mc77 questions

[yosh]

I'm building a pair of mc77s with Altran c-3837-1 input transformers provided in the kit from Purple Audio. I have two questions:

1. I installed the transformer so the black dot on the label is above pin 1 on the pcb. Is this correct orientation?

2. I'm clear on the wiring of the t-pad from the xlr jack, but from the t-pad to the altran I'm not sure. I assume that I use pads 6 and 8, but which does the com go to and which does the out go to?


Oh, and how did you who finished an mc76 do the metal work for attaching the switchbanks to the front panel? I could probably use screws through the front panel, but I don't want to ruin the look.

Thanks!

-Josh

 

[edanderson]

yes the black dot indicates pin 1.

look to the left of the input transformer, there are two pads marked + and -
input XLR pin 2 goes to "IN" on the attenuator
input XLR pin 3 and PCB pad - go to "COM" on the attenuator
PCB pad + goes to "OUT"

there may be some differences, but this pic should give you some idea of the wiring:

http://www.purpleaudio.com/images/MC76kit/MC77guts.jpg

ed

 

[yosh]

that's a huge help! thanks Ed!

-Josh

 

[smilan]

Hi, I know it's a very old thread but I hope someone can assist me with this project.
I have two MC77 v46 PCBs that I received from someone who never finished working on them.
The PCBs are almost fully populated, I just need to complete a few parts, adding xformers and pots and install everything in enclosures.
Since I don't have the original building guide (if someone still have it please send me a copy) I have few questions:

Relays and jumpers
On my PCB's there are bypass jumpers installed on RL 1, 2, 3 and 5 and none of the relays have been installed.
Do I need to get any of the relays or I can use it with the jumpers as an 1176 (not MC77)?
Is RL4 (that doesn't have a jumper) required?
What about J12 (that is currently not installed on my PCB's)?



Small PCBs

I do have the meter switches (without PCB) and the ratio switches (with a Purple audio ratio PCBs).
I don't know if the original kit came with other PCBs like the pots and LED PCBs on the commercial version of the MC77?


Power transformer

The MC77 schematic calls for a "center tapped 39.3 VAC x 2) secondary while other DIY 1176 using a 2X24VAC/500mA or 2X25VAC/500mA .
I can get a 2 x 25VAC xformer, does it work with the MC77 PCB?

Input / output Xformers
I received the PCB's with one output transformer (PA-96731) so I'm planning to order another one from Cinemag.
For the input transformer I'm planning to order CM-3303.
There's a reason to prefer the CM-O12 over the CM-3303 for this project?
The CM-3303 is cheaper and according to David from Cinemag is better than the CM-O12.

Meters
Does a Sifam AL29WF would work fine with the MC77?

 

[Rob Flinn]

The boards were supplied (from memory) with the switch banks, the i/o & mains transformers and the chunky zener for the psu. The were intended for making an MC76 although the circuit board is the same. Therefore your seller probably didn't populate the parts that relate to the MC77 which are in the main relays & the stereo link circuit. There is nothing to stop you building it into the MC77 though.If you PM me your email address I probably have the documents.

 

[smilan]

One more thing, since I have two MC77s, I would like to match Q1 an Q11 in both of the compressors.
I have here a bunch of 2N5457s, does this test with this tester is enough to choose the 4 transistors, or it would be better to test the transistors in this circuit?

 

[Whoops]

I have here a bunch of 2N5457s, does this test with this tester is enough to choose the 4 transistors?

Does if measure the Beta/HFE value?

 

[smilan]

Does if measure the Beta/HFE value?

According to the manual of this tester this tester measures Id(Drain Current) at Vgs (Gate to Source Threshold Voltage).
Can I determine the value of Beta from this information?

 

[moamps]

I have here a bunch of 2N5457s, does this test with this tester is enough to choose the 4 transistors, ...

Yes.

 

[moamps]

According to the manual of this tester this tester measures Id(Drain Current) at Vgs (Gate to Source Threshold Voltage).
Can I determine the value of Beta from this information?

No, Beta/hfe are parameters for BJTs, not for JFETs.

 

[smilan]

No, Beta/hfe are parameters for BJTs, not for JFETs.

I think you're right about that there's no hfe value for JFETs, but according to this article there's a Beta value for JFETs

 

[moamps]

I think you're right about that there's no hfe value for JFETs, but according to this article there's a Beta value for JFETs

Correct, but when beta/hfe/(hFE) parameters are written, then we mean BJT transistor parameters that describe the ratio of collector current to base current for different operating conditions.

 

[beatnik]

Is it possible to vary the Vgs parameter with that tester you have ?

The dedicated circuit you have posted tests the fet at various Vgs values, to ensure the curves are consistently matched across a range of voltages

If you test the fets only at one spot value, they might show good matching at that voltage, but be off at another point. So for example you have matching at 2dB gain reduction, but they are off at 5dB

I've always used a test circuit similar to the link you posted in conjunction with a vu meter and it gets the job done very well

 

[smilan]

Is it possible to vary the Vgs parameter with that tester you have ?

The dedicated circuit you have posted tests the fet at various Vgs values, to ensure the curves are consistently matched across a range of voltages

If you test the fets only at one spot value, they might show good matching at that voltage, but be off at another point. So for example you have matching at 2dB gain reduction, but they are off at 5dB

I've always used a test circuit similar to the link you posted in conjunction with a vu meter and it gets the job done very well

That make sense.
I can't control the Vgs voltage with my meter.
I'll build this test circuit and test my JFETs at various voltages.