Some TI-99/4a questions

[bluejay]

So, I have this beige TI-99/4a. It powers up but black screens (at least, that’s what happened when I tried to use a commodore video cable; but i received it as non-working anyways) I took it apart (TI did a damn good job on designing an annoying RF shield) and tested the power supply board. LED lights up, but the +5V output seems to be outputting +12V. I connected the power supply to the computer and powered it up again; I immediately noticed a loud buzzing noise, which seemed to be originating from the TIP31A transistor. It was also EXTREMELY hot. It was neither this hot or this loud when I powered it up without the power supply connected to the motherboard. Anyhoo, I got -4v from VBB, +4v on VCC, and +12v on VDD on the TMS9900. +3V on both RAM chips. Assuming these operate on +5v, it seems very low, but not so low that it should keep the computer from functioning. Does this mean the transistor is working fine? I have not yet found a pinout diagram for the TMS9901 so I was unable to test that.

 

I’m stumped on what might be wrong here. What might be wrong with this?

[HOME AUTOMATION]

As I recall, some of the TI mainboards have the 5v and 12v markings reversed! Perhaps this is a source of some confusion here.

I would be certain that the power supply voltages are correct ...before connecting it to the mainboard! A wrongly connected P/S could cause similar symptoms as would a short on the mainboard. Most of the components on an original 4A board can handle the kind of over-voltages that its P/S might produce, without causing any permanent damage, with the exception of the Sound chip(hybrid).

[bluejay]

Both 5 volt and 12 volt outputs were outputting 12 volts. However, then how the hell did all the VCCs end up with <5 volts?

[HOME AUTOMATION]

There are several possibilities...

 

Perhaps Q3 has become internally shorted or partially shorted to its other pins resulting in a resistive path to the rectifier bridge's +output. under load from the main board the voltage may drop across this resistance.

 

Another possibility is that the 7812's output is somehow getting on the +5 volt output. Perhaps through U3. The 7812's voltage would begin to drop when loaded beyond its one amp output drive limit.

 

I would make certain that the video/audio outputs are connected correctly before continuing, in order to avoid causing damage or perhaps, further damage.

[RickyDean]

This had been mentioned before, but some of the cables, you can purchase from ebay, that are meant to carry the video signal, via the rca connectors, are not wired the same. If you purchase one (I have 2, purchased many years ago) and the video does not work when connecting the yellow wire, you may try to test for a video signal on the white wire, that is how my 2 are wired. Cheap junk. Your symptoms seem to match what happened when I tried to connect the yellow rca male to yellow rca female. Figured I'd rewire it someday, but life happens.

Edited July 2, 2021 by RickyDean
additional content.

[AwkwardPotato]

If you're getting 4V on VCC on the 9900, that suggests there was some error when you measured voltages before that, since the 9900's VCC is connected directly to the 5V rail. What method did you use to probe voltages on the power supply/motherboard? There shouldn't be a significant difference between the VCC of the 9900 and the VCC of the RAMs.

 

4.7V to 5.2V is an acceptable range for the 5V output from the power supply. Anything lower than 4.7V may start causing instability.

[bluejay]

On 7/2/2021 at 9:24 PM, RickyDean said:

This had been mentioned before, but some of the cables, you can purchase from ebay, that are meant to carry the video signal, via the rca connectors, are not wired the same. If you purchase one (I have 2, purchased many years ago) and the video does not work when connecting the yellow wire, you may try to test for a video signal on the white wire, that is how my 2 are wired. Cheap junk. Your symptoms seem to match what happened when I tried to connect the yellow rca male to yellow rca female. Figured I'd rewire it someday, but life happens.

Yeah, I’m aware that my cable is wrongly color coded. I tried plugging in every plug into the video input, but nothing worked. 
 

 

On 7/3/2021 at 8:57 AM, AwkwardPotato said:

If you're getting 4V on VCC on the 9900, that suggests there was some error when you measured voltages before that, since the 9900's VCC is connected directly to the 5V rail. What method did you use to probe voltages on the power supply/motherboard? There shouldn't be a significant difference between the VCC of the 9900 and the VCC of the RAMs.

 

4.7V to 5.2V is an acceptable range for the 5V output from the power supply. Anything lower than 4.7V may start causing instability.

I used a digital multimeter, between GND and each of the power outputs on the power supply. I’m not sure what I could have done wrong, but I can try measuring it again. 
 

I think VCC on the chips is lower than 4.7V, because it flickers between 3 and 4 until finally settling down on 4.