TI-58 fault - repairable?

[Pennine_Lad]

Hi

Any insights into whether this TI-58 is repairable appreciated. Completely dead with battery supply, no display. 

Some main readings on images from downloaded service manual. Voltages much lower than spec.

The flat round component on internal PSU board hot to the touch after less than a minute powered up and can smell warm resin. Internal image link courtesy of datamath.org.

Thanks for any insight!

David



datamath.org internal image of TI-58

[Jeff_Birt]

Can you post a picture of this 'flat round' component that is getting hot?

[Pennine_Lad]

(08-23-2023 08:02 PM)Jeff_Birt Wrote:  Can you post a picture of this 'flat round' component that is getting hot?

Yes, it is the component on the PSU sub-board marked with red dot.

[Jeff_Birt]

I cannot find any information about the 'power supply module'. That flat round component looks to be a transformer which causes me to believe this power supply module is a small DC-DC convertor. The transformer getting warm is a sign it is trying to supply more current than it should, likely due to a bad component on the PPCB (a bad IC or perhaps shorted tantalum cap).

There is a note in the service manual that the charger should NOT be plugged in without battery pack present. On many old machines this will cause damage as the battery pack helps regulate the charger voltage. That may have been what has happened in this case.

[teenix]

Everything looks nice and clean, so corrosion is probably not an issue.

If there is another component that gets warm also, that 'may' indicate the faulty part, or frustratingly, just a symptom of the actual fault which could be bad. I would not turn it on again without some sort of servicing.

As you mention all voltages are lower, it may be that the power supply module has failed and is not oscillating properly. This can cause high battery currents through the coil and heat it up. The components around the coil usually fail before the coil, so possibly the transistors or as Jeff pointed out, the capacitors. There are also a couple of diodes there, possibly zener type, which could have failed also.

From the image, the power module looks like a sandwich assembly so I don't know how hard it will be to try to service it. The capacitors and maybe diodes will be marked and easy to get. The transistors may not be so easy if they have part numbering similar to HP.

A bit off topic, but I always wondered why, on circuit boards of this era, the protective coating always has a bubbled effect on the traces.

Edit: I don't know much about TI models, but it appears the power module was outsourced by TI and is why there are no circuits for it. Unless there are traceable numbers for the parts, it might be hard to repair (if it is the faulty part).

cheers

Tony

[Garth Wilson]

My first suspicion would be a shorted capacitor, followed by a bad IC.  I think it's pretty unlikely that a diode or transistor would go bad without being abused.

(08-23-2023 10:25 PM)teenix Wrote:  A bit off topic, but I always wondered why, on circuit boards of this era, the protective coating always has a bubbled effect on the traces.

It was common to put the solder plating on the copper before applying the soldermask; so when things were soldered, solder would run down under the trace.  Today it's normal to do soldermask over bare copper (SMOBC), before solder-plating the pads.

[Pennine_Lad]

Thank you for the suggestions and comments. The blue tantalum capacitor on the mini PSU board was a short, the others OK. Had a modern one of the right value and rating so replaced.
Current no longer very high, voltages on test points in correct range and display now working.
May replace the other tantalums for good measure.

David

[Jeff_Birt]

Glad to hear you got it sorted out.

[teenix]

Nice one, good that it turned out an easy fix.

At times, those little red diodes can sure put a smile on your face.

cheers

Tony