[BUILD] 1176LN Rev D DIY

[thesystem]

from what i remember if the unit isnt grounded properly it can go into ossilation.  Someone with more knowledge than me should comment on it, as I dont really understand WHY it exactly happens.

Hmm OK, is there an easy way to find out which part isn't properly grouded? I was very careful with this so I wonder where it went wrong.

 

[jplebre]

This would be a good place to start.

refer to mnats wiring pages http://mnats.net/1176_reva-d_hairball_wiring_power.html and doublecheck your wiring.
Make sure you use shielded cable where pointed out and to cut the shields where indicated (eg. connect the shield only on one side of the cable, snip on the other).

If you have a new rev PCB and follow this to the letter, you should be completely hum free and oscillation free.

Also, check for continuity on your enclosure, with the DMM on the earth lug of the IEC. If a panel does not have continuity, try scrapping the pain on the screw lugs

let us know if this helps

 

[thesystem]

Thanks jplebre, our posts must have crossed each other  as I didn't see it before I posted my message I will run through the wiring pages again and check case continuity. Will let you know if I can find a mistake!

 

[sr1200]

For the record I am a HORRIBLE speller... hahaha

 

[thesystem]

So I ran through the whole wiring again and I found one mistake, switched two wires on the output pot. I fixed that, turned the 1176 back on and... it's still there! I found out where the sound comes from though, it's coming from the output transformer. Checked continuity every where, that's all fine.

What can I try next to find the mistake? Thanks for helping!

 

[jplebre]

just to check, did you solder the 2 wires together on the secondary side? (yellow and orange assuming you are using the ed anderson - hairball O/p trafo)

I'm sure you already did check most components if they are correct in the o/p section (including the e-lytic cap polarity).
Also, cold joints.

Sorry if I'm just stating the obvious.

Do you have a oscilloscope or can you build a signal scope to trace the origin of the signal? voltages match these in that section? http://mnats.net/files/1176REVD_VOLTS.pdf

Wouldn't really know how to diagnose trafo problems.

 

[sr1200]

I take it you ran the ground from the board to the chassis of the transformer yet?

 

[thesystem]

just to check, did you solder the 2 wires together on the secondary side? (yellow and orange assuming you are using the ed anderson - hairball O/p trafo)

I'm sure you already did check most components if they are correct in the o/p section (including the e-lytic cap polarity).
Also, cold joints.

Sorry if I'm just stating the obvious.

Do you have a oscilloscope or can you build a signal scope to trace the origin of the signal? voltages match these in that section? http://mnats.net/files/1176REVD_VOLTS.pdf

Wouldn't really know how to diagnose trafo problems.

I did solder the orange and yellow wires together on the ea 5002. But now I'm wondering if that's where i did it wrong. You say on the secondary side, do you mean that both the yellow and orange wire should come from the same side? Because I now have one orange wire from the primary side and one wire (yellow) from the secondary side together. These wires are orange and yellow. There's another wire which is red (at least to me) which comes from the secondary side ans is now connected to the output XLR. Could this be the mistake?

I don't have an oscilloscope and don't really know how to build a signal scope. Really don't know a whole lot about electronics  :-[ Where do I put my probes when measuring the voltages?

I also connected the negative pole of C25 to the chassis lug of the transformer.


EDIT: Here's a picture of the output tx:

 

[jplebre]

since you connected C25 to the chassis. just place your negative probe on a screw hole, and probe with your red one.
make sure you place the probes on the right places on your DMM (black on Common, red on V/ohm - not A/uA/etc)

start by measuring the rails as indicated on the pcb (there's an arrow with +30 and -10).

If this is good, move onto to checking the voltages as indicated on the PDF. Those symbols on schemo indicate they are NPN so they have 3 connections which are (top to bottom): Collector (top of the circle), base (back of the circle), emiter (arrow pointing out). Check the datasheet for your 2n3707 or 2n3708 (depending on what you used) for the pinout (will say something like ECB) and probe the right legs of the transistor.

from your picture, I assume you didn't use the 5088 alternative, as these would have to be sitting differently on the board.

 

[thesystem]

OK, thanks for the explanation. I will do the measurements tonight and report back with the results!
I didn't use the 5088.

 

[thesystem]

I measured the voltages, here are the results:

rails:

-9,79
+29,75

          b   c     e
Q2: 0,55 - 0,89 - 0,01
Q3: ---    - 20,2 - 0,61
Q4: 1,01 - 4,35 - 0,42
Q5: 3,18 - 24,8 - 2,62
Q6: ---    - 27,2 - 2,2
Q7: 4,49 - 14,84 - 3,94
Q8: ---    -  ----    - 14,21
Q9: 3,45 - 16,85 - 2,87
Q10: ---  -  ---    - 16,26
Q11: ---  -  ---    - 9
Q12: -9 - 28,47 - -3,83
Q13: -3,55 - 28,41 - -3,81
Q14: ---    -  ----  - between 20 & 25

Q2,3,12,13 and 14 are way off! On some transistors, when I put my probe on one of the legs, the transformer would go into oscillation heavily. With Q14 it would 'shoot' the voltage every second from approximately 20 volts to around 25 volts, also with oscillation..
The rail voltage and the other voltages seem to be OK.

Now that I have these numbers, can we deduce some information from this? Or do I have to narrow it down some more? Thanks for the help so far!

 

[jplebre]

I would start by looking at the capacitors polarity on the input section, direction and type of transistors (mind they are 3391 and not 3707/3708, and the Q1 Fet which is actually drawn the opposite direction of all other NPNs in the circuit). Then would look for cold joints (if you have a chopstick - NON CONDUCTIVE!!!! - you can nudge parts to see if there's any culprit). I have a wooden japanese chopstick on my kit for this kind of stuff.

If that fails. would start pulling some resistor legs out (one leg will do) and measure them.

At this point if someone with some more knowledge could pitch in would be great as I would really just rinse and repeat the above until I found the problem, which can get messy

 

[thesystem]

Thanks for your help so far jplebre! I'll try the things you mentioned. Could you explain a bit further on how to test for cold joints using a chopstick? I take it the unit has to be on, but I don't understand how I can see if I have a cold joint with a chopstick.

 

[jplebre]

pocking to see if the noise you experienced or oscillation happens
checking for cold joints I usually do visual inspection first, reflow joints that look brittle, lifted, etc

 

[mnats]

EDIT: Here's a picture of the output tx:

I noticed that the trimmer potentiometers in this image appear to be mounted incorrectly (so that all three pins are shorted together). Yet you say that you have one unit that "works perfectly". Did you mount the trimmers on the other one the same way? What reading do you get when you test continuity between the three trimmer pins?

Now that I have these numbers, can we deduce some information from this? Or do I have to narrow it down some more? Thanks for the help so far!

Perhaps it isn't obvious but where you find incorrect voltages will ordinarily be the area(s) where the trouble lies. The first line of the first post of this thread links to my 1176 FAQ and to my knowledge there hasn't been one problem posted here that falls outside of what is described in the first answer in that FAQ.

Now I'll stop rephrasing what I've already written in the FAQ.

 

[jplebre]

I noticed that the trimmer potentiometers in this image appear to be mounted incorrectly (so that all three pins are shorted together).

Damn Mnats! You are good! Or my eyes suck!

 

[sr1200]

Ill agree with the above statement... I didn't notice that until you said it either, wow.  I guess when you've seen enough of these you can spot things out of the ordinary right away.

 

[jeffroberts]

I tried to search and read through as much as possible before asking, but haven't been able to figure out an answer for my issue. So far I have stuffed the psu section of my rev d circuit board And wired it all up as shown on the mnats avel lindberg wiring guide. When testing the power rails, no dc voltage was measured at This test points. I double checked all component values and Wiring, etc and all seems to be correct. However, when i measure ac voltage out of the transformer, I get 26V from The yellow lead, 13.2V from The black lead and 6.3v from the center tap. I think everything is Wired right and the input of The transformer is Measuring 120v so I'm not sure What can Be wrong.

 

[Echo North]

I tried to search and read through as much as possible before asking, but haven't been able to figure out an answer for my issue. So far I have stuffed the psu section of my rev d circuit board And wired it all up as shown on the mnats avel lindberg wiring guide. When testing the power rails, no dc voltage was measured at This test points. I double checked all component values and Wiring, etc and all seems to be correct. However, when i measure ac voltage out of the transformer, I get 26V from The yellow lead, 13.2V from The black lead and 6.3v from the center tap. I think everything is Wired right and the input of The transformer is Measuring 120v so I'm not sure What can Be wrong.

Measure the DC between the center tap and each of the outside wires (black and yellow I believe).  Use the Center tap as your common reference.  Then test your rail test points using your CT as the common reference.

What do you get?

 

[jeffroberts]

Wow. I was measuring voltages with reference to chassis ground. Stupid mistake. When measured Against center tap, dc test points read 29.8 and -9.8. Thanks for the quick reply