20M amplifier problem solved !!

Hello all ,
i have a problem with one amplifier 20M in my Festival 130 stereo magic 1967 under serial number 55001.
if I put the amplifier in the jukebox and choose a record, the amplifier has a loud noise even when the record is not playing , and gets louder when the mute is of when de record plays .

if I turn up the volume, you hear the music, but also the noise.
When I get above half of the volume (then the amplifier is quite loud, then the noise is suddenly almost gone.
turn the volume down and the noise gets louder again ??
does anyone have an idea where to look for the problem?
the noise is white noise and resembles if you have no TV channel and only noise


thanks in advance for any help of ideas .

regards ....nico



3 mal bearbeitet. Zuletzt am 13.02.21 22:14.

Hello Nico,

it's a common problem: Those AC151r germanium transistors get old and noisy.
If the noise dissapears, when you disconnect the volume control, the noise is generated by one or more of T1, T2, T3, T4. Also the contact of the small muting relay is oxidized and needs cleaning.

If the noise remains, T5 will be your candidate.

Those 5 transistors can be replaced with BC560 or similar PNP types without further modifications. If T3 is replaced, the AVC may react stronger than before.... better leave it in place, if it's not really necessary to change it.

Regards
Jens

thanks jens for your excellent information !!

i am on it and wil change the T1-2-3-4-5- and the relais if necessary thanks again .

I wil order some parts and let u know what te result is

Can i change the T5 for the same BC560 if necessary ? because its the same AC151 R -4

Sorry i need to reed better hahah u said 5 transistors so yes



2 mal bearbeitet. Zuletzt am 24.01.21 12:53.

hello jens,
just an update of the 20M amplifier.
the BC560 transistors that you specified do not work in this amplifier.
In the meantime, the white noise has disappeared by replacing the T5 AC151r with the same AC151r. there is still some distortion in the sound, which can also be caused by one of the other transistors, I'm still looking at that.
greetings nico



1 mal bearbeitet. Zuletzt am 31.01.21 17:56.

Hello Nico,

I can assure you, these transistors work in this amplifier. I did exactly this replacement several times.

Did you adjust P1 for -14.6V at collector of T10 and P2 for 40mA through R49 (without any signal)?

Regards,
Jens

ok jens i will have done something wrong, and not have seen it myself.

Can you tell me where the P1 and P2 and T10 are on this old spare PCB? , because from the drawing I cannot properly find these 2 potentiometers.

No markings can be found on any the PCB's
the problem i have is i only have the drawing of the 130d etc.

The amplifier on minimal volume is distorted and to loud , thats not good and maybe your solution wil help to solve it .

greetings nico



2 mal bearbeitet. Zuletzt am 01.02.21 19:44.

this is the old PCB

Hello Nico,

you have a private message.

If the amplifier is to loud, maybe the AVC is not working:
- C8 or C15 without capacitance
- T3 defective
- D1 or D2 open

or the volume control is lacking its ground connection.

With good transistors and proper adjustment, this amplifier sounds good and produces less noise than the newer 35M or 70s.

Regards
Jens

thank you very much for your message jens, this will do everything to get it right.

great that you wanted to share this information , and I will certainly keep it very safe and well hidden , and as soon as I have time I can move on, and wil let u know what the result is .


awesome .

greetings nico

Hello all ,
I just want to report that the amplifier is now working fine again.
Special thanks to Jens, who provided very good information about the schematics and adjustments.
without jens it probably wouldn't have worked out so well as it did .

I am A happy Camper