Amek M2500 thread

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jensenmann

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Hi group

I decided to create an Amek M2500 thread to collect and preserve informations about these desks. Feel free to post whatever you think is appropriate and needs to be shared. I´m going to fill the thread with bits and pieces as time and knowledge allows.

I´ve attached a picture of my desk. It´s the broadcast version with 24 transformer balanced micpres and 12 transformer balanced stereo line channels. It´s completely torn apart since years. Initially I wanted to refurbish it, but due to moving and servicing so many other desks besides having a job I haven´t much done other than recapping. After all my controlroom in our new house is still an empty shell with nothing inside than the console frame....

A few month ago a friend called me since he bought a M2500, too. He needed some help with installation, so I passed by to have a look at his desk. It turned out that there were micpre cards and group cards missing. After getting most of the gremlins out of the desk I made a PCB layout for both cards in the original format to bring his desk back to full potential. Today the micpre PCB prototype arrived. I´ll keep you updated how they turn out.

Here´s the helpful-links department:
http://www.audiomaintenance.com Our member slenderchap stocks spareparts for Amek consoles
https://groupdiy.com/index.php?topic=52462.0 Our member sahib´s Amek based DIY projects

Here are more threads about M2500s:
https://groupdiy.com/index.php?topic=73910.msg935598#msg935598
https://groupdiy.com/index.php?topic=75137.0
https://groupdiy.com/index.php?topic=62928.msg797117#msg797117
https://groupdiy.com/index.php?topic=61152.msg774717#msg774717
https://groupdiy.com/index.php?topic=35002.0
https://groupdiy.com/index.php?topic=37078.msg457230#msg457230
https://groupdiy.com/index.php?topic=61363.0
https://groupdiy.com/index.php?topic=73147.msg926248#msg926248
https://groupdiy.com/index.php?topic=29080.0
https://groupdiy.com/index.php?topic=25238.0
https://groupdiy.com/index.php?topic=72155.0
 

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gyraf

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A piece of information:

The M2500 "Fader Reverse" card relay on the channels is not designed for being mounted vertically/on-edge like it is - so over time quite a few "intermittent" type errors develop around here. IOW, if you have something unstable in channel, you'll want to push the faderreverse-button a couple of times to get around this. Cleaning the relay works for a while, but not long. Haven't found any pin-compatible relay that will work in this position, we were once considering making a subboard with standard telecom relays to get around this..

/Jakob E.
 

jensenmann

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Thanks, Jakob!

I have more to add to the relay topic. Indeed the fader reverse relays are a problem, in my desk as well as in my buddy´s desk. Even worse is the price because they are obsolete. I went through a bunch of different brands and should add that you´d better stay away from Aromat brand relays. Their PCB pins are so soft that if you press the relay into spot a pin can easily get pushed inside the relay. The last few relays I had to replace cost me 3 Aromat relays which went straight into garbage because of that problem. Omron LZN4 12V of course is fine as well as NEC NF4 12V. Make sure you get sealed versions, they work longer than unsealed ones.

There´s a british company making replacement PCBs with a pair of small signal relays: https://cephosrelays.com/omron-lzn4-relay/ I have ordered a pair recently. I´ll keep you updated how they turn out. In the long run I´m going to make a replacement PCB, too, as it might be cheaper to DIY than buying them prefabbed.

The other relays are obsolete, too (Hamlin Reed Relay CCPRI/J). There are five of these on each channel. They tend to make problems but mostly the reason is that they do unsolder themselves! Resoldering often helps. Audiomaintenance.com stocks replacements, though they are not cheap. Years ago I was thinking about doing a replacement but it turned out that the problem is the coil resistance. I found only one manufacturer who´s making something similar, but they have a different footprint. That´s Pickering Electronics 101-1-C-5/3 relay. Now guess which relays AML is using... Since these relays are no stock part, you have to order >1000pcs to get a decent price from the maufacturer. That makes no sense for a single person considering that you still need an adapter PCB, etc. After calculating everything I decided to buy them from Colin (Audiomaintenance). In the end he´s offering them for a fair price.

I had a conversation with Graham Langley about the relays years ago and he told me that the brands were chosen for lowest current. Esp. if you replace the Hamlin relays with current type 12V relays you´ll run into problems on the 12V (actually 11V) rail. My buddy´s M2500 has all CCPRI/J relays replaced with currently available relays on adapter PCBs, done by the last owner. He  blows the 8A fuse on the 11V rail every few weeks. It makes sense to stick with the AML solution as those Pickering relays are nearly the same resistance as the original Hamlins.
 

jensenmann

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The first preamp card is working. I had to modify the layout a quite bit for being able to add bypass caps to the electrolytics and rails. It´s pretty tight on that board, anyway. The Zetex ZTX214 transistors are obsolete, so I replaced them with BC560C. And for benchtesting I made a small jig on perfboard with all connections and the gain pot. That makes testing a lot easier without using the desk.
 

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jensenmann

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Next one is a replacement for the bus amp card. Again as above, the layout is modified to accept bypasscaps for rails and electrolytics. The new board contains more space for both the input and output caps. They can be doubled in value which is a huge improvement for the lowend response of the desk. Instead of TL072 I´m using OPA2134 because they have lower selfnoise.
 

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ilfungo

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Hi this discussion is a great news!!!
I'm working on a Amek M2000 PSU to rebuilt.
I have 2 psu
one in good condition
and the second ready to rebuilt.
I'm working on GL48 redraw because the boards are in very bad shape...
Can someone suggest me some advices or upgrades for  this board?
Thanks!
 

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jensenmann

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Increase C3 and use low ESR long life electrolytics with high ripple current rating here. Add a 100nF ceramic bypass cap parallel to C3.

Make sure that the power resistors are mounted with a distance to the PCB to prevent browning and overheating the PCB. E.g. use standoffs. It won´t do no harm if you increase their wattage. In theory wirewound resistors with bifilar winding are the best solution in this spot.

Use a thyristor in the crowbar circuit with higher current rating. In case of PSU failure it has to drain a huge current (short circuit current of the transformer winding + discharge current of the big electrolytics). The original part has 4A/20Apeak current rating which is not enough. 8A is more appropriate and a heatsink won´t do no harm either.

Often the TO3 sockets have contact problems with age. Maybe you can find another solution to mount these....

Make the PCB traces wider for better current handling.
 

jensenmann

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The sliderails break easily after all these years. Their plastic has hardened hence it breaks like glass if you slide a channel in with too much force. I broke a bunch of them in my desk already. A friend with a 3D-printer made me spare parts. If you guys need some spares, too, then feel free to contact him and say hi from me: https://audiosohn.com
 

ilfungo

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jensenmann said:
The sliderails break easily after all these years. Their plastic has hardened hence it breaks like glass if you slide a channel in with too much force. I broke a bunch of them in my desk already. A friend with a 3D-printer made me spare parts. If you guys need some spares, too, then feel free to contact him and say hi from me: https://audiosohn.com


Hi
there is another fader  part that create the same problem over the years...
With a friend we have redesigned it , if someone need the solid work file ready for 3D print,just ask...
 

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gyraf

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(this thread about the desk, I think you mean to ask about the mic preamp, which is somewhere else?)
 

jensenmann

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Another update regarding relays. The Cephos replacement board works just fine. Thanks Brexit, shipping and EU import tax the prices are stupid, though. So I did a replacemnt PCB using Omron G6K-2P 12V relays. The good news is that their current draw is a lot lower than with the original relays. That helps to save roughly 1A on the 11V rail in a 36 channel frame (9mA vs 40mA per relay * 36 channels). Since all my prototype boards went straight into the desk I have no pictures currently. I´ll add them later...
 

Matt Syson

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Regarding a 'copy' of the GL48 PSU circuit board. Use 2 ounce copper and better duplicate the htraces carrying high current on the reverse side of the board, as the copper traces heat significantly. I have a fair collection of 'toasted' boards!
Matt S
 

jensenmann

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Regarding a 'copy' of the GL48 PSU circuit board. Use 2 ounce copper and better duplicate the htraces carrying high current on the reverse side of the board, as the copper traces heat significantly. I have a fair collection of 'toasted' boards!
Matt S

I´ve seen M2000 PSUs with two and three fans. Two fans are always on the rear for cooling the regulator boards. But they don´t blow air into the case, only onto the regulator heatsinks. If the PSU doesn´t have the third fan on the side mounted then everything inside will heat up significantly. It´s a good idea to install a third fan. Some cases are already pre punched for that purpose.
 

Matt Syson

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Just looked at a couple of my '2000 supply' collection and confirm that some have 3 fans fitted. Speaking of which the two rear fans should not be of the 'low noise' versions as low noise is only possible by low airflow so insufficient cooling. A 'side mounted' fan could be a lower (lower airflow) version as it is 'wafting about generally and mainly persuading the heat of the transformer to exit and help with capacitor cooling too.
There are at least 3 versions of power transformers I believe, with two 'laminated' and at least one torroidal version. The rear fans must blow onto the heatsinks (inwards). I forget now whether it is the 'ball bearing' or 'sleeve bearing' version fans that are the longest lasting.
Note that the '2000' supply boxes were used for automation supplies from around 1990 onwards (the first of the Mozart desks) where they had a 3 rail XP power module fitted so not actually a '2000 supply at all.
Matt S
 

LouD

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Hello all!

What a great thread, special thanks to jensenmann for starting it. I've been meaning to post for a while now but just getting to it as my slow season approaches. I'm not sure if I can keep up with you guys on the technical end, but thought my project may be interesting and prove useful for others with M2500s.

So, a bit about my console:
It started out as an early revision M2500 with M2000 modules (no line trim knob or hi/lo shelving eq switches) with 36 channels. When I was searching for consoles years ago, the seller contacted me with photos and service history. The console was super clean and seemed to be well cared for, having been recently re-capped (around 2009). He also upgraded the +/-18v rails to two Acopian power supplies. The 2000 PSU now handles only the logic, 48v and lights.
The asking price was way out of my league though, so at the time all I could do was dream about it. Fast forward to 2018, I re-connected with the seller and he was still selling the board, now with 10 less channels (and backplanes!!). Long story short, he made me an offer I couldn't refuse and was on a plane that day to pack her up.
This was perhaps a little impulsive, as upon commissioning the console I realized that the channels he sold, 1-10, contained the bus amp cards for busses 1-10. The backplanes and associated wires were also cut. So, faced with the reality that I'd be patching from channels 11-36 and only having 14 busses, I decided to "shift" the channels to the right and rewire them all to be channels 1-24. I've also rewired the "Line 2" inputs on the patchbay to be normalled to my DAW outputs (Line 1 is my tape machine). So needless to say, I've got a bunch of open patches and a mess of wire under the patchbay. In the space where channels 1-10 used to be, I cut out the backplane mounting hardware and welded in a support beam effectively relocating the left side support. I built in rack rails and mounted my outboard preamps there.

It was a lot of work, but it all seems to have worked as planned. The console now has typical issues to be worked out as one should probably expect on an old girl like this. It seems that the Fader Reverse relay issue can be attributed to some of them and I'm guessing that a re-cap is in order as well. Beyond that I'd be interested in any modifications / upgrades that would be recommended, given the improved components that are now available. Does this sound like a good place to start? I'd be very interested in your thoughts/suggestions!

I'll attach some photos for your enjoyment.

Thanks for looking!
Lou
 

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jensenmann

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Thumps up for your brave dedication. That´s quite an amount of work to rescue this old lady.

Recapping for sure will make the biggest difference in sound. While working on my buddy´s desk we prepared two channels, both recapped in the same way, but one having bypasscaps soldered parallel to all electrolytics. The later one turned out to sound a bit more detailed and cleaner in a listening test. So the owner decided to have bypasscaps wherever possible as he liked it that way better. If you prefer less character (i.o.w. distortion) and more detail then solder small film caps (something 10-100nF-ish) parallel to all electrolytics.

In original state the lowend is a bit wooly and undefined because the busamp cards have a rather small cap at the summing input. These cards are used on the master as well. Increase C1 to as large as physically fits. The output cap C6 can be increased to 220 -330uF. That will give you tighter and more defined lowend as it lowers phaseshift. If you want to lower noise a few dB on the busamp card then use an OPA2134 or TLE2072 instead of the TL072.

Replace all tantal rail decoupling caps with electrolytics as they tend to fail as a short. There´s a pair at the fader-end of the channel PCB and another pair between the busamp and micpre sub-PCBs. Increase their value to 100uF/25V. Another tantal sits next to the 5V regulator (C3). This one should be replaced with 10uF. You have to remove the fader in order to do that. The EQ card houses another pair of tantals (depending on the EQ card version) for rail decoupling. Use 100uF/25 there, too.
 

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