Rotel 840BX Bias Adjustment - diyAudio
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Old 19th March 2012, 05:45 PM   #1
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Default Rotel 840BX Bias Adjustment

I would like to check the Bias on my 840BX. The service manual states to test with the + lead on TP3 & - lead on TP1 and adjust VR601 to 4.0mv.

Then repeat on TP4 &TP2. I have the wiring schematic in the manual but there is no TP1 thru TP4 that I can find. Any suggestions? I can forward the manual to anyone that can help me out.

Thanks,
Bill/Texun1

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Old 19th March 2012, 05:53 PM   #2
AndrewT is offline AndrewT  Scotland
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I suspect the Test Points are near the output device emitter resistors.

Look around the outputs and see if test points are fitted.

If the output stage is common emitter then the test points could be located around the output line and the collectors of the output devices.
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Old 19th March 2012, 06:32 PM   #3
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checked with the schematic ... you are correct they missed it 4.0mv is the voltage drop you will measure across to any of the emitter resistors of the amplifier ...

that will give you a bias of about 20ma which i find quite low at least this is what the service manual says

Kind regards
sakis
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SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
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Old 19th March 2012, 07:04 PM   #4
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Quote:
Originally Posted by sakis View Post
checked with the schematic ... you are correct they missed it 4.0mv is the voltage drop you will measure across to any of the emitter resistors of the amplifier ...

that will give you a bias of about 20ma which i find quite low at least this is what the service manual says

Kind regards
sakis
Thanks for the response gents. I did find the test points and my readings are 4.1 & 4.3. Sakis you said this is low. Is there a higher setting that you would recommend.

Regards,
Bill
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Old 19th March 2012, 09:29 PM   #5
tvrgeek is offline tvrgeek  United States
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Low? Odd. I was just wondering if it higher bias and that was one of the keys to why it sounds so darn good when the basic circuit and parts are mid-line at best. 125mA is more typical but I have no idea if the heat sink can take it. I have a pair of RA-951's too. Same listenable sound but with a few modern touches like a protection circuit. I am keeping my paws out of my RA-840II as it just sounds so darn nice.
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Old 19th March 2012, 09:47 PM   #6
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125ma will be far to high for this amplifier ( assuming that we are talking 125ma per device) i don't think that the heatsink will ever make it

still calculation of 18ma to my opinion is low still this is the recommendation of the manufacturer ..... i presume that this much is only required to reduce crossover distortion and not to bias the very first watts of the amp in class A

Kind regards
sakis
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SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
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Old 19th March 2012, 09:58 PM   #7
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This all started when I wanted to just check a few things on this old amp to see if she was still in basic good health. I started with the DC offset at the speaker terminals. Ideally it should be very close to 0.000Vdc. Well one channel was -57 and the other was 1.4. Most amps have a pot that can be adjusted to help bring these values a little closer. As far as I can tell this is not an option on the Rotel. It seems to me the only option would be a set of new matched transistors.


Next I wanted to check the bias while I had it opened up. That was the reason for the post. Once I found the test points everything was pretty much on spec. I agree this old girl sounds great for her age. I did bump the Bias to 5.0 and will keep an eye on the temps.

Last edited by Texun1; 19th March 2012 at 10:08 PM.
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Old 20th March 2012, 11:19 AM   #8
AndrewT is offline AndrewT  Scotland
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...........If the output stage is common emitter then the test points could be located around the output line and the collectors of the output devices.
if this is the case then the bias voltage is very much lower than for an EF output stage.
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Old 20th March 2012, 07:44 PM   #9
tvrgeek is offline tvrgeek  United States
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Quote:
Originally Posted by sakis View Post
125ma will be far to high for this amplifier ( assuming that we are talking 125ma per device) i don't think that the heatsink will ever make it

still calculation of 18ma to my opinion is low still this is the recommendation of the manufacturer ..... i presume that this much is only required to reduce crossover distortion and not to bias the very first watts of the amp in class A

Kind regards
sakis
From what I think I understand, it is not the class A region, or even crossover distortion, that we run higher bias unless you are going for several watts, but it has more to do the the linearity of the devices themselves.
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Old 20th March 2012, 08:01 PM   #10
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Agree ... no probs with that i just stated my opinion
the OP has pretty good understanding of the troubles related to temperature if bias is increased and generally stability issues ... but also owns the amp for quite some time so he may understand if it sounds better or worst with a click of bias more .

Another thing i wanted to add is that offset and other problems might originate from capacitor related issues ...sampling a couple of them for evaluation will be an easy thing to do ( to my world replacing them all will be the best to do ) since amplifier is old enough for a thing like that ...

As a million times said before ...given the opportunity you always have the option to put something better bigger in the same space

kind regards
sakis
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