Trident TSM-40 WOES

[digitall2000]

thank you all in advance for any help or advice you can offer. 

ok here is the situation i find myself in.
the owner brings me in to fix his jittery otari mtr-90 mkIII.  well he also owns a great sounding
trident tsm-40.  so we are sitting there listening to the 24 track and the system makes a strange
noise, the trident's meters slam against the right side (full scale) three or four times and a small
puff of smoke comes out of the console before he gets to the other room to shut dowm the trident.
the trident is left on 24/7

the owner nonshalantly says "yeah that was happening 8 months ago but it stopped". 
all but the puff of smoke that is. he said that hadn't happened before.
the next day i come in we pull out the psu for the trident and set it up to monitor it.
when the console weirds out the +18v psu regulator overheats and it goes into thermal
shut down, like it is dead shorted. while the -18v side seems to be stable.  when the reg.
cools down we can turn on the console again and it would run for a while and the same
weirdness would happen.  thinking that this is is probobly being caused by the power
supply decoupling caps (a lot of the modules still have old caps) we start pulling modules.
pull all routing modules,try again, same thing happems
pull all gruop modules, try again, same...
pull all eq modules, try again, same...
when we get to the last 30 modules input channels only, along with the monitoring.
slate/oscilator, and 2 track return section we try again and the console is stable
(he needs to record drums later that day).
so there are no eq modules, no routing modules, and no group modules
and yes i am in the process of recapping the power supply bypass caps

so i would like to ask some questions

1. what happens to op amps that are running on a dual rail psu and one rail goes down?
-18v rail is stable .......+18v shut down turns on and shuts down again a few times

2. would the fact that electrolytic caps are self healing allow us to power the console up again?

the last guy that worked on the console used a LM3886 (power amp) as a voltage regulator for
both 18v supplies, about four years ago

3. should replace this with something more suitable?

so here are some related issues

 

[digitall2000]

left to right

1. exploded TL071 (have never seen this before)
2. fried TL071    (this either)
3. had replaced IC 2 and socket then during the fiasco IC 1 fried
4. this was a tantalum cap. that is across the -18v rail and ground

anyone have any ideas on the causes of such catastrophes?

thanks for any advice that can be offered up
lance

 

[pucho812]

I have seen a chip explode before, but in my case, the cause was the fact the chip went in backwards and  when powered on poof and crack.  Fried I have seen that too. Oddly enough both were in a malcom toft design.  as for the tant I would assume age. 

As for the power supplies  I would replace with either one from GRS or build one out of off the shelf stuff at mouser. Would make things easier.

 

[digitall2000]

well all of the tantalums on that board, and many others in this console are the same age
and the only way i have seen them explode like that is being reversed voltaged or over voltaged
that particular tantalum cap is on the parts list but is not on the schematic and sit across the
-18v rail and ground and was the smoker when the +18v rail was flapping about

 

[ruairioflaherty]

As for the power supplies  I would replace with either one from GRS or build one out of off the shelf stuff at mouser. Would make things easier.

A known good power supply certainly makes life easier but if regulators are going into protection it's just as likely that it's one or more issues on the board itself causing excessive current draw.

 

[pucho812]

A known good power supply certainly makes life easier but if regulators are going into protection it's just as likely that it's one or more issues on the board itself causing excessive current draw.

Was thinking that as well.

 

[noulou]

The first thing I did on our trident tsm was to replace ALL tantalums on ALL the cards that are on the power lines with electrolytics.
I also got all sorts of explosions before that.

I wouldn't touch the signal coupling capacitors though. They are a big part of the sound of that desk.

Nikos

 

[Rob Flinn]

I wouldn't touch the signal coupling capacitors though. They are a big part of the sound of that desk.

Nikos

In my experience  the signal caps are the ones most likely to be out of spec when checked with an esr meter.  The lower the value the more likely they are to be out of spec.      Caps across the Power rails are much more likely to be OK.

 

[mjrippe]

In my experience  the signal caps are the ones most likely to be out of spec when checked with an esr meter.  The lower the value the more likely they are to be out of spec.      Caps across the Power rails are much more likely to be OK.

If the signal caps are tantalums they are not likely to be out of spec, electrolytics would be suspect though.  However tantalums should NOT be used on power rails as they can often fail shorted (if they do not literally explode first).  Neve 54 series boards are famous for this.  Fortunately they have low value resistors for current limiting which tend to act as fuses.

 

[digitall2000]

yes odd on this console all the different module boards have a jumper wire right near where the psu rails come
in, like they intended to do just that. but cheaped out.  i am now in the process of installing 10 ohm 1/4 watt resistors instead
of the jumpers, as per a suggestion by jim williams over at http://forum.analogconsole.com