EMI RS124

[DaveP]

Something no-one has yet mentioned on the plate to plate feedback.

When there is no compression, V1 plate volts are low so that DC flows from (V2) 145V to (V1) 75V, as the compression kicks in, the voltages pass through equality then they reverse when V1 is higher than V2.

I guess this is acting in much the same way as a regulated supply as far as DC is concerned.  This will tend to increase the CR over stock.  The feedback will reduce gain but also proportionally decrease the output impedance of V2 which will increase the drive to the 6AL5 which may decrease the attack time a little over stock.  The noise and distortion will also be reduced by the same proportion.

I think that the increased gain stability from the feedback also helps to reduce the possibility of thumps.

Full marks to Winston for a brave decision
best
DaveP

 

[gary o]

Im trying to read the revisions on the side .......

From left to right here what I think I can read & guess

The first box I cant read at all
11.5.63C5 & C6........ from .022u to .072u ??

27.3.62 V3 alternative type added ?

10.4.62 screened quad added on input pot & further colour coding added ?

11.5.63 ........added V3 2was 7    7 was 2  t note added ??

3.7.63  relay lead moved from hot to cool side of R12 , fuse moved

5.12.63    C4 was 1 uf balance neon button etc  C5,6 were .072 ????

2.4.65      P M N 2    R16 (27....deleted      R17was 132 ohm      C5 & C6 were 0.5    R25,26,27,were added

Can anyone clarify

thanks

 

[MagnetoSound]

Nice analysis, Dave.

The feedback will reduce gain but also proportionally decrease the output impedance of V2 which will increase the drive to the 6AL5 which may decrease the attack time a little over stock.  The noise and distortion will also be reduced by the same proportion.

You're talking about the AC component here, right?

 

[MagnetoSound]

Can anyone clarify ...

The first box I cant read at all

Same here.


11.5.63C5 & C6........ from .022u to .072u ??

Presumably a 0.05 added in parallel to the stock caps.


27.3.62 V3 alternative type added ?

Alternative types, in fact. Subs and/or equivalents to 6AL5 marked on the schemo.


10.4.62 screened quad added on input pot & further colour coding added ?

I see 'screened quad cable added ...'


11.5.63 ........added V3 2was 7    7 was 2  t note added ??

I get the note re: P1, but stumped on the rest of this ...


3.7.63  relay lead moved from hot to cool side of R12 , fuse moved

Thereby shifting the recovery network to the other (cap) side of attack resistor R12.

Fuse moved to hot side of AC line?


5.12.63    C4 was 1 uf balance neon button etc  C5,6 were .072 ????

Cut the TC cap value down to 0.5uF ...

(balance button to feed heater supply into V1 grids via R12)

... and looks like they altered the previous mod to the sidechain drive caps (took the 0.02s off and left the 0.5s?)


2.4.65      P M N 2    R16 (27....deleted      R17was 132 ohm      C5 & C6 were 0.5    R25,26,27,were added

R16/17, part of output pad network?

C5,C6 changed SC drive caps again back to stock!

R25,26,27 = last but not least, DC feedback added and V1 cathode bias adjusted!


Phew!

 

[gary o]

Thanks Magneto.......

11,5,63..............V3 2 was 7      7 was 2 ..... seems they swapped C5 to pin 1 of V3 & C6 to pin 5 of V3.....I think I read this a while back on one of the not real RS124 schematics or PDF......

2,4,65.......Yeah can see R17 on schematic .........R 25/26/27 already shown on schematic doesnt look like were drawn on in 1965.....or were they ......am I missing something there.....

I am currently re building the compressor I built from Rothus schematic......I couldnt get that to work as reported earlier in this thread......that is prob my fault but I couldnt work it out even with members help .....so Im turning it into Winstons RS124, not building PSU for or the neon circuit just wanna see if I can get it working......

 

[DaveP]

Thanks Magneto,
The revision dates must be in ascending order after the drawing date of 5/12/60.

The first rev. is 2/2/61 I think.
(I'm working on this but it can't be as important as the later mods)

2nd rev. 14/3/61.  C5 & C6 increased from .022 to .072 (in other words added .05)

3rd rev. 27/3/62  Alternative types added.

4th rev. 10/4/62  Screened quad cable on input pots, further colour coding added.

5th rev. 11/5/63 No. code added V3 2 was 7, 7 was 2 + note added (V3 pin code was corrected)

6th rev. 3/7/63  Relay lead moved from "hot" to "cool" side of R12.  Fuse moved.

7th rev. 5/12/63  P.M.N.-1  C4 was 1uF, "Balance, neon, push- button, etc. C5,6 were .072. (.022 were obviously removed)  P.M.N. is probably the initials of the engineer who designed the changes.

8th rev. 2/4/65  P.M.N-2 R16 276 ohms deleted. R17 was 132 ohms.  (these are previous pad resistors not stock numbers) C5 &C6 were .05 (returned to the stock .022 after experimenting)
R25, 26, 27 added (these are the 130 ohm cathode res. and the two 100k feedback res.)

The 130 ohm res shifts the gm to a straighter part of its curve in the same way as the 6.2V bias on the Fairchild does on its 6386's.

Got to go now, wife wants the internet!!
best
DaveP

 

[Winston OBoogie]

...P.M.N. is probably the initials of the engineer who designed the changes...

Mr. "Production Modification Note" 

 

[DaveP]

Thanks Winston, that makes more sense.

I've enhanced the print with corel draw and as far as I can make out, the 1st revision is:-

2/2/61 (the initials are not the same as those on following rev)

Fitted S2
Ry (relay) Then what follows are the resistors R20 to 24.

The final line is I think "hold setting"

So the whole revision is probably just about the mods to the release settings.
All the rest of the changes are covered in later revisions.

The UK 240V mains transformer must have been fitted from the word go as it has a bridge rectifier instead of the voltage doubler on the 115V stock transformer.  Interesting that they continued to use the original HT caps but recommended Plessey replacements for later.
best
Davep

 

[DaveP]

That would be a good move Winston, there are a lot of new guys on the forum nowadays.
Thanks from me for all your pioneering work earlier on, its a great leagacy.
best
DaveP

 

[fazeka]

On the schematic in the notes block next to PMN2 dated April 2, 1965, I see the initials "LP" in cursive.

"LP" = Len Page (EMI Technical Engineer)?

 

[Rob Flinn]

On the schematic in the notes block next to PMN2 dated April 2, 1965, I see the initials "LP" in cursive.

"LP" = Len Page (EMI Technical Engineer)?

Yes that's Len Page.  You'll see his initials all over EMI/REDD documents.

Speaking of him, the Fairchild manual I have was written by him too and I tried uploading it but the forum won't let me because of the size restrictions - it's a PDF and about 8,000KB.
I'll find somewhere to put it.

Could you not send it to the group gmail account ?  [email protected]

 

[gary o]

I have made my quick point to point version of the RS124 but am experiencing same from problems I did when I made the Rotheu circuit ot oscillates all the time sounds like a square wave from analogue synth.........when I turn the power off the tone raises up & up then drops then jumps up and octave its quite a good sound ha, it also passes distorted audio, when the circuit was like Rotheus schematic it made similar sounds it was suggested that I had positive feedback....I dont understand how I just copied the circuits as per schematics funny how results are similar I must have something basically wrong here .....or defective part/parts........or

Im using a Sowter 1:4 input transformer type 8540 ( I use this in my DIY BA6A works great)
The output tran is a Sowter 8650.....I read here somewhere it is good for the VK1 & Altec 436 circuits
I havent built the power supply circuit like the RS124, Im using a power supply I use to power all sorts of pres & limiters the B+ is ball park.....

Do you think the above factors are causing the oscillation......?

Its in a bread board state bit of a wiring mess but I dont think its that thats causing it

 

[MagnetoSound]

I havent built the power supply circuit like the RS124, Im using a power supply I use to power all sorts of pres & limiters the B+ is ball park.....

Do you think the above factors are causing the oscillation......?

Its in a bread board state bit of a wiring mess but I dont think its that thats causing it

Oscillations are usually down to poor grounding and/or decoupling causing positive feedback across the power rail. If you are tying all your circuit grounds together on breadboard, make sure there is a very short path to power ground or the impedance may be high enough to let signals flow from one part of the circuit to another.

In other words, connect all circuit grounds to the neg of your main filter cap and keep the wire short and fat.

 

[gary o]

Thanks Dan ...  Im not as knowledgable as most here about electronics Im afraid so bear with me ....yeah my point to point circuit is ( not actually on a bread board but in experimental satge) I have all the earths coming together at one point & a thick wire goes to the - of my power supply.

trying to narrow down the problem....
Ive since removed disconnected the 6AL5 from the circuit disconnected C5/6 from audio circuit disconnected R10/11 from V3 anodes, I also removed the input transformer, daul pot & leads going to V1 grids its still makes the oscillation......

Looking at the schematic the node where R13 10K & R14 6K8 join says 255V thats where I inject my + supply of 258VDC ......that should work right ?......I have built many other circuits this way without problems cant work this one out

thanks for reading

 

[MagnetoSound]

You mean you have nothing on V1 grids?? You should at least have them tied down with 1M or similar to keep them at 0v.

 

[gary o]

I had the 50K daul pot as per schematic but the oscillation.......I have nothing connected to them at the moment to see if any difference with oscillation......no difference......

Tried a different output tube.......no difference....

Dont have another 6BC8 so for quick experiment I tried a Ecc83 ......similar oscillation.......

 

[MagnetoSound]

If you pull V1 out does it go away?

 

[gary o]

ah good point ......it doesnt go away........

thanks Dan

 

[gary o]

I ve re connected side chain put all tubes in ....Im now using a separate power supply to supply the centre tap of OP tran with 127 VDC & my adjustable power supply is feeding R10 with 240 VDC thats as close as I can get to the schematics 255V I was hoping this would be close enough to get it to work ?

I have 87 VDC on the 6BC8 anodes instead of the 75V on schematic
I have 146 V on the 6CG7 anodes

The square wave sounding oscillation is gone & it passes audio but very very distorted.......any ideas

 

[baadc0de]

User ladislav from dripelectronics forum has successfully modded the 436c PCB (no trace cutting as I know) to incorporate RS-124 changes and some other mods.
http://dripelectronics.com/index.php?option=com_kunena&func=view&catid=12&id=2957&Itemid=41

He has also designed a preliminary control panel to change the one provided by DIY-Racked and if anyone's interested into making one of these or modding his 436 from drip to RS-124, the first thread offers insight into how to do it properly. The second thread, here:
http://www.diy-racked.com/diy-talk/viewtopic.php?f=7&t=167

Is a black market style interest thread where you can post your interest to get one of the front panels or cases.