Purple MC76 P2P problems - SOLVED !
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Rob Flinn
Well-known member
[quote author="edanderson"][quote author="mnats"][quote author="Rob Flinn"]I seem to be having a problem understanding the calibration procedure, can someone (Ed SSLtech) clarify it for me.[/quote]
I'm neither of the people you asked for, so you can take my advice with a grain of salt. Since the MC76/77 is essentially an 1176, try the calibration instructions in section 5.3.3 of the UREI manual. R54 in the commonly circulated manual refers to R44 - the fixed, selected resistor in the Rev C/D/E and MC76 schematic, but other than that I don't think there is a more lucid, comprehensive explanation of the calibration process that has ever been written.
FWIW[/quote]
i am one of the people you asked for, and i agree with mnats. if the calibration procedure in the MC76/77 manual doesn't make sense to you (and i agree it is confusing), just use the cal instructions in the 1176 manual.
r44 on the more recent MC77 PCBs is a resistor, trimpot, and jumper in series. this is so you can use a 100r resistor (so that the composite value is never 0 ohms) in series with a 2kohm trimpot to dial in the value of r44. the jumper is there so that you don't have to remove the resistor during cal.
ed
ed[/quote]
Ed sorry to come back to you on this again. I have been too busy to look at this for a few weeks.
Are you saying there is no meter scaling adjustment to be made ????
Also could you please explain your last paragraph above in more detail. I'm just feeling confused about this calibration procedure despite having done this several times before to Gyraf & real silver face 1176's. When I adjust the meter scaling pot it has no effect at all. Does this mean I have left out a jumper or is this correct ??
I'm neither of the people you asked for, so you can take my advice with a grain of salt. Since the MC76/77 is essentially an 1176, try the calibration instructions in section 5.3.3 of the UREI manual. R54 in the commonly circulated manual refers to R44 - the fixed, selected resistor in the Rev C/D/E and MC76 schematic, but other than that I don't think there is a more lucid, comprehensive explanation of the calibration process that has ever been written.
FWIW[/quote]
i am one of the people you asked for, and i agree with mnats. if the calibration procedure in the MC76/77 manual doesn't make sense to you (and i agree it is confusing), just use the cal instructions in the 1176 manual.
r44 on the more recent MC77 PCBs is a resistor, trimpot, and jumper in series. this is so you can use a 100r resistor (so that the composite value is never 0 ohms) in series with a 2kohm trimpot to dial in the value of r44. the jumper is there so that you don't have to remove the resistor during cal.
ed
ed[/quote]
Ed sorry to come back to you on this again. I have been too busy to look at this for a few weeks.
Are you saying there is no meter scaling adjustment to be made ????
Also could you please explain your last paragraph above in more detail. I'm just feeling confused about this calibration procedure despite having done this several times before to Gyraf & real silver face 1176's. When I adjust the meter scaling pot it has no effect at all. Does this mean I have left out a jumper or is this correct ??
Rob Flinn
Well-known member
OK so I figured out that there are 2 holes for jumpers next to the 100R (not in the pic of the completed pcb that people were linking to). Thing is when I hook this up the meter scaling now changes the meter setting, but I can't seem to get it to display the correct -6dB when the o/p is 50% of the i/p. Any ideas ??
edanderson
Well-known member
with the jumper not connected, the needle should move down from 0db when in bypass to minus something when compressing. when you connect the 2k trimpot and 100ohm resistor, the needle should move lower, allowing you to hit -6db (or -10db, whatever amount you are compressing)
you probably will have to re-do the 0VDC null on r75 once you have connected r44. unfortunately, the calibration steps interact, so make sure that you have 0VDC across r75 with no gain reduction and the meter at 0db AFTER you have connected r44.
if you can't get all the way down to -6db, try a different 2n5457 fet. they vary greatly and some simply cannot be made to show sufficient gain reduction within the limits of the trimpot. be sure also to make sure that the PCB is very clean around the FETs and associated connections... just a bit of stray flux or oil can mess it up because the FET buss is very high impedance.
ed
you probably will have to re-do the 0VDC null on r75 once you have connected r44. unfortunately, the calibration steps interact, so make sure that you have 0VDC across r75 with no gain reduction and the meter at 0db AFTER you have connected r44.
if you can't get all the way down to -6db, try a different 2n5457 fet. they vary greatly and some simply cannot be made to show sufficient gain reduction within the limits of the trimpot. be sure also to make sure that the PCB is very clean around the FETs and associated connections... just a bit of stray flux or oil can mess it up because the FET buss is very high impedance.
ed
Rob Flinn
Well-known member
[quote author="edanderson"]
you probably will have to re-do the 0VDC null on r75 once you have connected r44. unfortunately, the calibration steps interact, so make sure that you have 0VDC across r75 with no gain reduction and the meter at 0db AFTER you have connected r44.
ed[/quote]
Ed
Why is it necessary to disconnect R44 to null the circuit, when from what you're saying you have to re do this null setting once it is reconnected ??? I'm sure you must be right, but I'm just curious.
Also the problem I seem to have is not one of the meter not showing enough gain reduction. It's like an on off switch,in that it either sits at zero (no reduction) or switches off (full gain reduction) !
you probably will have to re-do the 0VDC null on r75 once you have connected r44. unfortunately, the calibration steps interact, so make sure that you have 0VDC across r75 with no gain reduction and the meter at 0db AFTER you have connected r44.
ed[/quote]
Ed
Why is it necessary to disconnect R44 to null the circuit, when from what you're saying you have to re do this null setting once it is reconnected ??? I'm sure you must be right, but I'm just curious.
Also the problem I seem to have is not one of the meter not showing enough gain reduction. It's like an on off switch,in that it either sits at zero (no reduction) or switches off (full gain reduction) !
edanderson
Well-known member
[quote author="Rob Flinn"][quote author="edanderson"]
you probably will have to re-do the 0VDC null on r75 once you have connected r44. unfortunately, the calibration steps interact, so make sure that you have 0VDC across r75 with no gain reduction and the meter at 0db AFTER you have connected r44.
ed[/quote]
Ed
Why is it necessary to disconnect R44 to null the circuit, when from what you're saying you have to re do this null setting once it is reconnected ??? I'm sure you must be right, but I'm just curious.[/quote]
once r44 is connected, the meter zero and null adjust become much more sensitive. so, if you haven't already dialed in the zero, it will be harder to do once you've connected r44.
ed
you probably will have to re-do the 0VDC null on r75 once you have connected r44. unfortunately, the calibration steps interact, so make sure that you have 0VDC across r75 with no gain reduction and the meter at 0db AFTER you have connected r44.
ed[/quote]
Ed
Why is it necessary to disconnect R44 to null the circuit, when from what you're saying you have to re do this null setting once it is reconnected ??? I'm sure you must be right, but I'm just curious.[/quote]
once r44 is connected, the meter zero and null adjust become much more sensitive. so, if you haven't already dialed in the zero, it will be harder to do once you've connected r44.
ed
Rob Flinn
Well-known member
Ed
OK.
I have tried following the UREI manual to calibrate these MC76 & neither of the pair I'm building are giving me the readings I'm looking for.
For example Paragraph 2 page 21:
"Turn the attack control on & observe a drop in the meter" Well I'm not getting a drop at all !
This leads me to believe that I have made a mistake somewhere because they both behave the same. Is there any possibility someone could do a high res pic of there MC77 rev 44 board populated as an MC76, so I can check it against mine ?
The other thing I just thought was perhaps I have the i/p transformer the wrong way round. If correct it should be reducing gain ?????
OK.
I have tried following the UREI manual to calibrate these MC76 & neither of the pair I'm building are giving me the readings I'm looking for.
For example Paragraph 2 page 21:
"Turn the attack control on & observe a drop in the meter" Well I'm not getting a drop at all !
This leads me to believe that I have made a mistake somewhere because they both behave the same. Is there any possibility someone could do a high res pic of there MC77 rev 44 board populated as an MC76, so I can check it against mine ?
The other thing I just thought was perhaps I have the i/p transformer the wrong way round. If correct it should be reducing gain ?????
Purusha
Well-known member
Rob, what FETs did you use? I did a mistake in mine and used the FETs for the G-1176 and also didn't have any GR.
Rob Flinn
Well-known member
[quote author="Purusha"]Rob, what FETs did you use? I did a mistake in mine and used the FETs for the G-1176 and also didn't have any GR.[/quote]
I used the correct fets, & I do have gain reduction. The problem I have is in calibrating the meter. What ever I seem to do it will not calibrate to any of the procedures !
I used the correct fets, & I do have gain reduction. The problem I have is in calibrating the meter. What ever I seem to do it will not calibrate to any of the procedures !
Rob Flinn
Well-known member
Right going back to this again after shelving it due to work.
Is there anyone who has managed to calibrate the meter to show the correct gain reduction ?????????
I have 2 units both of which I can only get to show 10dB gain reduction when it should be 6dB, this is with the scaling preset at one end of it's travel. The only way I can get the meter to show -6dB is if I set the zero control make the meter in the pegging region when it should show zero ? It seems like the presets do not have the range to
I have been setting this up with a dual dB meter, which I have previously set up loads of 1176's with no problems.
Can someone give me some pointers here ??as to what I've done wrong, as I've spent hours farting around with this, and just want to get them in my studio, where there is 4U of space waiting for there arrival
Is there anyone who has managed to calibrate the meter to show the correct gain reduction ?????????
I have 2 units both of which I can only get to show 10dB gain reduction when it should be 6dB, this is with the scaling preset at one end of it's travel. The only way I can get the meter to show -6dB is if I set the zero control make the meter in the pegging region when it should show zero ? It seems like the presets do not have the range to
I have been setting this up with a dual dB meter, which I have previously set up loads of 1176's with no problems.
Can someone give me some pointers here ??as to what I've done wrong, as I've spent hours farting around with this, and just want to get them in my studio, where there is 4U of space waiting for there arrival
SSLtech
Well-known member
Did you match the FETs?
Keith
Keith
Rob Flinn
Well-known member
[quote author="SSLtech"]Did you match the FETs?
Keith[/quote]
Yes, with PRR's multipoint fet matching circuit.
Keith[/quote]
Yes, with PRR's multipoint fet matching circuit.
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