Tele-master AS-20 amplifier. No output sound, just noise

[Francesc V.]

Hi:

I bought this amplifier from the 70s to try to repair it. So far it has no output sound, just noise. You can volume it up and down.

I've checked fuses and all are ok.
I've continued with capacitors:

• The main black one should have 4700uF, but it has lost half of its capacity during these years.

• The ones that I've numbered have still a really low ESR and the full capacity after ¿40 years?

The 4 transitors are perfect. Desoldered and tested individually.
There are also components which I don't know exactly what they are. The two components I've marked in a yellow rectange.......are these diodes? (there is one next to capacitor "2" that I think doesn't read correctly). This however wouldn't explain that both outputs have no sound :-)



Keep on analysing..........so far, quite lost but happy :-)

Regards,
Francesc.

[petehall347]

Re: Tele-master AS-20 amplifier. No output sound, just noise

maybe bias compensation diodes . could be a diode string inside each one . diode test them and see .
no sound is most likely corroded controls .

[jons]

Re: Tele-master AS-20 amplifier. No output sound, just noise

If you can volume that noise (what kind fo noise?) up/down then problem lies in the preamp section. Post full innards/back pics, maybe it has DIN inputs and you are using wrong cable?

[stj]

Re: Tele-master AS-20 amplifier. No output sound, just noise

well it's stereo - we can use that to narrow it down - do both channels act exactly the same?

[Francesc V.]

Re: Tele-master AS-20 amplifier. No output sound, just noise

maybe bias compensation diodes . could be a diode string inside each one . diode test them and see .
no sound is most likely corroded controls .

Yeap.......I tested so far all PCB components, and didn't see anything wrong. I mounted again and took contact cleaner spray. Once I cleaned up the potenciometers, specially the 4 position selector it started to sound again in both channels.

The amplifier sounds ok, however I am just wondering how changing the filter capaciter would change the sound.

The black capacitor says 4700uF / 50V, but the voltage after the rectifier is 53V and it's capacitance it's only 2000uF. Would you change it or just leave it as it is? (any recomendation/type?? Nowadays capacitors with this capacitance are 4 times smaller )

Thank you all for your feedback.

[petehall347]

Re: Tele-master AS-20 amplifier. No output sound, just noise

4700uF 63v

[UNITRAX]

Re: Tele-master AS-20 amplifier. No output sound, just noise

Check if you can change the power supply from 220 to 240VAC. There is often a switch on the back panel. If not, see if the transformer has additional taps with a voltage slightly lower than the current one.

[alfatv]

Re: Tele-master AS-20 amplifier. No output sound, just noise

Did you check switches and potentiometers? Could be oxidized and not making contact.

[Francesc V.]

Re: Tele-master AS-20 amplifier. No output sound, just noise

Everything mounted and reviewed. Main capacitor changed and stacked with glue.

Potentiometers cleaned with "clean-contact spray"

The new main capacitor makes a difference. I have the feeling that it sounds better. Maybe it is.





Thank you all for your valuable feedback,
Francesc.