Tascam PA-20B Oscillation?

[dswiston]

Alright, so I have a Tascam PA-20B. A little tiny guy, which is what I was looking for. However, the amp is acting strange. I'm not sure if its oscillation or not. The oscilliscope sometimes shows 65mV @ 5.4MHz when the whole thing is setup but without an input signal. It also shows random things like 1.4MHz @ 5mV at times as well. The scope may not be able to catch any higher freq oscillation, 5MHz is about the max I can resolve with it. I let it sit over night on w/out an input and sometime randomly at night one channel started giving a "bad" sound so I got up and turned it off. Another odd behavior is sometimes a channel will seem to crackle and go out, and if I tap on the woofer of the speaker, it will usually come back in.

You might think this sounds like a lose wire in the speaker, but let me assure you its not. I have had this amp do the same thing on a few sets of speakers, speakers which work perfectly on a different amp. Also, when I tap the woofer, the amp is no where near the speaker to be mechanically effected by the tap.

The tascam looks to be using a chip amp of sorts inside. If anyone has some suggestions or ideas let me know. I am not familiar with the "tap the woofer to get it to work" fix but I figured someone else has encountered it before.


Thanks!

[Eva]

The crackle issue and the sound coming in and out as you tap the woofer is probably due to bad solder joints in the amplifier PCB. The oscillation issue may be also due to these bad solder joints, or due to bad decoupling capacitors, or even due to bad design...

[dswiston]

Thanks for the suggestions. I hadn't thought of bad solder joints but the unit is relatively old. I'll open 'er up and take a look. Hmm, a little back emf helps w/bad solder joints. . . Could be caps too, I'll check that.

The unit is probably not of the highest quality either. . . but its a nice for a little desktop amp.