Gold Tecsonic MX-900 help needed!!! Left audio missing

[tec9]

Hey all-
I recently picked up this box, and I am having a problem with no audio from the left channel. Here are some details:
When I first picked up this box, I BELIEVE the left audio channel was kinda there, but very scratchy. I sprayed the volume and balance pots, and now the left channel is just about dead. When the volume is all the way down, you can slightly here the audio, but it will not go any louder, when you turn the volume up. I also noticed that the led VU meter for the left side is dead. Any ideas on where to look next??
-Mark

 

[floyd]

have you tried the aux inputs or headphones to see if all functions are affected? sounds to me like a problem in either the pre amp section or the amp itself, also try pressing record and stop several times on the tape deck because sometimes the record switch sticks causing similar symptoms.

 

[tec9]

I just checked the headphone jack, and when I have headphones plugged in, I get a very weak output from the left channel. It can be heard, but not very loud at all. I also tried pressing the record and stop buttons several times, and had no luck.
I'm thinking the pre-amp is the culprit at this point, and it could be tough without schematics.
Any other thoughts or advice on this??
-Mark

 

[docs]

What I did to check this was to jump the left and right channels on the amp to see if you get anying in the left. Check google for a schematic for the amp chip number and carefully see if you can get anything through the left. This might sound extreme but it saves you looking for hours at other things all the way back to the amp chip. If you can get sound that way, preferrably with a sound injector, then move backwards loooking at try joints and do some continuity testing with a multimeter.

 

[tec9]

Forgot to add that, the balance has zero effect, and when you have the mono/stereo switch set to "Stereo" the working channel sounds goofy and cuts somewhat on and off.
-Mark

 

[docs]

Sounds like the switches need cleaning to me.
Use some contct cleaner on them all thoroughly give them a roody good clean.

 

[tec9]

I have cleaned all the switches, and I still have the same problem with no amplified audio on the left channel. I have tried all the suggestions so far, but have had no luck. While troubleshoting this box, I have two guesses on what my next attempt should be-
1- The main amplifer IC (2 channel power amplifier) If I touch the solder pads on this with my finger when power is on, I can hear a hum/ buzz
2- I found 2 IC's that are 5 band EQ/ Amplifier IC's. There are two of them, so I would imagine that one is for the left audio and the other is for the right audio.
I was thinking of maybe ordering replacements, can anybody offer any more suggestions on what I can try??
Thanks-
Mark

 

[floyd]

sometimes the headphone jacks get dirty have you tried turning the headphone plug when its plugged in to see if there's intermittent contact ,just spin the plug at the end of the wire while you are listening to the headphones to see if there is static or bad contact.

 

[tec9]

sometimes the headphone jacks get dirty have you tried turning the headphone plug when its plugged in to see if there's intermittent contact ,just spin the plug at the end of the wire while you are listening to the headphones to see if there is static or bad contact.

I tried that, and I had no luck....I spun the headphone plug, and also put contact cleaner in it. No static or intermittent operation on the dead channel.
-Mark

 

[Oklahoma Wolf]

If you get buzzing on the left channel while touching the power amp chip leads, it's probably ok. I would try and find a datasheet on it anyway and try signal injection just to be sure, but so far to me it's sounding like the problem is upstream from there.

Before going any further, I'd go on a careful hunt for bad solder joints and clean any switches or controls that you might have missed. Most of the time, it's one of these two gremlins causing these kinds of problems. There's nothing worse than spending money for a part that may or may not be easy to find only to realize you just blew that money for nothing (been there, done that). Just doctored up a Toshiba that had similar symptoms... while I recapped around the amp chip to be safe, I don't believe the caps were the problem. Rather, I think an extremely dirty record switch was causing it.

Signal injection should work on the preamp chips as well as it would on the power amp. I'd also break out the DMM and check for voltage at these chips... if they aren't getting power, naturally they wouldn't work.

 

[tec9]

Just a quick update on this-
I jumpered the pins that are the inputs on the amplifier, and both channels play fine. The LED indicators are even working. It sounds good! So at this point I know the problem lies somewhere in the pre-amp circuit. I have cleaned all the switches, and I have reflowed several solder joints that were cold/ cracked. Here is a breakdown again of the problem
No left audio (only can be barely heard with volume turned all the way down)
The balance knob does nothing to the left or right channel.
If you have the switch for stereo/mono set to "stereo" the right speaker cuts in and out.

I'm going to dig deeper tomorrow....but not sure how crazy I want to get with this. Seems like it would work fine, as long as it was in the mono mode.
-Mark

 

[riker1068]

Some more helpful info