GS 5 THRESHOLD PROBLEM

[kt315]

look my topic again http://www.geotech1.com/forums/showt...ly-mod-offered

[Jose]

Hi jose....did you have the same problem with a GS 5 ????

I did not have the same problem with the threshold but other problems, such as high temperature in the TX stage, so I decided to feed the detector with 12 volts, besides being more practical, since you can use a gel battery.
For that reason, I use a low-drop regulator in the -10 volts line.

[dbanner]

My thoughts are that the -12v regulator(79L12) sets the dc bias of transistors in audio vco stage. What would happen if this were to change? Would this affect the threshold in some way? Lm2990t-12 is low dropout.

[kt315]

I did not have the same problem with the threshold but other problems, such as high temperature in the TX stage, so I decided to feed the detector with 12 volts, besides being more practical, since you can use a gel battery.
For that reason, I use a low-drop regulator in the -10 volts line.

change all TO-220/TO-92 stabs on impulse chinese converters. set a converter between batt and board. i see you just are seeking up problems on bare places.

[Jose]

My thoughts are that the -12v regulator(79L12) sets the dc bias of transistors in audio vco stage. What would happen if this were to change? Would this affect the threshold in some way? Lm2990t-12 is low dropout.

It is possible, you can try a low-drop regulator, (-12v) or connect the two resistors of 47K, directly to -10 vol to clear doubts.
I think this scheme was added to solve the problem of changing the audio level, which originated when activating the high or low conductivity mode.
See the proto scheme.

[Jose]

change all TO-220/TO-92 stabs on impulse chinese converters. set a converter between batt and board. i see you just are seeking up problems on bare places.

A DC / DC converter is possible, but it takes more current from the battery.

[daverave]

ive been doing more testing on the audio pcb...test points 1 & 2 do not vary according to the lower supply voltage....but i found that U4 -12v regulator stops regulating just under 14 volts supply....i checked the regulator and found it to be a standard 79L12 and i would have thought that it should be a low drop out type which i would have to order and see by substitution if it solves the problem.

[Altra]

Dave you could try replacing the 79L12 with a 79L10 or 79L09. I think the only difference would be the maximum volume. But since you have a audio booster it shouldn't matter.
Probably have to readjust the threshold and centering trimmers. Easy and cheap to try.

https://www.ebay.com/itm/10x-79L09-L...ieX:rk:12:pf:0

[daverave]

i may try 79l10 but i cannot understand why there are no -12v LD TO92 listed on the internet....do any exist ???

[dbanner]

can't seem to find 79L10 in TO-92 style either, only 79L09 which has Vin min of -11.5V. The 79L12 has Vin -14.5V min.

[waltr]

I just did a parameter search at Mouser and DigiKey and found no LDO -12 regulators even in SMD. Best has a drop out of 1.7V.

There is simply no big market for these so no one makes them.
+5V and lower Voltages is where the market requires LDOs and there are many available.

[daverave]

ive been some searching on the internet and i came up with the same at 1.7v drop out....maybe the -12v regulator is not the problem but not sure...i wonder if all the GS 5'S had this standard 79L12 regulator or a low drop out type which is now unavailiable ???

[daverave]

i just found out the standard 79L12 was used in all the GS 5 ' S so again ive been barking up the wrong tree...thanks to all those who have offered help and advice to me of which im very grateful.

[Ferric Toes]

Hi Dave,
That is correct. There are two regulators associated with the audio output, but their purposes are different. The 79L12 supplies a bias via a diode and resistor network so as to compensate for the variation due to temperature change in the Vbe junctions of the audio output transistors. This was added after initial trials of the GS5 in Australia where the ambient temperature could rise to over 30C.My modified audio pcb was labelled PPD105 Version 2. The earlier board, PPD105, did not have this additional circuit as drift in this country was not an issue, requiring only a small adjustment of the threshold control.
In version 2, the primary audio supply was from the -10V rail from the main board (LM337T) so you will get threshold drift once this starts to run low on overhead voltage. The 79L12 that feeds the temperature compensation will also fail to regulate when the battery voltage falls much below -14V. It might be an idea to disconnect the 79L12 and connect the output connection to the -10V rail so that both audio and temperature compensation run on the lower voltage. From what I recall from the GS5B the battery indicator meter was set to cross from the green into the red at -13.8V. this indicated that there was about 30 minutes remaining before charging was needed. Battery pack consisted of 12 x NiMh 3800mAh cells.

Eric.

[daverave]

Hi Dave,
That is correct. There are two regulators associated with the audio output, but their purposes are different. The 79L12 supplies a bias via a diode and resistor network so as to compensate for the variation due to temperature change in the Vbe junctions of the audio output transistors. This was added after initial trials of the GS5 in Australia where the ambient temperature could rise to over 30C.My modified audio pcb was labelled PPD105 Version 2. The earlier board, PPD105, did not have this additional circuit as drift in this country was not an issue, requiring only a small adjustment of the threshold control.
In version 2, the primary audio supply was from the -10V rail from the main board (LM337T) so you will get threshold drift once this starts to run low on overhead voltage. The 79L12 that feeds the temperature compensation will also fail to regulate when the battery voltage falls much below -14V. It might be an idea to disconnect the 79L12 and connect the output connection to the -10V rail so that both audio and temperature compensation run on the lower voltage. From what I recall from the GS5B the battery indicator meter was set to cross from the green into the red at -13.8V. this indicated that there was about 30 minutes remaining before charging was needed. Battery pack consisted of 12 x NiMh 3800mAh cells.

Eric.

Hi Eric...i will try what you said and connect the -10v to the output connection of the 79L12 regulator...i did find for some reason that if i were to decrease the supply voltage down below 12 volts then the threshold level would start to decrease slightly...not really sure why that should happen.