EMT A100 Mixing Desk - refurbishment/upgrade

I was lucky enough to get my filthy fingers on an EMT A100 mixing desk recently  8)

Of course there are one or two issues with it - so as there is little to no useful info about these on the net,
this here place might be a good starting point for gathering more information/detail about this particular mixer.

The issues with my unit:
1. high noise floor
2. some channels not working

I chose to recap the noisiest channel as well as the master section - all with Panasonic FC caps. The noise level did not change - so my guess is that it's one of the opamps that's causing this. Which brings me to my main question:

The opamps used in this console are:

1x5534 per channel on the mic-input
3x RC4739 on the filter sections of the channel as well as on the master
11x TDB0155 everywhere else (faders, sends, pan etc)

As I figure that both the noise floor and those non-working channels might be due to faulty (old) opamps, I am considering swapping some. And looking at the datasheets of those RC4739 and TDB0155 I wonder if there is any improvement to be had by upgrading?

Those RC4739 seem to be very slow, but any upgrade for those would involve soldering some kind of adapter for matching modern opamp-pin outs.
Those TDB0155 on the other hand look better on the datasheet - but there are veeery old as well. And they seem to be the same as the LF355.

My knowledge of electronics is like a patchwork though as I'm only learning by doing, so I have marked each opamp on the schematic (see below) and would love to get your oppinion which of these opamps are in sonically crucial positions and could benefit from an upgrade.

Considering that there is ten channels = 110xTDB0155 opamps in there, upgrading might not be cheap.
I'm not looking to hear supersonic fairy dust - just hoping to improve any noticable issues such as noise and distortion.

Hopefully some of you can give me a nod in the right direction?
The schematic is in horrible quality, I've improved it slightly in Photoshop, but it's still bad - my apologies, strangely it's the only one I could find on the web...

Thanks 
Lynn

The input module (NE5534 green, TDB0155 blue, RC4739 red)


The master module:

I've attached the schematic as well now, seems easier to download this way...

...and the master.

I think rule of thumb is always start with the powersupply. Recapp if needed and make sure its running 100%

The first thing to do is to try to isolate the source of the noise. Is it quiet with the channel faders down?  If it is then the noise problem is obviously pre-fader. Is is noisy on mic but not line in which case look first at the mic pre.

Cheers

Ian

Thanks a lot for the input!
The PSU had been recapped - I usually do those first, to be on the safe side

Anyway, I have solved the noise issue - it was indeed a faulty opamp at the input trim of one channel.

I am still wondering, should I just stick with these old LF355/TDB0155 opamps?

Thanks
Lynn

bootyfox said:

Thanks a lot for the input!
The PSU had been recapped - I usually do those first, to be on the safe side

Anyway, I have solved the noise issue - it was indeed a faulty opamp at the input trim of one channel.

I am still wondering, should I just stick with these old LF355/TDB0155 opamps?

Thanks
Lynn

Click to expand...

How does it sound to your ears? If it ain't broke, don't fix it.

Cheers

ian