Noisy XL500 pulse after battery pack upgrade

[Mtlexplr]

I have an original Garrett Seahunter XL500 pulse, I bought back in 1984. Has been a good unit and has had no issues all these years. Last year I upgraded the 7 sub-c NiCad battery pack to a newly made 7 cell sub-c NiMH. I took my time and built the new pack exactly like the original. Voltages were the same after assembly and I installed it into my case.

After charging the unit, the battery pack was up to normal voltage. I then took it on a trip and when I first turn it on, it makes all kinds of noise like the threshold is set too high. It eventually starts to quiet down and worked well enough, I fould a nice, large
mens 14Kt wedding band.

I wonder what is causing the noise on start up? It never did that before replacing the battery pack. I did read somewhere, that some people leave the unit to run for 5 minutes before using it to let the components warm up.

I am also wondering if it would be a good idea to replace any of the components inside, such as the electrolytic caps, due to age related decay? I have precision caps here and wonder if they can be upgraded with tantalum or other caps to make it function
better?

I also saw some postings in past where some resistors were replaced/added on the board to make it run better.

Any ideas for better performance on this unit with simple replacements or additions? I don't want to change things that need adjusting with a scope. I have a scope, frequency meters, multimeters and a well stocked bench, but like to keep things simple.

[SVEN1]

Thought this may interest you, might be of use for you and Homefire says stay with nicads http://www.treasurequestxlt.com/comm...0-pulse.61483/

http://www.treasurequestxlt.com/community/index.php?threads/sea-hunter-xl-500-pulse-coil-wiring.50310/

[Mtlexplr]

Sven1, thanks for the help! I read the links and even in the schematics under the coil wiring link, it shows the battery as Ni-MH. In fact, Garrett only supplies Ni-MH batteries in their battery pack replacement. I would be interested in knowing why Homefire says to stay with nicads? The voltage is exactly the same, and all I did was upgrade the cells to better mA ratings for longer run time. Going "backwards" to rebuild with Nicads seems like a waste or time and money. If there is some kind of issue with the voltage, I could always install a small voltage convertor to make sure the voltage needed is dead on.

I'm still looking for the simple upgrade that makes the unit more sensitive. It was only a resistor swap or add on, but I can't find the information on it. Also want to know if swapping out the electrolytic caps for newer, higher precision ones would be of help. They all look fine, but they are 34 years old. The board and all components look fine, with no corrosion anywhere.

[homefire]

Sounds like your batteries are putting out more then it wants for the first few minutes eaaa?

Maybe over driving something ?

Was it working ok prior to the battery swap ?

Don't Fix it if it ain't broke. Leave the caps alone for a while. Only ones that I would even think about swapping would be electrolytic s. Test them first.

I added a 8th cell and didn't have any issues way back when.

NiMh Cell SUCK ! Despise the things. They don't like me I guess. Everything I've purchased the things has died.

Any thing I tried using them in ended re stuffed back with Good ol NiCads.. Hell used 20 year old NiCads doing that. Best battery ever made prior to the Li.

Noise What kinda noise ?

Yea I changed out a timing resister way back. It help some but not really worth doing so.

I have a working PCB here it wouldn't take much to talk me out of.

My little DIY Barracuda runs circles around the EX500. Living in the New Mexico Desert I really don't have much use for those machines any more.

[Mtlexplr]

Hi Homefire,

It was working fine prior to the battery swap. If it is possibly over driving it, I could install a cheap voltage regulator inside between the battery pack and board. I would prefer not having to replace the battery pack again if possible.

The batteries last plenty of time and charge properly.

I will change the electrolytics. Just to be on the safe side.

The noise is just like the audio threshold is turned way up. No chatter or anything weird, just a loud threshold audio that can't be turned down for at least 5-10 minutes. Then it works fine. Sounds like it might be over driving it like you say.

The rest of the pcb and wiring looked fine while I had it opened.

Would you say the DIY Barracuda is the best choice for saltwater use, mainly for gold? I was looking to build the Mirage PI from Sven, but he does not have any boards available anymore. I like sticking with PI units only for saltwater.

[waltr]

Does it do this if the batteries are not fully charged?
If so then it is not the battery Voltage.

[Mtlexplr]

Just checking the setup right now, but the batteries are dead, so need to wait till they are fully charged.

I think Homefire is correct, that Nicads are better than NiMh. My Nicads were 33 years old, and even if I did not turn on the unit for a year, I could still turn on thepower and the battery would have some charge. Just turned on the unit now and the batteries are completely
dead. Some much for "upgrading" a perfectly functional battery pack with "better" NiMH. Looks like I will place a quick order on amazon for some new sub-c Nicads and rebuild the battery pack back to "normal".

[Mtlexplr]

Just did a quick test, after letting the batteries charge for about 45 minutes.

The meter initially says the battery pack is fine and I can set the audio threshold to barely audible. Then the threshold starts climbing and I have to reset it several times down before it finally holds steady. It detects a silver coin and gold ring as it should with the air test.

Just plugged the charger back in to let it fully charge and then will test it again to see if the threshold continues to climb or hold steady?

[6666]

What is the battery voltage after it comes off the charger ?

[SVEN1]

Just had a thought are you using the original plug in charger for the XL Nicads for the NiMHs? Or using a NiMH charger?

[Mtlexplr]

Yes, just using the original Nicad charger that came from Garrrett.

[kt315]

garrett charger is simplest one. just transformer and one diode.

[Ferric Toes]

The original nicad battery is 8.4V by 7 cells. The two transistor constant current regulator to control the charging current is on the control board inside the detector housing. This is from an original Garrett schematic blueprint dated 28the January 1983.

Eric.

[kt315]

i meant just charger housing inside. i opened it and rewinded primary winding, from american 110V (127??)
into 220V AC EC standart.

i say again MTLE you have to sure retune frequency/PPS on 60*N Hz/PPS. ie good for you would be 120, 240, 480 etc Hz
getting around off the electric nets interferency.

[Mtlexplr]

Checked the voltage of the battery pack after coming off the charger. The voltage was 9.46 volts. Threshold wanted to climb constantly and had to be readjusted several times. Let is sit for 5 minutes and was finally able to get the threshold on the audio adjustment to hold steady.

Detector worked properly. Looking to replace this old workhorse with better PI detector. Placed a post asking for advice on which ones would be best, but never got a response. There are lots of choices,Baracuda, Hammerhead, Mirage, etc, with each having their own pros and cons. Of the different models that can be built, is there not one or two that stand out as really good units for beach/underwater work on gold? I am leaning towards the Mirage design by Sven that has been modified by Elliot.