Problem found:
I checked and compared each resistor on the board between channels and found that R35 and R39 (soldered on the board) have about 0.5-0.8Meg instead of 1Meg like in a good channel. I have seen this problem before, unsoldered R35 and R39 have 1Meg, that's why I replaced the OP AMP. Since the board had no suspicious areas, I did not see any problem and looked for leaks in the components.
There is a leakage of about 1-3 Meg on the board between pins 5-6-7-8 of the OP AMP and R35/39, at least.
I removed OP AMP, film capacitor, cleaned the top of the PCB with ethanol, the cotton swab turned dark brown, like after pine resin.
I cleaned and installed all the native parts back, and everything works fine, let's hope.
I noticed that many resistors on the PCB are placed above the board tracks without gaps, so they can short them.
Measurements:
Driver boards with 7 transistors in parallel: 0.64-0.65V at 150 ohms, 4.2mA 67.5Vce 0.28W per transistor - they are quite hot, 2W per board.
MOSFET 0.003V at 0.1R, 30mA per transistor (measured in "Volts" range)
Power consumption at 230VAC:
Start up: 38W (BIAS LEDs on)
Operating mode: 57-58W (BIAS LEDs off, AMP LED on) - in this mode the feedback works and the BIAS control depends on the mains voltage. I use a 150W light bulb for protection, and it flashed very brightly periodically.
If you remove the BIAS boards and the tubes, the driver transistors will generate square pulses at about 1Hz, so you cannot test the amp without the tubes. You need to install BIAS boards, and then you can check without tubes.
Difference in the circuit (Unico SE):
R38a/R80a are shorted instead of 4.7R.
Questions:
1) When amp turned on, a faint "pum" sound is heard from the speakers. Is this normal?
2) 5 seconds after switching on, very quiet music is heard in the speakers. In the operating mode, the sound becomes normal. Does the amplifier really not have 100% mute at startup?
I checked and compared each resistor on the board between channels and found that R35 and R39 (soldered on the board) have about 0.5-0.8Meg instead of 1Meg like in a good channel. I have seen this problem before, unsoldered R35 and R39 have 1Meg, that's why I replaced the OP AMP. Since the board had no suspicious areas, I did not see any problem and looked for leaks in the components.
There is a leakage of about 1-3 Meg on the board between pins 5-6-7-8 of the OP AMP and R35/39, at least.
I removed OP AMP, film capacitor, cleaned the top of the PCB with ethanol, the cotton swab turned dark brown, like after pine resin.
I cleaned and installed all the native parts back, and everything works fine, let's hope.
I noticed that many resistors on the PCB are placed above the board tracks without gaps, so they can short them.
Measurements:
Driver boards with 7 transistors in parallel: 0.64-0.65V at 150 ohms, 4.2mA 67.5Vce 0.28W per transistor - they are quite hot, 2W per board.
MOSFET 0.003V at 0.1R, 30mA per transistor (measured in "Volts" range)
Power consumption at 230VAC:
Start up: 38W (BIAS LEDs on)
Operating mode: 57-58W (BIAS LEDs off, AMP LED on) - in this mode the feedback works and the BIAS control depends on the mains voltage. I use a 150W light bulb for protection, and it flashed very brightly periodically.
If you remove the BIAS boards and the tubes, the driver transistors will generate square pulses at about 1Hz, so you cannot test the amp without the tubes. You need to install BIAS boards, and then you can check without tubes.
Difference in the circuit (Unico SE):
R38a/R80a are shorted instead of 4.7R.
Questions:
1) When amp turned on, a faint "pum" sound is heard from the speakers. Is this normal?
2) 5 seconds after switching on, very quiet music is heard in the speakers. In the operating mode, the sound becomes normal. Does the amplifier really not have 100% mute at startup?
Congratulations to a successful repair.
This amp does not have a speaker relay. Some slight noise at switch on is normal.
Do not worry about the switch on mute, it is ok.
This amp does not have a speaker relay. Some slight noise at switch on is normal.
Do not worry about the switch on mute, it is ok.
Hi all,
I have an unico 90 amplifier where the right channel is 3db quieter than the left channel.
Swapping the tubes crosswise has not been successful. Has somebody a service manual or a schematic for me?
Thanks in advance
Emil
I have an unico 90 amplifier where the right channel is 3db quieter than the left channel.
Swapping the tubes crosswise has not been successful. Has somebody a service manual or a schematic for me?
Thanks in advance
Emil
Thanks for uploading the Unico P service manual, it is really helpful! I’m just starting a repair now and opened it up to find multiple blown fuses, so the hunt for issues begins after checking them. 🙂
I have similar problem with my Unico (2x80w). A local HiFi specialist using oscilloscope found the problem. In my case The 2db difference between Left-Right is caused by the volume potentiometer. Unison Research is using Alps motorized potentiometer made in Japan. It's very easy to find it in Europe and costs 30 Euros. I will also do a full Recap. The technician is trying to find the original Itelcond capacitors. These are Industrial grade capacitors and very hard to find them outside Italy. A great alternative probably is Mundorf. I will also replace the 2xECC82 Tubes. After all these my 15 years old Unico will be new again. I am waiting all the parts to arrive and I will share with you the results.
outlawviper:
I am currently experiencing the following phenomenon. Most of the time both channels are the same, but after a few days the left channel is 3dB louder again. This changes back and forth. The Alps potentiometer could possibly be to blame. I will also change ECC82.
I am currently experiencing the following phenomenon. Most of the time both channels are the same, but after a few days the left channel is 3dB louder again. This changes back and forth. The Alps potentiometer could possibly be to blame. I will also change ECC82.