One reason we love old equipments/parts/technologies is they are more "graspable", easier to understand. They are not a magic black box and we can "see" them working. That's why we love relays, flip-dots, and other electromechanical devices so much, despite their ridiculously poor performance or efficiency. This is a bit less so for vacuum tubes, though "microphonics" (?) effect can be observed in some conditions, but for transistors, which are usually metal-canned or epoxy-potted, forget it. You can't "see" a circuit working, unless you add LEDs for examples (extra kudos for Tim with his #LCPU - A CPU in LED-Transistor-Logic (LTL)). But still, this is far from a completely direct observation of the circuit's working.
Then I got a batch of OCP71 on eBay.
They are a variation of the vintage OC71, Germanium PNP in glass housing, with 2 modifications to make them useful as photo-sensitive transistors, or even phototransistors : no black paint, and the filler silicon is transparent (unlike the diffusing and opaque sludge of the standard OC71). This makes any circuit using them sensitive to light, so expect a lot of drift and weird bugs in "normal conditions". Already the OC71s' characteristics are altered when exposed under intense light (the black paint can't block everything, particularly infrared).
But the photo-electric effect is reversible ! This means that with the right equipment, one could observe the junctions' emissions when the circuit is working. The new problem is that it is not visible by the naked eye.
For example, https://www.rp-photonics.com/band_gap.html says that Ge is an indirect bandgap semiconductor with an energy of about 0.67eV at ambient temperature, translated to a 1.84 μm wavelength. To visualise the circuit, one would need a camera that is sensitive to the 2µm-1.8µm band. This is in the medium infrared, larger than 1µm (the near infrared that your TV remove uses) but smaller than the far IR, or thermal infrared, that is used by thermal cameras for example. Germanium is in theory active around 95THz or an equivalent black-body of 1600K. So it will not be enough to remove the IR filter of a CMOS camera, or use a temperature imager...
The cameras for 1.8µ are much less easy to source cheaply. Worse, mid-IR is a band with military interests : PbS at 3.34µm was used as the first "heat seeking" sensor in Sidewinder missiles, for example. And the exotic chemistries make such cameras expensive AND probably suspiciously considered.
I would love to have a small, cheap camera to record images of a circuit made of the transparent OCP71 transistors and make videos, or even live demos (to kids and old kids) but sourcing the old glass-germanium PNPs was only the easiest part.
I need your help to find, source and get such a camera, if it exists. I have found some but they are either "out of band" or very expensive ("industrial grade"). And I don't see the point in buying expensive gear that I'm not going to use a lot or regularly.