• Please note that there are a few updates and clarifications made in the Audiokarma Rules, mostly relating to advertising and the addition of the new "Paying it Forward" & "Giving back" forums in the AudioKarma Audio Marketplace section.

Dead L Channel in Realistic STA 850... HELP!

W

wilhausmin

New Member
28F18CAA-AFFF-4483-AD1D-2970ECC95F47.jpeg 1C497CBA-263E-477F-AD07-84CBC06C037B.jpeg 6C9E7C9C-6671-4F04-8445-E88F6083D355.jpeg Hi all,

I just got this receiver from my dads garage and want to get it up and running. I’ve checked all my inputs but still have a dead channel regardless of source (phono, fm, aux).

There is not read for the output on L when pumping the volume. I did notice that when no headphones are plugged in and no speakers are selected the L channel gets some reading. I am assuming it’s an internal hum? There’s a barely audible one when headphones are plugged in on L, and it doesn’t get louder regardless of volume level.

I’ve checked the fuses and they look good.

But.... I also have no idea what I’m doing or what I should be looking for.
I’d appreciate any help!
 
Register to hide this ad
The service manual & schematic are available on hifiengine (need to register, it's safe).
The question is whether you can follow the sm and locate parts, test points etc... Most
channel problems are caused by dirty switches or dirty connections, cleaning often helps.
If you have a multimeter then you can continuity check the switches to work out which
has failed. The next step might involve taking voltage measurements on a live unit, how
comfortable are you with that (lethal voltages inside)...

Suggest you download the service manual and find yourself a multi-meter, I will try and
find some test points for you.
 
Try to post some hi-res pix's, the above photos are low res, need to check for track damage, solder joints,,,
 
Try connecting a source (eg, CDP, ipod,,,) to TAPE IN rca's
and set tape monitor switch ON. Check L&R channel for audio (headphones).

With amp powered off unplugged (repeat, unplugged) AUX selected, tape monitor off,
measure the resistance,

Between the right AUX RCA inner/female and pin 30 (or R904 or R906) as highlighted
Between the left AUX RCA inner/female and pin 33 (or R903 or R905) as highlighted

expect less than 4 ohms in both cases.

upload_2019-8-4_14-17-11.png
 
mbz said:
The service manual & schematic are available on hifiengine (need to register, it's safe).
The question is whether you can follow the sm and locate parts, test points etc... Most
channel problems are caused by dirty switches or dirty connections, cleaning often helps.
If you have a multimeter then you can continuity check the switches to work out which
has failed. The next step might involve taking voltage measurements on a live unit, how
comfortable are you with that (lethal voltages inside)...

Suggest you download the service manual and find yourself a multi-meter, I will try and
find some test points for you.
Click to expand...

I’d really appreciate the guidance. I tried looking at the schematics and it’s pretty alien to me...
 
I took a multimeter to those points and both are below 4ohms.
I had a tape deck connected and am still missing the L channel. thanks again for your time!
 
Thanks for the high res pix, while some hacks are present the board is generally ok, just needs a clean up.
4 ohms is fine for those test points, along with the tape monitor test it confirms function selector basically ok.

You've probably got a failed transistor or... resulting in the failed channel, you will need test equipment to track
it down, an oscilloscope would be handy otherwise you need to diy...

Suggest build a simple audio probe.
You will need an old speaker/driver typically like those found in old hand held/portable radios of yesterday.
Do not use a good spkr, just a junk item. Connect one/any side of this speaker using a length of insulated wire, to
any black speaker post of the amp being tested. Connect the other terminal of this speaker to a capacitor, used
to block VDC, this cap should be 0.1-1uf and 250V or higher, any film cap/greencap handy will do. Connect the
other side of this cap, via a length of insulated wire, to an aligator clip, or minni grabber or similiar. The idea being
to put the clip on a test point and listen through the spkr for audio. The sequence being, amp power down, clip
onto test point, supply audio, listen for audio through test spkrs (may be very quiet), power down and move to
next test point.

Try the following test points,
Pin 30 /R904, 906 as before. This is the working channel so you should hear audio

Pin 33/R903/905 as before. Continuity test suggest you should also have audio.

R422 (red/red/red bands) as per pix below, you will need to turn the volume up. Expect audio/working channel

R421 as per pix below, audio?
 
R421, R422 which is infront of cable or connector, near balance control. Resistors should be coded red/red/red/...

Take care not to short anything out with the clip/only clip to resistor...

upload_2019-8-8_15-2-51.png
 
I'm starting to feel a little in over my head! I'll keep my eyes out for some junked speakers in the meantime. Thank you so much!
 
Sorry for the sequence am I doing: amp power down, clip
onto test point, then power on here? supply audio, listen for audio through test spkrs (may be very quiet), power down and move to
next test point
 
wilhausmin said:
View attachment 1580949 View attachment 1580950 View attachment 1580951 Hi all,

I just got this receiver from my dads garage and want to get it up and running. I’ve checked all my inputs but still have a dead channel regardless of source (phono, fm, aux).

There is not read for the output on L when pumping the volume. I did notice that when no headphones are plugged in and no speakers are selected the L channel gets some reading. I am assuming it’s an internal hum? There’s a barely audible one when headphones are plugged in on L, and it doesn’t get louder regardless of volume level.

I’ve checked the fuses and they look good.

But.... I also have no idea what I’m doing or what I should be looking for.
I’d appreciate any help!
Click to expand...

Well if you don't know what you are doing, you shouldn't be poking around until you learn a few things. You could get shocked :yikes: Or you could short something out and really have problems. However, with that said (you've been warned), look closely for any dark, or burned components. There are some red colored resistors around the big heat sink. If any of them look "dark" or burned, you have a blown channel. I recently rebuilt one of these 850 and it had a blown channel. There were numerous "bad" components and some do not look "bad" at all. Fortunately on the underside of the board, everything is labeled such as "Left Main" and "protection", etc. and the components are labeled including Base, emitter, and collector of transistors., etc. Look for anything like this photo shows.
 

Attachments

  • Burned resistors.jpg
    Burned resistors.jpg
    63.2 KB · Views: 23
wilhausmin said:
I’ll definitely take a look! What do you think about the 850?
Click to expand...

Well, once I was done with new output transistors, driver transistors, some resistors and diodes, and all new capacitors, it sounded very good! But yours may not be as bad as this one was, and may be a bit easier to get going. The owner was "emotionally attached" to this because it was her late Father's and wanted it repaired no matter what. So she didn't mind putting a couple hundred into it. One bad thing about this model, is you must remove the heat sink, output transistors, and BIAS diodes as a unit to gain access to a few things "hidden" by that. Otherwise you play hell for getting to just a couple of small items. But it's not hard to do, just a little time consuming.
 
Also, look for any of the electrolytic capacitors to have a "domed" top. (the small upright "can" looking things with a darker colored (sometimes black) stripe). The light blue one in about the middle of that photo (C511) is blown.
 
  • Like
Reactions: mbz
You need to take a better and a close-up photo of about the same area as mine, and one if possible of the left side. Your photo is too blurry, but it looks like you may have at least one burned resistor.
 
mbz said:
The next step might involve taking voltage measurements on a live unit, how
comfortable are you with that (lethal voltages inside)...
Click to expand...
Warning in post #2

The test sequence basically comes down to powering off/unplug the unit before you connect/disconnect
any clip etc... Once you are happy with the connection then ok to power up and supply/test for audio.
 
You must log in or register to reply here.
Back
Top Bottom