Radiotehnika U-101 repair, modification?

Mote

Member
2010-07-06 10:21 am
Estonia
I am a noob in electronics but I want to learn. I have this broken Radiothenika U-101 amplifier lying around, I thought that fixing it should be a good lesson for me. I ask for advice rather than do something I dont understand because my goal is to fix the amp, not to destroy it completely. Car audio legend Perry Babin has helped me with some car amps, thanks to him I have learnt many things and I hope that there is someone like him on solid state section of diyaudio.com. I apologise forehand to potential helpers for my dullness :D

OK to the point:
Radiotehnika U-101 is amplifier made in former soviet union. It is not very powerful nor does it have great sound quality but to me it looks good. The one I had has suffered quite a lot of damage, two of output transistors are removed, some wires are soldered out and I have no idea where they are supposed to go. I have the schematic but either the schematic is bad or i am stupid enough not to be able to understand it. Just in case I bought a working U-101 to compare mine to it. I get the feeling that something has done to power supply and the wires are soldered to the transformer incorrectly. I hoped to be able to tell if they are as they should be by comparing the two amplifiers but unfortunately power supplies are slightly different.

So I want to take it step by step and start with power supply. It consists of transformer, diodes and capacitors. Schematic sais that power supply should provide voltages: +26V, -26V, +31V, -31V. I remember that once I tried to turn on the power supply and measured voltage on the output of the power supply, I got the reading of more than 100 V where it was supposed to be 31V. Big capacitors in the power supply can tolerate up to 50V of voltage so I quickly turned it off. There was no load connected to the supply, thats why voltage got so high or is there something wrong with it? There are two diode bridges in power supply and 6 big capacitors. These capacitors are probably 25 years old, I would like to replace them with modern ones. The old ones are 2200uF, what if I replace them with bigger ones, lets say 4400uF? On the working amp the caps have already been replaced.
 

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Mote

Member
2010-07-06 10:21 am
Estonia
Thanks for reply analog_sa. My meter is not ex-soviet :D. You seem to know more about this stuff so please share your knowledge with me. You said that the less original parts get recycled, the better. I would like to start with replacing capacitors in power supply. Should I replace them with 2200uF caps or are bigger ones better (I have heard that bigger caps help amp to produce better bass)?
 

eostrix

Member
2011-01-15 9:17 am
hey, i just finished upgrading mine u-101 and my suggestions is if you want to make a good amplifier from that thing, then you could replace U4 and U7 plate to LM3886 amplifier and remove input selector plate - it is not needed to have many inputs with broken signal in there. Also i suggest to replace all those old electrolytic capacitators to new ones, because lots of them should be dry.

in this site you could find original repair manual for this amplifier.

VEF Radiotehnika RRR - Radiotehnika U-101