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 nonchalantly 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 probably 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 happens
pull all group 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/oscillator, 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?

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 (image #4) is on the parts list but is not on the schematic and sits across the -18v rail and ground and was the smoker when the +18v rail was flapping about

top to bottom

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

[Jim Williams]

Trident didn't use local 10 ohm power rail fuse resistors like the other brands so any psu errors will fry opamps rather than the fuse resistors. One could cut those power feed traces and install them.

Seems you are ready for a proper power supply for that console. GRS systems makes nice stuff or one could cobble a couple of 24 volt 12 amp Power One linear supplies and trim it down to + - 17 volts.

[digitall2000]

thanks jim
these modules in this tsm-40 do have the jumpers where the power rails enter the module.
as i have been researching this matter i read numerous helpful tips from you so thank you for sharing your knowledge.
in one of your posts you recommended 10 ohm 1/4 watt resistors in this spot, am i remembering correctly?

i am trying to talk the owner into a new supply

what are your thoughts on the self healing notion?
the guy that worked on the console 4--5 years ago is sure it's the old power decoupling cap. but he is the one who used the lm3886 for a voltage regulator.

also what about the exploded tantalum on the negative power rail?
someone told me to replace the because when they fail in that scenario it is usually catastrophic

thanks again,
lance

[Jim Williams]

Tantalum caps cannot tolerate any reverse voltage. Replace those with low impedance electrolytics, big ones.

[digitall2000]

thanks jim

[waltzingbear]

while a LM3886 is a little odd to use as a voltage regulator, it is not unheard of. after all. all voltage regulators are amplifiers.

I'm sure he was thinking he was improving the power supplies, and he may have been, we don't know what was there before.

But that isn't your major problem.

You needed to measure the current to find the problem, the old tant caps are not the problem. You may want to replace them, but they are not the cause of the problem. They are a problem when they blow up, not until. Occasionally they will short and draw current, but as was noted, not in a TSM without blowing up.

You have something else drawing too much current without blowing up. That's why you need to measure the current and find the problem that way.

Cheers
Alan

[jensenmann]

Tantalums behave like a resistor before they die. It´s quite possible that the raised current consumption is caused by several tantalums which are close to death. Since they are all the same age it´s quite likely that many of them are in the same state of failure.
It´s generally a good idea to recap consoles that age. Besides having less problems it will even sound better.