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Old 17th September 2013, 07:19 PM   #1
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Default rotel bias

Hello, I recapped my Rotel 840b that I owned since new. Now it won't bias correctly. Using the test points it should read 4.4mv. I'm getting 65mv with vr601,2 turned down. I have 41.1v on the rails(does the schematic show this?) also the correct v on 4 and 7 of ic 402 and 404, so i think Q901 and 902 are operating correctly. Offset voltage is 19mv.

This amp worked before I messed with it. I just have a DMM. I did check similar threads nothing seemed to apply to my problem. Just learning here. Any help to get this thing to bias...well, (I'll come paint your house or something.)

Thanks
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Old 18th September 2013, 06:40 AM   #2
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Let's just check basics:
Make sure nothing but power is connected to the amp, turn all volume down or even short the main amp input connections with 1k resistors or such. This eliminates disturbance to your setting.

Did you increase the capacitance values greatly from those specified and which caps, if not all in the amplifier, did you replace?

Are you saying that 65 mV reading (for bias current) is the lowest amount you can adjust to? Can you still vary the current easily?
Sometimes, we hear comments about the trimpots failing when adjusted but this is unlikely in both channels and you can check this by operating the control and watching the rate of change on the meter anyway.
C605,606 usually can be best replaced with higher voltage rating parts, like 16-25V for better reliability, though this should not affect bias.

In answer to your query, +/-40V is the nominal rail voltage according to the voltages marked on the schematic. That will vary with your local mains power supply, so 41V is cool and within tolerance for domestic appliances, as it should be.

Let's see if this is OK so far.
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Old 18th September 2013, 08:19 AM   #3
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You can download the full service manual here. Unfortunately, it has no indication where TP1,3 and TP2,4 for either channel bias setting are located. Presumably, it will be the usual measurement across one of the OR22 Emitter resistors of the ouput transistors. No doubt, it's all marked on the PCB.
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Old 18th September 2013, 11:43 AM   #4
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Hi Ian, First, thanks for your time here. The only value I increased was c903,904 from 8200mf-50v, to 10000mf-63v. 65mv is the lowest, both pots rise from there easily. So..are you saying to engage, say.. the cd input, then short across the input with the resistor? The test points are easily found in front of the power transistors. Also the PCB is lettered on both sides.

Thanks
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Old 18th September 2013, 11:50 AM   #5
Mooly is offline Mooly  United Kingdom
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I take it this 4.4 mv is across the emitter resistors. 65mv would be frying the output transistors. Are they getting hot ???
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Old 18th September 2013, 11:59 AM   #6
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Thanks Mooly, No there not hot.
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Old 18th September 2013, 12:16 PM   #7
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First, just turn the volume down to zero, switch to any unused line input and disconnect all but the power cord - that keeps it simple.

Yes, Mooly is correct if the voltage across any emitter resistor is 65mV. that's 6 amps DC per channel and would even have the transformer glowing, so not sure what you are measuring.

Can you please describe where TP1,3 and TP2,4 are? At least point out what you are measuring, either on the service manual's PCB assembly diagram or on the schematic, if you can follow it clearly enough. The problem is, you have the PCB with the marks but we don't.
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Last edited by Ian Finch; 18th September 2013 at 12:28 PM.
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Old 18th September 2013, 12:44 PM   #8
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by mark adams View Post
Thanks Mooly, No there not hot.
Two things came to mind.

1) Your measuring the wrong points or misunderstanding how you measure the bias. Its normally across one of the emitter resistors. I wondered if you were reading from ground to some test point.

2) Your meter is inaccurate at low level (unlikely) or you have it on a wrong range/setting such as AC.

If the transistors aren't hot they aren't passing the current your readings seem to suggest.
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Old 18th September 2013, 12:48 PM   #9
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O.K., TP1 connects to the emitter of Q625. TP3 connects to the trace that is shared with R629,625,633,627,635,631, and a jumper from fuse 601. TP2 to E of Q622. TP4 trace is from fuse 602 shares R632,628,636,626,634,630.
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Old 18th September 2013, 12:59 PM   #10
Mooly is offline Mooly  United Kingdom
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OK, so the test points are indeed over a single emitter resistor. If the transistors aren't hot then there isn't a true 65mv over that 0.22 ohm.

I would recheck your readings and it might be easier to solder wires to the test points and bring those wires to the meter. Any tarnish etc on the test points or the meter probes can affect the reading (and its easy to slip with disastrous results). And remember you are measuring across the test points and not from ground to them.

I think something is going wrong with your measurement technique tbh... or the meters not right in some way.
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