Woo hoo - Collection Completed!! TRS-80 inbound

[stephen_usher]

Yeah, it's a worry that the PSU has been plugged into the wrong spot, not sure whose bright idea it was to make all the sockets the same pinout!

Thats just asking for trouble.

Yeah, apparently all three connectors on the back are 5 pin DIN and unlabelled. Add to this the original cables were all the same colour.

[rubber_jonnie]

Some very quick tests and it looks like the 12v circuit is done for, so that's a good start at least.

I'm getting 2.3v from Q6, pins 2,3&10 are reading about 1.3v on Z2 and should be reading 12v, and R13, R18, C11 and C15 should also read 12v but read about 1.3v.

Given the thermonuclear amounts of heat coming from Q6 and the fact that it's only putting out 2.3v, I'll wager it's dead, and given that the 5v supply is driven by the 12V supply, that would explain why there is no 5v supply anywhere on the board.

Time to order a replacement MJE34, or at least see if I have an equivalent in my parts drawers.

[rubber_jonnie]

Yeah, it's a worry that the PSU has been plugged into the wrong spot, not sure whose bright idea it was to make all the sockets the same pinout!

Thats just asking for trouble.

It sure is.

[rubber_jonnie]

Thats just asking for trouble.

Yeah, apparently all three connectors on the back are 5 pin DIN and unlabelled. Add to this the original cables were all the same colour.

They are all indeed 5 pin DIN

They are actually labelled:

PXL_20220826_094300189.jpg (204.11 KiB) Viewed 1810 times

And I suspect that the power/ON labels may have been marked with silver paint at one time, but it would be so easy to plug power into the video or cassette port, because when this plate is installed, it's almost impossible to actually read them as they are so small.

The power is actually next to the power on button, and given this belonged to somebody's father and has been in storage, I am hopeful that he may have been careful about it due to the cost of these things in the 80's.

[rubber_jonnie]

I can't say I'm entirely surprised, as they are distant cousins, but the Dragon 32 has the same keyboard layout as the TRS-80 Model I:


Dragon is at the front:

PXL_20220826_100111555.jpg (341.28 KiB) Viewed 1807 times

And the only real difference is the TRS-80 has the extra numeric keypad.

[Tomswork]

The ribbon cable on the keyboard can be trouble some as well I put a good header wire in with a connector in the middle


Tom

[rubber_jonnie]

The ribbon cable on the keyboard can be trouble some as well I put a good header wire in with a connector in the middle


Tom

I've checked and it does have continuity across all wires, but I'll certainly be looking into it once I get the machine powered up.

[stephen_usher]

I looking forward to the next episode where you investigate the power supply. We needz voltages! Hehe

[rubber_jonnie]

I looking forward to the next episode where you investigate the power supply. We needz voltages! Hehe

The PSU is a rather simple affair providing 21.2V AC and 17.6V DC.

That works just fine, it's Q6 and Z2 (you can see them here, Q6-Power Transistor has the heatsink and Z2-Voltage Regulator is just below C11) and I believe one or both has a fault, as there should be a 12v feed into Z1 (Next to Z2), but its only about a 10th of what's expected, about 1.3v.

PXL_20220827_160353729~2.jpg (410.29 KiB) Viewed 1755 times

All of the input voltages are correct at Q6, but instead of 12V out, it's giving just 2.4V. No 12v means no 5v, but I didn't get the parts today, and now won't get them until Tuesday due to the bank holiday, so I have been working on the Dragon instead.

[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.