MOTM

Hi all,

I just assembled a '420 and am getting a strange problem. I wonder if 
anyone has any thoughts on where the problem might lie:

In HP mode the filter works correctly - and sounds great - with no 
resonance, but if I turn up the resonance, I just fade in more 
unfiltered sound instead of increasing resonance. So that with the 
resonance all the way open, there is effectively no filtering going on.

When I put it in LP mode I hear nothing from the inputs at all.  If I 
turn the resonance up I get a regular clicking. If I open up the res 
the clicking slows down, and if I adjust the filter freq, I can hear 
the filter acting on the clicking, but nothing from either of the 3 
inputs is going through.

NF is a weird mish-mash of both the above.

After I first assembled it, it was doing this, then I double checked 
some of the joints and via holes and it started working fine, then I 
inserted a cable into the fm jacks and it went back to having the above 
problem. I've gone over all the stuff I did to get it working right the 
first time and no luck.

I'm inclined to think that, because I had it working fine at one point, 
the problem is minor and that I'll find it in time - for example 
something was jarred loose by inserting that cable - but I'm wondering 
if anyone has any suggestions on places to start?

Certainly nothing is loose to the touch, and everything seems to be in 
the right place, but obviously I'm missing something, or it would be 
working.

Any help much appreciated,
Matthew

On the off chance that paul hasn't helped you out already (he's darn
good at this, helped me
track down the problem I had with my MOTM-300, which was, of course, a
bad solder joint
*somewhere*) I would suggest some basic things.  Check all of your pots
for proper wiring,
check for shorts on IC legs, and check for missed solder joints.  Yank
the board out of the
frame and hold it up against the light.  If you see any light in places
you shouldn't (really,
probably nowhere except cable tie points, but I don't recall exactly)
then you've got a problem.
That's a *great* way to find missed vias and solder joints.  Good luck!

-Geoff

Matthew Hiscock wrote:

> Hi all,
>
> I just assembled a '420 and am getting a strange problem. I wonder if
> anyone has any thoughts on where the problem might lie:
>
> In HP mode the filter works correctly - and sounds great - with no
> resonance, but if I turn up the resonance, I just fade in more
> unfiltered sound instead of increasing resonance. So that with the
> resonance all the way open, there is effectively no filtering going on.
>
> When I put it in LP mode I hear nothing from the inputs at all.  If I
> turn the resonance up I get a regular clicking. If I open up the res
> the clicking slows down, and if I adjust the filter freq, I can hear
> the filter acting on the clicking, but nothing from either of the 3
> inputs is going through.
>
> NF is a weird mish-mash of both the above.
>
> After I first assembled it, it was doing this, then I double checked
> some of the joints and via holes and it started working fine, then I
> inserted a cable into the fm jacks and it went back to having the above
> problem. I've gone over all the stuff I did to get it working right the
> first time and no luck.
>
> I'm inclined to think that, because I had it working fine at one point,
> the problem is minor and that I'll find it in time - for example
> something was jarred loose by inserting that cable - but I'm wondering
> if anyone has any suggestions on places to start?
>
> Certainly nothing is loose to the touch, and everything seems to be in
> the right place, but obviously I'm missing something, or it would be
> working.
>
> Any help much appreciated,
> Matthew
>

Nope, haven't heard from Paul yet. *hint* *hint*

:)

I've done the hold-up-to-the-light trick, actually - looks fine - and 
I've checked the obvious stuff like cables going to the right places 
and resistors and ICs in the right place.

My hunch is that it's an IC leg short. From a look at the schematic, I 
would think U3B, U5B or U4A... but I'm hesitant to just start redoing 
the joints without knowing a bit more. The resistors in the LP section 
look fine to the eye, but it's possible the problem is there.

About ICs - am I right in thinking U3A would be the side of the IC with 
the little circular "dimple" and U3B the other side?

Matthew

On 25-Feb-06, at 10:00 AM, Geoff wrote:

> On the off chance that paul hasn't helped you out already (he's darn
> good at this, helped me
> track down the problem I had with my MOTM-300, which was, of course, a
> bad solder joint
> *somewhere*) I would suggest some basic things.  Check all of your pots
> for proper wiring,
> check for shorts on IC legs, and check for missed solder joints.  Yank
> the board out of the
> frame and hold it up against the light.  If you see any light in places
> you shouldn't (really,
> probably nowhere except cable tie points, but I don't recall exactly)
> then you've got a problem.
> That's a *great* way to find missed vias and solder joints.  Good luck!
>
> -Geoff
>
> Matthew Hiscock wrote:
>
>> Hi all,
>>
>> I just assembled a '420 and am getting a strange problem. I wonder if
>> anyone has any thoughts on where the problem might lie:
>>
>> In HP mode the filter works correctly - and sounds great - with no
>> resonance, but if I turn up the resonance, I just fade in more
>> unfiltered sound instead of increasing resonance. So that with the
>> resonance all the way open, there is effectively no filtering going 
>> on.
>>
>> When I put it in LP mode I hear nothing from the inputs at all.  If I
>> turn the resonance up I get a regular clicking. If I open up the res
>> the clicking slows down, and if I adjust the filter freq, I can hear
>> the filter acting on the clicking, but nothing from either of the 3
>> inputs is going through.
>>
>> NF is a weird mish-mash of both the above.
>>
>> After I first assembled it, it was doing this, then I double checked
>> some of the joints and via holes and it started working fine, then I
>> inserted a cable into the fm jacks and it went back to having the 
>> above
>> problem. I've gone over all the stuff I did to get it working right 
>> the
>> first time and no luck.
>>
>> I'm inclined to think that, because I had it working fine at one 
>> point,
>> the problem is minor and that I'll find it in time - for example
>> something was jarred loose by inserting that cable - but I'm wondering
>> if anyone has any suggestions on places to start?
>>
>> Certainly nothing is loose to the touch, and everything seems to be in
>> the right place, but obviously I'm missing something, or it would be
>> working.
>>
>> Any help much appreciated,
>> Matthew
>>
>
>
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>
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__________________________________________________
www.bodega-audio.com
Bodega/Bootleg Sounds. Fat East Coast. Distillery Mastering.

It's virtually impossible to fix this sort of thing without having the unit in 
front of me. It could be 1 of 100 things.

I will say that 99.999% of the time, I "fix" bad modules by just looking at the 
board and visually spotting something.
It's like proffreading your own English paper in high school: you don't want to 
see a problem so you don't. You hand
it to a friend, and they spot 3 errors in the first paragraph :)

About 80% of the time, I can spot the problem in under 15 seconds. It's usually:

a) not *enough* solder on something
b) the wiring is wrong ("I've checked it 10 times!!")

In 7 years, only 3 modules had a bad part.

Paul S>

About ICs - am I right in thinking U3A would be the side of the IC with the
little circular "dimple" and U3B the other side?
 

The general-case answer is not necessarily. It depends on what the IC is and
how the person (or software) who drew the schematic and laid out the PCB
assigned the pin numbers and part designations. The only way you can tell
for sure is to look at the schematic and see which pin numbers are used for
sections A and B. Pin 1 is near the dimple and the numbers run along the
edges. If it's an 8-pin DIP then pin 8 is near pin 1, on the opposite side.
 
However, in the specific case of U3 in the 420, it appears from the
schematic that A is on the dimple side, since U3A uses pins 1-3.
 
If it's an intermittent problem--that is, it changed when you did something
mechanical (inserting the cable)--then odds are good it's a bad solder joint
somewhere. I'd probably start by going over the entire PCB solder side with
a magnifying glass. Yes, that means you'll have to mechanically disassemble
the module, but that's why I go over the entire PCB with a magnifying glass
before I assemble it further. :-)
 
Good luck!
 
--Adam