electrotricks: ROTEL RA 980BX STEREO AMPLIFIER REPAIR ASSISTANCE

Monday 10 June 2019

ROTEL RA 980BX STEREO AMPLIFIER REPAIR ASSISTANCE

Fault: No output from left channel. The right Channel is working fine.

Added some photos and circuit for reference

While testing the amp noticed that the left channel is giving switch on off thump while powering ON and OFF the amplifier. It happens when there is DC voltage flow through the speakers. It shouldn't be there. And the thump is there only on the left channel. While testing the amplifier section found that all the transistors where idling at mentioned voltage. All Base pins where around +/- 0.5volt. and the output is almost 0.012mvdc.

As the amplifier had an external Pre -IN, Pre -out coupling it was easy to use an external pre amp to test the amplifier section.

While testing with an external pre amp found that the amplifier section is OK. There were no switch off thump too while working through the external pre amp.

After testing the amplifier stage and confirming it is OK, turn to the next stage. The internal pre amp (tone control) section.

The internal pre amp of Rotel RA 9808X is built with two AD712JN ICs. Where each AD712 is used for the left and right channel separately. Hope Rotel did it to reduce the bleeding between the left and right channel. Its possible to reduce the bleeding between two channels. If we use two op amps separately for each channels than using the half of each.

While testing the pre amplifier it is found that there was a DC voltage of 15V at the pin no 7 (output pin) of one of the AD712 which is used for the left channel. It shouldn’t be there. The output, non inverting and inverting pins of an op amp which is used in the tone stage circuit should be at 0vdc ,if it’s working in dual supply rail (+/- supply). And there was no voltage found on the pin 7 of other AD712 which is used for the right channel. While further treating found that the supply voltages to AD712 where ok its there it +/- 17.5V.

As there was nothing else to check, replace the AD712 with a new one. Then power the amplifier and found that it’s working perfectly. There were no voltages where found on the pin no 7. And also there were no switch of thump too. Also replaced the power filter capacitor and coupling capacitors near the AD712 of each channel as it is found bad.

If AD712 is not available then Instead of AD712 you can use TLO82. It is an equivalent to AD712. Or you can use NE4558 , it will work too but there will be some difference in frequency response of the amplifier, but not that much noticeable. If your customer is not an audiophile then it’s ok.