Woo hoo - Collection Completed!! TRS-80 inbound

[rubber_jonnie]

If all the inputs on Q6 are fine but the output wrong would that automatically make Q6 the suspect?

How about driving the +12V with a bench power supply and seeing if the rest of the circuitry works? That's a quick test you can make without the new components and you can measure the current draw at the same time.

Yep, I have already thought I should do that, just haven't got round to it yet :)

Maybe tomorrow :)

[rubber_jonnie]

If all the inputs on Q6 are fine but the output wrong would that automatically make Q6 the suspect?

How about driving the +12V with a bench power supply and seeing if the rest of the circuitry works? That's a quick test you can make without the new components and you can measure the current draw at the same time.

So I tried it, and it didn't work....

The machine seems to need -5v, 5v and 12v to run, though I can't find where the -5v goes to...

I only did a brief test as I wanted to do the D32 mod, so may come back and try again later today.

As it's sunny I'll give the case a wash and put it out to dry, and I should check that there aren't any obvious shorts. The keyboard connector isn't looking too hot, so I'll replace that at some point, but I'll leave it whilst I'm working on it.

[rubber_jonnie]

Case now washed and out in the sun to dry :)

PXL_20220828_111235411.jpg

And here is the serial number for those who are interested:

PXL_20220828_111249363.jpg

[stephen_usher]

If all the inputs on Q6 are fine but the output wrong would that automatically make Q6 the suspect?

How about driving the +12V with a bench power supply and seeing if the rest of the circuitry works? That's a quick test you can make without the new components and you can measure the current draw at the same time.

So I tried it, and it didn't work....

The machine seems to need -5v, 5v and 12v to run, though I can't find where the -5v goes to...

I only did a brief test as I wanted to do the D32 mod, so may come back and try again later today.

As it's sunny I'll give the case a wash and put it out to dry, and I should check that there aren't any obvious shorts. The keyboard connector isn't looking too hot, so I'll replace that at some point, but I'll leave it whilst I'm working on it.

The -5V will be the bias voltage for the 4116 chips.

When you powered the +12V rail did the +5V come up, as that's derived from the +12V?

[rubber_jonnie]

So I tried it, and it didn't work....

The machine seems to need -5v, 5v and 12v to run, though I can't find where the -5v goes to...

I only did a brief test as I wanted to do the D32 mod, so may come back and try again later today.

As it's sunny I'll give the case a wash and put it out to dry, and I should check that there aren't any obvious shorts. The keyboard connector isn't looking too hot, so I'll replace that at some point, but I'll leave it whilst I'm working on it.

The -5V will be the bias voltage for the 4116 chips.

When you powered the +12V rail did the +5V come up, as that's derived from the +12V?

You're correct re the RAM chips, they are not the same as in the diagram I found, but now I have the correct datasheet, I have checked and the RAM is getting -5v, but not 12V on pin 8, it's getting around 8v.

5V is really going nowhere, I see mV on pin 9, and checking further, there seems to be a short from the 5v rail to GND.

Every single 74LS IC buzzes direct to GND on the 5v pin. Comparing to other machines, I'd expect it to buzz and then register some sort of resistance and stop buzzing, but it just buzzes like a dead short.

Having pulled every IC I can that's socketed, the short is still present. Since I have 2 replacement VRMs coming, I may just pull them since it won't hurt, and if the short goes away I'll know it was one of those. If it doesn't I'll socket them and carry on digging.

[stephen_usher]

:dualthumbup:

[rubber_jonnie]

:dualthumbup:

Well, the VRMs aren't shorted, but something the hell is.

On a positive note (Bad electronics joke) the short doesn't seem to extend to the keyboard, but I don't know why since it should be connected to both 5v and GND over there too...

Unless the ribbon has failed on either 5v or GND, which would explain it.

I have eliminated C10, C14 and CR1 by disconnecting one or both sides. CR1 is I think part of a crowbar circuit, so a short there might have made sense, but it's not that either.

Unfortunately, the next bit is in line is the 32 .1uF caps dotted around the board, so I will check those for shorts one at a time. They all look rather odd, as in faded and sort of dusty, so I wouldn't be surprised if one or more has failed.

I I can't find a shorted cap, then I will have to start trying for bad ICs. It's going to be a bit of a trawl.

[rubber_jonnie]

:dualthumbup:

Well, the VRMs aren't shorted, but something the hell is.

On a positive note (Bad electronics joke) the short doesn't seem to extend to the keyboard, but I don't know why since it should be connected to both 5v and GND over there too...

Unless the ribbon has failed on either 5v or GND, which would explain it.

I have eliminated C10, C14 and CR1 by disconnecting one or both sides. CR1 is I think part of a crowbar circuit, so a short there might have made sense, but it's not that either.

Unfortunately, the next bit is in line is the 32 .1uF caps dotted around the board, so I will check those for shorts one at a time. They all look rather odd, as in faded and sort of dusty, so I wouldn't be surprised if one or more has failed.

I I can't find a shorted cap, then I will have to start trying for bad ICs. It's going to be a bit of a trawl.

For now, I've removed the keyboard (I need to replace the ribbon anyway due to corrosion) and lifted one leg of all the caps that the schematics state as being on the 5v rail. I still have a short 5v to GND. Meh!!!

I need to figure out how I can work out where the short is (Obviously I will look for physical bridges using a magnifying glass/Microscope) as there are a lot of ICs it could possibly be, and none of what's left is socketed.

[stephen_usher]

It depends upon how good and accurate your ohmmeter is. The further from the short, the longer the traces and the higher the resistance.

So, you could start measuring the resistance on the +5V and GND pins of chips and find the one with the lowest and start looking around there.

[rubber_jonnie]

It depends upon how good and accurate your ohmmeter is. The further from the short, the longer the traces and the higher the resistance.

So, you could start measuring the resistance on the +5V and GND pins of chips and find the one with the lowest and start looking around there.

Sounds like a reasonable plan to me, good idea.