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Post by mrholmes on Oct 17, 2014 6:36:35 GMT -6
User smallbutfine talked in a different thread about modding an old Behringer MDX 1200. He also explained that it is some kind of Drawmer copy. The unit is at e-bay around 15-20 Euros depending on its condition. You can see them very often. I can read some schematics, and I can use a soldering iron. What makes sense to replace in the unit? As far as I know smallbutfine worked in this business!! The idea is that smallbutfine gives advise what to replace in the unit. What makes economically sense, what will be overkill. My goal is to invest as little as possible in parts. If you have such an old MDX 1200 chime in. One problem is still to solve: I was not able to get schematics.
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Post by svart on Oct 17, 2014 7:37:51 GMT -6
I used to have two of them. Yeah, they were drawmer copies, right down to the ICs used. Behringers of this vintage are actually decent. This was before they started going super cheap on everything.
I no longer have them, but generally, you need to recap the power supplies and add decoupling to each audio opamp at a minimum. Doing this FIRST gives a good basis for any mods you feel you need later.
Chances are that just improving the decoupling will be enough. If not, the next change would be to the audio path capacitors. I use Nichicon ES bipolars for everything. People will tell you all kinds of things, but as long as you use a good quality cap, you're going to be happy.
I would save the opamps to last. The problem with changing opamps is that if you change the type (going from JFET to BJT or to hybrid) then you change the impedances of the resistor networks around them. You'd need to change all the resistor values related to that opamp if you intended to keep noise to a minimum and signal integrity to the maximum.
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Post by Deleted on Oct 17, 2014 8:12:31 GMT -6
Oh - big mistake on my side... I made a typo/misremember kind of thing, my units are MDX 2100, not 1200, the composers not the autocoms. I hope this is not a problem, i did not want anyone to make a bad investment, but these units are really cheap, so, i guess we have no prob due to this... No matter, we can go thru these as well. Autocompression does lead to the guess these are more a rip off of DBX designs(?), not bad either. The 2 kinds of mods i made with Behringers dynamics boxes are: 1. Drop-in opamp replacements 2. Higher supply voltage and/or PSU caps upgrade 3. and - if desired, the That2150s can be upgraded to 2180 or 2181 as well, but i would leave this open until you heard the beforementioned mods. Obviously, the second one depends on which opamp chips we use for a replacement and if these work on higher supply voltage, providing more headroom. The LM339 comparators can take up to +-36V, no prob. The opamps we can concentrate on should be the TL07x and the Behringer BE0xx (I have to lookup what these were exactly, but very obviously Behringer re-labeled some of the opamp chips. As far as i know, these are TL0xx chips, too. Most probably quad opamps, due to the package type. Since we have no schematics, we assume we will upgrade in- and output stages, leading to better specs, using more of the still inexpensive alternatives for a simple drop-in replacement (or what we have at hand/in stock)... This makes sense on many low-end units, not so much on mid-class or high-class units, because there, oftenly opamp choices are made to cut costs and many designs run perfectly with higher spec opamps without change of the surrounding circuit. What we have to watch out if changing opamps, is, that we do not exceed the PSUs power specs. These may be weak anyways. Easy rule of thumb, if we are going to estimate power requirements, in chip designs like this, is: Add up power requirements single chip power requirement as of spec sheet x no. of chips in the design plus a safety margin.
Looking at the pics, one thing is very mod-friendly: These PCBs are single sided. Later Behringer PCBs are very mod-unfriendly, hard to desolder, and easy to mess up. But we have to take care of temperature still, because these PCBs are not as heat resistent as the later FR4 PCB material. Not so these shown in the pics.
When changing opamps, we should not be picky about loosing the chips we desolder. These are not very expensive chips. It is easier to cut the thru-hole legs and desolder each one after another for less thermal stress on the board. Also, solder precision IC sockets in afterwards, so you are able to check what fits your ears, or even go back to the original chips if it turns out you liked them better...
Question: What are the regulator types? I assume they are LM7815/LM7915? We see 2 bigger electros behind the regulators. Replacing these with good quality japan ones would be nice anyway, with some higher capacity, these are also parts of the circuit, where oftenly budget decisions are made for cheap units and we could probably gain something relevant. If in doubt, Panasonic FC/FR or similar are pretty good and easily available. What are the smaller IC packages?
To be continued...
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Post by Deleted on Oct 17, 2014 8:18:35 GMT -6
PS: Svart did make some excellent remarks about modding these units already, and btw, i like Nichicons just as well for modding, esp. in the PSU section...
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Post by Deleted on Oct 17, 2014 8:27:46 GMT -6
I just saw, the THAT chips are 2159. Actually, these were exclusively made for Behringer, and surprisingly, in tests they measured very good, better than standard 2150 - so i would leave them alone...
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Post by Deleted on Oct 17, 2014 8:38:52 GMT -6
Leave one unit completely unmodded. This is the only way you can objectively compare before/after. Subjectively, most modders think their modded unit sounds better than before, even if this is actually not the case, or noise specs are worse...
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Post by Deleted on Oct 17, 2014 9:13:43 GMT -6
For opamp decoupling, just solder 0.1uF MKT on the copper side of the pcb directly to the +Vs pin to ground and between the -Vs pin to ground. For the TL074 this would be pin4 and pin11 to ground...try the same for BE037.
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Post by Deleted on Oct 17, 2014 9:23:11 GMT -6
As far as I know smallbutfine worked in this business!! Well, this might have been a misunderstanding. No, i didn't work in this business. I did work in quite alot of fields around electronics, up to observation/analysis of mobile phone repairs in large quantities, but music and studio electronics i wanted to keep fun!
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Post by Deleted on Oct 17, 2014 10:41:48 GMT -6
Forget about my speculation about the design, as far as i could find out, these here are also drawmer rip-offs, DL241-alikes ...with CA3046 and and a pair of opamps forming a half-discrete detector circuit...unfortunately i do not have the schematics...
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Post by Deleted on Oct 17, 2014 10:49:43 GMT -6
mrholmes : Since low cost is important to you, a bag of local opamp decoupling caps e.g. 100nF MKT and PSU recapping electros get you into business...everything else i would put aside until this is done...
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Post by Deleted on Oct 17, 2014 11:11:28 GMT -6
It's a bit blurry in the pic, but i assume the smaller ic's are JRC4560 opamps. Here use decoupling 0.1uf from pin4 and pin8 to ground...
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Post by mrholmes on Oct 17, 2014 13:03:28 GMT -6
It's a bit blurry in the pic, but i assume the smaller ic's are JRC4560 opamps. Here use decoupling 0.1uf from pin4 and pin8 to ground... Tomorrow I have time to make pictures which come in a logical order. Best is if you explain it step by step. I do prefer save bet advises for example better capacitors , which one etc.... Easy mods first.....
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Post by mrholmes on Oct 17, 2014 14:58:08 GMT -6
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Post by Deleted on Oct 17, 2014 16:48:57 GMT -6
Yepp, these would do...like any other brand...
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Post by Deleted on Oct 17, 2014 16:53:16 GMT -6
Normally Reichelt is a good address in germany, too. Sometimes cheaper in the end, if you need a couple of things...
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Post by mrholmes on Oct 18, 2014 7:56:34 GMT -6
Yepp, these would do...like any other brand... Ok got it. I need for every OP amp two of them? Are all those IC are Op Amps?? Did some new pictures lets start with the PSU.
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Post by Deleted on Oct 18, 2014 18:53:43 GMT -6
OK, if you post some pics from the PSU section on the PCB, we can discuss upgrade possibilities. And yes, google the IC datasheets. As i see it, there are JFET quad opamps and BJT dual opamps on the board. (Plus comparators that are used for the front panel stuff, these will be left out of the considerations...) Each opamp IC package has a +Vs and -Vs supply voltage pin to decouple...so 2x0.1uF caps per IC package... these are around 12 cent each, buy in quantities, a few more than you would need for both units, to get better prices, these have plenty of use anyway... If you buy at Reichelt, you also can get Panasonic FCs as RAD FC part numbers, these are standard good sounding recapping electros for what comes after the regulators... If the 2x 1000uF i saw in the pics are before the regulators, up them to 2000 or 2200uF in this design, if these are not available from Panasonic, try to get Nichicons, these are oftenly available in physical packages that you can still cram into the 1 rack unit format.... We will see if there are more pics posted, so we can see everything... to be continued...
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Post by Guitar on Oct 19, 2014 17:08:17 GMT -6
cool thread. I'm going to modify an MDX2100 I just picked up.
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Post by mrholmes on Oct 20, 2014 14:11:19 GMT -6
PSU picks behind the transformer we have diodes. The big electrolytes are 1000 µf 100 Volts the small ones are 10 µf 50 Volts. ML7915A is an negative Voltage regulator??? Tomorrow I will do picks of the copper side of the PCB.
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Post by Deleted on Oct 20, 2014 18:02:22 GMT -6
ML7915A is an negative Voltage regulator? Yes, it is. Copper side pics are not that important, we do not have schematics still. So, pics from the component side will be enough, i think, we do very basic mods, most of the time this will result in instant improvement. Hard to do anything wrong with recapping PSU section and local decoupling. We can make suggestions for opamp changes, too. One last possible mod would be upping the supply voltage from +-15V to +-18V, depending on the opamps maximum supply voltage, but this is another story for later...you may like the unit already after proper decoupling and some re-capping and be happy with it...
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Post by Deleted on Oct 20, 2014 21:11:00 GMT -6
Caps are pretty cheap and can do alot of good if added to a circuit where they have been omitted due to cost reasons. There are different approaches to this. Normally, it is good practice to stabilize the power rails at EACH integrated circuit in a design with a 0.1uF metal film and a 10uF or 22uF electro in parallel. You can even use up to 3x 0.1uF metal film caps for an IC, one for each power rail, and one between +Vs and -Vs. Douglas Self wrote something about this cap for NE5534 and NE5532 opamps...can't find it right now.... The low capacitance metal film caps should be placed as near to the IC as possible, hence solder it directly to the pin on the copper side. Use the next available ground pin or directly the nearest ground rail on the PCB for the other side of the capacitor. Maybe svart can chime in how he did it on his units, maybe he remembers? Try replacing the 10uF i see on the backside of the PCB with 22uF ones. I strongly guess they are something like the stabilizing caps for power rails, that will be in parallel to the small metal films we are going to solder in. You can easily check this, if one pin is + or - 15V and the other one is ground, try a cap with 22uF when recapping.
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Post by Deleted on Oct 20, 2014 21:47:57 GMT -6
These caps also have a type of filter effect. The high capacitance cap (the electro) is kind of highpass filter. Upping the capacitance from 10uF to 22uF therefore *possibly* clears up the lowend response. It is *not* like more is better in this place generally. It depends on the voltage sag/current draw of the IC used. So, there are values commonly possible between 4.7uF and 47uF, depending on the type of IC. Sometimes, 10uF caps are used where 22uF performs better, because of cost reasons. Therefore, try. Sometimes, more is worse! Then go back to 10uF. But i wouldn't worry about this too much... The low capacitance metal film cap lowers the ESR of the parallel electro extremely and adds near nothing to the ESL. It is used to filter out high freqency noise. And, in proper designs, it is NOT optional, but mandatory. Therefore, many PCBs of well implemented designs using ICs look like they packed them full of caps. In higher than audio frequency application, you even commonly parallel another decoupling cap, like, say 10nF. Not necessary for audio circuits, though.... Just a short explanation why we do this stuff....
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Post by Deleted on Oct 20, 2014 22:09:14 GMT -6
The low capacitance metal film cap lowers the ESR of the parallel electro extremely... Well, in fact, modern electros already have very low ESR, so this is not true anymore....just wanted to correct myself on this...
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Post by Deleted on Oct 20, 2014 22:15:56 GMT -6
cool thread. I'm going to modify an MDX2100 I just picked up. Congratulations. They are a PITA to disassemble and desolder...but i guess, you already know this is common to the more modern Behringer cases and PCBs...
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Post by mrholmes on Oct 21, 2014 3:07:18 GMT -6
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