SX-838 Signal meter is stuck in high position

G

Gobstopper

Active Member
Hello Everyone,

I recently scavenged a SX-838 from a bulky waste pile. I cleaned all the switches and pots, and the unit works reasonably well with the Aux input. I just noticed the other day that the Signal Meter is stuck well to the right of the "5" mark, even when the unit is unplugged and has been off for some time(picture below). Turning it on and playing with the tuner doesn't do anything to it either. While unit was off, I removed the front plate and tapped the plastic encasing the meter and the needle briefly deflected downwards The tuning meter responds to moving the tuning knob. FM reception is so-so, I can tune a station and about a second later the stereo light comes on, but the sound gets static-y and more "stereo".

Are these issues related? Any way to get the signal meter back down?

Thanks in advance,

Gabriel
 

Attachments

  • _MG_0517.JPG
    _MG_0517.JPG
    132.1 KB · Views: 75
Register to hide this ad
It appears that the meter is jammed. Repair is tricky, and requires a sort of watchmaker. Best to find a replacement, although you don't have much to lose if you can work on a delicate mechanism.

Perhaps the pointer is rubbing on the scale, easy to fix. Perhaps the bearing is cracked, not easy at all. Or maybe foreign matter is in there.
 
I would be more concerned with the reception.
Check out the cleaning an analog tuner section in Tuners.

The issues are related in that the receiver wound up in a waste pile. The previous owner probably gave up on it.

The signal meter problem can be separate from actual reception problems. Which is why I recommend trying to clean the tuning capacitor first to see if that improves your "Stereo" reception.

From there it may need a complete alignment at which time you probably want to have a tech look at it anyway. Then weigh the cost of repairs versus relegating it to a waste pile.
 
Thanks for the responses. I will take a look at cleaning the tuner. There was a bunch of crud on the flat metal surfaces inside, so it's possible gunk also fell in the tuner area as well. The signal meter issue isn't a big deal, I just wanted to see if there was an easy way to fix it.

It's frustrating that it's so difficult(in terms of cost) to align a tuner
 
So I tried cleaning the tuner and not much changed. Maybe if I describe the issue a bit more clearly someone might have an idea of what is causing the problem.

So: When using the FM mode on the unit, I will come across a station. At first it will be kind of muted, but not too much noise or static. Then about a second or so later, the stereo light comes on and at the same time the sound gets louder and brighter, and there is all kinds of noise and static along with the broadcast from the station.

Is this most likely an alignment problem, or is something more serious going on?

Thanks.
 
There is a test - on the back panel is a jack called "4 CH-MPX" out. It is the signal straight out of the FM detector chip, unaffected by any mute or stereo decoder chip weirdnesses.

You need to run a cable from it to the TAPE 1 PB right channel (input) jack.

Then by turning on the TAPE MON 1 switch (down) the right channel will be raw unmuted FM. Left channel will be silent unless you use a Y cable to connect 4 CH-MPX to the TAPE 1 PB right and left channel (input) jacks.

See how that acts as FM is tuned. (btw REC SELECTOR to "SOURCE").

Interstation hiss, tuned station (how loud?) etc

can a/b by flipping TAPE MON 1 up and down, without affecting anything else in the tuner or elsewhere...
 
Hi Mark,
Great suggestion!

I did as you suggested and found the following:

The direct, unprocessed-out sounded very clear when tuned in to a station, the stereo light kicked in about a second later again, but no difference in sound(expected I guess). I didn't hear any static or hiss in between stations. When I switched back to the regular output(by toggling the Tape Mon 1 switch), the volume actually was lower and all the static and noise was present again.

Where to next?

Thanks.
 
Now it starts getting a bit more complex.

We have to figure out if the ha1156 stereo decoder chip itself has a problem.

That interconnect cable going into the 4 ch jack has to be turned into a signal probe (add a 0.1uf DC blocking capacitor in series with the center conductor) and then the audio on pins 4 & 5 of the ha1156 stereo decoder chip have to be checked.

Follow the link to see what I am talking about, and where to get the capacitor... you probably have the cable... :D

The audio may be at a different level, but we are looking for the static and noise.

That static is on BOTH channels?

Now, if it is, staticy, we can try fiddling with ONE adjustment, with care you should come pretty close to the "sweet spot" without the aid of instruments - I have done so in the past when no instruments were available and it turned out to be close to the actual adjustment. Skill ? Luck? Both? :dunno:

You probably can't end up worse off, IF you follow directions, on that ONE adjustment.
 
Hi Mark,
I'll try the capacitor probe this evening when I get home. As for static, yes I'm pretty sure it's in both channels(working from memory).

Thanks again.
 
VR1 is the closest adjustment to the pin 19, 20 & 21 terminals going off the board. It adjusts the subcarrier frequency and thus the capture of the stereo signal by the 1138 chip.

Try adjusting that to get a better lock on a better sound. There will be a "range" of that adjustment that "clicks in", and you want to end up leaving that in the center of that range of adjustment.

Don't touch anything else, all VR2 does is a small separation adjustment, and VR3 is the signal meter deflection. VR4 and VR5 are AM...
 
Hi Mark,
I had to rush home today(concern over riots in Oakland) so I didn't have a chance to pick up the small caps you had suggested. I did try adjusting VR1.
I found that by turning clockwise (starting at about 30 degrees from where started to end of range) I was able to get rid of the noise and static, but that it also prevented the stereo indicator from turning on. Also it seemed that the level dropped a bit? Adjusting the VR back gave me the Stereo indicator, along with all the noise and static. I'll pick up the cap tomorrow and try out the chip probe.
 
Tried the chip probe this evening. Getting the lead wrapped around the RCA center pin was harder than I had expected..
When VR1 was in the original static-y position, I heard the static-y signal on pins 4 and 5 (and 2). When I changed VR1 so no static(nor stereo signal) the sound coming out of pins 4 and 5 (and 2) was clean.
 
There are 3 possibilities:

1. C37, C38 or C39 have a problem.

2. The internal "auto in" signal is compromised (another cap C31 is a possibility) or Q10 is leaky.

does the "FM muting" button have any effect upon these staticy / non-staticy symptoms?

3. the chip (part of it) itself is bad - although it passes a mono signal - when it tries for stereo - bleeech...

The fact that the 4 ch out is clear somewhat exonerates the rest of the FM, as well as the alignment of the FM sections.
 
I tested the FM muting today, no effect on either the regular FM output or through the Tape Mon 1. Are there any further tests to isolate the problem?

Thanks.
 
FM muting works when tuning in between stations, as the signal strength drops, the sound should be muted, and the stereo decoder suppressed. If I am correct the 4ch output also should be quieted.
 
Hi Mark,
You are correct, the FM muting does work for both outputs. I had confused the on and off positions and was listening in the wrong spot.
 
markthefixer said:
There are 3 possibilities:

1. C37, C38 or C39 have a problem.

2. The internal "auto in" signal is compromised (another cap C31 is a possibility) or Q10 is leaky.

does the "FM muting" button have any effect upon these staticy / non-staticy symptoms?

3. the chip (part of it) itself is bad - although it passes a mono signal - when it tries for stereo - bleeech...

The fact that the 4 ch out is clear somewhat exonerates the rest of the FM, as well as the alignment of the FM sections.
Click to expand...

How does the stereo signal sound when in between stations and not muted, is the static similar to the static when it has a stereo signal it is trying to decode? softer hiss, or harsher crackling?

right now I am trying to figure out if we should replace a few capacitors that might be bad, while still having a good chip, or to replace the chip, or to do both. The disadvantage is they come from different places, 2 shipping charges.

I think we have exonerated transistor Q10.
 
The noise between stations is more of a hiss, the standard noise one expects. On a tuned station, after the stereo light comes on, the sound is much more crackly and scratchy along with hiss. The most prominent effect is the distortion of the signal, almost like it's horribly clipping. So to answer your question, I'd go with a softer hiss in between stations. I'll see if I can record the output to an MP3 later this evening.
 
Gobstopper said:
The noise between stations is more of a hiss, the standard noise one expects. On a tuned station, after the stereo light comes on, the sound is much more crackly and scratchy along with hiss. The most prominent effect is the distortion of the signal, almost like it's horribly clipping. So to answer your question, I'd go with a softer hiss in between stations. I'll see if I can record the output to an MP3 later this evening.
Click to expand...

not necessary, your verbal description is enough.

let's try replacing the capacitors. This type have a tendency to go bad by shorting out, they are electrolytic caps, solid aluminum type. These replacements are "forever" replacements, with no degradation mechanism to worry about. mouser.com parts numbers. IF they had the chip, I would advocate egtting it because it would be cheaper than a second postage - and the place that has the chip - I don't like their caps. they would be too big to fit - these will be a bit large, but can be shoehorned in.

C37 0.33uf 50v 667-ECQ-V1H334JL $0.14 ea
C38 1.0uf 50v 667-ECQ-V1H105JL $0.36 ea
C39 0.47uf 63v 667-ECQ-V1474JM $0.22 ea
 
You must log in or register to reply here.
Back
Top Bottom