Beogram 8000: A Fun Evening With the Record Detection Circuit (the Conclusion)

This is a follow up to this post. After I made the post about the failed transistors in the record detection circuit of a Beogram 8000, I received this forwarded input from the owner of the 8000 who has a friend who understands circuits:

“I think he [the Beolover] replaced every transistor he came across until he got to tr16.  Then he measured it, it was OK, so he didn’t replace it.  I think he should replace TR16.  The reason is TR16 operates with 6 ua of base current, maybe his curve tracer is a different current.  Clearly he has the right waveform going into TR16, and noise is still coming out.  With the beta he measured this should not be happening, so something is wrong with the measurement.  This thing where all the transistors wear out is disturbing.  Of course this circuit calls for very high beta in every stage, but it shows us how transient our technology is.  A transistor is only a transistor because of the sandwich of 3 different types of impurities, which we know move around. After 40 years, it seems they move around a lot.  Clearly these parts worked at one time, but they all changed enough to stop working.  It’s rare to see something this old with good enough documentation and people willing to keep it going.”

Point taken. I also wondered why this circuit was still not working properly even with TR16 measuring o.k., and I have no idea what exactly my little transistor tester does in terms of applied voltages etc...when it does its magic.

Only one way to find out: I restored R72 to 1M and put R70 back into the circuit, and then replaced TR16 with a 2N2222 that yielded an hfE of 225 on my transistor tester. It still did not detect the absence of a record. So I rummaged through my parts collection and finally found a new BC547B that yielded an hfE of 430, and put it into the circuit, and that fixed it:

The yellow trace is the measurement point between R72 and R71, and the green trace is HH (collector of TR16). This qualifies as a good logic LOW, and the micro controller apparently thought so, too. It swept the arms across the platter in search of smaller records, and then returned home and turned off, as it should!

So the conclusion is, yes, the circuit can work as designed (indeed, a satisfying result!...;-)

One question remains: Why does TR16 need such a high hfE? Sure, the base current is pretty low due to the 1M resistor. In this particular case, the minimum voltage in the sawtooth at R71/72 is about 6V, i.e. the minimum base current into TR16 should be about 5.3uA. The current across CE should be limited to 5V/33kOhm=151uA if the transistor is fully on. That is only about a factor 28, i.e. a hfE of 225 should be plenty to pull the collector of TR16 down with 5.3uA.

This got me motivated to look a bit deeper into measuring hfE values. It turns out they are strongly collector current dependent. This figure is taken from the BC547B datasheet provided by ONsemi:

So we see that at the low 151uA current that TR16 sees, the current gain is only about 60% of the maximum value at about 20mA. The 2N2222 has a similar curve (see here), and that (together with the strong variation of hfE values in general between individual transistors of the same series) may explain why the 2N2222 was not able to perform properly in this circuit.