SX-838, odd whooshing sound. Not present when bypass pre-amp

[Gobstopper]

Hello Everyone,
I've been off the forums for sometime, and decided to go back and revisit my old SX-838 which I thought was mostly fixed for a bit (other than the FM), but started up with new symptoms. It's hard to describe, but a few minutes after turning the unit on and playing music through it, there is an odd whooshing in the background(aux, fm, tape). It's not static, and is independent of me fiddling with any switches or knobs. Last time I tried it a few years ago, I could hear some bleed through of FM while using other inputs, not sure if related or not. If I plug a source directly into the main amp(via jumpers in the back) the sound is clear. Thus, the problem is upstream of the main amp. Does anyone have advice on where I should start probing voltages or other things to look for? My plan going in was to test voltages on the control amp board(AWG-029), but I looked in the service manual and there aren't any voltage listed as there are for the main amp. I also looked through the forums but didn't find any too many specifics on troubleshooting the boards upstream of the main amp.

Thanks in advance,
Gabe

 

[tsd71]

There are old tired caps and trannies that need to be upgraded on the control amp.

 

[Gobstopper]

There are old tired caps and trannies that need to be upgraded on the control amp.

I found some crappy solder joints(the unit was the first one I started "repairing" many years ago), so I know I changed out some caps and trannies at the time, could probably find the specific pieces. Other than the idea of a blanket replacement of parts, any other troubleshooting tips?

 

[tsd71]

I'm pretty sure if you replace the caps and transistors on this board your issue could be resolved. Transistors if they are dead or dying when they heat up they can cause these exact symptoms. Also if it's been sitting for a few years it needs a good Deoxit bath.

2SA725=KSA992
2SC1312=KSC1845

 

[Gobstopper]

I checked my old postings, and I had replaced many caps and the listed transistors a few years ago on the control board, and haven't used it since. I fixed up some of the solder joints, and now the background whooshing isn't present unless I move the bass knob. However, there is a ton of hiss coming in again (http://audiokarma.org/forums/index.php?threads/sx-838-hiss-jumpers-and-bad-caps-or-not.314914/)
The high-cut filter removes a lot of it, so it's upstream from there. I will try cleaning the pots and looking at what boards I didn't mess with last time(and double check those that I did)

Thanks

 

[Gobstopper]

update: I took the control board out. I had replaced the 2sa725 and the 2sc1312 transistors in the past, but it looks like I was incorrect about replacing any caps. I'll go ahead and order some parts. Should I order replacements for the Q1's (2SK30) while I'm at it?

 

[tsd71]

Leave the 2SK alone, are the transistors installed correctly and what did you use for replacements?

 

[Gobstopper]

Thanks for the response. I had to look up what I did: I switched out the 2sa725's and 2sc1312's with 512-KSA992FBU and 512-KSC1845UBU respectively
I got the part numbers from MarkTheFixer when I did it back in 2010. I'm pretty sure the transistors are in correctly, but will double check, I thought it was working better for awhile, but seems to have gone downhill in the intervening years.

 

[Gobstopper]

I found the list of capacitors for the control board and am getting ready to order the parts. I was looking at the power supply board, which is right next to it, and was wondering if I should swap out the capacitors on that board while I'm at it. Last time I took the voltage readings(2010) things were okay, but I don't know if a pre-emptive cap change is considered okay or frowned upon. I'm attaching a photo of the board. The foil on some of the caps looks a little loose up top, but don't know if that is normal. There is also a dimple in the ceramic cap(circled in red) Also circled in red is an ancient blackened area near the two resistors at the bottom of the screen. Should I test anything in there before ordering parts?

 

[larryderouin]

At the age the 838 is with original caps, a pre-emptive capacitor replacement is pretty much recommended along with the problem transistors.

 

[Gobstopper]

Thanks Larry. I'll add some more caps and some transistors to my shopping list!

 

[Gobstopper]

Swapped out all the caps on the control board and, for my short test at least, it was soooo much quieter than before! Next up is the power supply, then after that replacing some caps and some transistors on the power amp that I ignored last time. This unit may have a second pass at life(if I can get the tuner aligned for a reasonable price nearby)! I had one question on some caps I ran across on the control board. These were the 25v 10uf (C7,8,29,32, etc) and the 50V 2.2uf(C39, C40). I hadn't seen these before and didn't realize they were caps when first looking at the board. What kind are these? I also wasn't sure how to double check that the polarity markings on the board itself were correct since these don't have the negative side marked like they are on the electrolytics.

 

[larryderouin]

Those are tantalum caps. If space is tight, use Nichicon UKL Series electrolytics. .
2.2uf 50v
10uf 50v