated for less than 8 ohms particularly well. There's probably more I should tell you and more details I need to know... Good luck with the repair and I hope this helps!

Also, you can get genuine Sanken transistors from www.digikey.com.

re: G-7700 help

Hello,
First of all, thank you for your help. yes problem recurrent problem. The DC offset is near zero but wavy. Bias current 3.3mV, in the range of 4mV. All capacitors and resistors and transistors on the board have been renewed. 4.7 ohm resistors were renewed. I'm not sure if the transistors are fake. I'm not sure about STV3, it looks normal in measurement. Voltages are normal.

re: G-7700 help

That bias level should be fine. The wavy DC offset might be a hint that it's oscillating but this could simply be a trim pot that needs cleaning or replacement ( and hopefully, not the input FET). Do you have an oscilloscope? If not, often an analog VOM that is passive will respond to signal on a low AC volts scale. Is this happening to both channels? Are your speakers less than 8 ohms?

re: G-7700 help

I measured, the speaker is 7.8 ohms. I have an oscilloscope. I also have the Sansui G5700 and has the same malfunction. In the G5700, the transistor does not fail, but the relay cuts. Both amplifiers have the same error on both channels. I'm looking forward to your suggestion. I'm sorry to keep you busy. thanks.

re: G-7700 help

What could be happening is that your speakers are reactive. In other words, they're 8 ohms but at some frequencies, they reflect power back into the amplifier. Example, a 1 millihenry inductor as a load would have an impedance of 8 ohms at about 1.27kHz but the peak current would occur at the zero crossing point. Loudspeakers can exhibit this behavior, too. If your speakers have a 60 degree shift at 8 ohms (perhaps the woofer just below its resonance) in the case of the G7700, nearly 5A can come back into the amplifier from the speaker. This would make for 300W of heat dissipated in the each half of the output transistor array. Although it would only last a few milliseconds each time, it could easily destroy transistors depending on their safe operating area and thermal response. (If you want to learn more about thermal response, check On Semiconductor technical note AN-569) There's way too much for me to explain here. One good option is to simply place some really heavy duty transistors in the amp, if this is the case. I'd go with the Sanken 2SA2151 and 2SC6011. Though the heatsink is the same, the average dissipation is still low enough that during music playback it probably won't run too hot and the suggested devices should handle the peak thermal load better. Your explanation of the speakers tripping the protection in the G5700 leads me to believe the problem is the speakers are quite a reactive load...

re: G-7700

I love my G.

re: G-7700

how do I open this?

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