My audio is really distorted on the right channel only.
I seem to recall the distortion beginning shortly after installing the unit when I was pushing/pulling on the control knobs…I have since adjusted all the settings (balance, fade, bass, treble) and nothing seems to improve the right channel. The audio is great when the balance is on full left, but begins to get distorted when switched to center/full right.
We can rule out the speakers…and ideas?
Thanks,
Joe
https://www.youtube.com/watch?v=siDIpcYLWJk
Starting out full left…sounds good.
Move to right channel…not so good.
Could a wire have been damaged? can you test the wires? or simply a bad connection?
Thanks for the reply.
I can confirm all of the external wiring is in good shape. It has a pair of flat trailer type connectors for the speakers (one left, one right). I can swap the connectors (connect right output to left speakers) and the distortion will switch to the left speakers… which is why I said we can rule out the speakers. Power/ground is good.
I have the schematic diagram for the radio but it is pretty damn hard to follow. I don’t know where to start.
At first I thought it was a problem with one of the setting (bass, treble, etc), but if that is the case I would think both the left and right channels would be distorted…
https://picasaweb.google.com/lh/photo/vrhyTta3FJwNc4fYBCLF2tMTjNZETYmyPJy0liipFm0?feat=directlink
i hope i’m wrong… but, in those days, loudness was like a boost, that was not to be used over 25% of the volume. at 1/3 volume, you started do distort and do damage. Those figures were just my personal beliefs (result of experience). Just check your loudness button for starters.
that’s about as far as i can go. There are some way better car stereo guys around here that know what it could be.
Well, sounds like maybe a dirty rheostat, or part of the stereo circuitry may be damaged or has a bad solder joint / connection in there. At least you have narrowed it down to half of your schematic.
Clean all the knob rheostats (deoxit is good for rheostats). Even though you swap the speakers the right-channel portion of the rheostats feeds the distorted channel. Trebble, Bass, Balance (crap, forgot to circle it ~ just above and left of volume circle) , Fader, and even the volume (right channels circled in red). I would start with that, since it is probably easier instead of fiddling with the board components.
If you do have to go to the board components, I’ve put a black box around what should be the right channel components. If it isn’t a rheostat then I would check pre-amp component on the far left of the schematic next. Also take a good look at the individual components on the boards and look for any that appear burnt, severely overheated, or leaking, which indicates probable failure.
Please note I’m not a radio expert, and its been a few years since I’ve done schematic work, but this seems fairly straight forward (to me).
Hope this helps!
I’m rereading your posts… I need to watch the video when I get a chance, but so far I have this much:
You said that the volume, fader, bass and trebble do not affect the distortion, but it comes in as you adjust the balance.
Ok, one thing about the Bass, Trebble, Fader and Volume is that they appear to have individual rheostats for Left and Right channels. So even though they appear to be ok for the left channel, the right channel rheostat could still be causing the issue.
Balance appears to be a single rheostat. It could be dirty or damaged on the right half and causing your issue.
Does the loudness switch change anything? In theory, it could be a dirty / damaged switch contact.
I’d still go with cleaning / inspecting all the rheostats. They are often a culprit, and given that you were fiddling with knobs when this all started I think they would be a good starting point for fixing it.
Again, I hope this helps you out some.
Thanks very much for the reply Mike.
The radio is out of the car at the moment so I can not test the loudness switch. I will start with the electrical contact cleaner and report back.
I edited my post that you were quoting. Thats the trouble with thinking and typing and hitting submit!
The contact cleaner did not improve the distortion. Any way to bypass the balance rheostat?
Thanks,
I can’t see the schematic very well, but, for anyone who can, are there any electrolytic capacitors used in the circuit? If so they should surely be replaced, as lifespan of electrolytics is typically billed as 10 to 20 years. They can continue to work for longer, even 40 years or more, but it’s kinda like how you can drive with oil that has 20,000 miles on it. Can and should being two different things. 
The world I’m most familiar with is old tube amps, not old solid state electronics. But in the tube world, distortion is frequently caused by an electrolytic cap that’s used to filter out low frequencies failing, and thus not filtering out those frequencies. In many cases it’s used to filter subsonic frequencies, which are of no use to human ears and wreak havoc on an amp when it’s trying to produce them for no benefit.
So I’d look there for sure, and replace every single electrolytic cap in there, if there are any. Electrolytics that old are more or less guaranteed to fail soon and likely they have already to some extent if not fully.
Oh and just to clarify, electrolytic caps can be identified in a schematic like this:
Caps are two bars parallel to each other with a space between them, which are perpendicular to the signal path leading to the bars.
Electrolytic caps are like that, but also have a + sign on one side of them, indicating the polarity of the cap. Other types of capacitors you’ll see in a schematic like this are not polar, but electrolytics are.
As far as how to identify them on the board, they’re typically cylindrical in shape, and will have either a + sign or - sign printed on one side, identifying which side is which. They also normally have a sort of indented ring near one end of the body.
Other types of caps (which can last more or less indefinitely) you’re likely to come across would be ceramic discs, usually brownish in color, or some types have color coding on them like resistors do, with less disc shaped bodies, but not cylinder shaped. Those caps are types you don’t need to replace unless they prove to be faulty through testing. 40 year old electrolytics should be replaced, always, period.
Joe,
Most of the time, a problem such as you are describing is the output device or devices are bad in the offending channel. With discrete push-pull transistor output stages it is as easy as removing the transistors and testing them with an ohmmeter to find the bad one. With IC (Integrated Circuit) outputs (I think that is what I am seeing in the posted schematic) you pretty much have to shotgun (meaning blindly replace) the IC and see what you get. Here’s another thing you can do. Most output stages have ballast resistors (power resistors). There should be two per channel. With the unit powered and on, you measure the voltage on each one. If one is much different than the others (and often hot and/or smoking) this points to the bad transistor/IC. You can also put a finger on each output device and see if one is way hot or cold, different than the others. This too can point you to the offending device.
I realize this might be beyond what you are familiar or comfortable with, but this is the/a process to find, isolate and fix a bad audio output.
Hope this helps.
Bob
If you have a friend nearby who has an oscilloscope (or if you’ll be in Chicago anytime soon, I’d be happy to use one of mine) tracing the audio signal from preamp to output with a 'scope, that’s truly the ultimate way to track down the source of the problem. Never fails to pinpoint exactly where the noise is introduced. I’m guessing if you had access to a buddy with a 'scope you’d already be going down that road, but can’t hurt to suggest it.
Best of luck getting this all squared away!
Scott (tmh) - think I have that right, good ideas on both the capacitors and the scope. Just wanted to add one of the techniques I have used to find these sorts of problems and fix them. Old electrolytics can certainly be an issue and O-scope tracing (preferably with a sine wave input) works very well.
Resurrecting an old thread, I know…
Double ditto on all the recommendations to check the electrolytics; in the past I have been lucky enough to spot the offending cap fairly easily, as it leaked it’s dielectric (usually a black “goo”) onto the PC board. The li’l sucker literally "Took a S*** " !
Electrically similar to an elctrolytic but of different design, is the tantalum capacitor. These guys also have a record of dying in power amplifier service. Once while running the bench in an old CB shop down around Speedway/Swan in Tucson (25-something years ago); I was chasing an intermittant failure that finally showed itself in a spectacular fashion. When it hard-failed, it souded like a .22 going off, and I had little pieces of “stuff” embedded in my arm. Stuffed in an electrolytic as a replacement, bench-tested and charged the customer. Problem solved.
Tantalum caps look like a “teardrop” with two wire leads into the circuit board. The outer “shell” is generally either blue or an orange-ish brown. While I didn’t see any tantalums in your pictures, I did see the “Uniden” logo on one circuit board; and the ones I’ve found on older uniden’s were all blue. The name “uniden” actually triggered my memory of this occurance; as the radio was an old Cobra 138XLR single-sideband rig, manufactured by… uniden.
Radio is still sitting on my desk…apart.
Thanks for all of the replies. I will play with the radio again soon and report back.
Since you have it apart, could you check some part numbers? It’s obvious from pics you’ve posted that this unit has been repaired previously. On the lower right side of the schematic, there are four integrated-circuit power amps, IC501, IC502, IC601, IC602. Could you verify that the left and right channels have the same component part numbers as listed on the schematic?
IC501 is MB3713H
IC502 is MB3712H
IC601 is MB3713H
IC602 is MB3712H
If any of these four IC’s have been replaced with other part numbers (especially if you find NTE replacement parts), that would be valuable info. If you just plain give up and want it fixed, let me know. Ship it to me, and we could document the troubleshooting process on this thread, I’ve got all the necessary equipment available to me at work.
Huh. Hadn’t thought about NTE replacements in YEARS; though I believe I still have some yellow boxes with “ECG” stamped on them. And a bench… benches are always good. Should get all the crap off my bench and get it going again. Maybe dust off the radios, run some fresh LMR400 up to the antennae…
And did you test your suspected-bad component on the ECG Tube Tester Machine at the drugstore before you purchased the replacements in the yellow boxes? LOL!
I brought up the issue of cross-referenced replacement parts since I’ve been bitten in years past. Electrical characteristics and heat tolerances are just some of the ways these replacement components can differ from the OEM-specified part. Since AZCAT’s photos clearly showed evidence of past repairs (modern ty-wraps used, soldering iron burn marks on some wiring, re-soldering flux residue), I’m curious as to what components were used.
