I had a Delco '60s car radio that I put five temperature sensitive transistors in. I used it to see if newly hired techs knew what they were doing. Some worked hot, other worked cold, and all five had theirown temperature range. I had one hotshot spend half a day on it before he quit and walked out of the shop. It was funny watching them with a can of freeze mist in one hand, and a heat lamp in the other as they tried to decide which part was bad. A VTVM to measure the collect voltages would have shown them the faults in under a minute.