UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Help needed with

frsimen · 1st Feb 2023, 6:43 pm

The main VCO is being driven to its highest frequency from what you have observed. That could have many different causes, so the problem will take some tracking down.

For the time being, don't worry about the lack of response from the spin wheel and up/down buttons. The 9081 should start up on roughly 384/192/96/48MHz, the exact frequency depending on the channel spacing selected. The lack of output on the lowest frequency range could indicate a fault in the 120MHz loop, but that is at least partially working otherwise the counter wouldn't work.

I suggest you turn your attention to the phase detector board initially. Check that there are no shorts on any of the supply rails, leaving open circuit resistors or chokes on the board. There may still be a faulty tantalum capacitor lurking somewhere. If all the supplies are present, have a look at the various test points on that board for signs of pulse activity.

The output vernier control does seem to get very difficult to turn on these. A small drop of lubricating oil on the metal shaft made a big improvement to mine.

Paula

1st Feb 2023, 6:43 pm	#42
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: Help needed with - Racal Dana 9081 The main VCO is being driven to its highest frequency from what you have observed. That could have many different causes, so the problem will take some tracking down. For the time being, don't worry about the lack of response from the spin wheel and up/down buttons. The 9081 should start up on roughly 384/192/96/48MHz, the exact frequency depending on the channel spacing selected. The lack of output on the lowest frequency range could indicate a fault in the 120MHz loop, but that is at least partially working otherwise the counter wouldn't work. I suggest you turn your attention to the phase detector board initially. Check that there are no shorts on any of the supply rails, leaving open circuit resistors or chokes on the board. There may still be a faulty tantalum capacitor lurking somewhere. If all the supplies are present, have a look at the various test points on that board for signs of pulse activity. The output vernier control does seem to get very difficult to turn on these. A small drop of lubricating oil on the metal shaft made a big improvement to mine. Paula