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Backup battery and Main battery for Tandy 1400FD

seaken

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Jun 20, 2016
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Shokan, New York
I have an old Tandy 1400FD that has dead batteries. This was the first computer I ever purchased for myself. I think I got it about 1988? It has been sitting in it's bag unused for several years. Both the main 4-cell battery pack and the RTC backup battery are dead. I was able to get some activity from the machine after keeping it plugged in overnight. The low battery LED comes on and the floppy drives get accessed with a seek (?) and the floppy lights come on and off. But there is no LCD display and no boot from floppy.

I figured this was going to require replacing both batteries. I have the Technical manual so I followed the instructions to remove the top, power supply, and floppies. That gave me access to the RTC backup battery. I expected to see a mess with a coroded battery, as I have so many times with some old motherboards with a Varta battery. But this motherboard is very clean and I see no signs of any corosion or battery leakage. The probelm I am having is that I don't quite know how to replace the backup battery. It seems to be soldered to the motherboard. I have clean access to the positive and negative leads. But I am not sure if I can just clip it off and then splice in a new battery.

Should I pull the motherboard and unsolder it? Or can I just clip it and solder on another battery at the legs that are left over after clipping it off? There does not seem to be any "External" backup battery terminals like I have found on some of my other motherboards. On those boards I just clip off the Varta battery and use the External header for a battery pack. But now it looks like I will have to be a little more technical. And I am not very good at soldering.

What is your recommendation? Can I clip? Or should I practice soldering on an old throw-away board first and then attempt to unsolder and solder in a new battery?

And can I still get a compatible battery for this thing?

Thanks,
Seaken

Here's some pics:

IMG_20220807_153554629.jpg

IMG_20220807_153545788.jpg

IMG_20220807_154005026.jpg
 

T-R-A

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If you're not extremely experienced in soldering, I'd suggest clipping the leads as close to the body as possible and then either solder another compatible (doesn't have to be exact, just ensure the chemistry, voltage and amp-hours are exact or as close as possible) battery, or even better, use a holder with wires mounted away from the board and just solder the wires to the ends you just clipped.
 
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seaken

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Shokan, New York
Thank you @T-R-A and @booboo. I think I will try clipping it and soldering on a wire lead to an external battery pack. I watched a video where VWestlife used a 3-cell pack from a cordless phone. I have done similar with regular AA batteries on my desktops but I have not tried this with rechargable backup batteries yet. I think I am more comfortable with the cordless phone battery than I am buying the barrel battery. I know some comments were questioning the use of the cordless battery due to charge/discharge differences. I will do a little more reading and try to find a good battery pack to use for this.

I have had no luck so far wth the main battery pack. It is a 4.6v battery pack. It looks like a 4-cell that are approximately a "D" size battery. What do you think are the chaces that I willfind something to replace this main battery pack?

Seaken
 

T-R-A

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Thank you @T-R-A and @booboo. I know some comments were questioning the use of the cordless battery due to charge/discharge differences. I will do a little more reading and try to find a good battery pack to use for this.

I have had no luck so far wth the main battery pack. It is a 4.6v battery pack. It looks like a 4-cell that are approximately a "D" size battery. What do you think are the chaces that I willfind something to replace this main battery pack?

Seaken
Again, the chemistry of the battery is just as important as the voltage/current rating. If it's lithium, stick with lithium. If it's NiCad, stick with NiCad. It's generally not a good idea to go putting in something completely different from what you had. If it's 4 batteries in series, then try to find 4x1.2v batteries (4.8 total volts shouldn't be a big deal)
 

seaken

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Shokan, New York
I double-checked the Technical Service Manual. The Main Battery is a 4.8 V, 4400 mAh Nickel-Cadmium battery 4 cells.

Ok, so I'll look for 4x1.2v Ni-Cad battery.

Thanks,
Seaken
 

T-R-A

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seaken

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Shokan, New York
Hmm, okay. I had not thought of building my own battery pack. But this could work. I may need some guidance on how to put the wires together to allow the right connection to the existing wires going to the power supply. Right now there is a yellow/black pair of wires. Do I match red to yellow and black to black?

If I'm reading this right I only need 4 of those AA size 1.2 rechargables and each is 1100mAh so 4 will give me the 4400 mAh?

Seaken
 

T-R-A

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Hmm, okay. I had not thought of building my own battery pack. But this could work. I may need some guidance on how to put the wires together to allow the right connection to the existing wires going to the power supply. Right now there is a yellow/black pair of wires. Do I match red to yellow and black to black?

If I'm reading this right I only need 4 of those AA size 1.2 rechargables and each is 1100mAh so 4 will give me the 4400 mAh?

Seaken
Actually, I had a brain fart on that. You'd need all to be 4400 mAh batteries (something you likely won't find in AA size). A lower value would work, but you wouldn't have as long a charge.
 

pearce_jj

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D-cell type are still available as used in some home alarm systems, for example: https://www.fireprotectionshop.co.uk/p/4.8v-emergency-lighting-battery-pack-side-by-side.html

I've used a radio-control model 4.8V NiMH battery - with appropriate connection adapter - but I found that although the NiMH cells will power the machine fine (unsurprisingly), the PSU fuse blows if attempting to charge it.

You might be interested to know that there is a Lo-tech XT-CF board for these machines - available here. Although developed on the later 1400FD, it apparently does work on the LT with the second floppy drive disabled, for some reason - see here - with 1400LT ROM version 2.51.70.
 

seaken

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Thanks for the help guys. I can't find any packs that are assembled in the same way as the Tandy pack. I measured the battery cavity and I don't think the 4 side-by-side packs will fit. I did find a long stick of four. I was thinking maybe I could split it in the middle and fold it over to make the same type of pack setup as the Tandy pack. Otherwise I would have to solder my own pack from individual cells. I would prefer not to have to do that.

Here's the stick style I found

I also can't find the 4400 mah capacity batteries in a pack. They all seem to be in the 5000 mah configuration. Can I use the 5000 mah batteries if my Tandy specs say 4400 mah?

Here's the Tandy pack in and out of the case:
IMG_20220808_095115865.jpg
IMG_20220808_095134179.jpg
 

Unknown_K

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It seems to be obsolete (not surprisingly) but specs are 3.6V 50mAh. If you're adventurous, there are available on AlieXpress but the price is insane. Better find a comparable in another form factor as T-R-A suggested: https://www.aliexpress.com/item/2251832682505460.html
Sanyo CADNICA N-50SB3 is 3.6v 170mAh Nickel Cadmium rechargeable. I wouldn't use any other type of battery unless it works with that recharge circuit. Since it is just a backup battery I would find a close mAh replacement with the same voltage and chemistry and just solder a socket into the machine, so the battery is easily replaceable (cheaper also then OEM).
 

pearce_jj

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Re main battery - the mAh capacity really doesn't matter - it just defines the run-time. More mAh will give more run time. Provided it's a 4-cell NiCd type it should work fine, albeit with a life expectancy of only a few years.

Re internal battery - I soldered some wires to the terminals of mine and put a little rechargable elsewhere (away from the system board). Like yours, the original had not leaked at all.
 

seaken

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I clipped off the RTC backup battery and soldered on some wires to a 3.6v NiCad battery pack that I placed under the keyboard.

I put it all back together and as of now I get nothing. But I am going to let it sit on the charger for a day and see if it does anything. I did that a couple of times before. I was able to get lights and the floppy seek a couple of times. I'm hopeful the overnight charge will get me similar results even though I know the main battery is dead.

It is odd that I cannot use the computer without a battery. I can't just plug in the AC adapter. Apparently I need a good battery to make it work.

Thanks for the tips. I am going to get that 5000 mah stick of batteries and try my hand at modifying it to fit the case. I will of course just attach it and charge it first to see if it brings the computer to life. If not, then I have problems other than batteries.

Seaken
 

seaken

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Oh, that reminds me. I have to troubleshot my power cord on my coffee grinder. I was going to pull it apart and try to solder it today and I totally forgot. The "Braun" reminded me.
 

seaken

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I was able to rebuild the main battery pack. I split open the long 4-cell cartridge into two 2-cell pieces. Then I trasnsferred over the wires from the old pack and taped it all together. The good news is that it fit in the same space as the old battery and it took a full charge from the internal power supply with the AC adapter plugged in overnight.

The bad news is that it did not work to bring the computer life. I'm disappointed. And I'm not sure where to look next. Maybe I didn't get the replacement RTC battery properly soldered? Maybe the internal power supply was going bad and replacing the batteries was going to be fruitless.

I measured the voltage of the new main battery after I charged it and it is just over 5v. When I plug in the AC adapter to the internal power supply jack I hear a click. Presumably that is a relay? With it powered on I get 12v from the yellow/black wire that goes over to the main battery to charge it, and presumably feeds the computer power from said main battery when unplugged.

I put my multi-meter probes on the 15-pin connector at each colered wire and only one showed any voltage. But I am not quite sure I am doing it right.

When I measure the AC adapter voltage it shows 15v. I read in the Tech Manual that that is normal for no load. It goes to 12v with a load.

So, I am getting SOMETHING out of the internal power supply - it is charging the battery, clicking, and some power is getting out to the 15-pin molex. But when I plug everything in and turn it on I get nothing from the computer - no screen change, no indicator lights, not even the low battery light. And no sign of any try to power the drives, etc. Nothing.

I measured the voltage at the RCT battery where I soldered on a replacement NiMH battery from a portable phone. It is at .44? Maybe I should try replacing that battery again? Or should it work anyway without any RTC battery?

I watched a video where a guy totally replaced the power supply with his homemade triple power pack power supplies. So, maybe there is hope that this thing can be powered.


Here's some photos of my battery pack excercise:
 

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seaken

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In my reading I keep see referrence to the capacitors and how the bad caps need to be replaced. I'm not sure what I am looking at but I think I am now seeing some evidence of leaking caps on this power supply board. On the back of the PCB ther are stains and a thin film of what looks like maple syrup. It is sticky and I can get it discolor a white paper towel with a brownish yellow stain.

I'm not too thrilled about trying to desolder old caps and resolder in new caps. And even if I did can I be resonabley sure that this would help bring the power back?

This may end up too far beyond my skill level. What a shame, I was hoping to bring this old computer back to life since it was the first one I purchased on my own over 30 years ago. Oh well. You win some you lose some.
 

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T-R-A

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Most capacitors leak from the topside and there's usually some evidence of bulging when they're really bad. I don't see evidence of that in your photos. Perhaps the bottomside "syrup" is from a soda spill long ago.

Having said that, it's not unusual for old caps to go bad and show no real signs. And you're correct---even replacing all of them is no guarantee that the P/S would work.
 

hornbetw

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The "syrup" is probably rosin flux that wasn't cleaned off the board. It shouldn't cause problems, but you can just use some isopropyl alcohol to clean it off the pcb.
 
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