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
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
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.
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.
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.
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
Thanks
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.
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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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.
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.
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.
Thats correct and a good check on your meter. Try your bias adjutment again and make sure the meter is securely connected across the test points. Also you can do a physical check and make sure those test points really do go where the manual says. And you can also measure across any one of those 0.22 ohms to set the bias. All will give the same result. Thats worth looking at too. Its remotely possible (but very unlikely) that there is a problem with one set of outputs.
When you measure... its no signal and no speakers connected.
When you measure... its no signal and no speakers connected.
This may be of some help at showing crook meters, flat batteries etc. because 4.4 mV +/- 10% say, is a big ask for most DMMs. Measure across a pair of emitter resistors, one from the top pair of output transistors, the other from the bottom pair. That's in series or from collector to collector - in the same channel. - If your first reading was correct you should see 2 * 65mV or 130mV but I'll guess that's not what you read.
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Ian and Mooly, I think I did something really stupid here. I replaced the original emitter resistors they are marked 0.22 ohmsk which is .22 kohms or 220 ohms correct? But as I check the old resistors I get a .3 ohms reading. It seems the k after the ohm marking had nothing to do with the value. So 220 ohms is wrong. There is no K on the schematic either. Again I installed 220 ohms. Does this make sense? I changed them due to age.
Ian and Mooley, I put the old resistors back. It dialed right in. I'm playing it now and it sounds fine. Sorry if I wasted your time. You guys have probably have forgotten more about this stuff than I know. My approach to these projects may be flawed as well. I replaced resistors and diodes while recapping. How would you go about an older project like this? Anyway, thanks for your time!
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