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

frsimen · 18th Feb 2023, 7:14 pm

Hello Mark

That's good to hear. The old IC25 was faulty, so check for the following.

There should be a 12V positive going pulse at TP16, about 500ns wide repeating every 400us, although that will depend on the exact setting of other controls. There should be a waveform of the opposite state at TP9 too, that is negative going 500ns pulses with +12V the rest of the time. If these pulses are present the divider circuitry is working properly. You then need to confirm that TP19 has a broad 12V with a 0.5ms 0V every 3ms. The timings again may vary with the step settings etc. If that pulse is present, the divide by 160/161 circuit will be have a chance of working too. With all this correct, the problem lies in the phase detector.

It's quite possible that the pulse at TP16 is still missing. If that's the case, check for pulses at TP15. If they are missing, replace IC24.

If they are present, check pin 11 of IC6 for pulses. If they are missing, check for pulses on IC6 pin 12 and 0V on IC6 pin 13. If both are correct but there are no pulses at pin 11, replace IC6.

If you do have pulses from IC6 pin 12, check that they are still present at IC 8 pin 12. If they are, check for pulses at TP8. Those will be 10us wide positive going pulses every 300us.

If there are no pulses at TP8, check IC8 pin 13. If nothing there, check IC8 pins 9, 10, 11 and 12 again. They should all have pulses. If they do, and there are none at pin 13, replace IC8.

With pulses at TP8. check that they are still present at pin 10 of IC10. If they are, check pins 15 and 14 of IC10. Both should be at opposite states, that is, pin 15 should be 0V with brief 12V pulses, and pin 14 should be at 12V with brief 0V pulses. If you're not seeing that, either IC10 or Q8 are faulty. You can check Q8 with the diode test method. If it measures good, change IC10. If bad, change Q8 and check for pulses on pins 14/15 again. If they are still missing, change IC10.

The key measurements are the pulses at TP16 and TP9. If they are both correct, the divider chains are working properly.

Report back your findings and we can move on from there.

18th Feb 2023, 7:14 pm	#110
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: Help needed with - Racal Dana 9081 Hello Mark That's good to hear. The old IC25 was faulty, so check for the following. There should be a 12V positive going pulse at TP16, about 500ns wide repeating every 400us, although that will depend on the exact setting of other controls. There should be a waveform of the opposite state at TP9 too, that is negative going 500ns pulses with +12V the rest of the time. If these pulses are present the divider circuitry is working properly. You then need to confirm that TP19 has a broad 12V with a 0.5ms 0V every 3ms. The timings again may vary with the step settings etc. If that pulse is present, the divide by 160/161 circuit will be have a chance of working too. With all this correct, the problem lies in the phase detector. It's quite possible that the pulse at TP16 is still missing. If that's the case, check for pulses at TP15. If they are missing, replace IC24. If they are present, check pin 11 of IC6 for pulses. If they are missing, check for pulses on IC6 pin 12 and 0V on IC6 pin 13. If both are correct but there are no pulses at pin 11, replace IC6. If you do have pulses from IC6 pin 12, check that they are still present at IC 8 pin 12. If they are, check for pulses at TP8. Those will be 10us wide positive going pulses every 300us. If there are no pulses at TP8, check IC8 pin 13. If nothing there, check IC8 pins 9, 10, 11 and 12 again. They should all have pulses. If they do, and there are none at pin 13, replace IC8. With pulses at TP8. check that they are still present at pin 10 of IC10. If they are, check pins 15 and 14 of IC10. Both should be at opposite states, that is, pin 15 should be 0V with brief 12V pulses, and pin 14 should be at 12V with brief 0V pulses. If you're not seeing that, either IC10 or Q8 are faulty. You can check Q8 with the diode test method. If it measures good, change IC10. If bad, change Q8 and check for pulses on pins 14/15 again. If they are still missing, change IC10. The key measurements are the pulses at TP16 and TP9. If they are both correct, the divider chains are working properly. Report back your findings and we can move on from there.