CSCOPE VLF.TR 990B COIL.

[DOOLEY]

hello all,
i need HELP !!!

recently purchased a cscope cs990b on ebay,
it came with the standard 8" isocon coil hardwired // attatched.
after much testing inside with various coins i was disapointed with only 5" depth on a copper 2p coin,
however i do like the detector as it has a salt // beach mode,
and that it's non-motion and has nice audio.

so,
the decision was made to build a larger coil for it,
now i normaly play with PI,
and never had much interest in VLF,
so this is a whole new game to me.

so,
bought a 12" whites coil cover to use as my shell,
measured resistances of old coil,
and inductance with capacitors removed,
pulled the old coil appart gently and counted windings.

even did a video on phone of the oscilloscope with the old coil running with
various targets,
noting amplitude change and phase shift,
oh yes,
thought i was being very clever and very methodical.

i had read dave emerys, don bowers and carls pdf's on making concentric search coils
some time ago,
had a re' read and set to with the build.

here's my problem,
(if anyone's still reading and not got bored yet....lol.)
can't get it nulled below 0.4v
think ive tried everything,
bucking coil 1/4 of the TX turns,
my bucking coil is sepperate to the RX COIL,
so can move it around a bit,
can get the null down to 0.4v,
then it phase shifts to the right,
and the amplitude starts to rise again.

what am i doing wrong ???
tried shielded and un-shielded also.

i'm wondering if it's anything to with the TX being a center tapped hartley oscillator ???
has this made my bucking calculations wrong ???
why can't i null below 0.4v ???

OLD COIL SPECS :

RED =TX-D
BLACK =TD-0
WHITE =RX
GREEN+BRAID = COMMON
TX-D AND TX-0 TO BRAID WAS 4.6 OHMS EACH AND 500uH
BETWEEN TX-D AND TX-0 WAS 9.2 OHMS AND 1864uH
TX WAS 7" DIAMETER
RX WAS 3.5" DIAMETER

RX =1970uH
AND 12.5 OHMS

TX = 72 TURNS TOTAL
RX = 96 TURNS
BUCKING WAS 25 TURNS ?? THAT'S MORE THAN A 1/4 ???
CONFUSING ISN'T IT.
BOTH RX AND TX HAD 47 nf capacitors accross them
TX tuned to aprox 17khz
RX tuned to aprox 16.63khz.

NEW COIL :
TX = 12" dia and 46 TURNS TOTAL, CENTER TAPPED TO COMMON LIKE THE OLD ONE.
AND 1968uH , A LITTLE HIGHER so used 40nf to get 17khz
RX = 6" dia and 90 TURNS , 3375uh WITH A 27nf cap to tune to 16.63khz
like the old one.
yes,gave it more RX turns for sensitivity.
BUCKING IS 11.5 TURNS and 6" dia,
ok, 12 turns and a bit to move around.

tried the bucking coil wired to TX "hot"
and from TX finish to common,
as don bowers mentioned but to no avail.


thanks in advance for someone pointing out the obvious answer that
i'm failing to see,

yours...... in total dispair and at a total loss.........
john.

[DOOLEY]

some pictures of old coil inside,
may be of interest.





new coil using a whites 12" coil cover.





oh , and here's the circuit of the original board in the coil,
all very strange,
looks to be nulling the RX using both ends of the tx ???

[mykaitch]

Ah coils! It is day 10 of my coil building. Much learned. Glue is bad - no glue until all done!
Best method is to make 'spools' with craft board - you can build and tune the whole thing then bung it in the case.
I did not likes burning a groove so I pushed cocktail sticks through,all the way around, and wound on them.
If the coils are in wrong dir they won't null. Also, they MUST be on the same plane.
My best so far is 2.5mVbut today found that my test osc is faulty so will fix AM.
Next time I will make a 1" coil - thank heaven for little coils

[mykaitch]

PS: that won't work!!! BUCK must be wound directly over the Rx coil.

[DOOLEY]

hello mykaitch,
hum,
yes,
ive been reading up on you trial and tribulations also recently,
all the info on building concentrics make it all sound and look
so easy doesn't it,
but in practice................

hum,
on the same plane, that does make sense,
read a post by carl a while back saying bucking coil
can be outside / inside / bottom / top of RX coil,
long as it's close to it,
i'm a bit unsure now.

inside the old coil,
the plastic former in the middle had 2 grooves,
RX was wound on to the bottom groove,
and the bucking coil was wound on the other,
so effectivley the bucking coil was sat on top of
the RX coil,
and that worked for the manifacturer.

i'm still wondering if it's something to do with the TX being
a hartley oscillator with center tapped coil.

and the other puzzling thing is the built in circuit board
using 2 x 220k resistors and a 100k trim pot from the TX
to the RX.

interestingly though,
i took a video before i pulled it all apart,
and,
aluminium showed phase shift to right and an amplitude increase,
AND, this is just weird.....
iron showed amplitude increase also and what looked to be an
ever so slight phase shift to the left.

[sinclairuser]

hi dooley, something does not sound right about your first post here's why, last week i got hold of a mint cs220 with the 6" isocon coil the early ones with only 3 wires, i get 11" on a copper 2p in air and get a nice clear signal on my 8"pound coin test pit.
so considering my model is inferior something is up with your original coil.
actually info on these cscopes is not easy to find, my original post asked for information on tr200 as that is whats on the coil, but the detector its self looks like a cs220, so i'm unsure as to the proper model but it works well and is mint so i will keep it, i think i will post pics of it and its board see if i can find out what it really is.
the coil like i said has 3 wires which puzzles me how can a dd coil only have 3 wires?, thanks for your post i'm getting really interested in these detectors this little one has impressed me so much that i'm going to look out for other cscope bargains your coil info may come in handy.
thanks ally.

[mykaitch]

hello mykaitch,
hum,
yes,
ive been reading up on you trial and tribulations also recently,
all the info on building concentrics make it all sound and look
so easy doesn't it,
but in practice................

hum,
on the same plane, that does make sense,
read a post by carl a while back saying bucking coil
can be outside / inside / bottom / top of RX coil,
long as it's close to it,
i'm a bit unsure now.

inside the old coil,
the plastic former in the middle had 2 grooves,
RX was wound on to the bottom groove,
and the bucking coil was wound on the other,
so effectivley the bucking coil was sat on top of
the RX coil,
and that worked for the manifacturer.

Sure, but they have no doubt perfected a method, maybe even tune it with a cap,
all I know is that way never worked great for me (I use the same wire for all coils),
maybe they don't. Ever measured the NULL on the orginal ? Also, when I did do
it that way I found that by moving the buck coil I could get a good null.


i'm still wondering if it's something to do with the TX being
a hartley oscillator with center tapped coil.
I doubt it, all you need from the osc is a good sine wave - you can even do it with a square wave
and a couple of filters - which I did with a 555 once.

and the other puzzling thing is the built in circuit board
using 2 x 220k resistors and a 100k trim pot from the TX
to the RX.

interestingly though,
i took a video before i pulled it all apart,
and,
aluminium showed phase shift to right and an amplitude increase,
AND, this is just weird.....
iron showed amplitude increase also and what looked to be an
ever so slight phase shift to the left.
I am not sure about this, bit isn't this where disc comes in ?

Yeah that's the bit that caught me. I have made about 10 coils with separate buck coils but it wasn't until I followed the TSGL thread (somewhere here) that I found it easy!!! Really. Even glue on the Rx coil BEFORE the buck causes poor null. Wind the Tx coil about 200Hz too low. Wind the Rx coil correctly and then the buck directly on top. The Tx F MUST rise a little if connected correctly. The NULL will come in at around 3mV although I hope to get 1mV one day..
It took longer to mske the formers (NOT JIG) than to wind and adj. Do not worry if the final TX f is a bit off. Like I said, around an hour start to finish.
Be REALLY careful with the ends of the coils. I am on number three because I keep snapping them and also, I just discovered my test osc is faulty so I have wound a whole batch of dud coils (and I wondered why I needed 50% more turns. Idiot! (me)

[DOOLEY]

hello sinclairuser,

i think she was working ok,
more sensitive to some targets than others,
the distance was just rack of the eye,
didn't measure exactly.

but aggread on both your points:
not enough information out there on old cscopes for us
tinkering and repairing people.
wouldn't have thought a bit of sharing of a 30-40 year old design would harm
the company.

and yes,
i also think very nice detectors, non-motion,nice sound,
going to enjoy using at the beach should i ever get this
bigger coil sorted.

your detector being a lower model number don't mean it's
inferior,
just poss got less nobs to twiddle // play with.

the 3 wire coil bit i think is easy to explain,
TX-D ( driven)
TX-0 (FEEDBACK TO OSCILLATOR)
RX wire
and braid is used for common,
so only 3 wires and the braid used.
bet it'll be same hartley driven coil that cscope appear to
have used for years.

i recently picked up on here a schematic which i think is obviously
an old cscope.
it's not my 990,
hasn't enough in it.
link is below ( i hope )


oh, and below is the scematic for the 1220b
that i also found on here.
Cscope 1220B_2[1].pdf

the next one is supposedly for the 660.
cscope 660[1] scematic.pdf

unfortunatly non match the 990 exactly that i'm playing with,
but,
there are similarities, tx, rx, and power look to be the same.
and i cant find any info on their coils either.
posting a few pics of my 990 board below.

[DOOLEY]

how do mykaitch,

hum,
ok, will deff try doing the RX AND bucking on a cardboard former close
to each other,
but i'm still doubtfull after one of carl's posts said it don't make much differance,
but willing to try.
yes,
all the pdf's about making e'm do make it look easy..........
poss to easy...........
huh,
maybe i'm just slow..........
LOL.

THING THAT GET'S ME IS,
REGARDLESS OF MY PITYFULL NULLING,

HOW IS IT POSSIBLE THAT ON THE OLD COIL,
FERROUS AND NON-FERROUS GIVE AN INCREASE IN AMPLITUDE.

[DOOLEY]

HELLO ALL,

just had another play with the coil nulling,
and had an idea......
what if i had1 nulling coil fed from the "hot" TX-D
AND
an identical nulling coil fed from the feedback TX-0.......

well, ive tried it,
and turns prob not right but by moving them around i can null right
across zero and back out of the other side !!!!

ah !! success !!...........

NO !!!

even when nulled to about 20mv
(apparently anything less than 50mv is good for this detector)
i'm not really getting any phase shift,
and as this detector , i think,
needs phase shift,
i'm not getting much sensitivity at all,
amplitude quite hapily going up and down,
but no phase shift.

so,
adopted the 2x220k resistor idea from the original in coil board,
ONE FROM TX-D to RX
ANOTHER FROM TX-0 to RX,
still, not allot of phase shift.

if this carries on im just going to build a double D mount it
in housing and say bugger it !!!!

[sinclairuser]

hi dooley, when i said 3 wires i was counting the screen, it has black,red and screen, the only thing i can think of is red tx, black rx and screen is both negatives tied to ground, i do know its a dd coil.
someone said that coils are interchangeable on these old cscopes, and there are unused pads where the coil connects so maybe, going to post pics tommorow.

[satdaveuk]

Hi Dooley
I am a bit confused because you have headed this thread VLF.TR990B and then proceed to say you have a CS990B which are different machines , looking at your pictures you have the CS990B.
I may be be wrong but apart from there both none motion VLF machines that's where it ends.
Have owned the VLF.TR990B for many years and it works as good as the day I bought it.
The only downside to these machines is they look cheap and tacky, but far as performance they are great, and really nice once and a while to go out do old style detecting.
The knack of detecting with these machines is low and very very slow and then your pulling goodies that others with expensive all bells and whistles have walked over.
There was one young lad on here a couple of years ago started modifying the housing stem did away with the meter so just sawed that part off just to make it look more of the business.
My opinion was what used to be a classic is now worthless, he should have left it alone, Its like using a Beatles album as a frizzbe.
Going back to coils I am quite happy with the performance of the shop coil it came with so never considered building one.
Its worth keeping in mind that when manufactures increase or change design of coils they often change the inductance also for optium performance of that particular design on a pacific machine.
I learned that in the early days when we were building/experimenting with TGSL build.
If you look under Tesoro (4 Pin plug type) which were built for the detectors of that time they are all different depending on size/type for instance I have 2 here one is the web type the other is the brown type differences are 5.9MH-6.4MH the other is 5.7MH-6.2MH resistance 25ohms/22ohms, The only thing in common is the 500uH between RX/TX .
Have also experienced this with PI format like the Barracuda original , My findings were the smaller the coil for optimum performance it was better to increase inductance and with the original shop coil which was 8"OD to keep at 350uH as per manufacture spec.
So I learned that if your going to build a DIY coil its better if possible to replicate a original manufactures coil and then if that's successful its easer to experiment.
What I normally do if I have access to a original is do the normal resistance/inductance, pinout checks then hook it up to the multi purpose oscillator project I posted here a couple of years ago together with a frequency counter and scope you can easily measure what frequency the coils are running at on the meter both TX/RX and independenly , you can also see and take note of the null point with the scope then together with all that information you can duplicate the two coils and then produce a exact copy incl the null point.
Can only really be done with either non tuned coils or with coil components RX/CX etc removed.
Done many coils this way with success unlike the early days wasting money and hundreds of hours experimenting with hit and miss coil builds.
Suggest you build the oscillator its common cheap components easy build , everything is there incl lay files.
All the best
Regards

[DOOLEY]

hello satdaveuk,
oooopsss !!
yup,your right, my mistake,
cs990 is written on the coil,
thought the vlf.tr bit was correct for the detector,
my mistake.

yes to the tacky bit,
only used briefly and the handle creaks and can imagine im going to get wrist ache
due to no arm rest and the angle of the handle.
so,can see why someone repackaged it,
but yes would be a shame to butcher it.
suppose i'm butchering in a way wanting to fit a lager coil.
but could soon revert back to normal with a 2nd hand coil.

was trying to replicate coil,
just larger for hopefully a bit more depth and coverage.
this whole nulling things got me beat,
done "0'0"'s
done "d'd"s
finding concentric a little bit of a challange,
driving me a bit mad.

only thing i can think of is nulling a coil that uses a center tapped TX and harley is different
to nulling a "standard" single coiled TX COIL.

going to do a DD this evening and see if detection distance makesme happy.

sinclairuser,
yes,
got an old scematic of a cscope which only uses one coil for TX
so makes sense only 3 wires,
but bet that wont work on the detectors requiring a feedback for oscillator.
hum,DD,
thats the answer and the easy way out.
i'll see what distance i get with one.

[sinclairuser]

what ive discovered today from a lad in the club is, the versions like mine with the 3 wires can take the bigger coils with the extra two wires, but not the other way round so you could be right about needing feedback on the more complex type, if you would not mind have a look at that schematic you have and see if it has the extra pads on the coil connection, i can get a bigger coil for mine, but my board is unmarked so i dont know which pad is which.

[DOOLEY]

hello sinclair user.

if your coil is:
red=tx
black=rx
and screen is common,

then i think your right about fitting the differnt coils,
colours are different,
but it should work, you just wont be using the feedback.

so:
red is still tx,
screen still common,
WHITE=RX
green also to common,
and BLACK=feedback would be unused.

thats my best educated guess anyways.

i'm still pondering my problem though.......
all boils down to either :

i'm doing something wrong to null this new coil.
or,
when a center tapped coil and hartley oscilator is used for the tx
then the "usual" theory of the 4:1 turns for the bucking coil doesn't apply,

or is there something weird with the hartley that requires a different approach
to nulling,
hence the need for cscope to have a circuit board in the coil with a couple of 220k resistors
and a 100k trimpot ???

all the articals i have read regarding building concentrics have only used ONE coil
for the TX,
not 2.

don and carl,
where art thou in my hour (week) of need.
lol.