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Old 3rd February 2009, 01:28 PM   #11
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Old 3rd February 2009, 01:40 PM   #12
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i pull your leg, you pull mine, sounds reasonable.

Seriously, check out the number of contacts between input and the power amp section in the service manual of the RA980BX that i posted.
Contacts can be a problemo major, depending where one resides, when i look outside the window at home on a clear day i can see oil refinery flare pipes in several directions, i stopped using silverwear ages ago.

A poor man's integrated Krell biased too high ?

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Old 3rd February 2009, 09:44 PM   #13
Bartt is offline Bartt  Netherlands
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Thanks for the reactions; didn't expect to have so many allready! I really appreciate it.

The amps is about 14 years old; should the caps be renewed? I remember from years ago (10?) that I had the same problem.

I'm pretty sure it's not just my ears of brains. It can't be that much of a difference. By the way, I don't listen for hours, but walk away and come back when the stereo played for a while.

Maybe I'll check the bias then, but I have no idea how to. Where can I find information about that? I do have the service manual.

I'll checked the termperature when it's idle. From the outside it feels warm, not hot. Does that say anything?
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Old 5th February 2009, 11:33 AM   #14
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IIRC, the RA-980BX was manufactured between 1992 and 1995.

There are 2 testpoints (TP) at both output stages, indicated in the top view of the amp in the service manual.
With a digital multimeter you can measure the voltagedrop across one of the white MPC70 emitter resistor blocks.
The trimpots VR601/VR602, which adjust the bias of the output stage, are also indicated in the top view drawing.
The SM says to adjust to 10mV, that's 45mA current through each of the 6 output devices.

With 6 power devices, the output stage of the 980BX is pretty sturdy for a 100W amp.
3 pairs of B817/D1047 handle 600W max dissipation, 6 times the continuous power in 8 Ohm is rather good.
The reason that Rotel chose a Pd/Power factor of 6 is because it lacks sophisticated protection circuitry, merely switches the power amp input to ground with a relay when the output current reaches 15A or when the heatsinks become to hot.
I'd check that relay if i were you !

First action would be to verify if the output stage is indeed biased at the advised 10mV, and not lower.
10mV is a low value, much better for an AB output stage would be 20-25 mV.
20mV is an extra 15 watts of idle dissipation per channel, each of the output devices can easily handle an additional 2.5W of heat.
You'd have to try it to see if the size and ventilation of the heatsinks are adequate enough for an increased bias.
Your Vivace loudspeakers are a very easy load for an amplifier. Afair, the minimum impedance of the first series 3-way LSP's is somewhere around 6 Ohms with an efficiency of 89dB/w.m

Checking the vitality of the PS electrolytics is more difficult.
Replacing four 10.000uF/63V would be a sound investment for a decent integrated as the 980BX.
Electrolytics have become both better and relatively cheaper, an exchange would cost around €35 if you order some parts at Digikey.
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Old 14th February 2009, 07:32 PM   #15
Bartt is offline Bartt  Netherlands
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I adjusted the bias and what a difference!! It's sounds so much better! The only thing is that one of the emittor resistors looks a bit black. I checked it and they all seem to be 0 ohm on my multimeter but on they should be 0,22 Ohm K (that's 220 Ohm?). So are they all burnt? On them is written: "MPC70 0.22 Ohm K fil 2502".

Also the bias seems to drift. When I set it at 10 mV it 'll be 20 or so in a while and next time I switch it's different as wel, though it won't get past 25 mV or so. Left and right are different (like 5-10mV difference). Is that a problem?

And about the caps: I ve seen lots of brands on the internet. What would be a good choice in my amp? What about these: Nichicon Muse KG 10000uf 63v at 90 euro? I couldn't find any at digikey btw.
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Old 14th February 2009, 09:53 PM   #16
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Originally posted by Bartt
[B]I adjusted the bias and what a difference!! ]

The bias shouldnt really make much difference to the sound unless it was so low that crossover distortion was occurring.
Once you have a standing current then it should sound pretty much the same.

I usually set my bias up with a scope and signal generator until the crossover distortion just disappears.
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Old 14th February 2009, 11:32 PM   #17
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Bartt, I think you should replace the emitter resistors, your bias shouldnt be drifting so much, Ive never seen this on rotels. As for the caps, I think they are still good, I have some rotels with 20 year old caps which are still good, they use high quality slit foils, far superior to the common muse series. If you do want to replace I suggest you contact rotel and replace with originals.
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Old 14th February 2009, 11:51 PM   #18
SQLGuy is offline SQLGuy  United States
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Originally posted by homemodder
Bartt, I think you should replace the emitter resistors, your bias shouldnt be drifting so much, Ive never seen this on rotels.
I don't buy this. I would first like to know how Bart did the adjustment.

In general, the amp should probably have been idling for at least 20 minutes before you start. If you had to turn the idle up a lot, then let it sit again for a few minutes to stabilize, then recheck, readjust, etc, until the bias is where you want it, and is probably drifting around within 1mV or so +/- of where you want it.

Then you should run the amp at pretty good power (say 50W or so) for a few minutes, and then allow it to settle back down for five minutes at idle. It should return to the point where you had adjusted it before.

If adjusted the amp before it was warmed up, or didn't allow enough settling time between adjustments, then I could certainly see the types of deviations you're talking about.

Others can probably provide some better tips on the bias adjustment process. This procedure has worked well for me, but I may well be missing some points as well.

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Old 15th February 2009, 12:10 AM   #19
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I recommended the replacing of the resistors as these are a weak link in all rotel models, they simple dont handle the power too good, I have 9 rotel amps and had to do this at some time or the other especially the more powerfull models, the value undergoes some change after some years use if you turn up the power and Bartt has said that some seem to be a bit black. The zobel resitor should also be checked as these fail after some use and become open circuit and the amp can have slight oscilation with some speaker loads which can cause the heating of the output transistors. It is important to check this as this amp uses no output coil.

The bias has to be set with the amp warmed up as specified by the owners manual.
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Old 15th February 2009, 03:15 AM   #20
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I haven't come across low value wire wound resistors that drift significantly in value with time and use. Could it be a problem with the junction of the lead out wires of the resistor and the resistive element ?

Bartt: that is 0.22 ohms and maybe your meter cannot measure low value resistors which is why it reads 0.

A friend with an Audiolab amp accidentally discovered that the amp sounded much better when it was 'hot' . That happened one day when something was placed on top of it hindering air flow. After a while we realised that the amp sounded much better than it usually did.In about half an hour it had become quite warm.
Obviously the dc bias had gone up . I couldn't do any electrical checks and before I got another opportunity , he sold it .

I have a Rotel RA970BX . I bypassed the input circuitry and connected the input selector switch directly to the volume control and that was connected directly to the power amp via a Solen cap ( original was a Blackgate ). It has completely transformed the sound of the amp. Sounds very nice now. Maybe I should bypass the input selector also. Right now the input switching is useful but maybe I can have one input direct wired.
That would make it parallel with all other inputs IF it's plugged in while the others inputs are used. I can live with that. Just have to make sure that input isn't connected when the others are used.
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