ROTEL RB-870 low output in bridge mode

[Swampy]

Morning, afternoon, or evening to you.

Amp came in to workshop with fuses blown of R channel power supply, replaced bridge rectifier and all appears well.

During testing I measured output @ 65 watts for each side into 8 ohm resistive load @ 1kHz running stereo mode no problems.

However, only measuring 18 watts in bridge mode with mains current draw > 2 A. The sine wave input, when measuring over dummy load, looks squared on the bottom.

Never worked with bridge mode amps before, but if I'm getting 65 watts per side in stereo that must place the tangle somewhere in the input stage right?

rotel_rb-870_sm.pdf




Chur

[mozwell]

Looking at the diagram, in briodge mode, you need to use the right input, and connect the speaker load between the two speaker positives, with the left speaker output connecting to speaker positive.

The right input goes into IC701 pin 2, which is set up as an inverting amplifier. Pin 1 of this should have an out of phase signal, of the same amplitude as the right input.

So, measure on the pcb that has IC701, between terminal 1 & the RCA ground, and between terminal 2 & the RCA ground, do you get the same signal level on each, but with terminal 2 inverted and of the opposite phase to terminal 1 ?
Also check pin 4 & pin 8 on IC701, do you get +/- 17VDC ?

[g1]

As well as what Mozwell said about proper hook-up to the load, if you are using a scope you can't connect the ground to either red post when checking bridged operation. I would suggest scoping each red to ground to make sure you are not clipping, and connecting a multimeter between the red posts to measure the AC voltage and calculate the power output.

[Swampy]

As well as what Mozwell said about proper hook-up to the load, if you are using a scope you can't connect the ground to either red post when checking bridged operation. I would suggest scoping each red to ground to make sure you are not clipping, and connecting a multimeter between the red posts to measure the AC voltage and calculate the power output.

Right. So first mistake was hooking up scope over load, figured that one out after taking out mains fuses.


Only measure signal on pin 1, no signal on pin two. R701 and 703 measure good with DMM. IC gone bad?

[Swampy]

Right. So first mistake was hooking up scope over load, figured that one out after taking out mains fuses.


Only measure signal on pin 1, no signal on pin two. R701 and 703 measure good with DMM. IC gone bad?

I lie. Signal all good on 1 and 2, also verified at C601 + 602. Measured 58 watts into 8 ohms from R - to ground with mains current draw > 3 A...the unit is only built to dissapate 500W from the mains @ 240 V = 720 Watts and only half output power?

It's gotta be my technique right? Straighten me out here fellows.

[g1]

Measured 58 watts into 8 ohms from R - to ground with mains current draw > 3 A

If you are back in stereo mode then I think the amp has a fault. Does the left side give the same result?
If you are still in bridged mode you should not have a load connected to ground.
At this point make sure everything is ok in stereo mode.

[Jazz P Bass]

Yeah, something is certainly not quite right.
A 58 watt output requiring 3 amps from the mains is a a bit too much current.
What does the idle current measure?

[Swampy]

Yeah, something is certainly not quite right.
A 58 watt output requiring 3 amps from the mains is a a bit too much current.
What does the idle current measure?

Idles @ 30mA in both modes.

In stereo mode both left and right dissapate 65 watts into 8 ohms, draw 0.76 amps at full output. Power amp is operating fine.

[g1]

Sounds good. When you last had it in bridged mode did you have the load between the 2 red posts? It sounded like one side of the load was grounded.
Try this: load connected between 2 red posts. One channel of scope from L red to ground, other channel of scope R red to ground (again, load does NOT connect to ground).
The 2 signals should be 180 degrees out of phase on the scope. Place a multimeter across the load, at full power (just before clipping) measure AC volts and calculate power output. Also check your current consumption.