Sansui 771 hiss

[quadmatrix82]

Hi sansui experts, I recently purchased a mint 771 receiver that wasn’t working, after a recap and replacing all the 2sa726/2sc1313 transistors and zener diodes it’s working very sell however I do notice quiet loud hiss on without music playing I am wondering if the transistor substitute chosen are causing this issue?

I used ksc1815 in place of 2sc1313 on the tone board note gain was approx 211 instead 560 of originals - should I replace these with ksc1845 instead given the higher hfe available?

I used ksc1845 on the equaliser board in place of 2sc1313 approx 430 gain vs 560

Any thoughts or recommendations here would be appreciated Thankyou

 

[CrSens]

My recapped 771 also has slight hiss, I was never able to resolve it. It's not the replacement transistors causing it, that I know. My suspicion is on some high value carbon resistors in preamp stage (creating very high gain for one stage circuit), but I never got around to replacing them.

 

[quadmatrix82]

My recapped 771 also has slight hiss, I was never able to resolve it. It's not the replacement transistors causing it, that I know. My suspicion is on some high value carbon resistors in preamp stage (creating very high gain for one stage circuit), but I never got around to replacing them.

Thanks I might have a look at resistors nothing looked bad upon visual inspection but might test some

 

[CrSens]

Thanks I might have a look at resistors nothing looked bad upon visual inspection but might test some

It's the nature of the carbon resistors, they can get noisy, especially in high gain circuits. They might measure fine for value.

 

[mrk229]

Hi sansui experts, I recently purchased a mint 771 receiver that wasn’t working, after a recap and replacing all the 2sa726/2sc1313 transistors and zener diodes it’s working very sell however I do notice quiet loud hiss on without music playing I am wondering if the transistor substitute chosen are causing this issue?

I used ksc1815 in place of 2sc1313 on the tone board note gain was approx 211 instead 560 of originals - should I replace these with ksc1845 instead given the higher hfe available?

I used ksc1845 on the equaliser board in place of 2sc1313 approx 430 gain vs 560

Any thoughts or recommendations here would be appreciated Thankyou

You did not mention if the hiss is always there or just on certain inputs.

I would put the 1845's in place of the 1815's. I don't have a 771 any more to look and see if these resistors on the F-1504 board are good old carbon comps or not. Worth a peek if you are changing the transistors.

 

[The Fuxtor]

I'm unfamiliar with the 771, do they use the BA312 chip by chance?

 

[CrSens]

I'm unfamiliar with the 771, do they use the BA312 chip by chance?

No.

 

[CrSens]

If it's anything like my 771, it is present on all inputs, and varies widely by the volume control position, starting low, than getting higher as you advance to 9-12 o'clock, than drops down as you continue to max.
Muting button completely stops it.

 

[quadmatrix82]

You did not mention if the hiss is always there or just on certain inputs.

I would put the 1845's in place of the 1815's. I don't have a 771 any more to look and see if these resistors on the F-1504 board are good old carbon comps or not. Worth a peek if you are changing the transistors.
View attachment 3310611

Thankyou, the hiss is there all the time even when volume is zero but raises with volume adjustment.

I’ve had 771 and 661 before and they do hiss a bit but this seems more elevated than others I’ve had previously.

I don’t see any carbon resistors they seem to be metal film types

I replaced the tantalum caps 1.5uf and 2.2uf as I had previously left those i used tantalum and ukl Nichicon low noise replacement this has helped a bit I believe.

I’ll try replacing ksc1815 with ksc1845 and report back

 

[Dom123]

Been listening to my 771 this week. It's dead silent with no hiss. Sounds phenomenal to my ears
I used KSC1845's throughout (were applicable), no KSC1815's in there.

I also changed out the diff pair (2SA640's) on the driver board last week with KSA992's, and I must say the sound is even better.
I think they were the last of the original TO92 transitors remaining in the amp.

 

[mrk229]

The Sansui manual for the 771 lists those resistors as C.R. type (Carbon Resistor). I imagine they are carbon film. I found some interesting information on resistor noise here.

Resistor Noise | Resistor Fundamentals | Resistor Guide

What is Resistor Noise? Noise is an unwanted phenomenon for resistors. For some applications the noise properties are important. Examples are high gain amplifiers, charge amplifiers, and low-level…

eepower.com

I grabbed a chart from that page that interestingly enough shows that carbon film resistors are only a little better than carbon comp types for noise.
They don't tell you if metal film resistors are the "Thin Film" type, but that is my assumption.


Might be worth swapping those high value ones I highlighted earlier, if you feel like experimenting.

Also there are several transistors in the drive stage that are known to have gain deterioration with age.

 

[quadmatrix82]

The Sansui manual for the 771 lists those resistors as C.R. type (Carbon Resistor). I imagine they are carbon film. I found some interesting information on resistor noise here.

Resistor Noise | Resistor Fundamentals | Resistor Guide

What is Resistor Noise? Noise is an unwanted phenomenon for resistors. For some applications the noise properties are important. Examples are high gain amplifiers, charge amplifiers, and low-level…

eepower.com

I grabbed a chart from that page that interestingly enough shows that carbon film resistors are only a little better than carbon comp types for noise.
They don't tell you if metal film resistors are the "Thin Film" type, but that is my assumption.
View attachment 3310799

Might be worth swapping those high value ones I highlighted earlier, if you feel like experimenting.

Also there are several transistors in the drive stage that are known to have gain deterioration with age.

View attachment 3310798

Thanks for this that’s helpful, ok I’ll try the resistors and see how it goes…those transistors you highlighted I have changed already with mje15032/33 my amp didn’t have the 2sa640 but 2sa726 instead I replaced these with 2sa970bl

 

[quadmatrix82]

I

If it's anything like my 771, it is present on all inputs, and varies widely by the volume control position, starting low, than getting higher as you advance to 9-12 o'clock, than drops down as you continue to max.
Muting button completely stops it.

Interestingly with the muting button selected on my one becomes dead silent also no hiss after that I will investigate the resistors…

 

[mrk229]

I

Interestingly with the muting button selected on my one becomes dead silent also no hiss after that I will investigate the resistors…

The muting button is after the pre-amp so the noise is coming from the pre-amp. It will be interesting to see what happens when you swap the 1845's for the 1815's, and further if you do replace those resistors.

 

[quadmatrix82]

Ok so I replaced the resistors, the 820k read low at 800k the rest were good
I replaced tr01/02 with ksc1845 approx 450hfe instead of ksc1815

Reassembled it’s marginally better but definitely not as quiet with the mute engaged and volume cranked there’s noise there still which flattens out at half way volume

 

[mrk229]

Ok so I replaced the resistors, the 820k read low at 800k the rest were good
I replaced tr01/02 with ksc18145 approx 450hfe instead of ksc1815

Reassembled it’s marginally better but definitely not as quiet with the mute engaged and volume cranked there’s noise there still which flattens out at half way volume

That is somewhat disappointing, I was hoping for a light bulb moment. It certainly does not leave much else to look at. The muting switch is a -20dB attenuator which will lower both the the audio and the noise by that amount. Which tells me/us that hiss is coming from a source in front of the attenuator switch.

Seems like it is not noisy carbon resistors. The only other active devices you have not replaced are the 2SK24 FET's.

You have the +33 volt power supply rail, if you have a scope look for any HF noise riding on the DC voltage. Not likely but possible.

The audio from the selector switch goes through switching contacts in the mic jack, another really long shot but easy enough to try.
With the power off or the volume all the way down run a 1/4 plug in and out of the mic jack a bunch of times to exercise the switching contacts.

Another easy test put two shorting plugs in the Aux input jacks and see if the noise is still present with the selector set for Aux. (probably will be)

After that

 

[quadmatrix82]

That is somewhat disappointing, I was hoping for a light bulb moment. It certainly does not leave much else to look at. The muting switch is a -20dB attenuator which will lower both the the audio and the noise by that amount. Which tells me/us that hiss is coming from a source in front of the attenuator switch.

Seems like it is not noisy carbon resistors. The only other active devices you have not replaced are the 2SK24 FET's.

You have the +33 volt power supply rail, if you have a scope look for any HF noise riding on the DC voltage. Not likely but possible.

The audio from the selector switch goes through switching contacts in the mic jack, another really long shot but easy enough to try.
With the power off or the volume all the way down run a 1/4 plug in and out of the mic jack a bunch of times to exercise the switching contacts.

Another easy test put two shorting plugs in the Aux input jacks and see if the noise is still present with the selector set for Aux. (probably will be)

After that

Thanks for your help I appreciate it, certainly a mystery this one…I tried the short plugs and cleaning the mic input but it’s the same. I realised I haven’t replaced the 35v zener on the power supply board is that worth a shot? I don’t have any 35v on hand but probably two other zeners I could put together to make 35v if you think it’s worth a shot. I could reflow the fets also? The joints look good however? Does the equaliser board come into play here? I don’t have a scope unfortunately but can take voltages where needed

 

[mrk229]

Thanks for your help I appreciate it, certainly a mystery this one…I tried the short plugs and cleaning the mic input but it’s the same. I realised I haven’t replaced the 35v zener on the power supply board is that worth a shot? I don’t have any 35v on hand but probably two other zeners I could put together to make 35v if you think it’s worth a shot. I could reflow the fets also? The joints look good however? Does the equaliser board come into play here? I don’t have a scope unfortunately but can take voltages where needed

If you have a couple of zeners to get 35 volts plus or minus a volt or two it is worth a shot. The EQ board only comes into play for the phono input, which I would expect a little noise out of. I don't think you need to re-flow the FET's.

 

[quadmatrix82]

If you have a couple of zeners to get 35 volts plus or minus a volt or two it is worth a shot. The EQ board only comes into play for the phono input, which I would expect a little noise out of. I don't think you need to re-flow the FET's.

Ok thanks, I’ll try tomorrow nothing to lose!

 

[CrSens]

I was thinking also R31/32 (100K) could be contributing to noise. The last thing is. the volume pot itself, not sure how, but I've seen noisy volume pots.