Building Purple MC76 - troubles and solutions

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RE: input transformer

Yes, it's 500:200 ohms. That was accounted for.

Pin 7 is tied to the can externally, as you can see, and the silk screen shows that pin 7 belongs in the row of holes on the right side. Pin 1, indicated by a dot on the label, then ends up in the upper left, just like the silk screen shows. Trust the silkscreen!
 
Bump! regarding the bridging of jumpers for 240v operation, I don't even have my kits yet.. but I still want to know the answer!! hehehe Maybe Ed can confirm this finally. :thumb:
 
I am still wondering how to connect the torroid to 220VDC without going to
the board and using one of those

ckv_slide.jpg


Can someone tell me how they work? I couldn't find any schematics of the
switch. Can someone make a simple drawing of how the switch works? The
rest I can figure out on my own.
 
[quote author="hitchhiker"]Purusha! I did the same as Keith, and didn't take the ac to the pcb.

For 220v primary I figure the split primary is wired in series. I think the primary hook up would be the orange and yellow wires tied and the white and black to the 220v ac . Hook the transformer up that way and measure the secondary voltage. Mock up a switch affair to confirm the pcb makes the same result . If all checks out you are good to go![/quote]

I also think this is the way to go but would be nice to see how the switch works.
 
hitchhiker wrote:
Purusha! I did the same as Keith, and didn't take the ac to the pcb.

For 220v primary I figure the split primary is wired in series. I think the primary hook up would be the orange and yellow wires tied and the white and black to the 220v ac . Hook the transformer up that way and measure the secondary voltage. Mock up a switch affair to confirm the pcb makes the same result . If all checks out you are good to go!


I also think this is the way to go but would be nice to see how the switch works

To tie Orange and Yellow wires, the center pads must link to the top pads.

I'm thinking..would this be safer to feed 120v AC (from my 240->115v converter) into the unit firstly just to be sure? and what damage could be done feeding 220-30v in the 115v switch setting?
 
Well if you feed 230V into both of the 115V windings seperatly (means that every single 115V winding is seeing 230V) then the transformer should blow up. Some years ago i worked on a project at 3 am and was really tired so i wired up the transformer that way and the result was that i had to buy a new transformer as the old one was totally destroyed.

Flo
 
Mc76240Vhookup.jpg


Ok. I was able to feed the unit firstly 115v via an adapter and test both options:

Jumpers up gave 20-0-20v AC Jumpers down gave 39-0-39v AC

Therefore Jumpers up as suspected gives 39-0-39v AC under 230v mains
 
I have one question for the experts here :thumb:

I couldn't get the 1N4936 fast rectifiers for the Purples but I got ultra fast ones UF5404
which have same specs but they are ultra fast. Can I use these instead?
They come in the PSU section.
 
OK, next question regarding the MC76 DIY :roll:

Does anyone know how to hook up the T-pad attenuator (input) to the PCB?

MC76-attenuator.jpg


This is the last thing I need to wire and I am done :thumb:
 
What is the deal with R44 :?:

It says (T&C) ~1k on the schematic but calls for a 2k trimmer on the PCB :roll:

In the "Calibration instructions" it says to disconect for one of the measurments.

I have already installed the 2k trimmer but there are not enough detailed instructions about the 2k trimmer.

Please Help! :sad:
 
R44 in the rev D Mnats schematic is a 2K trimmer used to tweak the meter tracking. In Mako's notes for the Rev D he says to leave R44 off the board until you have calibrated the meter circuit according to the 1176 service manual on the JBL site.

Hope it helps-

Josh
 
on the MC77 PCB R44 is both a 2k trimmer and a 100ohm resistor. read the calibration instructions - you are supposed to make R44 be some value around 1k. you've already put in the trimmer. you can connect or disconnect the trimmer by installing or removing the 100ohm resistor.

read the calibration instructions, look at the PCB, look at the schematic, and most importantly... think about it. i'm sure you can work it out.

i'm not going to answer any questions about it.

ed
 
[quote author="JdJ"]R44 in the rev D Mnats schematic is a 2K trimmer used to tweak the meter tracking. In Mako's notes for the Rev D he says to leave R44 off the board until you have calibrated the meter circuit according to the 1176 service manual on the JBL site.

Hope it helps-

Josh[/quote]

Thanks man :thumb:
 
Could one of you guys tell me if your 600 ohm T-Attn (3 Deck) feels smooth.

Mine is not right :sad:

From about 8:00 to 12:00 it feels stuck and not smooth at all!

I tried to contact Andrew but it's the weekend... I hope he will replace it. It's a very important & expensive part of the KIT.
 
I also have the newer style 3 deck T-Pads, they all feel like silk with a very smooth rotation and very little resistance.
 
hi everyone,
i've hooked up my PSU section on my MC76 kit and i've taken measurements and everything seems o.k. as far as voltages (-10.71VDC and +30.43VDC), but a strange things occurs after a few minutes of the unit being on. the positive voltage measured off the zener stud diode starts to creep up slowly.
after about 5 minutes or so it went up to about 33VDC. negative rail is stable.

is this normal? is this a bad thing?
as I have no experience with this type of zener, i'm really sorry for this newb question

i didn't really want to leave the unit on for too long thinking it was going to continue to creep up. any insight to this would be greatly appreciated.
-grant
 

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