Lynx II tracing a sound problem

[RestoLynx]

Hi.

 

Looking for a bit of input on a sound problem on a Lynx II, pulled it out of a box after about 15 years and the sound was a bit distorted for a few days then just stopped, nothing from the headphones or the speaker.

 

So I changed all the sound capacitors as my starting point and still no sound, then I started following the schematic tracing the circuit, but discovered my board has two LM386s, meaning it is probably more similar to this schematic (after the volume control), posted in this thread, like this:

 

 

Hoping that everything before the volume control is the same as the original schematic; I put the two together and got this:

 

 

 

 

I put the speaker on C16 and got a the hear Chips Challenge, but it was very quiet. The path on my board from C16 goes: C16-> R42 -> Volume control -> R94 ->LM386. So I jumped C16 to the following and got:

 

• R42: Nothing
• Volume Control inputs: Nothing
• Volume Control Outputs: Nothing
• R93: Quiet output on right channel
• (R94 : Quiet output on leftchannel)

 

In my head that points to the signal at C16 (or C58) being very weak, as both LM386s are unlikely to be faulty. So what is my next step, my plan was to pull the RF shield to see if the hex invertor has a dry joint or if there is something going on there, but tbh I could do with some help and guidance, any ideas?

 

Thanks

Edited February 20, 2017 by RestoLynx

[+karri]

The most common audio flaw is from inserting a DC power plug into the headphone jack. This burns the speaker that will show the symptoms you just described.

[RestoLynx]

The most common audio flaw is from inserting a DC power plug into the headphone jack. This burns the speaker that will show the symptoms you just described.

 

Thanks karri, so I've read, but I'm pretty sure that hasn't happened as I've owned it from new and had to wire the power supply directly to the board about a year after I got it because the socket fell out! (Speaker appears to be fine as it is showing ~16Ohms)

[+karri]

Well, it is not only the ohms. It may be mechanically broken. Usually moving parts break more likely than solid chips. You could check the volume control and the speaker first.

 

I have no good ideas of where the error is. Just guessing.

[+Stephen Moss]

Check L12, L20 and R44 and the volume control pot for continuity/resistance/bad solder joints. Other than that without a oscilloscope to trace the audio signal I think it may prove to be a difficult fault to find if it is not the speaker.

[RestoLynx]

Check L12, L20 and R44 and the volume control pot for continuity/resistance/bad solder joints. Other than that without a oscilloscope to trace the audio signal I think it may prove to be a difficult fault to find if it is not the speaker.

 

Thanks for the input: Volume control pot tested fine (multimeter), I resoldered the connections just in case, I'll check the L12/L20/R44 and let ye know. What is the function of L12/L20?

[7800fan]

audio transformer.

[RestoLynx]

No joy

I pulled the shield and spent a bit of time working on it and got the following: I went through all the resistors and checked L12/L20 and they are all good, everything is reading as they should (R44 isn't on my board, it just goes to ground). As before I put a speaker across anything in the audio circuit with the following:

Before R40: I could hear the music clearly

After R40: very quiet, but there is music

After C16: Very quiet and a bit distorted

After R42: Nothing

So then i decided to reconnect the speaker and short from before R40 to R93/R94 and I get something out of the speaker, but it is quiet (headphones are clearer)

I'm guessing that the signal shouldn't drop off after R42 so badly that I wouldn't be able to hear anything at all through a speaker, I'm wondering is it the inverter that is gone and it's output isn't strong enough? How would I test this without a scope?

[+karri]

You could start by re-soldering the legs of all the chips in the audio chain. The reason could by solder getting cracked by old age.

 

If you have spare chips then change them.

[RestoLynx]

Ok, so before I break out the iron again (or buy an oscilloscope!), is there a way to test if the inverter (U7) is faulty/bad connections?

 

Should the levels of a speaker connected to U7 be similar regardless of which side it is on? So if it is quiet after U7, but super loud before it then u7 is faulty... correct?

 

Thanks for the help

[RestoLynx]

FIXED!!!!

 

It was a bad solder joint on the inductor (L21) that supplied power to the audio circuit. I missed it because it was supplying .48v and not 4.8v, so when I looked at the meter I thought it was good, also with a bit of power I was getting the weirdness above which was confusing.

 

Thanks everyone for your advice.

[Stairre]

@RestoLynx you just became my favorite person. I was prepping my Lynx II for a McWill LCD but had no sound and was trying to convince myself that I didn't mind... So I started poking around on the web and came across this post and your solution... Not only did it give me a starting point, but what are the odds... Mine had EXACTLY the same problem. Had it powered up and as soon as I touched L21 with my MultiMeter the probe bridged the bad joint and I instantly heard sound. I'm still relatively new at this stuff, so it probably would have taken me forever to even get to that point from a diagnostics standpoint if I hadn't stumbled across your post! So... Thanks!!!

[Ansel]

I had this same sound problem and found this thread. I had tested and traced everything with no luck. I read here about the L21 and resoldered it and boom my sound works.

This forum is great!!

Thanks' to all who contribute!!

[Pete_Codes]

On 2/23/2017 at 10:40 PM, RestoLynx said:

FIXED!!!!

 

It was a bad solder joint on the inductor (L21) that supplied power to the audio circuit. I missed it because it was supplying .48v and not 4.8v, so when I looked at the meter I thought it was good, also with a bit of power I was getting the weirdness above which was confusing.

 

Thanks everyone for your advice.

I had the exact same issue... Resoldered L21 and off you go... It's fiddly and a fine tip on the soldering iron is ideal... Also, make sure not to bridge to the shielding. Thanks!

[Judas-Prime]

On 2/23/2017 at 2:40 PM, RestoLynx said:

FIXED!!!!

 

It was a bad solder joint on the inductor (L21) that supplied power to the audio circuit. I missed it because it was supplying .48v and not 4.8v, so when I looked at the meter I thought it was good, also with a bit of power I was getting the weirdness above which was confusing.

 

Thanks everyone for your advice.

Dude.....Not to ressurect an old thread.... but i just did a BennVenn LCD mod, sound worked fine before...afterwards...no sound.... pulled my hair out for days tryin to track it down myself and on Forums....You saved my sanity!!!!!! Thank You!!! This solved my issue. I have a fantastic image and great sound now!!

 

Thank You Again!