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Hooked a 120V Tandy 1000 EX up to 230V, fuse blew out. Further damage?

digger

digger

Experienced Member
Joined
Jul 9, 2008
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395
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Amsterdam, the Netherlands
Hi everyone,

Before you judge me, please read the entire post.

So I finally received my Tandy 1000 EX that I ordered on-line from someone in Greece. I was excited to try it out, but the plug was missing from the power cord. At first that gave me some pause about the possibility that this might be a North American unit, since Tandy 1000 machines weren't that common in Europe. However, I saw a "warranty void if opened" label on the bottom that contained notices in 4 European languages, including German and even Dutch(!). So it must certainly be a unit intended for the European market, or so I thought.

So I spliced a grounded Schuko plug to the severed power cable, noticing that the previous owner had apparently connected an ungrounded plug to it, since the yellow/green ground wire was cut back lower with no copper exposed.

Anyway, I powered the machine up, it sprung to life, the PSU fan was spinning and the power LED lit up... For a few seconds. Then I saw a brief flash on the back of the unit towards the PSU and the system went dead.

I opened up the system, and the left-side of the cover that covered the power supply had a sticker with a warning to wait 10 seconds after powering off the system before accessing the PSU. That warning was also shown in the same 4 languages (English, French, German and Dutch). Then I removed that cover and took a look at the PCB of the power supply. In the corner, towards the back of the unit, where the power cable plugs into the PCB, I noticed a printed text on the PCB that said "120VAC". There was my first "uh oh", moment. I removed the fuse and took a closer look at it. It looked dirty, so it looks like it had indeed blown out.

Then, I finally took a closer look at the label on the back of the unit. And on the top of the label it indeed said "VOLT ... 120 V FREQ ... 60 Hz AMP ... 0.4 A"

D'oh! This was such a classic embarrassing n00b mistake. :( But I was really lulled into a false sense of security by those multilingual labels. Why on Earth would a North American computer have warranty notices and warning labels in multiple languages, two of which are only spoken in Europe? (Well, they speak Dutch in Suriname as well, but I doubt that would have been a large market for Tandy.)

So my question for people more knowledgeable (and competent) than me is this: how likely is it that the fuse completely protected the unit? The unit was running and being powered by 230V for at least a few seconds before the fuse blew out. If a PSU rated for 120V 60Hz gets 230V 50Hz, doesn't that also result in roughly twice the DC power that it feeds into the circuitry? So 5V becomes 10V and 12V becomes 24V? Or are there voltage regulators and such that would limit the DC voltage regardless? This is a machine from 1986.

I really hope it's only the fuse that got damaged, or at most the PSU. Tandy 1000s are rare and valuable these days! I'm glad I found this one at a somewhat reasonable price, and from a European seller no less. The PSU looks fine by the way. No bulging caps or anything.

Thanks for any advice or other information you can provide. And feel free to chew me out, or to laugh at my expense. :oops:
 
Check the blown fuse to see if it was the proper amperage. If so, it most likely did its job. Let your nose do some troubleshooting and sniff around the PS for really bad odors and if it smells bad, something else may have fried. If the "sniff" test is negative, then replace the fuse and then plug the unit into 110 VAC and see if it comes up. If the fuse pops again then there is some deep troubleshooting that needs to be done.
 
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Replace the fuse and power up the supply with it disconnected from the motherboard and drive. If it doesn't blow the fuse again (or it doesn't release magic smoke) check for good voltages. If the voltages are good then check the other components for damage.

Pull the motherboard (yes, I know EX systems are a pain in the butt to do this with those silly metal clips) and check for damage on both sides of the board. LOL, I use the "sniff" test as well when I see nothing physically wrong. Do the same thing with the 5.25" drive. Look for any damage.

Replace the motherboard and only fire it up (no drive). If it works, then disconnected the motherboard and only hook the drive up. If the drive retains its magic smoke then try both at the same time.

I once had an HX come to me that was surged by a lightning strike through the mains. I replaced the fuse and AC plug, did the checks above, and it worked. Those little Tandys are pretty resilient.
 
The multilanguage stickers are stock items, regardless of where the computer was sold. It avoids problems such as not having the warning in French when selling in Canada (pretty sure that bilingual labels are required by law). Why does my HP Vectra have a sticker saying "Made in France" but not "Fabriqué en France"? Or my tape drive say "Made in Norway", but not "Laget i Norge"? Probably because US regulations say that the country of origin must be stated in English.
 
digger said:
So my question for people more knowledgeable (and competent) than me is this: how likely is it that the fuse completely protected the unit? The unit was running and being powered by 230V for at least a few seconds before the fuse blew out. If a PSU rated for 120V 60Hz gets 230V 50Hz, doesn't that also result in roughly twice the DC power that it feeds into the circuitry? So 5V becomes 10V and 12V becomes 24V? Or are there voltage regulators and such that would limit the DC voltage regardless?
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On a switching PSU, the output voltages do not rely on the input voltage. So you can assume that the system itself and the secondary side of the PSU are still fine. However, it is VERY unlikely that only the fuse has blown. The fuse blows on current, not voltage. There is most likely a MOSFET or something that melted and caused a short, which then blow the fuse. There is certainly damage on the primary side.
 
On the bright side it'd probably be pretty trivial to replace the Tandy PSU with something like one of those picoATX power supplies, if you have any issues sourcing an exact replacement. It's only 28 watts and it doesn't need a -5v feed, which is missing from most modern PSUs.
 
You might be lucky just a blown fuse and perhaps a RIFA filter capacitor across the input that shorted out with the over voltage. But you should see if there's a transformer with a 230 volt tap on the power supply. Most likely not if Radio Shack didn't sell that computer directly to the Euro market and your Greek seller imported it.
 
Timo W. said:
On a switching PSU, the output voltages do not rely on the input voltage. So you can assume that the system itself and the secondary side of the PSU are still fine. However, it is VERY unlikely that only the fuse has blown. The fuse blows on current, not voltage. There is most likely a MOSFET or something that melted and caused a short, which then blow the fuse. There is certainly damage on the primary side.
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That's somewhat reassuring. You're right about the fuse now that I think about it, since it was clearly rated for max 250V. (It said so both on the fuse itself and on the PCB near the fuse holder.) So indeed the 230V alone would not have blown it out. Thanks! Within the context of this hardware, is the MOSFET the same as a voltage regulator? Sorry if I'm coming off as a total novice here. I studied electronic engineering for two years before transferring and switching majors to something more software-oriented. Also, that was decades ago, so please bear with me. :mrgreen:

Eudimorphodon said:
On the bright side it'd probably be pretty trivial to replace the Tandy PSU with something like one of those picoATX power supplies, if you have any issues sourcing an exact replacement. It's only 28 watts and it doesn't need a -5v feed, which is missing from most modern PSUs.
Click to expand...

Yeah, I definitely prefer a solution like that over using a 230V-to-120V transformer. It would be more efficient, cooler, more quiet and with cleaner power being fed into the machine too. I inquired in the Tandy 1000EX PSU issue topic about that ATX adapter board that tgotr was designing. That seems just what I need. Just bypass the original PSU altogether. And it looks like it's a completely reversible mod too. Alternatively, I've been thinking about splicing a female ATX connector directly on the wires that connect to the motherboard, but then I would be cutting into a proprietary and perhaps hard to replace original internal cable.

DeltaDon said:
You might be lucky just a blown fuse and perhaps a RIFA filter capacitor across the input that shorted out with the over voltage. But you should see if there's a transformer with a 230 volt tap on the power supply. Most likely not if Radio Shack didn't sell that computer directly to the Euro market and your Greek seller imported it.
Click to expand...

No, at this point it's pretty clear that this thing is a 120V PSU. The text "120VAC" is printed on the corner of the PSU PCB and "120V 60Hz 0.4A" is printed on the back label of the machine as well. I messed up. I should have examined it better first, pure and simple. But from what I've heard from all of you here so far, there's a good chance that the PSU bore the brunt of the damage and the rest of the system is probably fine. Let's hope so.

Thank you all for your information and advice so far!
 
digger said:
Alternatively, I've been thinking about splicing a female ATX connector directly on the wires that connect to the motherboard, but then I would be cutting into a proprietary and perhaps hard to replace original internal cable.
Click to expand...

The plug that goes 0nto the motherboard pins is a standard part (as is the connector at the PSU end), you should be able to buy a compatible one with no trouble if you don't want to cut the existing power supply cable.

The little adapter PCB is "nice", but it shouldn't be hard to just cobble it all together manually. To power-up an ATX power supply you just need to ground the PS_ON line (pin 14?), so that and a ground go to a rocker switch wedged in place of the original's, and then just match up the voltage feeds appropriately. I use a small ATX PSU as my bench/breadboard power supply by just looping pin14 to ground on the connector when I want it on.
 
If the original PSU is toast, why not simply un-solder the cables and connectors and re-solder them to a new PSU?

Maybe I'm missing something here--it wouldn't be the first time.
 
Chuck(G) said:
If the original PSU is toast, why not simply un-solder the cables and connectors and re-solder them to a new PSU?.
Click to expand...

I assume the OP wants it to be reversible if they ever get an OEM replacement PSU board.

The PSU is these things is a discrete PCB (not integrated into its own metal box), and on the one I’ve messed with at least it’s connected to the motherboard by a cable assembly that’s just two of those chunky white plastic multi-pin connectors (I want to say Molex but I think it has a different exact name?) connected together with discrete wires, allowing it to be unplugged from either end. Finding a plug to match the motherboard end should be pretty trivial if the OP doesn’t want to cut the original harness but, yeah, truth be told I’d probably just cut the PSU end off and splice the wires straight to the ATX outputs.
 
You're right. The existing connectors are apparently pretty standard. The model/type nummers for them I could find in the BOM of the Tandy1000EX-HX-ATX project on GitHub: https://github.com/tgotright/Tandy1000EX-HX-ATX

And indeed, a PCB, although pretty neat, wouldn't be necessary. Rigging up a conversion cable would suffice.

Another question, however:

If I were to go with a PicoPSU, wouldn't the lack of -12V cause problems when I upgrade this machine with a serial port? I only really need a serial port because I intend to connect a mouse to it.

Someone on Vogons developed an 8-bit ISA adapter for PS/2 mice. Maybe that card doesn't need -12V. I could then perhaps use one of those ISA-to-Tandy adapters to plug it in. I'll ask the developer of that project.

In the meantime, other suggested solutions for making a mouse work in this computer without the need for the -12V rail are welcome! Thanks again. :)
 
I looked up several Pico-ATX listings and all of them claimed to supply -12v @0.1a, which is a similar rating to the original Tandy supply. What are you looking at that doesn’t have that?
 
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