Woo hoo - Collection Completed!! TRS-80 inbound

[Tomswork]

Have you tried removing the 4116 dram the memory needs all the voltages inculding minus 1e one on the diodes is bad for shorting to ground as well


Tom

[rubber_jonnie]

Yep, all the RAM, CPU, ROMs and anything else socketed has been removed. Short is still present.

The CR1 diode has been lifted at one end too.

[Tomswork]

Hmm maybe the ttl around the video circuit it gets fried by the fun din connector problem


Tom

[rubber_jonnie]

Hmm maybe the ttl around the video circuit it gets fried by the fun din connector problem


Tom

Definitely worth a look there I think, cheers.

[rubber_jonnie]

Looking at the schematic just now and I think it would have to be a pretty specific set of circumstances for you to do any damage by plugging the power into the cassette port.

The video port, as per @Tomswork's suggestion is another matter. It could easily damage the video circuit and cause a number of transistors and a couple of ICs to go to short.

That is definitely my next port of call.

[rubber_jonnie]

Looking at the video port, I've removed C2 and C7 and I seem to be in a position whereby I no longer have a dead short between 5v & GND :)

I hooked up directly to 5v and powered on with a bench PSU and I am getting approximately 5v at the rest of the ICs.

I also did note when having a cleanup of the board, a solder 'splat' appeared from nowhere, and this may also be related, but not 100% sure.

So, I need to make a new keyboard cable, as the original was badly corroded and has started to fail, so I can see if I'm also getting 5v over there, and hopefully the parts I ordered will have arrived.

As for C2 and C7, I'm pretty sure I have spares enough for those.

[stephen_usher]

Tried powering the 12V section using the bench power supply to see if it's generating the +5V and -5V?

Actually, maybe the short was killing the whole PSU section. Maybe try a normal power-up?

[Tomswork]

Excellent time for the power brick see if all the voltages show up at the ram socket. Do you have the monitor for is or you going to go into a smart connector or composit

Ton

[rubber_jonnie]

Tried powering the 12V section using the bench power supply to see if it's generating the +5V and -5V?

Actually, maybe the short was killing the whole PSU section. Maybe try a normal power-up?

No, as I removed both the VRMs as part of the troubleshooting and the power transistor, new parts are due in hopefully tomorrow so I will try then. If I get time I may try hooking up two bench PSUs and the main PSU (for -5v) and see whats going where whilst I have all the RAM removed.

-5v works just fine though, it was the only thing that did, but it is a very simple circuit.

[rubber_jonnie]

Excellent time for the power brick see if all the voltages show up at the ram socket. Do you have the monitor for is or you going to go into a smart connector or composit

Ton

See my reply to Stephen regarding the voltages, as for video output I'll be hooking it up to composite on my bench VDU.