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AwkwardPotato

PSA: Failing TI Power Supplies!

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I don't know if this has been mentioned before, but here goes:

 

Recently I've been working on three TI systems for a fellow forum member (1x 99/4 and 2x 99/4A's). Out of these three machines, two have had damage caused as a result of the internal power supply failing. On one of the 4A's, there was significant ripple on the +12V line, which kept blowing VRAM IC's until I noticed it and replaced the power supply with a spare. On the 4, the +5V line was outputting +10V, causing severe damage that I haven't had time yet to diagnose. Ouch!

 

I'm not sure yet what's causing the power supplies to fail, although I suspect it's the electrolytic capacitors because a.) they're a common failure point in most systems from the same era, b.) these supplies are "coffee cup warmers" and c.) both of the failed supplies had a couple of capacitors where the rubber piece at the end was bulging, the typical failure symptom.

 

I'm going to replace all the electrolytic caps on one of these supplies soon and will post any updates here. :)

Edited by AwkwardPotato
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I’d like to see how this is done. Something for beginners who struggle a little at skills such as the dreaded de-soldier.

 

 

Sent from my iPhone using Tapatalk Pro

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If the +5V supply is too high, check Q1 (TIS93). ISTR having a PSU with an over-voltage +5V line and Q1 was the culprit (it was physically broken).

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Is there a way to mod the power supply to add a protection/fuse circuit so that if a cap does leak or blow it would keep it from damaging the motherboard?

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@Airshack - When I start working on the power supply I'll take pictures to document the repair. Luckily replacing the capacitors doesn't require any fancy desoldering equipment or skills; each capacitor only has two leads, so you can just heat one lead and pull the part out from the other side of the board.

 

@Stuart - Thanks for the tip, I'll check on Q1 when I get home. One thing I found interesting was that Q2 (or whichever one the heatsinked transistor is) looked like it had been replaced in the past (there was a lot of flux residue around the pins).

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Is there a way to mod the power supply to add a protection/fuse circuit so that if a cap does leak or blow it would keep it from damaging the motherboard?

 

This is a pretty interesting idea and I'm definitely going to look into how one could do it later. I guess you could put an appropriately rated fuse on every rail for the power supply, although I don't know how well that would protect the motherboard. To be fair though, once a protection circuit is designed, it might just be easier to replace the capacitors or whatever part(s) are failing because they're cheap and easy to replace.

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"The Zener diode's operation depends on the heavy doping of its p-n junction. The depletion region formed in the diode is very thin (<1 µm) and the electric field is consequently very high (about 500 kV/m) even for a small reverse bias voltage of about 5 V, allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material."

 

O.k. that sounds tough, but the circuit is simple.

 

 

 

post-64523-0-92981200-1553880150.jpg

 

Insert the max. allowable current fuse in-between the TI's P.S. and the regulators v-in.

 

Best to make R very low or bypass.

 

Current induced by over-voltage will not travel through the load, instead traveling through the Zener diode and fuse!

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@Airshack - When I start working on the power supply I'll take pictures to document the repair. Luckily replacing the capacitors doesn't require any fancy desoldering equipment or skills; each capacitor only has two leads, so you can just heat one lead and pull the part out from the other side of the board.

 

That would be cool, along with someone selling a re-capping kit for the TI's power supply. I've had one on the bench for about a year now and have never gotten around to searching for all the caps. Call me lazy, call me busy, but if a kit was available to purchase I'd no longer have an excuse.

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"The Zener diode's operation depends on the heavy doping of its p-n junction. The depletion region formed in the diode is very thin (<1 µm) and the electric field is consequently very high (about 500 kV/m) even for a small reverse bias voltage of about 5 V, allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material."

 

O.k. that sounds tough, but the circuit is simple.

 

 

 

attachicon.gifregulate.JPG

 

Insert the max. allowable current fuse in-between the TI's P.S. and the regulators v-in.

 

Best to make R very low or bypass.

 

Current induced by over-voltage will not travel through the load, instead traveling through the Zener diode and fuse!

 

 

So it may be possible to create a little circuit board with places to solder in Zener diodes (Guessing 3 (+12V, +5V, -5V)) with an input connector (4 pin male (molex?) from the motherboard) and output pigtail cable to female 4 pin (molex?) connector?

 

Assuming that someone with those skills could design the board to be created by PCBway allowing me to solder assemble them, do you think the cost would be worth the value to protect the motherboard in case of PS failure?

 

It seems to me that the installation would be easy for novice users (like me) to install. Having the protection circuit would give me peace of mind. If the PS did blow, it would be easier to replace or repair (recap) than de-soldering chips from the mobo.

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That would be cool, along with someone selling a re-capping kit for the TI's power supply. I've had one on the bench for about a year now and have never gotten around to searching for all the caps. Call me lazy, call me busy, but if a kit was available to purchase I'd no longer have an excuse.

 

Agreed! Having a parts list or kit available for purchase to recap would be awesome!

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Call me lazy, call me busy, but if a kit was available to purchase I'd no longer have an excuse.

 

I'll could look into selling kits later if they turn out to be the issue, but I could also upload a Mouser cart later (BTW, I checked the prices and all the PSU caps + shipping comes out to around $17).

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The best practice when repairing these consoles is to upgrade the old "coffe warmer" supply with red LED to the later switching style supply. I usually add a blue LED and resistor to the later switching supply so I can tell by sight if it's a switching supply in the console. I usually also refresh the electrolytic caps.

 

(FYI, always take notice of any regulator part number in any TI power supply. TI sometimes used a fixed voltage regulator instead of the usual linear series regulator.)

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The best practice when repairing these consoles is to upgrade the old "coffe warmer" supply with red LED to the later switching style supply.

 

 

I wanted to do this with my main console to cool it down a while ago, but unfortunately at the time I couldn't find a source (or at least an affordable one) for the switching/QI supply. I do agree though, getting rid of the primary source of heat in the machine would probably improve reliability a good bit.

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If the +5V supply is too high, check Q1 (TIS93).

 

Turns out this was part of the problem with the 99/4 supply! Not only was it physically broken as you described, but the transistor case was visibly deformed, a chunk had been blown out of it, and there was a small burn mark on the PCB! I don't know how I missed it the first time I looked it over :P

Edited by AwkwardPotato
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so, are capacitors a common source of high voltage levels in powersupplies? My main PEB's 8V is at 13V... which I'm sure is burdening all the cards more than desired...

 

[email protected]

 

No, they can't increase the voltage. If a capacitor's capacitance has fallen over the years then you're likely to have a higher ripple voltage, but it can't increase the overall voltage.

 

Was your 8V supply measured as 13V with no cards in the PEB? The voltage will likely be higher if there is no load on the supply.

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No, they can't increase the voltage. If a capacitor's capacitance has fallen over the years then you're likely to have a higher ripple voltage, but it can't increase the overall voltage.

 

 

Yup, the capacitor has been called the 'shock absorber' for electrical and electronic applications.

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I got the 99/4 & its power supply repaired and sent back to its owner, but I still have the flaky 4A supply. I realized while going through my parts bin that I had some modern switching replacements for the old linear regulators that I intended to use for a Commodore project, but now I'm thinking about swapping them into the stock 4A supply along w/ new caps to try and cool it down a bit.

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