Series 2400 Ground Mod?

Founded in 1973, Soundcraft first made its mark with the launch of the Series 1, the first mixing console in a flightcase.

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Series 2400 Ground Mod?

Postby Own Little World » Thu Nov 19, 2009 11:27 am

Hi all,

I have a Series 2400 which I am gradually nursing back to former glory (still).

Now then... it is very noisy in the earth hum sense and I am wondering how to tackle this.

Each input module has a central spade connector bolted to the board and connected to ground at on of the IC ground pins. These are connected to a large copper bus bar on the bottom of the console via fairly thick guage yellow/green wires. This suggested to me that the board had bee modified (it is an early 2400, about 1980, I think). However, when I test with a meter, it looks as if there is still continuity between this and the ribbon connector earth. Is this correct? There does not seem to be any evidence of a PCB trace having been cut.

Does anyone have any detail of this mod so I can check that it has been doen correctly please?

As it is, every channel which is switched on adds (audibly) to the overall console hum (which is obviously worse in the case of unbalanced sources than balanced sources). Is this common? Providing I ensure optimal input levels to each channel it is not too much of a problem (unless in quiet musical passages). I really think I should be able to improve on this.

Thanks for any pointers.

Alec
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Re: Series 2400 Ground Mod?

Postby dementedchord » Thu Nov 19, 2009 4:49 pm

what do the power rails look like??? make sure it's not the supply first before freakin out...
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Re: Series 2400 Ground Mod?

Postby Own Little World » Fri Nov 20, 2009 2:30 am

Hi dementedchord,

I recently rebuilt the (original) power supply with beefier smoothing caps and replaced one (or two) rectifiers as someone had let the smoke out. I haven't got a scope (I should really get one) so I haven't checked how clean the power rails are. However, this is not my first suspect at this moment unless you know of anything I am missing (likely).

I had a look at a couple of channels (1 input, 1 group) from a slightly more modern 2400 last night which I recently bought in error thinking they were direct drop ins (and these will shortly be for sale but that's another story). The circuit board designs are slightly different to mine but I can clearly see that a ground mod has been performed.

For example, on the input channel the trace between the signal 0 (ground) and the connector pin of the upper (smaller) ribbon connector has been cut (in this case by cutting a straight wire on the pcb front rather than scratching off a pcb track). The ground connection to the larger ribbon connector is maintained.

On my input channels however, there is continuity between the pcb ground and both ribbon connectors. I am presuming that unless the earth is broken elsewhere (e.g. one ribbon wire snipped? I will check tonight) then this must be wrong. I don't suppose that even if this earth is cut somewhere else that it could hurt to cut this line but I just need reassurance before I take a chisel to my vintage desk. It's not a nice feeling.

Ground hum aside, the desk sounds very nice indeed and I have done some great mixes recently. I intend to replace/upsize and bypass a few electrolytics on each channel and maybe replace the master chips with something more modern but leave everything else stock for now. The hum is a problem though...
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Re: Series 2400 Ground Mod?

Postby dementedchord » Fri Nov 20, 2009 5:07 pm

already rebuilt... cool... just needed to be sure... it seems that the obvious overlooked always bites ya on the ass... or maybe it just seems obvious after you've sorted it out???

as to your grounding... no real opinion at present... but it sounds as if the primary ground mod hasn't been done as you dont mention a major hunk of copper having been installed... i would look into doing it if i were you... as i am plannung on doing this for my little 500...
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Re: Series 2400 Ground Mod?

Postby Own Little World » Mon Nov 23, 2009 8:15 am

Hi again dementedchord. Thanks once again for getting back to me.

Now then...

It looks like the star ground mod has at least partially been implemented as there is a copper bus bar on the bottom of the console with thick guage wires to a spade terminal on each strip.

However, confusingly, it does not seem as if the original daisy-chained earth traces, trough the ribbon cable have been cut. Either on the channel strips themselves or on the ribbon cable.

This led me to believe that maybe my excessive hum noise may be due to unnecessary ground loops.

I mentioned before that I have a couple of spare channels from another Series 2400 (a 24 bus) on which the mod seems to have been performed and the traces cut as expected. I slotted this in yesterday and I tried to hear any difference in hum level between just this channel and just one of my channels. I have to say it was difficult to spot any difference with just one channel at a time. Obviously, it is the nature of ground loops that they multiply as more and more channels are used.

I am thinking that it can't hurt to cut this trace on one strip and see what happens. I no-one gets hurt I may do a few more.

Any thought sand recommendations still gladly received.
Alec
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Re: Series 2400 Ground Mod?

Postby Jim Williams » Wed Nov 25, 2009 11:15 am

Soundcraft issued a grounding mod sheet for the 2400 back in the 1980's. Look for it. It involves cutting the input module input grounds and running a seperate ground wire. Mods also include the master and patch cards.
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Re: Series 2400 Ground Mod?

Postby Own Little World » Thu Nov 26, 2009 4:32 am

Thanks Jim,

I have been looking for that info. It seems that Soundcraft are rennovating their site and there is currently no tech info on legacy products like there used to be - I hope this is just temporary.

If anyone else has this info to hand I would be most grateful.

Alec
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Re: Series 2400 Ground Mod?

Postby Own Little World » Thu Nov 26, 2009 6:16 am

Incidentally Jim (off topic a bit) I have a spare input channel that I recently bought on ebay from the states. It has been modded and I was wondering if it was one of yours.

Basically, it has most of the chips socketed with LF353s in, a few electrolytics replaced and upsized (phantom, mic & line trims and a couple of coupling caps), the uncoated polyesters have been replaced with (Vishay polyesters?) in a few places (e.g. HPF and HF EQ), a WIMA on the hi mid tuning cap and the ground mod has been done (straight wire to ribbon connector is cut.

It sounds great and I am currently using it on my bass guitar channel (due to its low end focus) while I upgrade my other channels myself.

Thanks again for your help,

Alec
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Re: Series 2400 Ground Mod?

Postby Jim Williams » Thu Nov 26, 2009 11:20 am

Not my mod. I just sent out a set of 2400 modules yesterday. The inputs have a mix of LME49720 and 6172 opamps. The EQ transistor buffers are removed. The EQ's use polyprop film caps. The hi mids sweep to 20k hz. The low mids sweep to 60 hz.

The mic preamp is redesigned into a trans-amp with the transistor pair wired into the global opamp feedback network. That lowers THD at higher gains and noise is reduced with very low noise input transistors and opamps. They will do -129.6 db EIN with a 150 ohm source, lower with 50 ohm sources.

I use 220 uf coupling caps, some are removed due to offset reductions. Those are bypassed with polyprop film caps.
The master mix is made direct coupled, no caps at all. DC to daylight bandwidth and very low noise and crosstalk. The sonics are like clear water. Without those grounding mods, stereo mix buss s/n ratios measured - 55 dbu, mostly 60 and 120 hz grounding loop noise.
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Re: Series 2400 Ground Mod?

Postby Own Little World » Fri Nov 27, 2009 4:38 am

Thanks for that info Jim.

Your mods sounds very extensive inded - more so than what I am currently doing to my input channels.

In terms of caps - I have chosen similar values to what you detailed above. I was initiall unsure whether I might be able to use bi-polars but have eventually gone (ordered) Panasonic FMs and NHGs for the phantom blocking caps. I have chosen WIMA polyprops where values permit and WIMA polyesters in the signal path and cheaper polyesters for bypassing.

Regarding the ground mod, I am going to cut a few traces on one module (from the look of it, it will be fiddly) and if the channel still work I will do more.

In terms of chips I am deliberating LN49720 for the master section and I have ordered 24 OPA134s for the input pres. Having recently rebuilt the power supply I am guessing/hoping I will have enough juice for the extra 60mA.

Out of interest is there anyone on this side of the pond who has a complete JW Soundcraft? That would be nice to hear.

Thanks for all your help.

Alec
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