Retired Texas TI99/4A engineer needs help please.

[Ian E G7OLT]

Silly question, what happens to U3 pin 12 as the 5V rail collapses?

[boater sam]

Silly question, what happens to U3 pin 12 as the 5V rail collapses?

Not silly at all. I will check, thank you.

[Alistair D]

If L4 was O/C or out of circuit the output current will be supplied by R7(3k3) in series wit D10 and R8 so the max current available will be minimal. After the load is removed C15(470u) will have to recharge via R7 etc to restore the 5V. R7 and C15 give a time constant of 1.5 seconds but that time is only to 0.632 Vmax. Reaching the full 5V will take a good bit longer than this.

Al

[boater sam]

I have washed and checked the board both visually with a magnifier and point to point with a continuity tester, found nothing.

Thanks to all for the valued suggestions, however the problem is still not resolved.

The two  0.56 ohm resistors R8 & R9, measure 0.23 ohms in parallel after allowing for the resistance of my meter leads. I don't have alow reading ohm meter.

The TIP31 transistor is properly connected. When it was OC is when the 100 ohm resistor R6 burnt up and the 5v rail was at 12v.

From turn on it takes 12 seconds to get up to 5.146v, the same time it takes to recover once any load is taken off, tried with a 30 ohm load and it is exactly the same, voltage drops to 9 mV as soon as any load is placed on it.
The 12v supply stays at 11.99v no matter what the 5v supply does.

L4 is a ferrite toroid wound with thick enameled wire and is connected properly.

Looking like I am beaten, hate that, and a buck module will be the only way out.
I will however persevere if I can get any further advice.
Thanks to all again, Sam.

[Refugee]

What resistance do you get from the emitter of the TIP31 transistor lead lead as opposed to the board track. It should be less than one ohm.
Then measure the collector tab of the TIP31 to the smoothing capacitor this should read very low as well.
A photo of each side of the board would be useful too.

[boater sam]

Resistances of the TIP 31 leads to the board are zero. I can find no defect in the soldering or the track work.
Pix attached.

[boater sam]

There is an anomaly on this pcb.
C5 is not fitted, looks like it never was.
The circuit shows it in the -5v regulator feed. In fact it is between pin6 of the TL331 and ground.

Testing it today and I have another catastrophic failure back to where i started. No regulation at all on the +5v, its at 25v and the TIC31 and the UA723 are duff. What state the obsolete TL331 is in I have no way of establishing.

I think that's it for this board, it obviously is beyond my ability to understand it or fix it. Patience with the 36 year old horror is over, I'll gut some space on board to piggy back a buck module on.

Unless anyone has any bright ideas.

Bugger, I hate being beaten.

[Alistair D]

Was the device originally fitted to Q2 a TIP31?

Normally I would expect the switching device to be PNP as in this link https://hackaday.com/2018/02/15/the-micr...regulator/ the schematic in the manual looks odd.

Al

[boater sam]

Yes TIP31 is original device as circuit.

[Nick]

What's the state of Q1?

If that's failed all sorts of things could happen - R6 failing being one of them.  A 2N3906 would be a good replacement.