Rejuvenating an old DM-602.

[chriscam]

First, this post isn't about bower and wilkins their 602, nope, its about a 16 year old korean made 5 channel disco mixer which I bumped into at a garage sale for only six and a half euro.




Reason for posting is that it came with both the original manual and schematics, something which nowadays doesn't happen too often anymore, allowing me to do some educated replacement and/or upgrading.

• swap all 47u/16v electrolytic coupling caps with 2.2u mkt models which I came by really cheap at a local army dump
• replacing the 4558 with the OPA2604.
• replace and upgrade the supply caps 220/470 with 470/1000 models

Anybody out there who did this before and like to share tips & thoughts ?

[Ian Finch]

Yes, I was intrigued for a moment there, Chriscam but then you would not post here, eh?
I have not rebuilt one of these, though there are plenty available as cheaply and some do already have quieter op-amps but the mixers still seem to have high noise levels from layout and wiring issues which will be hard to eliminate.

All the same, you certainly seem to be taking a sensible upgrade approach.
Thanks for sharing the schematic - as you say, rare! Are you going to use it as a live mixer and just want to improve its performance?

[chriscam]

Yup Ian, I'm a strong believer that quality isn't always analog to the # of dollars (euros, yens, roebels) put in.

Originally Posted by Ian Finch

Are you going to use it as a live mixer and just want to improve its performance?

Well, that depends on how it will sound after modification. For live gigs I'm using a crest console which is kinda overkill for standard disco work where I only require 2 lines and a mic.

I was planning on buying a small 19" mixer for some time but since there is no need for topnotch quality (source is mainly mp3) this little gem just seemed a perfect cheap trick. With ch#1 to 5 closed, eq in bypass and the master open it indeed generates a considerable amount of oldskool 741 noise , I wonder what the OPA swap will do with that. Good thing sofar is that it's just noise without 50hz components which makes me believe layout/wiring is fine.

I just received word from farnell that the opa's are in the post, once they're in I'll shoot some pictures of the inner works which, taking the 200euro srp into consideration (1994) don't look bad at all.

I'm not after making it crest worthy, just wanna see how far it goes with some extra polyester & modern opamps

[Xoc1]

Are you ever going to use the Reverb?
I removed the reverb from a similar mixer, and hardwired the signal bi-passing a lot of wiring, switches, & circuitry.
I would be tempted to do the same with the graphic!
What local decoupling is there for the op-amps on the pcbs, if any?
I would add decoupling caps to the voltage rails where you can in pairs of 100uf and bi-passes as close to the op amps as possible - where there is no ground maybe strap some of your 2.2uF MKT caps across the op-amp from pin 4 to 8. If you add a lot of capacitance its advisable to add diodes across the regulator ICs.
There are lots of DC blocking signal coupling caps in the signal path. I would think that at least some of these could be linked out as long as it does not introduce noise when you adjust the level sliders.
I'm slightly confused by the output section, does the signal actually go thru 2 signal coupling caps and a transistor? Looks pretty ugly!!!

[chriscam]

Hi Xtal !

Thanks for your elaborate response, it contains some very good tips !!

Originally Posted by Xoc1

Are you ever going to use the Reverb?

Me probably not but Luca, my 11 year old son who will be the DJ for most of the time, tends to like the digital artifacts. You know, a whole generation gets currently spoiled by autotune junk so what the heck, let him have his echo

I have not yet looked into the whole echo thing because, like the eq bypass, I can kill it's output by closing vr401. But you are right, if noise levels remain unexeptable high I can always try and disconnect the whole echo troll as it sits comfortably on a separate board.

Originally Posted by Xoc1

I would be tempted to do the same with the graphic!

Oh yes, it was kind of amusing to find the 0,68u and .33u caps in the lower bands to be electrolytics this together with the sheer amount of coupling C's makes me wonder if Inkel was into the capacitor business as well.

Originally Posted by Xoc1

What local decoupling is there for the op-amps on the pcbs, if any?

Ah, you hit the nail on the head. There is none. Except for the 0.1 in the supply they are non exisitent. Incredible huh ? I was planning on giving each opa it's own 0.1 from the solder side (ah, solder side, try that for SMD)

Originally Posted by Xoc1

There are lots of DC blocking signal coupling caps

Yup, I feel a strong urge to try and lose some, as as far as I can see most opamps are ground biased anyway. Should clean up the soundstage.

Originally Posted by Xoc1

I'm slightly confused by the output section, does the signal actually go thru 2 signal coupling caps and a transistor? Looks pretty ugly!!!

Oh yes, it's a genuine work of art