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-   -   Tascam PA-20B Oscillation? (http://www.diyaudio.com/forums/solid-state/72086-tascam-pa-20b-oscillation.html)

dswiston 19th January 2006 04:42 AM

Tascam PA-20B Oscillation?
 
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 19th January 2006 06:37 AM

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 19th January 2006 09:19 PM

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.

djearflaps 13th October 2012 11:14 PM

I have a Tascam PA-20B as well. Loving the tiny footprint but for some reason it clicks off in protect mode at low to moderate levels. It doesn't always click off, just randomly. It actually stays on most of the time and works great. Once it goes into protect mode, it can be powered off and back on and usually works normally.

I wonder if I should open it up...or just deal with the slightly intermittent problem.

Has anybody else encountered the same problem?

BTW I'm using 6Ω Yamaha NS-20T Speakers
It also did the same thing with my 8Ω Tannoy PBM 6.5s

Ian Finch 14th October 2012 02:38 PM

The protection fault seems, from other forum comments about the PA-20B, to be a common one and from the fault mode, is likely due to bad solder joints, internal wiring connections to the output connectors or component failure. If you are confident with soldering, it might be advisable to resolder the through-hole board connections of the amplifiers and the protection chip and circuit.(note post #2 from the very old thread) It could still be a component fault but this eliminates an obvious cause without costing much more than your time. Unfortunately, a schematic doesn't seem to be available so that we can be more specific and helpful. Perhaps someone else has a schematic or link to share.


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