I got a broken ROTEL RA-820BX3 pretty cheap.
When I started to measure voltages I discovered that they are about 4 volts lower then they should be, according to the service manual.
I measured now the voltages at the rectifier and found out that instead of 41 volt they have 37 volt. I assume that the power supply does not work properly now, or?
When I started to measure voltages I discovered that they are about 4 volts lower then they should be, according to the service manual.
I measured now the voltages at the rectifier and found out that instead of 41 volt they have 37 volt. I assume that the power supply does not work properly now, or?
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Yes I measure without input.
Then I took out the fuses which appear after the Transformer. Then I measure an AC of around 25 Volts. So I guess no problem with the caps...
Bias I have not measured yet...
So far I only put a sine tone on the input and I don't get any signal from speakers output. Only on the record output.
Nothing seems to be obviously damaged, no smoky resistors or similar...
Aha! It is not switching. But I might run in Trouble if put jumpers? Better to put a new relays inside I assume. I have a spare one like this:
http://www.schuro.de/Daten/Takamisawa/ry.pdf
http://www.schuro.de/Daten/Takamisawa/ry.pdf
Jajaja this funny with this schematic...but thats the one I found...If i measure directly at the speaker output I read almost 3mV on my multimeter.
If the relay is not switching, most probably there is a problem with the driver circuit (sometimes they also get stuck, but that's not the norm). It normally should engage a few seconds after power on, requiring only the main amplifier supplies to be present.
Relay driver circuitry is located just above the rectifier. Check the indicated DC voltages around Q638 and Q639, especially the -5.7 V. My primary suspects would include a leaky C652 and a dead Q639. Also check the thermistor for continuity.
In case the power indicator should be flickery, replace the little 10µ/50 electrolytic (C653).
Relay driver circuitry is located just above the rectifier. Check the indicated DC voltages around Q638 and Q639, especially the -5.7 V. My primary suspects would include a leaky C652 and a dead Q639. Also check the thermistor for continuity.
In case the power indicator should be flickery, replace the little 10µ/50 electrolytic (C653).
GoranB, there is a relay based circuit according to the schematic, but it is not a conventional DC protection. It appears to be simply for anti-thump and thermal overheat protection. It connects the current source made by Q607 to the VAS buffers Q609/Q611. If the relay or its control circuitry fails to energise, there will be no sound.
I found TH601. It is glued on the Q627
I will buy some transistors tomorrow and exchange Q638/639. Can I use BC 640 for 2SA608K?
If I need to exchange the thyristor TH601 (td5c350d), which substitute shall I get? My Ohmmeter reads 70k on this part, but I measured directly on the board...
I doubt the thermistor is faulty, but apparently it is a 50Kohm NTC type. You can test it easily - measure its resistance and heat it up with the tip of a soldering iron. You should see the resistance change.
You can use BC640 but bear in mind the pinout will be different so you will have to bend the leads to fit.
You can use BC640 but bear in mind the pinout will be different so you will have to bend the leads to fit.
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