dead console preamp trouble shooting

[heavyhanded]

Hi, I've lurked around on this board for a while and know there is a lot of knowledge here so I thought I'd try to tap in to it for some advice. Thanks in advance if anyone gives up some time to help me out.

I recently picked up an old Dynamix 20-4-2 console basically for free and after a lot of cleaning and some deoxit I've managed to get it close to fully functional.

The only things not working on it are about half of the mic preamps. All line inputs work on each channel as well as all other switches, faders, and pots. The dead preamps will pass a very quiet distorted signal if I crank the gain.

After exhaustive searching I can find practically no information on this board whatsoever (it's an old british made board that looks like this) and no schematic but I'd like to try to revive these channels if possible. However, where to start? All the connections look ok as far as I can tell... Would recapping be the first place to start? or replacing the opamps? This board appears to use TL072's for the preamps.

I guess I am a bit of an electronics noob but am good at soldering and willing to learn more and thought this might be a good opportunity. If anyone can steer me in the right direction I'd appreciate it. Also if anyone can give me any info at all on these old Dynamix consoles that'd be great too!

[guitarguy12387]

Got a scope?

I'd put a sine wave through it and follow signal from the input to the output and see where you lose it.

I'd change the opamps before recapping if there are sockets for the opamps. Be alot easier and tl072's are cheap.

My two cents...

[heavyhanded]

Unfortunately, I don't have a scope although I have been wanting to pick one up if I can find a cheap one suitable for basic audio work. Also the opamp is soldered to the PCB. I'll probably put in a socket though if I replace it.

These are kind of cheap desks and not really set up for modifying, there are ground wires soldered across all the channel boards so one has to cut the wires and jump them to get a board out. I'm thinking of replacing the fixed wires with connectors as I go along to make it more modular.

I guess I'll start with one channel and replace the TL072 and see how it goes.

[vince @ speck]

Is there an insert jack on each channel?

If so, maybe the mic pre signal goes through the insert jack and the jacks' "normaling" terminals are not making proper contact.

[Rick Sutton]

Originally Posted by vince @ speck

Is there an insert jack on each channel?

If so, maybe the mic pre signal goes through the insert jack and the jacks' "normaling" terminals are not making proper contact.

Vince, that's a really good point. A lot of the cheaper desks route the mic pre out through a normal on the line in jack and that is certainly a weak point. Insert a 1/4" plug into the line in about 20 times while listening to the mic pre and see if it affects the level of the mic pre.

[heavyhanded]

Vince, you were exactly right and Rick thanks for the advice because that worked great.

I am kicking myself for not thinking of this sooner.. I guess I thought that the line inputs would be affected the same way if it was the inserts so I ruled it out early and started looking for bigger problems.. I was going to rebuild the engine before checking the spark plugs.

Well you guys just gave me 10 more mic preamps thanks!

This old console is turning out to be quite a bargain actually.

[Matt Syson]

Hi
As a general note, most op amps are very reliable especially if not subjected to 'external signals' such as mic / line inputs or an output.
You do not necessarily need a 'scope to check things out as you can usually get about 90 percent of the way using a multimeter. Some simple tests on an op amp are often sufficient. Is the power there? For a 'split' (dual) rail design are the inputs and outputs all sitting within say 100mV of 'ground'. If so them the majority of circuits are likely to be working. Compare working channels to faulty channels.
You should suspect 'mechanical' problems way before electronic unless you know that the thing has ben stressed by a serious power supply overvoltage (unusual).
Matt S

[heavyhanded]

Well, now I have bigger problems anyway, I took the back off to clean and inspect the xlr connections and when I put it back together now nothing at all works and the master channel's VU's are cranked up.. Yikes! What did I do?

[Jim Williams]

I've rebuilt those. It used a single 071 opamp off a old English transformer. Pop in a socket and try a LME49710. Use the same part in the stereo mix sum amps. The inserts have a blend control ( ahead of it's time with parallel outboard mixing possible) so it always will pass a signal. The fader panels are a bitch with the buss bars, have fun with that.

Jim Williams
Audio Upgrades

[ripple_fx1]

Reseated a multipin connector and missed it by 1 pin, or found a spot of the same oxide that was on your inserts?

[heavyhanded]

I didn't unseat any connectors at all... on purpose anyway. The gutz of this beast are really hard to access though because it is in a big flight case with the power supply mounted on the bottom half of the case but there are soldered wires going to the board without much slack. Perhaps I loosened something accidentally.. I am checking connections right now but haven't noticed anything unusual yet. There are very few connectors on this thing though. Most connections are soldered, like the buss bar accross the faders as Jim mentioned. My fingers are crossed... Quite a setback!

[heavyhanded]

It looks like an old solder joint on the power supply pcb finally gave up.. I think I might have found the culprit.