EEVblog 1724 - BWD 603B MiniLab RETRO TEARDOWN

[EEVblog]

Teardown of the classic Australian Made BWD 603B Minilab that was saved from the UTS dumpster.

Schematic: https://www.kevinchant.com/uploads/7/1/0/8/7108231/bwd603b_schematics.pdf
Manual: https://www.kevinchant.com/uploads/7/1/0/8/7108231/bwd_minilab_603b_manual.pdf
BWD History: https://www.radiomuseum.org/dsp_hersteller_detail.cfm?company_id=12570

[langwadt]

@18:55 hard pull-down and wimpy pull-up is how TTL is

[free_electron]

TTL doesn't require totem pole. The inputs of a TTL gate are the emitters of a transistor with its base tied to vcc via a 400-ish ohm resistor.
When you are sending a logic 0 into a TTL gate you are actually pulling current OUT of the input.
That's where the concept of fan-in and fan-out comes from. You need to know how many inputs you will be driving LOW because you are pulling current OUT of them.

[MarginallyStable]

Whats up with the power switch? (Down for on?)

[EEVblog]

TTL doesn't require totem pole. The inputs of a TTL gate are the emitters of a transistor with its base tied to vcc via a 400-ish ohm resistor.

Correct, it doesn't require it, but that's how it's implemented, as a totem pole with the upper resistor dependent upon the series (7400,LS,S)

[Kleinstein]

For an educational instrument the simple resistor + transitor to ground solution makes perfectly sense as it is short circuit proof (at least towards ground). The rise time is not great and there is some delay from the transistor driven to saturation. For the educational market this is not a problem, as it is not about highest performance.