Replies to Original Minipulse Project Details

[Qiaozhi]

Please see the final schematic for this design (with all errors corrected) in the sticky post at the top of this section ->
Original Minipulse Project Details

[Qiaozhi]

Few issues with 4538 connection on mini PCB. Pins 8,1,15 are Vss

The MC14538BCP has pins 1, 8 and 15 connect to -5V.
So this is correct.

timing capacitor goes from pin 2 or 14 toVss, but on original PCB pins 1 and 15 are not explicitly connected to Vss (-5V in this case).

I also noticed this. The original PCB designer assumed pins 1, 8 and 15 are internally connected in the chip, which is true, but it is good practice to explicitly connect them to -5V.

Pins are internally connected inside chip itself (thru some very low resistance path), will work connected like this, but data sheet say pins 1,8,15 must be grounded externally, usually they are in most designs.

As above.

Also timing capacitor marked C24 on PCB, from pin14 U12b is missing on schematic.

Damn. It's on my hand-drawn schematic, but got missed off when I drew the schematic. Thanks for spotting that.

Just another rainy day to finish this, I already retraced PCB in Protel from pictures, only for component placement, now checking connections.

If you find any other issues, please let me know.

I've updated the schematic with the missing cap, and added the improvements mentioned in a previous post.

[Tepco]

Progress so far, relatively similar to original, just small cosmetic changes.

[Tepco]

Few issues with 4538 connection on mini PCB. Pins 8,1,15 are Vss, timing capacitor goes from pin 2 or 14 toVss, but on original PCB pins 1 and 15 are not explicitly connected to Vss (-5V in this case). Pins are internally connected inside chip itself (thru some very low resistance path), will work connected like this, but data sheet say pins 1,8,15 must be grounded externally, usually they are in most designs. Also timing capacitor marked C24 on PCB, from pin14 U12b is missing on schematic.


Just another rainy day to finish this, I already retraced PCB in Protel from pictures, only for component placement, now checking connections.

[sinclairuser]

very interested in a board would definately buy a kit.

[Qiaozhi]

I'm now working on a Minipulse II version with some good results.

The transmit rate has been increased to 1000pps with a pulse width of 100us. Quiescent current consumption is 86mA.

Although I read somewhere that this detector was sampling with a 20us delay, it was actually measured as 58us. Although, with the components fitted, the sample time should have been 30us. I have made this adjustable, and managed to sample at 22us using the original coil (543uH). The MC14538BCP (U12a) has a minimum possible delay of 10us. This is achievable with the delay adjustment, as long as you have a faster coil.

A self-adjusting threshold (SAT) has been added, which is a great improvement. When you adjust the Reject control, it no longer needs to be retuned. The SAT is quite slow, so there is no loss of depth.

A small change was made to the final integrator stage that has greatly improved stability and the rejection of external noise.

The pre-amp has remained as a UA709 (I don't want to overdo things with the original PCB) but will be replaced by an NE5534 in the Mark II design.

Mk II PCB layout coming soon, together with wiring to the external controls, headphone / charger socket, etc.
Please be patient, as I'm very busy and trying to do 15 things at once.

[Qiaozhi]

Please see the Project Details for an updated schematic (V1.2).
Errors fixed in this version are:
C11 was shown as 10nF. Should have been 100nF.
R39 and C20 were connected to 0V instead of -5V.

[Qiaozhi]

Does the second sample initiate a sort of GB?
If not I guess this design would not like Australia's mineralised gold bearing areas?
Cheers john

This detector was designed and built in the 1980s, and there is no GB feature.
The second sample is for Earth field elimination.

[johnandles]

Does the second sample initiate a sort of GB?
If not I guess this design would not like Australia's mineralised gold bearing areas?
Cheers john

[Qiaozhi]

The project files are now "sticky" (so that they'll remain at the top of this thread) and all the replies have been moved to another thread.
I've also updated the schematic to Version 1.1 to include the capacitor value changes and the feedback resistor for the preamp.

[Tepco]

I already mentioned reverse engineering PCB from pictures on this forum, so this is good reason to put it to the test if I get some time... Why not? People restoring oldtimers, tube radios etc, this is nice part of MD history.

[Qiaozhi]

Retain DIP trough hole solution, please!

That was my intention.

Qiaozhi, i can not see the feedback resistance of U2....

It was in my hand-drawn schematic, but it got missed out when I recreated it on the computer.
Well spotted.

I am interested. If possible I'd like to see UA709 replaced with more common NE5534 and of course some NJR counterparts of it.

OK, I'll do that.

On PCB picture feedback resistor is visible near LM709, capacitors C6 and C14 are 10pF, not nF.

Just rechecked the PCB, and C6 is labelled "3.3", so I assume this is actually a 3.3pF (3p3) cap. I assume you mean C4 (as C14 is an electrolytic) which is labelled as "10", so that's a 10pF.
Again, well spotted.

I will post an updated schematic in a few minutes.

If anyone else sees something wrong, please let me know. Like everyone else, I do make mistakes.

[Tepco]

On PCB picture 1M feedback resistor is visible near LM709, capacitors C6 and C14 are 10pF, not nF.

[Davor]

I am interested. If possible I'd like to see UA709 replaced with more common NE5534 and of course some NJR counterparts of it.
I strongly support keeping the PCB through hole, and preferably single sided.
BTW, your schematic is missing a feedback resistor at UA709

[Geo]

Qiaozhi, i can not see the feedback resistance of U2....