Noise and Hum after bus recap in PM2000

[lukev]

Last night I started to recap the busses in my PM2000.  I am only trying to replace the electrolytics in each of the eight bus channels.  I recapped 2 channels which was a piece of cake.  When I put them in the console I discovered that there is a low level hum and a bunch of noise on both of the recapped strips.  The noise kind of has a fast ticking to it.  The channels pass audio and the audio itself sounds good.  There is an extension in the high frequency compared to the bus channels with the old caps.  The noise is not that loud compared to the audio going through it, but it is slightly audible under the audio and this kind of noise is completely absent from the other busses with the old caps.  I am a bit confused here.  The replacement caps are all the same values as the originals, except a few are rated for higher voltage.  The caps are all smaller than the originals in physical size but other than that my understanding is that they should be the same thing.  I have never worked with bipolar electrolytics before, but my understanding is that they can be placed either way in the circuit.  There are eight caps on each bus channel strip. Here's the cap compliment in each channel and what I used as a replacement:

Bipolar Electrolytic (Panasonic SU for replacement)

100uf 25v  x2 replaced with ECE-A1EN101U
3.3uf 25v  x2 replaced with ECE-A1EN3R3U
47uf - 16v  x1 replaced with ECE-A1EN470U

Electrolytic (Panasonic FC for replacement)

220uf 25v x4 replaced with EEU-FC1V221

This seems like such a simple job so I am really confused as to what could be going on.  If you wise folks have any ideas as to what could be going on I would greatly appreciate it.  I am attaching the schematic of the channels for reference. 

 

[jwhmca]

Check polarity on the power caps?

 

[lukev]

I checked those.  Polarity is correct on the polarized electrolytics. 

 

[JohnRoberts]

I can't read that schematic very well.

Added hum and noise, may be symptomatic of a short or solder bridge somewhere.

Is there any way to divide and conquer? Isolate where in the audio path the sound is getting corrupted.

Perhaps with an audio sniffer compare a good channel to the problem channel. to see where it gets noisy.

JR

 

[lukev]

I added a more zoomed in version of the schematic to see if that helps.  I did check for shorts and I didn't see any thing on a first pass.  I will check that again.  I don't own an audio sniffer unfortunately. The best way I can think of isolating the issue is to replace the caps one at a time in another bus channel and test it after each cap replacement to see which cap is introducing the noise issues.  I will try tackling that tonight and report back with what I find unless anyone else spots something else that I did wrong first. 

 

[jwhmca]

Did you overheat or Physically stress any of the caps? i.e. you put one in wrong then pulled it out type thing...

 

[lukev]

I don't believe so.  I used a desoldering iron to pull the old ones which was very easy to do and soldered the new ones in at low heat.  There were no instances where I had to pull a cap once I put it in so I think the possibility of heat damage is very small, especially because the issue is exactly the same in both channels that I worked on.  I have done a fair number of DIY projects over the years so my soldering chops are pretty descent at this point. 

 

[JohnRoberts]

If you are confident that you performed the replacement perfectly, the only logical answer is a faulty capacitor. In my personal experience most often when I experience such results I did something wrong,

a faulty capacitor is not impossible, just unlikely.

Good luck... like I already said, you can debug by determining where the sound quality starts to deteriorate.

JR

 

[Brian Roth]

Are you listening to the "PGM out"?  If so, does the bus master pot affect the noise level?  That can tell you the general area where the noise is being introduced.

Bri

 

[pucho812]

if you put a cap in backwards and the cap is rated for high voltage then supplied it will not pop but it can cause some of the issues your describing. I would first test the power voltages with out 2 X buss modules in and see what you get. Then add the 2 recapped modules to the mix and test, see if one of the power rails drops? I would also do a visual comparison between a non recapped module and a recapped module check for caps that were put in backwards
.

 

[lukev]

I am listening to the Program outputs.  The noise does go away when when I turn the fader down.  I know this does not eliminate a ton of variables, but I did try putting the channel strips in different slots.  The recapped channels exhibited the noise issues in all the slots I tried while the strips with the original caps had no noise in any of the slots I tried, so I at least know the issue is isolated to the actual; bus channels.  I have already done a visual comparison as well as double checked the polarity with the schematic and the four polarized electrolytics are all placed with the correct polarity.  My understanding is that the bipolar electrolytics should work either way that they are placed.  I can test the rail voltage tonight to see if there is a drop when the recapped channels are placed in the console. 

 

[Brian Roth]

OK, that narrows it down to apparently stuff in the IC1 and IC2 positions.  Double check to see if the noise is also present at the "master out" 1/4" jack.  Do you have a scope so you can probe around in there?

If not, try lifting one end of the 3.3 uF cap (or one end of the 47K R that connects to that cap).  That splits the circuit into two halves to narrow things down some mode.

Bri

 

[lukev]

I tried out Brian's suggestions and here's what I found.  When I disconnected the 3.3 uf cap leg that connects to the 47k resistor the noise went away completely.  You mentioned checking the 1/4 inch master outs.  Are you referencing the insert send on the Program channels?  I checked those and the noise is audible there too, but much quieter. 

 

[Brian Roth]

On the schematic posted here, "master out" would be for insert send.  Levels there would be lower than PGM out since there is a chunk of gain in the post-fader amp stage.

So now the problem seems to be in/around the summing amp itself.  I dunno what to suggest.....I'll take another peek at the posted schemo (can't see it while replying) and report back in a sec.

EDIT.  Hmmmm.....the only "new" part would be the 100 uF cap at the input of IC1.  Did you deflux the PCB after soldering the new part in?  Maybe try putting back the original cap into that position?  I'm a bit stumped.

Bri

 

[lukev]

Alright.  I think I have finally tracked the issue down.  And it is of corse a stupid mistake on my part.  The noise, at least most of it, was coming from a couple channels that were patched into my H3000 and were assigned to the program channels in question.  After I messed around I found that there is at least some noise on all the channels.  Probably due to old caps in the power supply, but the noise was worse on the recapped channels because the H3000 channels were assigned to them.  The H3000 was not actually even turned on, but it was still introducing a bunch of noise for some reason.  When I unassigned those channels the noise floor dropped significantly.  The noise is still there, but I am now pretty certain it's not any better or worse due to the recap.  I guess to eliminate the noise from the console completely I am going to have to figure out where to get some new power supplies for it, but for now the noise floor is much less of an issue.  Thanks a ton for all the help.  I very much appreciate the suggestions and the invaluable expertise that is provided here.