DS5R Phantom power supply hum (cont.)

[Simon Taylor (staylor)]

Hello all.  I posted the following lastnight, but not sure it went out, since it was from a thread archived in November 2002.  You can find the original thread underst Alembic Basses & Guitars -> Archive through November 30, 2002 -> DS5R Phantom power supply hum.  I'd be interested to know whether this post actually went out to folks, or not through some quirk of the system.
 
Anyway, here's the post:
 
Hey folks. I finally got to sit down and do some testing on the power supply.  
 
I tried lifting the ground by plugging the DS5R into a 3 to 2 adapter. That did not yield an observable result.  
 
So, I moved on to Ron's tests:  
 
1. turn off power supply while hum is happening.  
Result: all hum/buzz was gone.  
 
2. selector switch to standby  
Result: with volume knobs up, this reduces the hum, but with vol knobs down, it's the same. I infer from this that there is no difference since there is noise coming through even with vol knobs down and/or selector switch in standby. When volume knobs are up it gets worse because the instrument is amplifying the hum along w/ everything else....phew, killed that horse!  
 
3. plug a 1/4 jack (no cable) into 1/4 output to force over to DC (battery) power, while using the 5-pin cable for audio path.  
Result: no hum, sweet silence when the strings are not ringing and beautiful Alembic tone when induced in the standard fashion.  
 
4. running the power supply and not using it 1/4 SII out to 1/4 Ampeg SVT4 Pro in  
Result: no hum, sweet silence identical to test (3)  
 
5. I'm not sure if I understand what you are saying for test number 5, Ron. If you are talking about lifting the ground as Nate suggested, then I covered that above.  
 
Anyway, it would seem from the first test that there is likely something wrong with the power supply. Are there any further tests I can run before shipping it back to you folks??

[Simon Taylor (staylor)]

It seems I can no longer add to the original thread that is now archived under Archive Through March 15, 2003 -> Archive Through November 30, 2002 -- must be something about posting to an archive of an archive.
 
Anyway, per your instructions, Ron, I did some further testing.  Got some results that I don't completely understand.  First a table of the resistance between pins on the bass (I think I actually measured the complete table through the 5-pin cable when it was plugged into the bass, but did the standby position both directly from the jack and through the cable ...):
 
Pins    Standby    Bridge   Both   Neck
----    -------    ------   ----   ----
1+2     0          0        0      0
1+3     0          0        0      0
1+4     0          0        0      0
1+5     0          0        0      0
2+3     0          0        1      0
2+4     0          0        3      3
2+5     0          0        3      3
3+4     0          3        3      0
3+5     0          3        3      0
4+5     0          0        0      0
 
It's been a while since I've used an Ohm-meter, but I had it set to x1K for these readings, so I think, for example that pin 3 to 4 in bridge pickup position was reading 3,000 Ohms -- does this make sense?
 
Anyway, I'm a little confused because my readings across pins seem consistent both from the 5-pin jack on the bass and through the 5-pin cable when connected to the bass.  So, I thought this would mean that the cable was good, i.e. there were no broken connections in the cable??  
 
However, I also tried your test of checking the ground connection through the 5-pin cable by touching a test lead from the sleeve of the 1/4 bass output to the sleeve of the power-supply 1/4 output while the 5-pin cable was connected from the bass to the power-supply.  I listened for a change in the hum, and did notice a significant change, though the hum did not completely go away.  So, this is why I'm confused -- you indicated that this result would demonstrate a bad ground connection through the 5-pin cable, however, getting the same readings through the cable when connected to the bass as directly off the 5-pin jack on the bass seems to indicate to me that the cable is good -- unless the ground path on the cable is not carried by one of the pins?  Anyway, if the cable is good, then I suppose we're back to the ground loop theory -- only, problem here is I did all the testing with the power supply ground lifted, i.e. the power supply plugged into a 3 to 2 adapter.  What am I missing here??  What can I test next??
 
Thank you kindly!!
 
Simon
 
PS:  I also did the test you suggested about connecting the 5-pin cable the PS and w/ pickup selector in standby and mono/stereo switch on PS in stereo position test the 1/4 plug on PS.  As expected, I read 0 Ohms from Tip to Sleeve.

[Ron Wickersham (rjw)]

Simon, i also think you are measuring 3,000 Ohms with your meter.  if you have a setting for lower resistance, such as x1, then it would be better for testing cables and ground connections.
 
but your test with the test lead connecting the grounds indicates that the problem is in  the cable.   any change in hum at all indicates that the cable shield is no longer good.  (well there is a tiny chance that the wire inside the bass between pin 1 of the 5-pin jack and the sleeve tag on the 1/4 inch jack is broken but since it's protected inside it's very unlikely).
 
so i think you've identified problem.  sometimes the cable shield goes bad near the end of the cable (usually the end at the bass) since it gets the most flexing, so you could try cutting off 12 - 18 inches and reconnecting the plug and see if that cures it.  or just replace the whole length of cable.  
 
-ron

[Simon Taylor (staylor)]

I tested the cable and don't think it can be the cable.  I disassembled the ends and found a short (0 Ohms) between both ends of pin 1, as well as between the two set screws in the jack assembly that make contact with the pinless terminal (don't know what it's called) to which the cable shielding is soldered.  Also, a short reading between pin 1 at both ends and the pinless terminal.  So, it looks like the shielding connections in the cable are fine.
 
So, I'm reading that as no break in shielding connection.  I'll open up the bass and check the jumper between pin 1 and the 1/4 'sleeve tag'.
 
Any other suggestions if that test fails to yield a fault?

[Ron Wickersham (rjw)]

Simon, sometimes deterioration in the cable shield conductors can still show a connection on the ohmmeter, but be failing enough to cause the hum.  since you could make a difference with the separate ground lead, we know the trouble is somewhere between those points.  you can touch the separate ground lead from one end of the cable to the 1/4 inch jack on that end and see if the touble is inside the guitar on that end or the power supply on the other end.  if that doesn't make a difference, then you could have the connectors open and touch pin 1 to pin 1 and see if that makes a difference.   of course, if it does then you will know the cable shield has degraded.   (some ohmmeters are less capable of showing this than others.  the definitive test is the separate ground lead.)  
 
you're very close to determining just which area is causing the problem.  since you can hear a large change when you go between the 1/4 inch jack on the instrument and the 1/4 inch jack on the power supply, you can jump from the shield end (pin 1) to the 1/4 inch jack on the instrument and see if that makes a difference.   likewise from pin 1 on the power supply end to the power supply 1/4 inch jack.  (a possiblity exists that the nuts holding the jacks on have become loose and that will make the ground connection poor - retightening the nuts will correct that...keeping in mind that you don't want to overtighten the nut on the instrument enough to crack the wood).   but in my experience the cable is most likely to be the problem, even if it checks out on the ohmmeter.
 
-ron