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Tandy 1400 HD -- PSU Bad I think

I uncovered the issue with the 'boots to 3 lights' issue. The small board has what appears to be a design flaw that places 2 hot components (transistors) in very close proximity to 2 others (one is a transformer and one is a varister). In this case, one of the transistors melted into the transformer and burned the varister over time, causing the failure. I still do not see a post, with moving all the 'good' components to one laptop (or the other, apparently my motherboards are equal, since the behavior is the same).
plug in adapter
internal click in the power supply relay
turn on power switch
caps lock, num lock, scroll lock and low battery lights all come on briefly
blinking light on hd (two blinks then a pause, repeats, never varies)
two beeps,
the floppy drive spins, then stops
the floppy drive then accesses (DOS 3.3 boot disk) the disk and stops after a few seconds
the display backlight is always on - there is never any post or other messages though (the display does not turn the greyish color that a working one does)
the fan does not come on
 
I have been splitting my time between other computers (I have an HP 9000/360 and a few compact macs that need attention).
Within the last few weeks, I was able to get a 1400HD that worked, except for the hard drive. Fortunately, both of my other 1400's had good hard drives, so after replacing the non-working hard drive with one of those, I now have a fully functional 1400HD. I am hoping to examine what happens, when, during the boot process. To say that I am a bit nervous about poking around on a functioning one is an understatement, but I hope that it will help me to understand what is broken on the others. I hope to have time over the Holidays for this and I will definitely post any results. If anyone has suggestions for what we need to look at and document first, let me know.
 
Hi, no idea why I didn't post this already, but I have the service manual for the 1400LT sitting right there on my wiki!

http://vintage-blog.peacon.co.uk/downloads/manuals/tandy/Tandy_1400LT_Service_Manual.pdf

It has complete detail detail on the internal PSU from board layout, outputs and ratings and indeed a full circuit diagram on page 195. As you said the voltages are -12, 5, 12 and 15V for VDU drive (inverter board then creates c.115V for it).
 
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Hey guys, I am in desperate need to know the values of the following caps from a 1400HD PSU
C52
C43
C14
C4
C53

Also R44
R47

My machines components for those can't be read.

Thanks
Carlos
 
does anybody know how to open one of these 1400's up. I also suspect a bad PSU. I would be able to start diagnosis if i could get into the machine.
I can't seem to fine a guide anywhere online, but I have found the service manual.
any leads are appreciated.
 
The Radio Shack Technical manual shows assembly, so use that as a guide.

By the way, I had parts for a 1400HD to build one. I started to recap the power supply and lost my notes to the cap values. I replaced 1/2 the caps when this happened. If you need a power supply (NOTE, the HD power supply is different) please look at my eBay list!
https://www.ebay.com/sch/colorcomputerstore/m.html?_nkw=&_armrs=1&_ipg=&_from=

I am selling the other parts of my 1400 if you need to replace/build yours..
* Top case
* LCD
* keyboard (VERY clean)
* Floppy
* HD

Cheers
 
Nice stuff out there colorComputerStore - you have lots of other fun items besides, checking it out now.

Sorry that I did not get a chance to open up mine and research your PSU question, but my 1400HD's have been kind of buried for a bit.
 
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dgre6133 - if I get to mine I can let you know, assuming yours is also a 1400HD.
 
dgre6133 said:
does anybody know how to open one of these 1400's up. I also suspect a bad PSU. I would be able to start diagnosis if i could get into the machine.
I can't seem to fine a guide anywhere online, but I have found the service manual.
any leads are appreciated.
Click to expand...

There's just a bunch of screws on the bottom, then the top lifts off IIRC.
 
I just picked up a booting, 1400HD off eBay. It came with a power supply that has been recapped and adjusted, so I look forward to taking it apart, checking it out and putting together a very good working model e.g good screen, floppy, HD (maybe even installing my XTIDE-IDE). I am hoping this will also provide enough of a roadmap for my to fix my other non-working 1400HD's.

I have powered it on, and booted it, but so far that is all the further that I have gotten.
 
I could use some help on this issue as I have 5-6 1400HDs dead in a stack I'd like to revive - possibly making a replacement PSU board for the original.

In an epic effort at procrastination on another project, I took one apart tonight and made some notes. Just about every large electrolytic in the PSU had leaked badly. I didn't try to fix the PSU board, but rather try and get the mainboard powering up off a series of bench supplies.

southbird above noted this - I've added my notes in black out to the side:

1 (White) UNKNOWN - this appears to be a control signal. It is pulled-up to 5V on the main board.
2 (Dark Blue) UNKNOWN - This is a control signal from the PSU. It is pulled-up to 5V on the main board. If I ground it, the low battery light stops flashing.
3 (Brown) UNKNOWN - This had a large electrolytic next to it. I believe it is a switchable +5V rail that is off during standby. But I'm not 100%
4 (Gray) +5 - I don't believe this is a supply rail. It has a pull-down on the main board. When I apply +5V to it and then remove it, the HDD spins down. When I reapply 5V, the HDD spins back up. I believe this is a control signal relating to standby mode
5 (Blue) +5 - Not sure what this is. It definitely does NOT have low impedance to +5V on the main board.
6 (Red) +12 * Transformer Upper Right (bottom view) - Yes this appears to be the +12V supply rail as it has a large electrolytic next to it and has zero ohms to the fan hot wire (12V fan)
7 (Purple) -12 * Transformer Lower Right (bottom view) - Unsure about this pin. There was NOT an inverted electrolytic near-by
8 (Orange) UNKNOWN Transformer Mid Right (bottom view) - This had an inverted electrolytic next to it indicating a negative supply rail. Applying -9V with a battery caused the positive lines on the LCD to come on randomly (which the contract knob was able to affect).
9 (Black) GND - Ground
10 (Black) GND - Ground
11 (Black) GND - Ground
12 (Black) Ext Black ** - Ground
13 N/C
14 (Green) Ext Green ** - This is without a doubt the always-on (even in standby) +5V main input. There is a very large electrolytic right next to it. And it has low impedance to most VDDs on main board ICs
15 (Pink) +5 - Not sure exactly what this is. There is no low impedance path to +5V on the main board

Ext Yellow +5 (probable floppy drive direct power) - I'm pretty sure this is Vin from the barrel jack. The 2 pin yellow/black from the PSU runs to a power regulator board underneath the HDD. The 2 pin green/black carries regulated 5V back to the main 5V input (pin 14).

pearce_jj said:
Right I've made a start on documenting the 1400FD, actually only the pinouts so far:

http://vintage-blog.peacon.co.uk/wiki/Tandy_1400FD
Click to expand...

Now a broken link. :(

Juror22 said:
I still do not see a post, with moving all the 'good' components to one laptop (or the other, apparently my motherboards are equal, since the behavior is the same).
turn on power switch
caps lock, num lock, scroll lock and low battery lights all come on briefly
blinking light on hd (two blinks then a pause, repeats, never varies)
two beeps,
the floppy drive spins, then stops
the floppy drive then accesses (DOS 3.3 boot disk) the disk and stops after a few seconds
the display backlight is always on - there is never any post or other messages though (the display does not turn the greyish color that a working one does)
the fan does not come on
Click to expand...

After I attempted to power the +5V and +12V supply rails I identified above and apply an inverted 9V battery to hot-wire a negative voltage on pin 8, I get the exact same behavior above - except the fan does come on. I get a backlight on but never a signal. The processor is clocking data/addresses and appears to boot from floppy just fine.

So if anyone has a working machine that can help, I would love you long-time!
 
I started a new job after my last post (June of last year) and have been in the process of moving ever since (started the job in August, but it did not come with a relocation package and it took months to pack, move and now I am in the process of unpacking). My 1400HD's (one is the one that boots and the other is a 'donor' machine that does not, but is otherwise is complete) were never packed away and came with me, when I moved. Even though I have them, I won't be able to work on them for some time, since I'm still setting up my den and trying to get unpacked, while working from home.

It looks like I'm only 4 hrs or so away from you eeguru, but I assume that you are currently under the same travel restrictions that we have here. I would loan you the working one that I have so that you could sort out the issues with the power supplies and maybe help all of us to get our other 1400HD's running again. I will not ship the working one that I have, but feel free to PM me if there is anything you would like me to examine, perform, or take pictures of and I will try to get it done.
 
Hi.
I just godt a 1400FD which was completelky dead and nothinmg happened when turning the power on. I thought it would be an easy fix - a blown fuse or a short on the PSU... Boy was I wrong...

I have tested the rails and found this link, that describes the pin out on the PSU and daughter board, just posting it here since the link in the previous post are all dead.
https://www.lo-tech.co.uk/wiki/Tandy_1400FD_Pinouts

I have replaced all caps on both the PSU board* and the little daughter board and replaced the F1 fuse(resistor, 10 ohms) - but to no avail. Almost all caps had leaked. But at least it godt the PSU somewhat going. It now exhibits the following symptoms:
  1. When connecting the 12 volt plug, 1 click from the PSU board
  2. When switching the power switch on on the PSU board, rapid and continuous clicking from the PSU board, LEDs constantly on, screen backlight flickers in sync with the clicking. No cursor on screen, no seek on drives, no beeps from speaker.
If I power on the PSU board when its not connected to the main board, I get only a few clicks, and in this configuration I have measured the voltages and I get exactly what the pin out on the lo-tech link describes execpt for: 1-WHITE = 0v, 3-BROWN = 0v, 4-GREY = 4,5v.

I have measured power connector on the main board for shorts, and there does not seem to be a dead short but does anybody have an idea as to how much resistance would be acceptable or what values to expect?

It seems that the PSU board is shutting of due to something drawing too much/little current?

I did test with the floppies connected, no difference.

This is one very tricky PSU - any help or input is much appreciated. THANK YOU!

Sincerely,
Jay

*ColorComputerStore, if you still need the values on thos caps, let me know and I will look at my board and post i here. J
 
Update - the computer works (posts)!

Turns out the fault was not on the PSU board, but with the user... :)

I had replaced F1, which I thought was a normal 10 ohms resistor that was just used as a fuse, so I just soldered a new one in. But looking at the BOM List posted in this thread I noticed that it was in fact a 3.15 amp fuse, which woukld indicate a significantly higher power draw than what a normal resistor can handle. So I removed the new resistor and soldered in a 3 amp car fuse, and now the clicking is gone and all voltages are being generated. However, the un regulated 5 volts, that goes directly to the main board via the grey and pink wire was low, only 3.3 v. So I tried to adjust it using VR1(next to the DC barrel plug), but maybe that VR is bad, since it is extreme hard to adjust, it jumps from 3.3 v to 5.5 and sometimes even higher. So I need to watch that.
I managed to get it stable at arround 5.3 v which I guess is ok for now. I will look for a replacement. Also the big coil that drives the LCD screen is giving off a high pitched hum, so I guess that will need replacing in a not so distant future.

So now I will make a 720k boot disk and try to boot the thing. If it boots I will get a lo-tech XT-CF card for it.


Sincerely
Jay
 
Very impressed with your post, all the work that you put in and with your results so far. Nice detective work in sorting out the fusing issue!
I look forward to hearing more about your 1400FD.
 
Juror22
Click to expand...
Thanx. I replaced the VR1 which was indeed bad, with a more ‘modern’ multiturn trimpot and that fixed the issue with the 5v direct rail.

But… just as I thought everything was golden and I was making the boot disk, the screen went dead. Backlight stayed on but the text disapeared and the contrast seemed to go to 100 %.

I checked the rails, and sure enough all of a sudden the minus 22v rail had dropped (or risen?) to minus 17,5v. I unplugged the screen and powered it on again, and now the minus 22v was back. So I initially thought the screen might be drawing too much current for the regulator to keep up, but the screen seemed fine, no leaked caps inside, actually looked brand new. So I started poking arround on the psu board again with a multimeter trying to trace back the minus 22v line. And it more or less is fed to the screen directly from the transformer on the psu board through a diode a transistor and a single cap. I held put a 10k resistor over all of the transformers points and that seemed to get the 22v working again(or actually its minus 24v being regulated down (up) to minus 22v via a 7922 just before leaving the board.

So - the machine works for now… but I dont really think it is fixed - maybe the coil in the transformer is just bad and is momentarily just a little less shorted inside now.

But I have to say, this machine is SUPER NICE with its design and form factor, but the internal psu is simply way too confusing, complicated and foremost unstable so I dont think this machine is a good choice for a beginner retro tinkerer like me.


Hope my experiences may help others with the same issues on their Tandy 1400FD/HD


Sincerely,

Jay
 
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