(Topic ID: 258210)

Stern SB-300 dull sounds/no treble


By semicolin

59 days ago



Topic Stats

  • 11 posts
  • 4 Pinsiders participating
  • Latest reply 46 days ago by eyeamred2u
  • Topic is favorited by 2 Pinsiders

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#1 59 days ago

I have a Stern Meteor that sounds like it's got a rolled up sock in front of the speaker. Sounds are dull and muted. Very little treble/high end, as if there was a low pass filter on it. It gets loud- overall amplitude isn't the problem. It just sounds really dull and muddy, without brightness.

Sounds themselves are fine. Everything plays when and how it is supposed to, but the quality is very poor.

Has been re-capped and the problem isn't the speaker itself. I could start tearing apart the audio amplifier circuit soon but I figured I would ping the crowd here first. Hell, maybe its supposed to sound like that.

#2 59 days ago

i have found ceramic capacitors drifting in value can cause the "tin can sound" is the best way I can describe it. The last board I remember doing it on was a capacitor stradling p3 and p4 of an lm3900 that was supposed to be 68pF was testing more like 500pF when i lifted one leg. Replacing it fixed it. I don't see the same type of circuit in SB300 board but if every single sound sounds wrong then I wold be looking at stuff at and past the 3340 chip where all the sounds come together which isnt too much stuff.

Like C14, C12, C4, C2. Assume the problem did not change when you replaced C5 and C9 electrolytic caps?

Make sure the 10vdc (tp3)supply voltage is good too.... ie .R5 or zener is not open circuit or anything like that.

#3 55 days ago
Quoted from barakandl:

i have found ceramic capacitors drifting in value can cause the "tin can sound" is the best way I can describe it. The last board I remember doing it on was a capacitor stradling p3 and p4 of an lm3900 that was supposed to be 68pF was testing more like 500pF when i lifted one leg. Replacing it fixed it. I don't see the same type of circuit in SB300 board but if every single sound sounds wrong then I wold be looking at stuff at and past the 3340 chip where all the sounds come together which isnt too much stuff.
Like C14, C12, C4, C2. Assume the problem did not change when you replaced C5 and C9 electrolytic caps?
Make sure the 10vdc (tp3)supply voltage is good too.... ie .R5 or zener is not open circuit or anything like that.

That's a good assumption- the electrolytic swap had no effect other than perhaps a bit of futureproofing and cosmetic improvement.

TP3 is steady at 9.75VDC. I'm not unhappy with that.

I didn't even recognize these glass-packaged capacitors as capacitors until really squinting down at the schematics. I'll focus on them next.

#4 55 days ago

Looks like you may have been on to something.

Looking through the glass of C12, it's dark and toasty compared to C3. If they were both installed at the same time, something went wrong along the way. Unfortunately, the capacitance meter is in my workshop on the other end of town, but I'll order the parts anyway. I go through a lot of these caps anyway.

Popped in an order at digikey for all of those axial ceramics. Can never have enough 0.1uF. Waiting on an order of radial ones from overseas, but why bend leads when I can make it look nice? Added a few resistors too... R3(3K9) and R4(10K) are way off label at 3K and 5K respectively.

And for the real hell of it I ordered a few MC3340P and TDA2003A chips from AliExpress. Possibly counterfeit, hopefully just old stock, but they're so cheap that I feel like I'll roll the dice.

#5 55 days ago

you can get TDA2003 from LCSC. Its a made in Asia brand but I have had no problem with any UTC parts including that amp. The ones from AliExpress will probably be fine tho too.

https://lcsc.com/product-detail/Audio-Power-OpAmps_UTC-Unisonic-Tech-TDA2003L-TB5-T_C83050.html

#6 50 days ago

Well, I pulled all of the passive components from R23 onwards, and it's still muddy as all hell. Perhaps my old ears deceive me.

#7 49 days ago

Maybe try a zener with a breakdown closer to or slightly above 10v. I know small differences in voltage of the 10v supply effects volume control circuit

#8 49 days ago

My Meteor sounds the same. Plenty loud and bass-y but seems rolled off above 1200hz.

#9 49 days ago
Quoted from barakandl:

Maybe try a zener with a breakdown closer to or slightly above 10v. I know small differences in voltage of the 10v supply effects volume control circuit

Good timing. I was just putting in a digikey order for some electrolytics. I'll put in a BZX85B10 as it has a much tighter tolerance than the original 1N758. We'll see how it goes!

A_Bord: when I get this licked, you can be sure I'll post the solution here.

#10 47 days ago

Replacing the Zener diode VR1 made a huge difference. Solid on 10.01V instead of a shaky 9.6-9.75V. Made a big difference in sound levels.

I still don't feel like I'm getting the highest highs out of it, but then again, it's an SB-300. Not exactly high fidelity. Still, a major improvement.

In any case, massive help. Thank you.

While I was under the hood I took out the scratchy 25K pot that was always cranked to the max with the 20K ballast resistor and replaced them both with a 50K pot. Solved any possible concerns I could ever have about volume. I get the full range now from nearly silent to unusably loud.

#11 46 days ago
Quoted from semicolin:

Replacing the Zener diode VR1 made a huge difference. Solid on 10.01V instead of a shaky 9.6-9.75V. Made a big difference in sound levels.
I still don't feel like I'm getting the highest highs out of it, but then again, it's an SB-300. Not exactly high fidelity. Still, a major improvement.
In any case, massive help. Thank you.
While I was under the hood I took out the scratchy 25K pot that was always cranked to the max with the 20K ballast resistor and replaced them both with a 50K pot. Solved any possible concerns I could ever have about volume. I get the full range now from nearly silent to unusably loud.

All good info, this would be good add to Pinwiki

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