The veritable 8640B generator, a marvel of engineering and still remarkable design today, it has reached an age where repair is often hampered by obsolete parts – sure, you can search for a donor unit or some NOS parts in some far away places, but in other cases, it is more practical to replace the circuitry with some new parts, especially, if the circuit is easily tested and verified.
This applies to the pre-scaler/frequency counter, which is an essential part of the 8640B, at least, if you want to use its internal counter (the 8640B is not PLL controlled).
Thanks to a kind contributor, Bodo, here a brief story of the repair:
(1) Symptom: the 8640B showed irregular frequency counts, completely unrelated to the expected output frequency. First consideration was a defective band switch (which has a cracked Delrin gear), but test with a spectrum analyzer revealed a perfectly good signal. Connecting a 0-10 MHz signal to the external counter input also gave perfectly good counting.
(2) After study of the schematic, the issue could be traced to the RF scaler, which is located in a die-cast cavity. Note that there are various versions of this board, but all feature some ECL logic ICs, with high power consumption.
(3) Further tests showed that the first divider, :2 was defective.
(4) There are several ways to fix this. Here, the complete divider chain was replaced by a U644BS, available in DIP8. This IC is quite common in old TV tuners. There were also several projects in popular electronics magazines of the 90s to use the U664 as a pre-scaler up to 1.3 GHz.
Picture of a random tuner from the web, using a U664B:
With the external input, the 8640B is counting up to 900MHz, sensitivity is better than -25dBm.
Note that the U866BS is self-oscillating (not a problem, because the 8640B oscillator is permanently attached for internal counting).
The modified prescaler board under test:
One side effect is the much lower temperature of the RF scaler (much lower power consumption of the U664BS vs. the old ECL logic).
Finally, this is the schematic, the signal is connected from a-INPUT to b-OUTPUT.