SEX kit from 2006 resistance check issues

[randykato]

Hi,

I'm building a SEX kit for a friend who dug it out of a closet. I'm not sure which version it is (it's not indicated anywhere I can find) but the manual is Rev 6-29-06. The forum posts don't even go back that far so I'm not sure if the posts I've been searching are even relevant to this model.

I wanted to ask about some resistance values that aren't matching the manual.

Terminal	My reading	Manual
Center pin input jacks	100 KOhm (min vol) - 230 Ohm (max vol)	0 Ohm (min vol) - 100 KOhm (max vol)
19, 39	0 Ohm (min vol) - 230 Ohm (max vol)	0 Ohm (min vol) - 100 KOhm (max vol)
20, 40	255 Ohm	295 Ohm
A4, B4	0 Ohm (any vol)	0 Ohm (min vol) - 100 KOhm (max vol)
C2, C5	> 1 KOhm	< 1 Ohm

I saw in other posts that there were errors in the manual, and saw similar 230 Ohm values that were deemed OK and T20/40 seem within tolerance, but the volume-dependent values and the C terminal values have me concerned.

Any input would be greatly appreciated.

Cheers!
Randy

 

[Paul Birkeland]

The center pins of the input jacks should read about 100K regardless of level control position.  The wiring is all nicely color coded, just double check that you haven't mixed anything up.


19 and 39 should vary with level.  At minimum value, this will be close to 0 Ohms.  At max volume, it will pop up to around 100K.

20 and 40 aren't alarming, there's some variation in the Speco that would cause this.  (Are you using the supplied iron, or the iron upgrade?)

A4/B4 go with 19/39.

For C2/C5, if you don't have tubes in the sockets, you may get the values your describing.

-PB

(I still have a SEX amp of roughly this vintage kicking around at home)

 

[Grainger49]

Let's see, this might help somewhat.  Here is a link to the old board:

http://www.audioasylum.com/forums/bottlehead/bbs.html

I think PB/CB is saying there is something screwy with the volume pot wiring.  The center conductors should go to lugs at one end of the pot, the other end lugs go to ground and the center lugs go to the tube grids.

So if the resistance varies when the pot is turned I'll say that the inputs went to the center lugs.  That is my first guess.

Just check the wiring closely against the manual pictures.

 

[randykato]

Thanks for the quick replies!

I've been going over (and over) all the wiring and everything is as it should be as far as I can tell. I'm just baffled by the strange variations and how the resistance change (which apparently isn't supposed to happen at all with volume adjustment) is happening in reverse at the center pins.

Re: C2/C5, I do have the tubes installed.

 

[randykato]

So... I removed the tubes and now everything checks out to spec...including no variation at the center pins with volume adjustment.

The manual said to install the tubes before resistance check. Is that wrong?

 

[fullheadofnothing]

Are your tube sockets oriented properly (i.e. is the notch pointed towards the back)?

 

[randykato]

Well...DUH!

Guess I should have paid attention to that little voice in my head that said to label "front" and "rear" of the panel to avoid just this kind of mistake.

Thanks so much for figuring that out - I'd have spent hours re-checking everything else but doubt I'd have thought about re-checking the socket orientation.

Now I've got a bunch of re-work to do.

 

[fullheadofnothing]

PB came up with the diagnosis when I read your post aloud in the lab.

Good luck with your repair!