Fairchild 627 from scratch

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DaveP

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I have been asked to make a Fairchild 627 equaliser, this is apparently a very rare beast (only 5 in existence?) so I will make it as a project on the forum as a record for the future. According to legend, one was used to master the Beatles She Loves You single in the USA.



The schematic and the following pics come courtesy of the Lost in Time website so I hope they won’t mind me reproducing them here.




This project has considerable problems with difficult or impossible to find parts, but I hope I have found work-around solutions to these, we shall see.

The first problem was the surround for the 6E5 magic eye tube, but I found it was possible to adapt a black GS lamp fitting to take the tube successfully.

The second, more serious problem, is the triple-ganged-pots. I have found some brass cog wheels which I hope will turn the three pots simultaneously. The values of the pots can be derived from the twin tee formula using the component values in the schematic and the frequency maxima and minima on the front plate. For the LF boost the pots are ~60k/30k, covering 20-100Hz, for the HF boost they are ~11k/5.5k, ranging from 4k to 10k, so I will be obliged to use 50K/25k pots and 10k/5k pots and adjust the associated caps by the appropriate amount, for example the LF caps will go from 105/210nF to 122/250nF, not a massive change. The HF caps will go from 2.4/5nF to 2.2/4.7nF, hardly any difference, but worth doing nevertheless.

There is no power supply shown for this equaliser and I have chosen not to put one in the case even though there is room. So I have made a separate power supply to keep hum and noise well away from the chassis. This provides 2.4A DC for the tubes and about 30mA @ 230V for the HT/B+. The maximum HT for the 6E5 is 250V, so I chose ~230V to be on the safe side.



Best
DaveP
 

xazrules

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omg!! i've spent two years on this project and the high frequency boost section never sounded right, i've replaced the three gang pots with a lorlin 4x3 switch, the freq selection and Q was spot on but even with no boost there was always 4db of boost at the selected freq.
the low boost was amazing, nothing comparable to others eqs.
eventually i was thinking about building it just for the lows section and use a passive pultec topology for the highs.
i can't wait to see how it's done by a real pro!!
for the three gang pots try contact audio maintenance, they could have them.
go Dave go

cheers
 

Ricardus

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JMan

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I too hope that @Slenderchap continues to offer this kit, as not all of us can do what DaveP does and I've been dying to build a pair of the AML version. I just hope it stays in the catalog long enough for me to put together the funds.

Regarding the main focus of this thread, I have never followed one of Dave's builds in real time before, and I'm pretty excited about this! In particular, I'm already very interested in the creative solutions for the hard to find parts.
 

DaveP

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Thanks to all for your interest and support, I will do my best!

Thank you Colin for the Colourbook download.

Some parts on the schematic have non standard values, which tells me that there is some fine tuning involved. (caps for instance).

Also some restistors state "select", which again suggests that each 627 needs a fair amount of setting up. The setting up may be more taxing than the build!

Best
DaveP
 

DaveP

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OK, here we go:-

This is a black E27 lightbulb fitting that I have cut to allow the 6E5 tube to pass.



The end cap has ribs inside that need filing down so that the tube can pass to the end.



This is how it fits on the tube.



The rest of the fitting can be cut off leaving just the threaded retaining ring. The ring will be behind the faceplate and the end in front, so that it will support the tube as it protudes through the face-plate.

 

Stevie342000

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The original article for this was in Audio Engineering March 1951 starts at P18. You can find at the American Broadcast History website. Pretty certain if you have not seen it before that it mentions in the article that a few of the components are selected on test or what would call these days trimmed.
 

DaveP

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This is the power supply.



The heater and HT supplies are taken to the 627 via a 5 core 3 phase cable, the pre-amp earth is separate from the heater earth.

These are the cogs for the triple pots.



As you turn the central pot, the other two go in the opposite direction, so they have to be wired in reverse. I bought the cogs from a US supplier who I suspect sourced them from China, they were not easy to find for 6mm dia spindles. 3mm dia were much more common for the model car hobbyist.

The next job is to start cutting out the parts for the chassis.

Best
DaveP
 

Stevie342000

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Stevie342000,​

Thanks very much for that link, it will be very useful.

Best
DaveP
I thought it might be, you can print the pages as a PDF as well. Glad to be of help, I always enjoy your builds, you do the obscure stuff, this one has been on my list prior to the Preservation Audio mention. Not sure if there is a link on their site either to the original article, do not recall if he references it, he says the issue are those ganged pots. I figured you could work out what the values were likely to be. Good work so far.
 

HerbertR

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Hi, so a cool project. Mega. I would prefer to replace the LF / HF Boost with stepped rotary knobs and resistors, with these blue 24 stepped China ones ore Blore Eds, that would be afordable. Would be great if someone could make an excel sheet with the three Resistor Lines and the achieved gain. Perfect for mastering.... That would be a super solution as replacemement for these 3Gang Pots. Pleaseeee
🥹🙏
 

DaveP

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Hi HerbertR,
The virtue of this EQ is that it is continously variable or as we say nowadays, its parametric. Making it switchable would negate this virtue. A switch would be very tedious to wire up I imagine.

DaveP
 

DaveP

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I have cut out the front panel 19"x7" from 3mm sheet from RS Components.
The centres of the fixing slots are 1.5" from the top and bottom and 0.5" from the sides. Holes are 0.25" dia.



The fixing angles will be attached to the back by recessed screws.



I don't normally work in inches, but vintage designs all have dimensions in inches so it's easier to reproduce them that way.

I have calculated the centres of the control knobs from photos in inches and will post them when I get to that stage.

best
DaveP
 

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