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

frsimen · 3rd Feb 2023, 12:45 am

Hello Mark,

The 5.2V marked by TP21 is referring to the voltage applied to R2 and R3. There will be some sort of pulse waveform on TP21, not a steady voltage.

The voltage you read on R41 will depend on the setting of the channel spacing switch. Try the switch in different positions and the voltage will jump back up in some positions. I doubt that there is anything wrong with R41.

Your next move should be to measure the voltage across the various tantalum capacitors, eg. C4(should be close to 12V), C22 (should be close to -6V) C16 (could be over 20V) etc. Check also the voltage at the junction of C11 and R33, which is used to reset the divide by N counter. You may see a blip when power is applied but it should drop shortly afterwards. If any of the voltages measure much lower than expected, the associated tantalum capacitor is suspect and may have damaged the feed resistor.

If all the voltages are fine, how you move on will rather depend on what test gear you have. An oscilloscope would be ideal, a logic probe would be better than nothing, although it may not work in the ECL part of the circuit.

Paula

3rd Feb 2023, 12:45 am	#45
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: Help needed with - Racal Dana 9081 Hello Mark, The 5.2V marked by TP21 is referring to the voltage applied to R2 and R3. There will be some sort of pulse waveform on TP21, not a steady voltage. The voltage you read on R41 will depend on the setting of the channel spacing switch. Try the switch in different positions and the voltage will jump back up in some positions. I doubt that there is anything wrong with R41. Your next move should be to measure the voltage across the various tantalum capacitors, eg. C4(should be close to 12V), C22 (should be close to -6V) C16 (could be over 20V) etc. Check also the voltage at the junction of C11 and R33, which is used to reset the divide by N counter. You may see a blip when power is applied but it should drop shortly afterwards. If any of the voltages measure much lower than expected, the associated tantalum capacitor is suspect and may have damaged the feed resistor. If all the voltages are fine, how you move on will rather depend on what test gear you have. An oscilloscope would be ideal, a logic probe would be better than nothing, although it may not work in the ECL part of the circuit. Paula