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.

[Marcurios]

Hi there Francesc !

Can i ask you about the amp, i recently scored the same amp but can't find a circuit diagram for it,
and someone has been tinkering with it before and changed the bias pots for 2 totatally different values.
Since i can't find the circuit diagram i was wondering what value the originals should have been.

Do you by any change know what the value of these pots is, cause from the pics i can see they're still original.
Or would you be willing to pop the hood and measure them for me ?

you would do me a huge favor !

Thanks,

Marcurios.

[Disoluto]

Hi,
i need find a circuit diagram for it too... please :-)
The electrical transformer on my TeleMaster has died and I need the schematic to replace it.
Thanks,
Disoluto
www.disoluto.com

[stj]

is the primary faulty or the output side?

[Marcurios]

Hi Disoluto,

the transformer is a 220~220 primary 40v single 12v single secundairy transformer, i suspect a 70 watts one, but it will get hot under normal use (70c), better to mount a 100w transformer to have some spare power so it runs a little cooler.

You could just take a 39~40v secundairy and use a small step down print to get 12v for the front light (cause that's what the 12v is for, it runs on AC the light), or you could get a higher voltage smal light so you dont need the 12v.

in my opinion a 220v Pri/ 40v Sec 4A wil be perfect. than you'll have 1,5A leftover power..

It probably died cause all the caps are bad, and the bias needs to be set reasonably low.
All the caps in mine where shot, and the bias pots as well, and a few transistors where off spec.
it caused the transformer to heat up immensely, it got like 100c.
so i replaced all caps, bias pots for multiturn, and now it (transformer) runs at 65c after a few hours.

[Francesc V.]

Hi everyone............I wasn't able to find the schematic for that one.......so sorry. :-(

[Marcurios]

Me neither, but i will draw one, i got it fully repaired now and will soon map the entire circuit board in LTSpice.
Once done, i'll post it here.

[Disoluto]

Hi Disoluto,

the transformer is a 220~220 primary 40v single 12v single secundairy transformer, i suspect a 70 watts one, but it will get hot under normal use (70c), better to mount a 100w transformer to have some spare power so it runs a little cooler.

You could just take a 39~40v secundairy and use a small step down print to get 12v for the front light (cause that's what the 12v is for, it runs on AC the light), or you could get a higher voltage smal light so you dont need the 12v.

in my opinion a 220v Pri/ 40v Sec 4A wil be perfect. than you'll have 1,5A leftover power..

It probably died cause all the caps are bad, and the bias needs to be set reasonably low.
All the caps in mine where shot, and the bias pots as well, and a few transistors where off spec.
it caused the transformer to heat up immensely, it got like 100c.
so i replaced all caps, bias pots for multiturn, and now it (transformer) runs at 65c after a few hours.

Perfect. I'll try installing a new one with those specs. I'll let you know how I progress. Thanks for your help.

[Marcurios]

Right on, it sounds pretty good and balanced for such an old amplifier.
be sure to insulate the transformer from the chassis, maybe with a 1cm piece of wood under it, to reduce AC induced hum.
The amp tends to induce a AC hum to the ground plane when it's bolted onto the chassis.
best would probably be to add a netfilter, ring transformer and a snubber to the secundairy output to prevent high frequency noise and oscillation.

[Marcurios]

Follow up;

I have changed just about every semiconductor and every single capacitor in the amp, cause i could not get the transformer to cool down to a reasonable degree.
I want the transformer to get no hotter than 45 degrees celcius, with the original transformer that is not doable.

I suspect that the transformer is just a bad design, the core heats up too much, even under a small load after hours.
it's idling at 10 watts with a bias on both channels of 10 millivolt with all new components, and i took apart all potmeters completely and cleaned them up like new.
Replaced all the small signal transistors for ksc1845 and replaced the 2sa647 and 2sd667 for new ones and replaced the main transistors 2sd476 for new ones.
Still the transformer gets like 70 degrees celcius after 2~3 hours of playing music at volume 4 with 8 ohm load.
I also had to enlarge the bias ceramic caps (220pF) for 1nF caps to tame high frequency oscillations a bit.

I also think that the transformer should be no more than 36 volts, it's made for 220v and our net voltage is currently 233v, that's why it delivers 39 volts with me.

Anyway, wanted to update this for others that search for info about Tele-Master AS-20 Amplifier.

The Amp is 20 watts, i suspect it delivers 16 watts per channel so a 80Watt transformer should be enough.
Going to order a 80w 36v transformer from Aliexpress to replace this one with to see if it stays cool with that, cause the Amp sounds exceptionally good now.