Failed PC amps

[atferrari]

After assembling an audio amp (LM386) for bench tests purposes and when looking for a speaker and enclosure I was surprised of how many failed amps for desktop PCs I had around!!

After opening them:

I noticed that in ALL, once plugged, /edit the AC from the secondary edit/ goes straight to the rectifier. No switch in between.

The ON switch at the front just applies that rectified voltage to the filter, feeding the amp.

Found from discrete amps to TDA and TBA ICs.

Found one PCB cracked by the pressure exerted on the ON switch and also a transformer heating madly even after desoldering all the components around.

To complete my performance I opened the pair of this PC and replaced the failed pot with fixed resistors. Intput level software control does the rest.

Two years ago I had replaced the filter cap when my daughter discarded it because of a severe buzz.

To disassemble the pot I had to forcibly cut it because the nut inside a deep circular recess did not allow to free the shaft. I wonder how it was assembled but evidently nobody expected this to be repaired.

They were all made in China.

Oh God, music is so nice...!

 

[cowboybob]

Outstanding recycling job!

Precious little of that even possible any more...

 

[atferrari]

Sorry for my mistake. Corrected in the original post.

The six amps I disassembled had transformers fed with 220V and what goes straight to the rectifier is the AC from the secondary.

 

[atferrari]

End of the batch

Repaired the last in my list (office desktop).

To correct an intermitent left channel I had to replace the input jack & cable. I simply used one from those I disassembled yesterday. Not bad after all.

To my surprise I found several electrolytic caps of .22 uf / 50V. What is the reason to use them?

Are they cheaper than non polarized polyester or multi layer?

I ow myself a tasty espresso to celebrate.

Everybody is invited tomorrow Sunday at 09.00 at Victoria Cream. Ask for Agustín.

 

[cowboybob]

To my surprise I found several electrolytic caps of .22 uf / 50V. What is the reason to use them?

Some manner of filtering. Or a pass-thru for the output. Can you give us a schematic or a pic showing location in relation to other components?

Thanks for the invite, but can't make it.

 

[atferrari]

In series after the pot

The pictures I took are good only to see where the dual pot was.

There were several .22 uF caps, two of them in series with the left / right inputs as follows:

Each chanel goes to the pot.

The attenuated signal leaves the pot passing through one of those caps and goes to a resistor in parallel (yes in parallel) with a cap of some kind (not these) .

This again attenuated signal is the input to the IC, a TEA2025B.

Considering this is the first time I repair somehing in a long time, I feel happy.

In about one hour, off to my espresso.

Be well.

 

[cowboybob]

Sounds like they're just DC isolation caps: in there to keep any DC riding on the input signal out of the pre-amp circuit.

 

[KeepItSimpleStupid]

They are significantly cheaper.