Trident 80b repair

Twenty five years have passed since Trident Recording Studios made the decision to build their own Multitrack Recording Console.

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Trident 80b repair

Postby Jeff » Tue Jul 25, 2017 5:42 pm

Hi all,
Sorry for the long winded first post, but I'm hoping I can find some help here.

The studio that I work at owns an 80b. We keep the console running 24/7 and it has been running fine until a couple weeks ago when it started smoking. We quickly shut it off, found the input module that had fried, pulled it, turned the desk back on and one of the 20v rails was down. We turned it back off, pulled about half the modules, turned it back on and both the rails were present. However when I measured the voltage at the edge connector, the positive rail measured 30v. We shut it back down, and I pulled the psu where I found a handful of toasted resistors and capacitors. The electrolytic caps on the pcb appeared to be original and measured bad. I recapped the pcb and replaced all the damaged components and the supply rails were back to normal (Well, 20.4v on both the positive and negative rails which seems a tad high, but not too terrible. I measured it without a load).

I put the psu back in line and the desk was functioning properly again. That is, until today when I got a call from one of the other engineers at the studio saying the +20v LED was not lit on the meter bridge.
I told him to shut it down until I could make it by. (They ran a session all day yesterday and the dest worked fine.)

The frustrating thing is that the problem is intermittent. When I got there today, I pulled a couple of modules so I could measure voltage at the edge connectors, turned the board on, and all rails were present and measuring correctly. Turned it back off and on again and the +20v rail is out. So once again I pulled the supply, took it to my bench and everything measured okay.


I've heard a few people say that 20v rails are too high on this console anyways, so perhaps I should lower rails a bit?

There is a short supply of good techs in this town, and while I am becoming proficient, I thought I'd see if anyone has encountered an issue like this on an 80b, or any console for that matter, before I go back and get going with this. The fewer red herrings, the better, and since this problem is intermittent, it could possibly be a tricky fix. Any suggestions and tips would be welcome and appreciated.

I also left the pcb transistors in because they all tested ok and replaced the chassis mount darlington transistors.

Thank you!
Jeff
Jeff
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Posts: 3
Joined: Sun Jan 08, 2017 1:00 pm

Re: Trident 80b repair

Postby Jim Williams » Wed Jul 26, 2017 9:53 am

You may have a bad module someplace taking the rails down. Trident lacks local power supply 10 ohm fusing resistors so all modules connect together, if one goes down they all go down.

There are a couple of jumpers on the inputs near the connector that can be lifted and replaced with 1/4 watt 10 ohm resistors. Then if you have a shorted module, the resistors burn up and then it's easy to find and fix. Mount them up off the pcb so they don't scorch it.

The same applies to the groups and master module.
Jim Williams
Audio Upgrades
Jim Williams
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Re: Trident 80b repair

Postby Jeff » Wed Jul 26, 2017 10:30 am

Great info! Thanks so much Jim.
As of yesterday, the desk seems to be working ok, but now it's that, "when is it gonna happen again?" Feeling. Those fuse resistors will definitely be at the top of the to do list.

Regarding the 20.4 volts on the rails, is it fine to leave it operating in that range, or would it be beneficial to lower them a bit? My assumption would be the variable resistors in the power supply pcb control the voltage output, but I know what usually happens when you assume things.... any info on that would be great.

Thanks again,
Jeff
Jeff
Analog Owner
 
Posts: 3
Joined: Sun Jan 08, 2017 1:00 pm

Re: Trident 80b repair

Postby Jim Williams » Wed Jul 26, 2017 11:26 am

+ - 20 volts is beyond the absolute maximum ratings of the common TL071 opamp. The absolute maximum voltage for those opamps is + - 18 volts. + - 17 is a safer setting. Do reduce those rails, nothing is gained from that.
Jim Williams
Audio Upgrades
Jim Williams
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Posts: 362
Joined: Mon Jul 14, 2008 9:58 am
Location: beautiful Carlsbad, CA

Re: Trident 80b repair

Postby Jeff » Thu Jul 27, 2017 10:30 am

Thanks for the great info.
Really appreciate it!
Jeff
Jeff
Analog Owner
 
Posts: 3
Joined: Sun Jan 08, 2017 1:00 pm


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