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Tracking low level hiss in sx-838, audio probe sensitivity?

G

Gobstopper

Active Member
I finally finished my sa-7100 with help from folks here on the forums and was able to revisit an admittedly small, but annoying, problem with my sx-838. I first posted regarding the issue a couple of years ago,

http://audiokarma.org/forums/index....l-with-it-or-possible-to-troubleshoot.714847/

but never resolved it. With some more experience underneath my belt, I thought I would take another go at it.

In summary, listening through headphones there is a much higher-than-normal level of hiss in both channels, compared to other receivers/amps from the same era. it's noticeable between songs and during any quiet periods in the music.

Removing the pre-out/main-in jumpers gets rid of the hiss, and if I only remove one jumper, hiss is gone in that channel but remains in the other. Engaging the muting filter and high-cut filters also gets rid of it, so I believe that would indicate the pre-amp or thereabouts.

My idea was to use an audio probe to trace the hiss and find the source, but my problem is that I can't hear it at the power-amp inputs or anywhere upstream of that. Only at the outputs of the power-amp and downstream(protect board, speaker switch and headphone jack).

Further tests included sending an audio signal directly into the Power-Amp, and also sending audio from pre-out to another unit. In both cases I couldn't hear the hiss.

Would it be reasonable to think that the hiss is still coming from the pre-amp area, but that it's low level enough that the audio probe can't pick it up? I'm not sure why the power-amp would amplify it, but a different unit wouldn't.

I still have access to a scope, and can try that, but wasn't sure if my inability to hear it with an audio probe would suggest that a scope wouldn't pick it up either.

Any suggestions would be very welcome. I found someone nearby who can do an alignment, so this unit has almost finished it's 10 year journey to full restoration!
 
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Possibly the 0.01uf cap on the audio probe is blocking the hiss, maybe change to 0.1-1uf film eg, MKS2... in any case
since you have access to an oscilloscope suggest you use that, will try and get some test points. The Pre out/Main in
results have me thinking that the external/co-operating amp is providing something (grounding???) that mutes the hiss.
Wouldn't read anything into muting & high filter switches as this is their purpose, ie, mute and rid HF.
 
Set sprs to A, volume to minimum, connect probe GND to chassis
and probe to L or R speaker post, slowly wind up volume and adjust
volts/div scale until you can "see" the hiss (for future reference)

Remove pre-out/main in jumpers

Insert probe into pre-out RCA and check for hiss, will need to significantly
adjust the volts/div scale since signal has not been through the power amp...

Insert probe into main-in RCA, check for hiss (hopefully none)
 
Thanks so much! Which board(s) for pins 7,8, 27,28? You mention a pic, but I don't see one attached. I will do the other steps as suggested and report back.
 
I would just shotgun the preamp board, transistors and caps, though it’s probably transistors causing the hiss.
 
If it was not done when the unit was serviced, there are several instances of 2SA725s and 2SA726s in the main amp and equalizer amp of the unit which are known noise-makers. There are also a couple of 2SC131s in the equalizer amp which are prone to generating cyclic noise, and 2SC1344s in the MIC amp which are reputed to produce static and random static bursts. There are also the 2SC1451s used as pre-drivers, and they are prone to creating static and going open-circuit, potentially taking out the drivers and output transistors.
 
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Thanks everyone. I recapped the unit several years ago (following the list from MTF and others), and replaced most of the transistors as well at the time (I think there are still some original transistors on the protect board). I'll try the scope on the suggested locations and also try cleaning the jumpers.
 
Admittedly it was rather late, but first pass with scope and I couldn't "see" the hiss at points other than downstream of power amp out. I tried turning up the volume knob to amplify hiss with jumpers in place, but re-discovered that if volume knob goes past a certain point, FM signal become audible, even if source not set to FM, so it was overshadowing the hiss on the scope. Not sure what this means...
As for scope, I'm plugging it into a power strip that is shared with receiver, and tried both AC and DC coupling, ground probe on chassis.

I'll try cleaning jumpers and corresponding RCA plugs. Would deoxit or something else be recommended?
 
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