Fix soviet Романтика 15-120С audio amplifier

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Réparation canal droit РОМАНТИКА 15у-120

Методика ремонта транзисторного УМЗЧ - Аудиоаппаратура - Форум по радиоэлектронике
Files: Original RU or automatic translation EN

What happened

The amplifier is a Романтика 15-120С. It sounds quiet nice but sometime the sound gets no so clear either on right or left speaker.
To fast improve sound quality, I unpluged and replugged AUX input 3.5 jack. Dumb but done. One day it resulted on no more sound at all.

I quickly found dead fuse F3. I took one of the fuse of the other channel and F3 died again. Dumb but done.

I put 2 working fuse on left channel, and 2 dead fuse on right channel. Switched on the amplifier. Left channel is working. Good.

After reading internet, I quickly suspected that power transistor of the right channel were shorted. It was the case.

Test a transistor

How to test a TRANSISTOR with a multimeter PNP or NPN
How to check fuses, diodes, transistors, voltage regulators
3 Ways to Check Capacitors in Circuit with Meters & Testers

Troubleshooting (eyes+multimeter)

Le fusible FU3 de l'alim +27V cramé.
Les deux transistors de puissance VT3 et VT4 en cours-circuit.
 => +27V s'est vidé en quasi direct dans le -27V (R23 et R24 de 0.22ohm sur le chemin)
R15 et R20 cramées sur la carte principale.
R17 et R23 cramées sur la carte module de la voie droite.

Power transistors that were in service before the problem

Left channel (working), original ones:

Right channel (not working), not original ones:


Remplacement des transistors
PNP A1941 => A1943
NPN C5198 => C5200
Les nouveaux sont un peu plus gros. Il me reste à les coller sur le radiateur (je n'ai pas pu reprendre les anciens mica).

Remplacement des résistances sur la carte principale
R15 et R20. J'ai mis des 1/4W en comparant visuellement...
Ces résistances servent à prévenir l'utilisateur qu'un certain niveau de puissance a été atteint à travers l'alimentation d'une LED.

Remplacement des résistance sur la carte module
R17 100 ohm je voulais mettre 1/4W mais le vendeur n'avait plus que du 2W.
R23 0.22 ohm. A la base la résistance avait une forme de ressort (j'imagine que ces résistance doivent avoir une petit propriété inductive ?). Le vendeur n'en avait pas. J'ai hésité entre 2W et 5W. J'ai mis une 2W de forme classique.

Power transistor replacing broken ones


Intervention 2 - January 2024

After first intervention the amplifier worked well for a while and then start to be unstable, with too much cracks on the right channel
It was then replaced by a small chinese amplifier. Романтика 15-120С  remained unused for about 3 years.
Then troubleshooting occured again in December 2023:

  1. By swapping the the amplifier module boards, it was identified that the defective(s) componant(s) where on the module board.
  2. Using multi-meter, VD1 was found faulty and replaced (but installed in the wrong way!).

Amplifier worked well for a day then started again to crack and even relay protection was triggered every few seconds due to this same right canal.

  1. Comparing right and left modules using multimeter to measure each resistance and diode showed something different around VD1 VD2.
  2. VD1 was put in the right way.

So far so good.

Intervention 3 - March 2024

Online Tone Generator - Free, Simple and Easy to Use.

The right speaker is working always good after intervention 2. But sometimes, the left channel is obvioulsy producing a lighter sound. I would say it is the case since I bought it.
By swapping the channels output from preamplifier, problem did not moved to the other side.
But for some reasons I checked preamplifier voltages vs. what is written on the schema:

I then swapped VT1 and VT2. The voltage values from left channel (top on the schema) exchanged with right chanel (bottom).
It didn't solve the problem but I will try to find a КП303Е transistor to get voltage in line with schematic (or not because sound is good anyway).

Sumup of operations:

  • I swapped right and left chanel at several location:
    • Before the preamplifier
    • After the preamplifier / Before the amplifier
  • I swapped КП303Е transistor (VT1 and VT2 on preamplifier board)
  • I swapped right and left channel модуль ОУНЧ
  • I swapped speakers

Problem always remained on the left channel.

Conclusion: it is due to some component on the last stage. Obvious old component : two power amplifier transistors and on old capacitor which is not on the way of the sound signals.

Now need to listen music!

Futur intervention : fixing balance potentiometer СП3-33-22 68к

In the futur : to make new PCB or use chinese made amplifier boards
Maybe i could use stuff like this to repair last stage: Amplifier Board - AIYIMA A2D847


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