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sx-838 background hiss, should I just deal with it, or possible to troubleshoot?

G

Gobstopper

Active Member
Hello Everyone,
Back again with some follow up questions on my Sx-838. It seems as the big jobs(replacing caps and transistors on control board and power supply) get done I find little things to nitpick about!

I am saving up for a tuner alignment, but in the meantime I as using my sx-838 as a headphone amp(my small marantz that was in that role dropped a channel). I was testing it out after cleaning the aux push button and now when playing music everything sounds fine, but when the source is not playing there is a low level background hiss(on all inputs) that is not present on my Concept 7.5D(as a comparison).

It gets a bit louder if I turn up the volume knob, seems to be the same in both channels, the Hi-cut filter kills the hiss, and if I remove the pre-main jumpers the hiss goes away. The jumper effect would imply that the problem is in the control amp correct? Given that I replaced the caps and transistors on that board, are there any other usual suspects to check? I poked around on the forums and it seems that resistors can cause problems as they age, but I don't know if it makes sense to replace every resistor on that board. I also read about using scopes to locate the issue, but I don't have access to one unfortunately. It's not very noticeable when I turn on the speakers, but as I mentioned above this is currently my bedside headphone amp.

It's not the end of the world, and only really noticeable when the source is quiet(between songs for example), so I can live with it, but thought I would check to see if there are obvious things to check.

Thanks in advance!

Gabe
 
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Did you replace the 2SC1312s on the control amp board? What about the 2SA725s? They are notorious for noise. If you did not use a low-noise replacement device the problem mat still be with those devices.
 
Could also be in the EQ amp. If it's before the jumpers, work back towards the inputs, checking all boards that have either 2sa725, 2sa726 and/or 2sc1312 or 1384. DeOxit and exercise ALL pots and switches, again if done already. Especially the source recording switch.
 
Thanks Watthour and Larry! On the control board I switched out the 2sa725's and 2sc1312's with 512-KSA992FBU and 512-KSC1845UBU respectively, but in 2010, well before this round of replacing parts. I will give a go at de-oxiting all the pots and switches again and I'm pretty sure I replaced all the caps and transistors on the EQ board as well(back in 2010), but will review to make sure.
 
Before running in circles, short the input and see if the noise goes away. If it DOESN'T, it's internally generated and go to the next step.

USING HEADPHONES, listen to the hiss - if it is centered in your head, it's mono and something common to both channels is causing it.
If it is stereo hiss it will have a much wider effect, and thus there is TWO hiss sources, one per channel.
You DID use UKL caps where appropriate??? And one of the four recommended cap types in the other locations?
 
Hi Mark, Thanks for suggestion. Shorting the inputs(ran an RCA from Aux R to Aux L) did nothing to hiss. The hiss seems to be centered in my head. Some additional tests showed that moving the balance pot does nothing, and increasing the treble increases the hiss. I followed the standard lists on this site for replacing caps and transistors.(http://www.audiokarma.org/forums/index.php?threads/need-sx-838-recap-list.439828/) Looking at my mouser packing list, On the control board I used UKLs for C (7,8,29,32), (15,16) and (39, 40)
and UPWs for C(4,3,37,38) and (13,14,35,36)
 
In your 1st post you said the hiss goes up some with operation of the Volume control. As the Volume and balance controls are BEFORE the Control Amp board, that's ruled out. The problem is in the Mic amp OR the EQ Amp.

Also it could be dirty switches in the Function switch boards aws-073-1/3, AWX-070 Switch and Volume assy board. The Switch and Volume assy is the last BEFORE the CONTROL AMP.
 
Last edited:
Gobstopper said:
Hi Mark, Thanks for suggestion. Shorting the inputs(ran an RCA from Aux R to Aux L) did nothing to hiss. The hiss seems to be centered in my head. Some additional tests showed that moving the balance pot does nothing, and increasing the treble increases the hiss. I followed the standard lists on this site for replacing caps and transistors.(http://www.audiokarma.org/forums/index.php?threads/need-sx-838-recap-list.439828/) Looking at my mouser packing list, On the control board I used UKLs for C (7,8,29,32), (15,16) and (39, 40)
and UPWs for C(4,3,37,38) and (13,14,35,36)
Click to expand...
mark meant short the the inner to outer rca connectors basically grounding the inner hot signal connection to ground . not patch left to right .
 
Gotta get some sleep. Totally missed that, pete.

Gob; take an old RCA and cut the ends off with about 3" of wire extended. Bare both wires and twist together on each end. Now you have a shorting plugs. Make 2. Insert in the AUX jacks and listen.
 
Thanks everyone for the suggestions. I was trying to google "short your inputs" but obviously I found the wrong answer. Will try to remember that it's safer to appear dumb rather than mess something up! I've never touched the Mic Board but re-did the EQ amp some time ago, hmmmm. I'll sacrifice some RCA cables, try it out and report back.
 
Okay, did the correct "short your inputs" and the hiss is still there. Any way to narrow down the source to a specific board?
 
The hiss is all over the thing, and you can't get it away with new parts - it's just your typical 70s design. Most such amps have it, even the high end units. It just seems most people on AK cannot hear it, or don't have good enough speakers. Get an 80s amp or newer, and it won't have any hiss.
 
are there any original transistors or electrolytic capacitors left in the unit?

You see, an e-cap with leakage will usually manifest as noise, as well as noise from old transistors.

the eq amp / mic amp etc is quite unique, it has different feedback networks to shape gain and equalization - it may even run FM through it!!
These feedback networks run through SWITCHES!!! and if those switches are misbehaving - well, YIKES!!!
 
Hi Mark,
Off the top of my head ,the protection circuit has most of the original transistors and some original caps (including the CSSAs) , the mic board has not had any replacements at all, (EQ amp was redone) and the Power amp still has Q11-14 as stock, and possibly the 2SB528/S2D358s still on there(but hiss not present when remove pre/main jumpers) and the Tuner board is mostly stock, minus some caps we replaced with films when troubleshooting my initial tuner problems 6 years ago. I've never had a chance to test the recording selector switch, but I suppose I could possibly do that with the tape monitor?
 
I agree with ZOOM, l have quiet a few 70's amps/receivers and they ALL have a slight background hiss to some degree, don't think you would ever get it as quiet as a later design.
 
(but hiss not present when remove pre/main jumpers)
suggests the hiss is NOT in the main amp .
did you try shorting aux ins and selecting aux ?
is the hiss equal on both channels ?
 
Hi Pete and everyone else for responding. I did the shorting plugs test (post #11 above), and the hiss is equal in both channels(balance knob does nothing). As to the comments on hiss being inevitable in equipment from this era; I don't expect complete silence(I know this stuff is about as old as I am!), but I have other 70s equipment to compare to and this is well beyond normal. Also, given that the hiss is gone when I bypass the pre-amp, I have a reference to compare against within the same unit. I'll keep at it some more before admitting defeat :) In response to MTF's question on original electrolytics still being present, I forgot to add that the main filter caps are still stock. I've always been a bit intimidated with replacing those monsters!
 
Fair enough, can see why you would want to track down if hiss is well beyond that of your other units. Let us know how you go.
BTW my sx 939 isn't my best unit through the headphone jack.
 
Thanks again, This unit was the first one I ever started working on, and it would be something of a milestone to finally have it back up to snuff. I will keep everyone posted!
 
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