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Tandy 1400LT: No Power

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itsvince725

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I received a Tandy 1400LT yesterday and it has some kind of no power problem. I tried two different power adapters and testing both with the battery pack installed and with it removed, but I'm just not getting anything out of it.

Any suggestions?
 
it takes a 15v negative tip supply, and is very sensitive to undervoltage conditions. And the battery pack is almost certainly shot now and not doing anything.

Can you verify your external power brick is putting out at least 15v and the polarity is correct? If not you may have blown the PCB fuse inside the power supply. The 1400LT PSU's seem to be prone to capacitor issues, mine kept shorting or blowing them until I more or less replaced all of them, and each time one failed it blew the PCB fuse.

If you look at page 195 (7-20) of the service manual you'll see a 2A fuse marked F500. If you're looking at the PSU PCB it's somewhere towards the bottom left, or at least it is on mine (there are at least 2 revisions of the main and power PCB's for the 1400LT.)

Download Tandy 1400LT Service Manual | ManualsLib

www.manualslib.com www.manualslib.com
 
ross.dykes said:
it takes a 15v negative tip supply, and is very sensitive to undervoltage conditions. And the battery pack is almost certainly shot now and not doing anything.

Can you verify your external power brick is putting out at least 15v and the polarity is correct? If not you may have blown the PCB fuse inside the power supply. The 1400LT PSU's seem to be prone to capacitor issues, mine kept shorting or blowing them until I more or less replaced all of them, and each time one failed it blew the PCB fuse.

If you look at page 195 (7-20) of the service manual you'll see a 2A fuse marked F500. If you're looking at the PSU PCB it's somewhere towards the bottom left, or at least it is on mine (there are at least 2 revisions of the main and power PCB's for the 1400LT.)

Download Tandy 1400LT Service Manual | ManualsLib

www.manualslib.com www.manualslib.com
Click to expand...
I don't have the PSU, I was using a universal laptop power adapter.
 
The PSU is internal to the unit. The "power brick" for lack of a better term is what sends 15V DC to the PSU, which then provides +12, -12, +5, and battery charging.
 
ross.dykes said:
The PSU is internal to the unit. The "power brick" for lack of a better term is what sends 15V DC to the PSU, which then provides +12, -12, +5, and battery charging.
Click to expand...
Ah, I had a feeling that was what you meant! Given the fact that putting in 15V produces absolutely zero response, there's a good chance I have a blown fuse.
 
Check for shorted caps, otherwise you're going to immediately blow the fuse again. Or you can do what I did, replace the PCB fuse with an automotive fuse holder, swap the fuse, and keep popping caps until you find all the bad ones! =)
 
ross.dykes said:
Check for shorted caps, otherwise you're going to immediately blow the fuse again. Or you can do what I did, replace the PCB fuse with an automotive fuse holder, swap the fuse, and keep popping caps until you find all the bad ones! =)
Click to expand...

I opened up the 1400LT today and took a look at the PSU, I didn't see a normal fuse so I guess the fuse is like...a resistor form factor?

It's a fairly small PSU so I guess I'll just replace all the caps and that fuse and see what happens...
 
It looks like a resistor, yes. Here's a pic of my board, it's the later revision so it may be different from yours. I've circled the fuse in red:

1663520881055.png
 
Word of caution, there are mica insulators between the case of the TO-220 transistors and the heatsink, I had to remove the heatsink from the power PCB to replace all the caps and clean the board and I almost lost one. All of my caps were slightly leaking so I had to clean all the goo from the board with isopropanol. There were also a lot of questionable solder joints so I reflowed everything. Been working like a champ since!
 
That's the spirit! =)

I installed an automotive fuse holder on leads because I had to have blown half a dozen fuses until I fixed all the leaky/shorted caps. However, if you do all the caps then you're likely good!

Make sure you record the position of the caps before you pull them out, one of them on mine (-12v I think) was installed in the reverse orientation than the rest and I almost missed it.
 
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