GS 5 THRESHOLD PROBLEM

[kt315]

you can NOT believe me and thinking thats Photoshoped.

[kt315]

You have to raise the voltage to 14-15V for it to be stable.
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Eric, in my elite i LEFT LM2931-ADJ on board. now i think to delete it. a converter gives 13V on output. from 3.7V to 13V. enought reserve on voltage.
you can believe me or not.

[eclipse]

The TDI SL has audio based on 4066 and 555 and 3 additional op amps. It doesn't use 79L12.

[kt315]

ive been using 4 li-ion batteries 15v
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i put only one word DV. ive been using 4 chinese li-ion batteries 15v

[daverave]

Hi Dave,
Basically, trying to run that circuit at 12V input is bound to cause problems. You have to raise the voltage to 14-15V for it to be stable. The Whites TDiSL runs at 12V and necessary circuit changes have been made, but I do not have the details. I believe from reports that the performance is not quite as good as the GS5B based TDi.

Eric.

Hi Eric...ive been using 4 li-ion batteries 15v and i was hoping that i could use the detector down to at lest 11 volts before recharging...ive had another idea of using a small SM minus LD adjustable regulator with a drop out of about 300 mV to just supply the audio board which in theory should work....ive had to order one and i hope it may do the trick with the threshold drift....do you think it may solve the problem ???

[Ferric Toes]

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.

Hi Dave,
Basically, trying to run that circuit at 12V input is bound to cause problems. You have to raise the voltage to 14-15V for it to be stable. The Whites TDiSL runs at 12V and necessary circuit changes have been made, but I do not have the details. I believe from reports that the performance is not quite as good as the GS5B based TDi.

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.

[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]

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.

[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 ???

[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.

[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.

[daverave]

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

[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]

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.