Help Needed: Excessive DC Offset/Imbalance - Rotel RB-990BX Power Amp - Page 11 - diyAudio
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Old 14th October 2012, 07:04 PM   #101
Mooly is online now Mooly  United Kingdom
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If you connect your meter to measure the offset and then touch any of the four input transistors with a finger to heat it you will see the offset change. Depending which transistor you choose will determine if the change is higher or lower... try it
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Old 14th October 2012, 07:14 PM   #102
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Here's another spice simulation with two different bias settings. Look at the voltage across Q617. The offset is unchanged between the two as expected.
Attached Images
File Type: jpg High bias.JPG (145.5 KB, 62 views)
File Type: jpg Low bias.JPG (144.6 KB, 59 views)
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Old 14th October 2012, 07:30 PM   #103
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Quote:
Originally Posted by Mooly View Post
If you connect your meter to measure the offset and then touch any of the four input transistors with a finger to heat it you will see the offset change. Depending which transistor you choose will determine if the change is higher or lower... try it
Understood... They have a shroud with heat sink compound
To ensure constant ambient temps... They have been reinstalled
With this on for your info... The 1016s & 2362s arrived sequentially on
Ammo tape so they are consistent BUT I have to check
that the 1016s
& 2362 have same gain code......

Stay tuned

Thanks,
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Old 14th October 2012, 07:41 PM   #104
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I wouldn't worry over it too much tbh (certainly not at this stage). And do check the other channel too for interest. If you feel it is a problem then you could look at getting "off the shelf" unmatched devices and matching them yourself.

That's me done for tonight. Any questions... just ask
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Old 14th October 2012, 07:57 PM   #105
dtm1962 is offline dtm1962  Canada
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Quote:
Originally Posted by Mooly View Post
I wouldn't worry over it too much tbh (certainly not at this stage). And do check the other channel too for interest. If you feel it is a problem then you could look at getting "off the shelf" unmatched devices and matching them yourself.

That's me done for tonight. Any questions... just ask
Thanks Mooly .... Will keep this in mind ... Transistor matching that is...for future undertaking.... Once I get the amp to an usable state...

Stay tuned for tonight's finale ... Will be home in 2-3 hours to do the final test...
Must remember to take out 4 amp rail fuses and add 6 amp prior to removing
bulb in AC line ... I didn't want to waste the 6 amp units as I had
a bunch for 4 amp units I was prepared to sacrifice

Stay tuned
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Old 15th October 2012, 11:40 AM   #106
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Stay tuned for tonight's finale ... Will be home in 2-3 hours to do the final test...

Stay tuned
Couldn't get to it last night..... will perform the final test(s) tonight...

Stay tuned....
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Old 15th October 2012, 05:46 PM   #107
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Quote:
Originally Posted by dtm1962 View Post
Stay tuned....
We're all tuned in... just take it slow and check along the way. No rush
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Old 16th October 2012, 03:47 AM   #108
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Quote:
Originally Posted by Mooly View Post
We're all tuned in... just take it slow and check along the way. No rush
Hi Guys:

I connected it up without bulb in AC Line and adjusted the bias accordingly.

I noted the 195mV DC offset and decided to power down and match the 1016's & 2362's in the Differential Input with hfe matched to within <10% between the 1016 pair & 2362 pair (& <5% between the 1016/1016 & 2362/2362).

I got the DC offset down to 80mV on the repaired channel versus <10mV on the good channel.

I ran it unloaded for 2 hours and noted the Bias stayed at 7.0mV +/-0.20mV
and the DC offset stayed constant at 80mV +/-1mV.

So for now I will leave it as is and hook it up to an old pair of speakers to let it run for awhile. I will stick a 100uF VDC EE Bipolar cap in series with the speakers, just in case it lets go in this testing mode. I will do that tomorrow once I get home from work.

I am assuming there is something is still not perfect with the repaired channel, but I assume 80mV is not really a problem as it would be less than 0.0016W of DC @ 4ohms

Let me know what you think.

Thanks in advance,
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Old 16th October 2012, 04:45 AM   #109
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Way to go! But I agree with you that maybe the repair is not entirely finished yet, 80 mV is not good enough, especially in light of the good channel's <10 mV.
I wonder what had caused the offset in the first place. Seems that we were mostly chasing a bad replacement part in this thread, that may have diverted our attention from the original cause a bit.

A few questions I have for putting together the puzzle:
Did you keep al failed and/or replaced components? If so, can you mark in the schematic which components failed (and how, i.e. failed open or short)?
Can you do comparative measurements again between the channels?
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Old 16th October 2012, 06:39 AM   #110
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That's brilliant

Now this 'ere offset. You haven't a fault, just an imbalance due to non matched input devices. It will never be a problem and will never cause a problem.

So that said, how do we improve on it if we want to. Well I notice that Q615 and Q611 are the same type. So next question... can we use these ? And you have two more in the other channel. So you see where this is going

It is only the balance between each of the input stage NPN's or PNP's that matter. You don't have to match NPN and PNP which is impossible anyway despite what you hear about "matched pairs". So if Q615 and the one in the other channel are the same and also Q611 and its opposite number then we have a solution

Q615 and 611 are not critical as they are used as current sources and can be swapped about as necessary or substituted with another device altogether.
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