what's this symbol in the fairchild 670 diagram?

Hi Guys,

Looking at the 670 circuit diagram, top right at the front end of the amp section? is a dual 100k lin pot,

what's odd about it is that it is drawn with what appears to be two wipers?

can some one educate me on this?

best

Pete

Which diagram are you looking at? As there is one on Kubarths site that is very wrong  (it's the 670 redrawn.png one)
The original manual shows a dual 100k lin tapped pot with 24K resistors hanging off the taps to ground. However, in our actual unit the pot is a single 100k and no tapping. ???

Tapped pot, in case you missed in that last post.  Used to be a much more common part than now. 

thanks guys,

great info, so i guess it was used to stop noise when turning the pot? removing the dc or something?

Yes it kubath drawing, well on his site is what looks like a realy old drawing, in what way is it wrong?

of course if you have a right version id love to see it.

all the best
Pete

On Kubarths site, the "Huge Hires 670 schematic" file is the same as I have. If you download that and compare to the others, you'll see the missing resistors on the pot taps, and the effect that has!! Also you will see where line representing the 240 volt rail gets conused with the ground rail where it goes up to the upper right amp section.

there are a lot of confusing things in life.
like

1)this thread has the wrong part number in the title
2)the 100k looks like 180k on some pronts
3 1/2) mispelled prints
3)the tap
4)the little wiper direction arrows look like more taps!
5)the print is fuzzy to read
6)if you look at the chassis pics, there is Another terminal protruding from the pot.
another tap?
no, a grounding lug.
7)there is a dashed line denoting a common shaft
8)there are weird screwdriver slots for top chasis, bottom chassis
9)insert your own drama here

ok then...

Yep, you're right CJ, there are a lot of confusing things in life. As I said up above, the unit we have doesn't have dual pots let alone tapped ones. The unit is on the bench at the moment, having just had it's smoothing caps, rectifier tube and regulating tube replaced.

But it's now gone 5.30pm and I'm going home in a bit, so I can't be bothered to trace out what's going on!  ;D

Hi Walrus,

ah yes you are right, missed those, so the drawing with the 24k res on the taps is the correct drawing?

So is there a work around? use of normal pots possible?

HI CJ,

you noticed i was dyslexic, good spot

thanks for the input,

pete

you can still find those, either evilbay, or clarostat might even still make them, check ohmite, unless they got bought by clarostat.

A/B is toast, i know the guy who did all the printing for A/B, the boxes, p/n's on the pot,

you don't think there would be any money in that, but then you add up the volume for A/B back in the 50's and it was gazillions.

The tapped pots were commonly used to effect a loudness contour in consumer hifi. At lower pot settings the frequency response was different that at higher pot settings.  

In that circuit it looks like they are just tricking up a linear pot taper to be two different linear ranges above and below the midpoint. An audio taper pot may work there.

JR

here ya go, 100k audio, with taps:
just funin...

fantastic, all super knowledge.

where is the tap placed? i guess it can be placed any where, is there a typical place that pots are tapped?

Why not just replace it with a step Attn. to keep the symetry. That must have been the deigners goal.

those are trim pots, you have the davens on the front panel.

we have all the part numbers, i think it would have to be a 50% tap.

40% is reserved for ultar linear taps on transformers only! 

no wait, that is the Threshold control
left and right should be seperate.
more confusion,  ???

thanks CJ, well if you have any email-able info on any thing fairchild 670 and you would be happy to share then that would be great.

Pete

Stepped att. without a knob then

not sure if i have missunderstood this, but its dual gang pot per channel not across left and right, its across the amp input which i suspect is push pull, so one gang per half 12ax7..if that made any sense...

sinestaraudio said:

not sure if i have missunderstood this, but its dual gang pot per channel not across left and right, its across the amp input which i suspect is push pull, so one gang per half 12ax7..if that made any sense...

Click to expand...

Yes

ok, now we are on the same page.

i have a ton of fairchild,
but is it not here someplace already?

i can just put up links, or wad it up to gspace,

did you check the GroupDIY upload site?