Hi everyone - I'm trying to fix this stereo as it has great sentimental value. None of the local repairers will help me as they don't have experience with this brand so I'm forced to try and fix it myself...
The issue is there is bad distortion on both channels whilst it is playing. But when I switch it off, the sound fades out over a few seconds, presumably running off the energy in the filter caps. As soon as the power is switched off all the distortion is immediately gone - ie. it plays crystal clear for that few seconds as it fades out.
I'm (obviously) not super knowledgeable - Any idea what could cause this? Would much appreciate anyone's thoughts.
Pic of the stereo below for anyone interested:
The issue is there is bad distortion on both channels whilst it is playing. But when I switch it off, the sound fades out over a few seconds, presumably running off the energy in the filter caps. As soon as the power is switched off all the distortion is immediately gone - ie. it plays crystal clear for that few seconds as it fades out.
I'm (obviously) not super knowledgeable - Any idea what could cause this? Would much appreciate anyone's thoughts.
Pic of the stereo below for anyone interested:
My guess is improper bias of some stage when full power is applied. One guess is leaky coupling capacitors but could be a large number of things.
See if you can get a schematic with normal operating voltages. Amplifier output before the coupling cap should be roughly half of what exists at the fuse for each output stage.
See if you can get a schematic with normal operating voltages. Amplifier output before the coupling cap should be roughly half of what exists at the fuse for each output stage.
Turn it on. Carefully check the supply voltages.
Switch off. Pull out one of the fuses on the board УНЧ-0. Turn it on. Check. If one channel works, look for the problem.
Switch off. Insert the fuse into place. Pull the fuse in the other channel. Turn on. Check. If a faulty channel is identified, look for faulty parts and connections.
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Describe what you mean by distortion? Noise, Hum, Buzz, Rattly, Other? What other equipment might be connected?
The DC rails' filter capacitors have dried out & lost the ability to filter the ripple. When you turn the unit OFF, the "raw" DC (straight after the rectifiers) is removed, allowing the unit to run off clean DC stored in those capacitors.
As above, if it works properly with new filters in the power supply, proceed to replace all electrolytics.
Thanks to everyone for the suggestions and assistance.
Thanks for the schematic. I checked and I am getting voltages very close to what is shown.
Thanks for the suggestion - this sounds very logical to me! I previously tested all the filter capacitors for the capacitance and they were pretty close to spec, but I have no meter to check the ESR - which I guess would be the issue. I'll order some equivalent new capacitors and see if this works....
Details http://rw6ase.narod.ru/index1/radiopriem/rl_ps/melodija101s.html
Schematics http://rw6ase.narod.ru/index1/radiopriem/rl_ps/melodija101sx.djvu
Manual http://rw6ase.narod.ru/index1/radiopriem/rl_ps/melodija101s_bds.djvu
First of all, check and replace those soviet-era electrolytic capacitors.
Schematics http://rw6ase.narod.ru/index1/radiopriem/rl_ps/melodija101sx.djvu
Manual http://rw6ase.narod.ru/index1/radiopriem/rl_ps/melodija101s_bds.djvu
First of all, check and replace those soviet-era electrolytic capacitors.
Small capacitors can be exchanged for capacitors of a similar capacity for an operating voltage not lower than the specified one.
Large capacitors are best left in place so as not to damage the PCB. Connect new capacitors in parallel with the old ones on the wires.
Large capacitors are best left in place so as not to damage the PCB. Connect new capacitors in parallel with the old ones on the wires.