jensenmann

AMEK M2500 with transformerbalanced Preamp
« on: November 19, 2007, 11:34:50 AM »
Hi there
Today my new (?) board arrived. It´s an very early AMEK M2500. Since it has been sold to me as defective I took it apart to have a quick look inside on some details. That´s when I realized that it has a different preamp than it´s been posted here in several threads. It´s transformerbalanced with a Sowter iron followed by a NE5534.
Does anybody have infos/schematics about that? The schematics which came with the board show the usual later preamp (which it definitely is not).

 :thumb:
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort


SSLtech

AMEK M2500 with transformerbalanced Preamp
« Reply #1 on: November 19, 2007, 12:02:04 PM »
Ah. The (also Graham Langley-designed) early version, I think.

Does it also have a couple of transistors between the Sowter and the TDA1034/NE5534? -If so, then it's the one which I'm thinking of, and it's probably the same as on the M3000.

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

jensenmann

AMEK M2500 with transformerbalanced Preamp
« Reply #2 on: November 19, 2007, 12:28:35 PM »
No transistors on the preampboard (GL122), only iron and NE5534

Also I realized that my board has GL124A EQ boards, not the C-Version which I have on the schemo. Is there any difference in the circuit?

Are those Allison VCAs available or is this a weak point?
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

SSLtech

AMEK M2500 with transformerbalanced Preamp
« Reply #3 on: November 19, 2007, 01:06:21 PM »
There was an earlier version of the 2500 EQ, which had only "÷Q" switches at the top and bottom... the later versions had "bell/shelf" switches as I recall.

Does yours have bell/shelf or just divide Q?

The mic pre sounds like an interim version, rather like the Harrison or the early SSL, although both of those used Jensen transformers instead of Sowter.

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

SSLtech

AMEK M2500 with transformerbalanced Preamp
« Reply #4 on: November 19, 2007, 01:07:49 PM »
I don't know of any source for Allisons, though the circuit should be modifiable to accept a THAT 218x if you ever have to...

Colin A. ("slenderchap" on these fora) may have mic pre documentation and insight into Allison availability... he used to work for Amek, after all.

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

jensenmann

AMEK M2500 with transformerbalanced Preamp
« Reply #5 on: November 19, 2007, 01:20:23 PM »
Again it seems to be a version inbetween. It has Q pots (4) but no bell/shelving switches. But now I found the schematics on another page barely readabl and very tiny. But it helps.

What makes me curious is that a lot of the coupling caps are with underrated voltages. 100uF/3V tants or 100u/10V ´lytics as coupling caps in circuits with +/-16V makes me think. Was this part of the boards famous sound or simply made for space/economic reason?

Now looking at the mikepre schematic it reminds me on the Midas Pro4 Preamp which uses Belclere input iron
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
AMEK M2500 with transformerbalanced Preamp
« Reply #6 on: November 19, 2007, 02:00:27 PM »
Quote
What makes me curious is that a lot of the coupling caps are with underrated voltages. 100 uF/3 V tants or 100 uF/10 V ´lytics as coupling caps in circuits with +/-16 V makes me think.

That's no problem. There will never be more than some dozend mV across the cap if sized correctly and used with audio signals.

Samuel

jensenmann

AMEK M2500 with transformerbalanced Preamp
« Reply #7 on: November 19, 2007, 02:02:52 PM »
well that´s exactly what makes me think. A tant wants to be biased with more than a few millivolts
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
AMEK M2500 with transformerbalanced Preamp
« Reply #8 on: November 19, 2007, 02:03:41 PM »
It's often claimed that life span is impaired without bias, but I'm not completely sure about that one. Distortion surely get's higher with bias.

Samuel

SSLtech

AMEK M2500 with transformerbalanced Preamp
« Reply #9 on: November 19, 2007, 03:53:38 PM »
Quote from: "jensenmann"
It has Q pots (4) but no bell/shelving switches.

Yes, ALL versions had 4x Q pots (the lower/outer knob of the dual-concentric pair) -but the important thing is the switches:

Each band should have a [÷Q] switch, and/or a [f x 10] switch?

Tell me exactly what buttons (switches) the highest and lowest bands have.

If they both have a [÷Q] switch, then these can be modified to work as bell-shelf switches instead... -It means a little bit of trace-cutting, but it works. You don't HAVE to do every module either...

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.


12afael

AMEK M2500 with transformerbalanced Preamp
« Reply #10 on: November 19, 2007, 04:06:42 PM »
Jakob has a schematic of the amek2500 eq on his page. If someone have the schematic of another vercion or the complete console schematic, I want to see it.
heavy metal is the law!!!

jensenmann

AMEK M2500 with transformerbalanced Preamp
« Reply #11 on: November 20, 2007, 03:25:42 PM »
Keith, there are no switches at all in the EQ section (besides the Polarity reverse), neither for Q divide nor for frequency multiply. But I saw the mod you´ve been talking about on the schematics which I found now. It seams really easy to implement the Bell/Shelf switches.
The partvalues of my EQ version (A) seem to be the same as in the C version.
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

SSLtech

AMEK M2500 with transformerbalanced Preamp
« Reply #12 on: November 20, 2007, 04:33:12 PM »
Wow... that's not a version which I've seen then. -Might be about 1980 manufacture date? -look on the P&G faders to see if there's a manufacturing datecode... Might be a week number and year: for example 35/82 would be Week 35, 1980 (late August/early sept. 1982 or thereabouts)

In the online schematics, (with the frequency X10 and divide Q buttons) the three lowest bands usually have the same (VERY wide) frequency ranges, I'm wondering then if yours might have different ranges, to compensate for the lack of range-switching...?

-from memory- the two mids and the LF section went from something like 30Hz to 800Hz, and with the "X10" engaged that goes from 300Hz all the way up to 8kHz... even the LF!!! -Without the X10 buttons, they might have changed the two capacitor values in the biquad section of each band, to 'shift' the ranges to slightly more appropriate center points.

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

Bluzzi

AMEK M2500 with transformerbalanced Preamp
« Reply #13 on: November 21, 2007, 12:47:50 AM »
Jens - You may already have but have you tried asking Mr. Langley at http://www.langley-design.com?

Maybe he can help?

Also Colin at Audiomaintenance?

Just a thought.

jim

Slenderchap

AMEK M2500 with transformerbalanced Preamp
« Reply #14 on: November 21, 2007, 01:55:54 AM »
I'll see what I can find for you.... send me an email.

Colin
www.audiomaintenance.com

jensenmann

AMEK M2500 with transformerbalanced Preamp
« Reply #15 on: November 22, 2007, 10:08:24 AM »
It seems that I found most of the schematics now. Thanks for your offer, Colin. I will drop you a mail as soon as I have a rough overview over the parts that I need. At the moment I´m reworking both PSUs. They are in bad shape, five of seven of the big caps are leaky and a lot of details need attention. When this is done I will fire up the desk ang go through all the channels.

I´ve take some pictures which can be seen here: http://www.twin-x.com/groupdiy/thumbnails.php?album=212

Interesting is that all channelstrips are named M2000 though they are M2500
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

SSLtech

AMEK M2500 with transformerbalanced Preamp
« Reply #16 on: November 22, 2007, 11:08:25 AM »
replace the main reservoir caps in the PSU, and MAKE SURE THAT THE CIRCUIT HAS THE FUSES BETWEEN THE RESERVOIR CAPS AND THE THYRISTOR CROWBAR.

I cannot overstate this... if the thyristor is wired directly across the caps, as it was in many of the original PSUs, the crowbar will explode without pulling the fuse, leaving the console unprotected, and over-volted. -that was a VERY common Amek failure mode on the older PSUs

Also, clean off and replace any old silicone grease beneath the power transistors.

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

SSLtech

AMEK M2500 with transformerbalanced Preamp
« Reply #17 on: November 22, 2007, 11:09:46 AM »
The M2000 used "winkie-blinkie" switches. We had an M2000 on loan from the factory in Salford while they completed the M3000 for us. (-It was behind schedule and we had a studio to open...)

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

jensenmann

AMEK M2500 with transformerbalanced Preamp
« Reply #18 on: November 23, 2007, 07:05:47 AM »
At the moment the Fuse is between rectifier and cap. I don´t understand the word crowbar in your post. But what I think you mean is that if the regulator fails, the cap will explode because the current gets too high?

Have you had some experience with the IC sockets? They look like pretty lousy quality. But replaceing them all would be PITA and completely unnecessary if they are not prone to failure.

Do you have other suggestions where things could need attention? Keep it coming, it´s good to have someone around with more experience than me. Since I´m only a hobbyist, I´m definitely not very secure about all the things I realize and do.


And you´re right with the silicone stuff, it´s more like dust, meanwhile. I already realized that and will definitel renew it on all regulators and rectifiers.


thanks :thumb:
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

SSLtech

AMEK M2500 with transformerbalanced Preamp
« Reply #19 on: November 23, 2007, 09:44:44 AM »
Fuse between rectifier and cap = BAD.

The crowbar is a thyristor across the unregulated DC rail. In the event that a regulator pass transistor fails hard short from collector-to-emitter (the most common failure),  the FULL, UNREGULATED (30V or so) voltage is then conected to the output.

The overvolt sense cannot shut down the transistor, because all it does is close down the transistor base voltage... which does nothing because the transistor is SHORT fromo base-to-emitter.

Recognising this, there ie a 4-amp thyristor across the rail. On paper it looks like the incoming power from the bridge rectifier goes -through the fuse- to the thyristor, and will blow the fuse, making everything safe.

But no...

The thyristor is rated at 4A, and probably explodes abotu 10A peak. The reservoir caps -when fully charged- supply a peak current of about 40Amps, utterly destroying[/u] the thyristor, and therefore removing the last protection that you had against overvoltage. (The recifier/secondary doesn't respond to an increase in current demand anywhere NEAR as quickly, since the secondary has too much inductance, and a SUDDEN change is seen as a high frequency event... iductance increases impsdance at high frequencies, so less current as a result). Now, you have a hard-short transistor, and no way to shut it down.

Move the reservoir caps to the OTHER side of the fuse, so that now the COMBINED current from the rectifier AND the reservoir caps goes through the fuse, and you have a design that works as intended.

That's all you have to do to makeit work. -It WILL NOT WORK as built at the factory.

You can safely test it with the console disconnected by brushing two wires together, one wired to the unregulated voltage, the other to the output. The way it's built from the factory, the thyristor will explode, and as long as the wires are touched, the full output passes unprotected. once modified, the fuse will pop and the thyristor will live, and the supply is safely shut down.

the blue sockets were pretty terrible, and aged poorly with heat, becoming crumbly. If they're not disturbed they hold the chips okay, but if they are over-handled, they tend to crumble and break. replace any that break, but don't go LOOKING for trouble... a "shotgun" replacement will tend to lift traces, since I remember that the PCBs tended to do that, although when I was working on them, my jedi soldering skills weren't as good as they are today. :wink:

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.


 

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