Arado VFA90 Schematic?

[Duncan]

Has anyone on this forum have a copy of the schematic
for the Arado VFA90 detector?
I have searched the schematics archives, and unless I
have overlooked something, I cannot find one.
Can anyone help, please?

Duncan

[digger barns]

not many about

Probably only a 100 or so of these ever made, and almost certainly all potted. Nice detector on the beach but not very deep by todays standards. About £180 30 odd years ago. Nice money. 9khz search coil mixed with really high ref coil 9 or even 18 mhz xtal ocs tuned by a single turn pot. I think its called pulling a crystal somehow. Never even seen one on EBAY. All been thrown away by now probably.This is a pity because this would make a nice DIY project with just the single coil.

[Duncan]

Arado VFA90 Schematic?

Thanks Digger for your VFA90 information. I suspected Arado would pot the circuit, like they did with half the circuit board of the 120b IB machine. Maybe they did this to improve mechanical and temperature stability? I was going to contact Arado to see if they would let me have a copy of the VFA90 schematic, pretending that I had a broken machine that needed repair, but obviously if they potted it, they would realise that I couldn't repair it anyway.
I wonder if they ever patented the detrector?

Duncan

[pi-beach]

VFA 90

I doubt if the detector was ever patented as it was potted in yellow epoxy both sides of the pcb just to hide everything.
I owned two of these and reverse engineered one of them at the cost of my health due to the noxious fumes trying to get the stuff off the components,
there were a lot of polystyreen capacitors these melted in the heat used to remove the epoxy and the values were lost along with the transitor markings,
the circuit was quite basic, I dont have a working schematic but am currently trying to reproduce it with modern parts, ive got the 4.5Khz 6" head osc up and running with 140turns of thin mag wire and still trying to re-develop the follower acoustic shaping circuits with decent gain to drive the mixer/outputs.
will return when I have repeatable results.........

[Duncan]

VFA90

Thanks for the information. I remember the PCB assembly of the 120b was potted in exactly the same way.

Perhaps the intention was to make it almost impossible for anyone to copy the detector?

I understand the principle behind the VFA90 in as much as it was a sophisticated BFO detector.

A low frequency oscillator (9Khz?) with the search coil being the inductive part of the frequency determining LC circuit, has its output frequency muliplied and mixed with a reference oscillator of much higher frequency (9Mhz?).

So that for a given change in search coil inductance at 9Khz should, in theory, give a proprtionately greater shift in beat frequency when multiplied and mixed at 9Mhz.

One thing is crucial to success with this sort of circuit - the search oscillator would need to be incredibly low-drift because even a small drift would be dramatically exaggerated by the time the signal meets the 9Mhz mixing signal. Fortunately, the 9Mhz reference oscillator can be crystal controlled.

One line of research could be trying to utilise a PLL system to keep the search oscillator stable until a target is detected?

[pi-beach]

The head Osc was zener stabalised at around 7Volts
but dident have to critically steady as the machine was open to variation in sounds
a bit like a minelab explorer with frequencies going up and down, our ears did the decoding as there was always a noise present, you just had to differentiate the noises
for the ferros or non-ferros in your head
your almost correct Duncan but the Quartz Xtal I removed was 27Mhz
that is related harmonically to 9Khz Search Freq, there was an ajustable pot core of unknown resonance, then a ferrite tuned HF coil with a dozen turns on it
it was over 30 years ago I took one apart I had no audio test gear at the time, but there was no way the maker could have tuned, serviced or repaired the detectors once potted.
I will try and get a piccy of the case aon here somehow as I still have the originag case and shaft + brochure but the PCB was no more after dissection and only have a dark photo copy of some tracks
hope you have a gud xmas
Rob in UK

[pi-beach]

Picture Test-1

Just Trying to see if I can send piccys in here of VFA 90 case etc.

[pi-beach]

Piccys

only rubbish photocopy of VFA 90 PCB Tracks i have
dident have digi cams in those days :-)
bottom left is headphone skt. middle = Vol/On-Off. far right = Multiturn 5k Tuning Pot made by (Spectral Reliance) hole above to right is pot core tuning.
top left is audio out and mixing top right is search coil osc.

[pi-beach]

ARADO IBA45

in case anyone wants to compare things this is the views of same case used in both models and pcb construction with yellow potting as the VFA90 was, *** for ref the PCB is the IBA45 *** using with less components.
I am in the process of trying to repair my 45 as audio output has died and it took 3 hours to get 3 components out using a very hot soldering iron chiseling away to turn the compound to dust, the stink is still bad after 30 odd years :-)
Rob.....

[Duncan]

VFA90

Thanks for the Pics Rob - very interesting stuff!
Regarding the 27Mhz crystal, unless the crystal was used in a 3rd overtone oscillator circuit, it would work on its fundemental frequency of 9Mhz.
If I get time in the New Year, I may begin experimenting with some circuitry to try and recreate the working principle behind the VFA90.
It states on the case of both detectors "patent applied for", so I wonder if there is some record or other in the patents office files decribing these?
Duncan in UK

[Alexismex]

Hello PI Beach,
The method i use in very hard compound it is :
-Warm very hot a small part of the yellow resin with hot gun (ideally with a small metal head)
-and in the next second cold rapidly with a freon spray with a small pipe (the use in electronic for detect hot parts defect)
-and the result is a crack suddenly easy to separate with a scalpel...
It is my method and has resulting in various board and practically no damage to the parts

[pi-beach]

Yes Duncan I think you are correct as the circuit was very basic and the coil had to much wire for 27Mhz so prob was tuned to 9Mhz makes more sense now :-)

Regarding patents, I still dont think the inventor would have lodged anything
especially as someone mentioned only about a hundred of these units were ever made
and I had two of them!

Thanks for the Pics Rob - very interesting stuff!
Regarding the 27Mhz crystal, unless the crystal was used in a 3rd overtone oscillator circuit, it would work on its fundemental frequency of 9Mhz.
If I get time in the New Year, I may begin experimenting with some circuitry to try and recreate the working principle behind the VFA90.
It states on the case of both detectors "patent applied for", so I wonder if there is some record or other in the patents office files decribing these?
Duncan in UK

[pi-beach]

Hi alexismex,
I hadent tried the freezer spray as it wasent around in thoses days
sounds like a good idea to crack up the IBA45 pcb, thanks.
for that

Hello PI Beach,
The method i use in very hard compound it is :
-Warm very hot a small part of the yellow resin with hot gun (ideally with a small metal head)
-and in the next second cold rapidly with a freon spray with a small pipe (the use in electronic for detect hot parts defect)
-and the result is a crack suddenly easy to separate with a scalpel...
It is my method and has resulting in various board and practically no damage to the parts

[digger barns]

IBA 45 Schematic

I have the schematic hand drawn, but no scanner. If someone here can let me have a UK address I will post it this week. For putting on here.Its quite a simple circuit and I may be able to find rhe coil details.

[digger barns]

IBA 45 Coil details

for a 7to 8 inch 2d coil Rx coil is 90 turns .35 wire. No tap. tuned with .033 cap
Rx coil is 90 turns .35 wire start then 37 then 8 CT then 8 then 37
So its 90 turns center tapped with 8 turns each side of center tap. Tuned with .033 cap
Back in them days it was common place for products to be patent applied for because it only cost £ 1 This lasted for a year and was usually not continued. I did one myself in 1977 for cordless headphones on a metal detector. It was an ego thing, but was pointless you could never defend these things against the big companies. So you just cover everything in yellow resin.