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DMW88

Rotel RB-980BX (2 channel) Issue

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Hi knowledgeable people, i thought i pick your brains on this issue i have with my power amp. When i connect my right speakers directly to the amp, there’s no sound in stereo mode. But when the amp is set to Bridge mode (mono), there’s sound. Obviously i have a preference in Stereo mode but am using Bridge mode to circumvent the issue.

 

Questions:

1. If the speaker binding posts has issues, then it shouldnt work irregardless of the amp mode.

 

2. In bridge (mono) mode, the amp is giving out 360w. However, Ive connected 1 speaker to each channel normally. I have concerns that the amp could cause damage to my speakers as they are only rated up to 100watts of power only.

 

Any thoughts are appreciated.

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Great old amps, those.  Worth persevering with to get it back to full functionality.

 

Answer to Q1 is yes.  Binding post fault would show up in either mode.

Switching an amp to bridge mode only manipulates the INPUT signals to each channel, it doesn't change anything at the output.  In this amp, bridge mode uses the left channel input.  Switching to bridge mode mutes the right channel input and feeds a reverse-phase left channel signal to the right channel amp.

You might have noticed the bass sounds a bit thin with both speakers connected in bridge mode as the out-of-phase signals will cause some cancellation to occur, most noticeable in the low frequencies.

 

Q2.  Leaving your speakers connected as a stereo pair when in bridge mode shouldn't do any damage.  Unless you crank up the volume way too far to compensate for the cancellation mentioned above!

You can only get 360W from the amp in bridge mode when there is just one speaker connected between the Left+ & Right+ terminals.  With the speakers connected as a stereo pair, the power amp section can only produce the power specified for stereo mode.

 

A couple of possibilities that I can suggest:

- Interconnect cable.  Reverse the L & R connectors just at the amp end, and see if the fault moves to the left channel in stereo mode.

- Right channel input connector. Maybe the solder has cracked where the connector is soldered into the PCB.  This occasionally happens in older gear after years of connectors being plugged in and pulled out.

- If the infamous glue used to secure large components in that period has been used to bond C602 to the circuit board, the glue may have corroded a component lead &/or wire link that it was in contact with.  I had this happen to a Zener diode that was very close to a filter capacitor in my RA-970BX.

- A dry solder joint/broken wire may have developed in the connections between the input board and the main board via the X1172 PCB.

- The protection relay might be worn out/sticky and is not releasing the right channel input from ground after the power-up delay.

 

The service manual is on Hifi Engine.  These amps are relatively easy to work on and I expect the fault would be quite easy to trace with a CRO.  Of course, it is my duty to state that only suitably qualified personnel should attempt repairs on mains-powered equipment.

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Thanks mate. Very insightful

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