Melcor Deconstructed

[Mendelt]

[quote author="mick"]
So my question is, this problem that keeps getting mentioned is it something that I should be able to hear and notice straight away or what ? because going through the gain min to max I don't notice anything strange happening. I don't have any test equipment just ears so if it's not audible I'm not able to tell if I do or don't have this problem ?[/quote]

I'm not really sure there even is a problem. As far as i understood the only thing that caused real trouble was the single diode for D2 and that has been fixed

I tried simulating the opamps in LTSpice and saw a bump in the frequency response in the megaherz range. I'm not sure if this is something to worry about but i guess not.
Deveng found the same bump and was smart enough to find a solution.

I'm not even sure about the mod with C2 to the -pin. I looked at some schematics of the 2520 and it has a similar capacitor to the collector of the negative input transistor. I tried this in the simulator and it doesn't look worse than the mods.

When i get around to building my melcors the first thing I'm going to do is try it with only the added diode for D2. That way i can try out the other mods to see what sounds best. I'm still trying to find the pictures of the butchered melcor by tommytones to see what the original melcor looked like. Or maybe someone has a blown melcor lying around for me to try ? :razz:

 

[Mendelt]

[quote author="deveng"]Mendelt,

I've been playing with the simulations on the 1731 and the 2*5*2*0 and they both have this bump. There's an explanation of this in the thread.

It's inherent in the design of these opamps but can be reduced with some manipulation. Just changing the local feedback cap values won't do it but I was able to reduce it by adding a series resistor (39k) feeding the lower half of the 3rd stage in my design. I've built and tested it and it stands up to an original 2*5*2*0.

Regards,
Jeff[/quote]

Have you tried this with the 1731 too? Was this with the 'C2 mod' or without?

I'd really like to know what the schematic of the original Melcor is so we have someting to work from. As the 2520 has a capacitor connected just like the original C2 I'm guessing the C2 mod wasn't in the original 1731

 

[mick]

Oh I thought it was something you'd notice, I must say that I'm using a 22 rev log pot on mine.

 

[Mendelt]

[quote author="mick"]Oh I thought it was something you'd notice, I must say that I'm using a 22 rev log pot on mine.[/quote]

There are a lot of people here who seem perfectly happy with their melcor clones. So if there is a problem it's probably subtle.
I was just commenting on some stuff I found using a simulator to try out different variations of this schematic. But this is purely speculation based on simulation results because I havn't tried the 1731 out in the real world yet.

If you want to find out what sounds best you're beter off trying out a few of the different options to find out. (and don't forget to report back here!)

 

[DrRick]

I have 30 or 35 of the real ones. Does anyone know the value of them? I tested all of them for gain and noise and they are all fine. I should add that my friend from whom I got them from in a massive trade told me he had checked all the ones in the trade and he's a very sharp EE (we were in some of the same classes "back in the day"-I woke up one day and found out I'm old!-we had no PC's in EE school but at leaste we had calculators) and he's always right when it comes to engineering ( right Vlad?).

Anyway, the 1731's are on a circuit card along with a 2N2102 and a 2N4036 which, if I remember correctly, are a NPN/PNP complimentry pair of TO-05 case bipolar transistors and a bunch of assorted other components.

Since I rambled so far off track:

What are these reaL Melcor 1731'a worth?
Does anyone know what the circuit cards they are on are or came from? The trade was 15+ years ago and I'm just getting around to playing with the stuff! (The non-tube stuff always gets pushed aside around here for some reason-LOL).

I'll post a picture shortly.

Peace,
DrRick

 

[DrRick]

Pictures of 1731 on circuit board:

http://f7.yahoofs.com/users/4478cb6bz5f11b43c/34e4re2/__sr_/67d1re2.jpg?phoLNeEBuK4BiqAE


http://f7.yahoofs.com/users/4478cb6bz5f11b43c/34e4re2/__sr_/67d1re2.jpg?phoLNeEBuK4BiqAE

 

[rodabod]

And IMNSHO it's about time The Jesus Lizard make a comeback, that'd be yummy to the max (and we'd forgive them those last few albums  :twisted: )

Yes..... that would be rather yummy.

 

[clintrubber]

And IMNSHO it's about time The Jesus Lizard make a comeback, that'd be yummy to the max (and we'd forgive them those last few albums  :twisted: )

Yes..... that would be rather yummy.

Brilliant discovery Roddy ! VERY nice ! 

That might sell fast I presume ?
Skipped the Scr Acid re in Chicago a few years back, I guess we'd better join this one !

Bye,

  Peter

 

[rodabod]

That might sell fast I presume ?
Skipped the Scr Acid re in Chicago a few years back, I guess we'd better join this one !

It may sell fairly quickly. The rest of the line-up for the festival is voted for by the fans, so you should check up every week or so. It's at a holiday camp where you stay in chalets.

Alternatively, they will probably play some single shows. Maybe in the UK... Maybe in the US. I was over in Chicago this year actually. It's great.

Anyway, I think this should be a nice opportunity for someone like me who was too young really to see TJL back in the day.

 

[clintrubber]

That might sell fast I presume ?
Skipped the Scr Acid re in Chicago a few years back, I guess we'd better join this one !

It may sell fairly quickly. The rest of the line-up for the festival is voted for by the fans, so you should check up every week or so. It's at a holiday camp where you stay in chalets.

Alternatively, they will probably play some single shows. Maybe in the UK... Maybe in the US. I was over in Chicago this year actually. It's great.

Anyway, I think this should be a nice opportunity for someone like me who was too young really to see TJL back in the day.

In my enthusiasm about the news I didn't read it in full, only later learned that the short period is in fact still several months. So let's hope they tour the rest of Western Europe as well. If they don't then that Minehead-holidaycampgig doesn't look too bad at all 

A few disclaimers from my side though  ;D
#1. only material from Pure, Head, Goat, Liar
#2. Travis Bean gtr

Enough ! 

Bye,

  Peter

 

[syn]

An update, hope not too late:

According to the opamp(s) i have "deconstructed" :

Rafafredd was right http://www.groupdiy.com/index.php?topic=89.100  on this .Below is his scematic edited, for clarity (hope you don't mind):

This Melcors come from a  RCA pre amp:

http://www.groupdiy.com/index.php?topic=32222.0

.01uF caps that are given gray, are on the opamp itself but not needed  if you decouple power rails  with 0.1uF, externally.

Cheers

Milos

 

[mitsos]

Thanks for confirming that... one more dead opamp for the cause...

I actually redid peterC's layout to accommodate the cap change. It was a PDF, don't know what I did with the file but it is rather easy.. just swap the cap pad with the jumper pad and you're done. I'm gonna have to look for it cuz Ive been wanting to etch some lately..

 

[syn]

Cool.
Cheers

 

[emrr]

Very nice to know which you have, but how do we ID the later API from the Melcor, among the RCA rebrands?    RCA made these modules on into the mid-'70's. 

 

[Walrus]

The three 1731's I have (2 labeled Modular Audio Products and 1 MDI) are all 0.5 inch thick,
whereas all the 2520 I have are at least 0.6 inch thick.

 

[syn]

0.5" high overhere as well.  As some moduls that i have are dated the end of the 1970. I guess all
before it are Melcors. Not much  help, sorry...

 

[emrr]

Interesting.  Then I appear to have eight 2520's and one Melcor, based on thickness.  I note the one Melcor had two RCA labels, one glued over the other, with different drawing #s on each.  The mystery is that the top label has the same number as the other eight.

 

[sonicwarrior]

I have finished my first Melcor 1731 from Bauman's PCBs. Hope I didn't make an error as I'm currently not able to test it. I used a 43K7 for R4 as I couldn't find a 43K9 and didn't want a big resistor cluster.

 

[peterc]

Jo

Check D2, it should be 2 diodes in series as per the schematic.

Its not clear from the pic.

Peter

PS: Edited R2 to D2

 

[sonicwarrior]

I guess you mean D2 not R2 (R2D2, hehe): Yep, I staggered them. Here is a picture from the side.

Thanks for the heads up!