These 8662A generators keep coming in, seems that after so many years (like, 30 years) of 24/7 service, some little things are coming up. But not really sure if the frequency of failure/repair really increases, because with all these 1000s of parts, most of the units have seen some repair already, and it is just in the ordinary course of things that there can be defects every once in a while.
This unit add issues with the reference sum loop (A6A5), showing up in intervals of about 10-20 MHz. Knowing that a 10/20 MHz reference is used for the ref sum loop assy, this was the first and most likely cause of the defect. So, I substituted the 10/20 MHz signal, but, to no effect. Therefore, the defect is most likely located on the A6A5 assy.
One option would be to switch the assembly with one of my 8662A/8663A units, but these are currently in use and I don’t want to pull out assemblies that are difficult to fix and need adjustments, etc.
So, next, checked the pre-tune circuit, and, issue found. TP14 is at a constant -2 V, rather than variable voltage as per requirement given in the service manual. Digital input is OK – maybe the fault is in the semiconductors – one transistor, and 4 FETs switching a current source.
With spares at hand, these suspicious parts were quickly replaced. Effect: no effect. Big mystery.
Now, connected an external resistor, to measure the pretune DAC (4 FETs), and, it works! Even more mysterious.
Finally, also checked the traces and the resistors, and found the ‘2K’ Trimpot to have about 40k!! With the wiper contact having some effect, but not much.
Desoldering the Trimpot – all seems to be OK with it. Maybe some kind of aged solder joint? Whatever it was, the Trimpot is now fixed, the A6A5 assy re-adjusted, and the 8662A working again!!