HELP! - Soundcraft 2400 Sudden Meterbridge weirdness!

[moamps]

....I do have some brand new 6u8 35v tants in stock, and I wonder if it would be safe to temporarily sub with those...

If it works with 4u7 as power supply bypass, it would also work with 6u8 for sure.

 

[Dr nEon]

Thanks John, thanks moamps..

I'm going to put the above mentioned dubilier DJ4X's in there and see...

My cheap chinese meter isn't to be trusted 100% for esr readings,  but looking at the datasheets ,  the dubilier's esr figure for a 50v 4u7 of around 2.5 ohms, is on a par with any of the currently available tantalums, which I could find.. So I think that looks OK.

Also, whereas I could order some panasonic FCs, for an even lower impedance than tants,  the FCs are rated for only 1000hrs at 105deg,  whereas these dubiliers are rated for 3000hrs at 105deg...  So I may as well stick with the dubiliers, as they look fit for purpose !

I will report back if there are any anomalies that may require the additional ceramic discs.  I plan to install the power ribbon back to only blocks of a few channels at a time, to observe carefully for any misbehaviour elsewhere which may have been caused by the power short.

Thanks again!

nEon

 

[Dr nEon]

STOP PRESS...!

Guys,  please take a look at the attached pics!    I think I just discovered something else...

I took a pic of the pcb before I removed the tants,  just so I could refer back to it when installing the new caps...  And I've just noticed something very odd..

In the pic on the left,  looking at the pcb holes, from L to R...  hole 1 and 3 both connect to the ground rail.    Hole 2 and 4 connect to -7.5v and +7.5 v respectively...  The first two terminals are the -7.5v tied together, the next two terminals are the +7.5v rail tied together.  The ground terminal is not visible in the pic.

Now, looking at the pic on the right, which I took before I touched anything...  On the left we have the neg rail tantalum,  with its positive leg to GND and it's neg side to -7.5v...    However, on the positive rail, we have the same thing, with the positive leg going to GND , and the neg going to the +7.5...  Surely this positive rail tant cap is the wrong way around!!

I would appreciate if one of you gurus could confirm to that this is definitely wrong  or if there's possible a reason for this arrangement which I'm not aware of...  I will hang fire from installing my new ones until I hear some words from an expert..!

My understanding is that these are decoupling caps, and the diode between the +/- rails is reverse biased for protection.  Therefore the positive rail cap should have it's positive leg to the +7.5v and it's neg leg to GND..

If it is - as suspected - the wrong way around,  it sure has lasted a long time like that...nearly forty years!

Any help much appreciated,

nEon.

 

[JohnRoberts]

Stuff happens... Measure voltages with caps removed and replace with proper polarity.

JR

 

[Dr nEon]

Thanks for checking that out, John!    My voltages are good, so I'll install the caps properly. 

I'm amazed that cap has sat there happily all these years time before finally failing... perhaps the 35v rating made it robust enough to handle the reversed situation.

Thanks again,

nEon.

 

[JohnRoberts]

Thanks for checking that out, John!    My voltages are good, so I'll install the caps properly. 

I'm amazed that cap has sat there happily all these years time before finally failing... perhaps the 35v rating made it robust enough to handle the reversed situation.

Thanks again,

nEon.

I had a production error back in the early 80s where an aluminum electrolytic cap was installed backwards in a production run. It was a relatively low reverse voltage maybe 7.5V across a 25V cap...  I did testing to see if there was a problem and they showed no apparent symptoms, but in the end I had them replaced... because when in doubt do what's right.  8)

JR

 

[Dr nEon]

... because when in doubt do what's right.  8)

JR

Absolutely! 


nEon

 

[Dr nEon]

Just to wrap this up, for the benefit of future people with this issue..

Pleased to report that, with the two 105deg aluminium electros installed in place of the two tantalums,  the desk has been running perfectly now, for a couple of weeks... Got a couple more mixes done,  and everything is behaving as normal.

With the logic rails fixed, now - as well as all of the meters all working normally - all of that very weird distorted breakthrough onto the mixbus from muted channels, has gone.    Thinking about it, the actual 'sound' of that distorted breakthrough, wasn't really actual distorted audio...it was more like pure dc crackling (imagine when you connect your charger croc clips to a car battery) which was responding to all the transients of each incoming instrument..        I assume that the half-powered logic circuit (which is naturally connected to every channel at a point before each channel's mute button) was misbehaving really badly, and resulted in weird screwed-up fluctuating dc levels which must have somehow influenced and imprinted onto the mixbus audio path. 

Interesting fault, but not too interesting when it happens in the middle of an album mixing session!

All good now, though, and many thanks again to all you guys who helped out!

Cheers
nEon.

 

[CJ]

backwards tants will feel warm when reverse biased,  if they don't blow,

 

[L71]

I realise that this is an old thread, but I’m currently in the process of acquiring what‘s needed to re-cap my 2400 and this thread may have saved me some grief. Not that there’s a problem with those tant caps on my board AFAIK, but I’m going to replace them anyway. So thanks Dr nEon & all who chimed in!