Acoustic 25-b

[dfiction]

Hey!

I'm repairing an acoustic b-25c amp that i rescued from the garbage.

This amp consists of two modules, a preamp PCB and a poweramp PCB. The preamp works fine, verified by the headphone jack. The power amp is not working, and exhibiting curious behavior. when i turn the amp on, no sound comes out the speaker. when i turn the amp OFF, normal sound comes out the speaker for about 5 seconds—and then there is silence. It's as if the amp works normally as the caps are discharging, but not when they are fully charged.

any ideas? schematics are attached.

by the way, can someone enlighten me as to what the LM13700 is doing in the power amp circuit? haven't seen that configuration before.

best,
daniel

 

[moamps]

This IC most likely has the function of a limiter/compressor and/or a saturated tube amplifier emulator. Try disconnecting resistor R14 (disconnecting the control circuit) and check that the amplifier will work normally.

 

[dfiction]

thanks for your quick reply!

I tried to lift r14 and there was no change to the behavior—same error as before.

 

[moamps]

Next, remove the left pin of capacitor C3 from the PCB and connect it with a piece of wire to the left side of R1. If the amplifier works, the power IC is ok, if not, the power IC needs to be replaced. Also if the amplifier is working then, the problem is somewhere in the control part that we can try to locate later.

 

[dfiction]

Thanks for suggestion number two. Tried that! same error. I will replace the power IC and report back. Gotta order some online...

 

[AnalogPackrat]

Did you check all four power rails on the power amp module? DC ok? Any ripple present? Preamp appears to run from another separate pair of rails.

 

[dfiction]

Hello Analog packrat—

great suggestion. I checked all the rails with my meter, no scope at home right now, using the AC setting to look for ripple. I checked the DC, too—12v rails are okay. But found something interesting—the 18v rails are at 22v, both + and - rails.

best,
daniel

 

[AnalogPackrat]

Well, the 18V rails are unregulated and depend on power transformer winding/wall voltage/load. The amp chip can handle +/-25V so should be ok. Any ripple on the 18V rails? Do the supply caps look ok? No swelling or goop leaking?

 

[dfiction]

hi everyone! well, turned out that all i needed to do was swap out the TDA2050! The amp is working like a charm. Thanks for all your help!

 

[AnalogPackrat]

Rock on

 

[dfiction]

whoops. looks like i posted too soon.

amp "works" now, but i'm getting some very nasty low frequency clipping. maybe it's factory behavior, or maybe it's the ebay sourced TDA 2050 i'm using.

i attached a video that demonstrates this. it's a little nasty, i would say it makes the amp unusable. i had to detune the string to induce the behavior but it basically happens every time i tap on the pickup, when using a piezo mic—which is my desired application.

https://www.st.com/resource/en/datasheet/cd00000131.pdfi'm looking at the datasheet and comparing it to the schematic, it seems that the 18v rail is unregulated—220UF is on both +18v and -18v in the datasheet, but here it's just .1.

any ideas?


View attachment IMG_1055.MOV

 

[AnalogPackrat]

What happens to the DC rails when you induce the clipping?

 

[dfiction]

I just have a meter on hand here—my lab equipment is not in this home at the moment for the next while.
i'm looking at it with DC setting, the rails stay at 21v, using AC mode, there's about a .8V surge.
wish i could figure out more from the limited tools i have.

 

[AnalogPackrat]

Does the signal between the preamp and amp boards exhibit clipping?

 

[CJ]

those tda chips suck donkey,

obsolete, most evilbay chips are fake and will pop like orovill redenbocker

try this drop in replacement from Natl Semi>
.

https://www.digikey.com/en/products/detail/texas-instruments/LM1875T-LB03/3901928
er, make that TI, first guy doesn't have a chance,

and drain the caps before soldering as a precaution,

you can bend pins from hor to vert and vert to hor if needed,

 

[dfiction]

@CJ , thanks for your link to the LM1875.
looks like the 2050 is a 32Watt amp and the 2040 is a 25Watt amp.

I actually have a 2040 leftover from 2009—back when it was in production. Maybe that's just as good as the LM1875?

 

[CJ]

Should be fine, make sure you drain the caps,