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

frsimen · 30th Mar 2023, 5:07 pm

Unfortunately, the diode that is needed is an MV1634 which, annoyingly, has significantly higher capacitance than the MV1622 and a significantly lower capacitance than the MV1642. As Chris has said, the other diodes are all well away from the needed capacitance. Thank you for looking, Rob.

I've made some measurements of the capacitance swing available from various diodes (normal rectifiers) and some LEDs that I have here ,using some better test gear than my previous rough and ready measurement. Three 1N4007 diodes in parallel are very close to the same capacitance and capacitance swing as the original diode. I suspect that the Q factor will be lower, but I can't measure that easily. I only tried a couple of LEDs and neither showed any promise for this application.

I tried the fine tuning facility on my 9082. That gives a total swing at 300MHz of 300kHz. Scale that down to 300kHz, that would represent a swing of 300HZ, very much less than was being measured on your 9081.

Before fitting a replacement diode, try a 22pF capacitor in place of D3. The fine frequency control won't work but it will be useful to see if the rapidly changing reading on the counter stops. If it doesn't, there will be some more fault finding to do to sort that out.

Paula

30th Mar 2023, 5:07 pm	#284
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: Help needed with - Racal Dana 9081 Unfortunately, the diode that is needed is an MV1634 which, annoyingly, has significantly higher capacitance than the MV1622 and a significantly lower capacitance than the MV1642. As Chris has said, the other diodes are all well away from the needed capacitance. Thank you for looking, Rob. I've made some measurements of the capacitance swing available from various diodes (normal rectifiers) and some LEDs that I have here ,using some better test gear than my previous rough and ready measurement. Three 1N4007 diodes in parallel are very close to the same capacitance and capacitance swing as the original diode. I suspect that the Q factor will be lower, but I can't measure that easily. I only tried a couple of LEDs and neither showed any promise for this application. I tried the fine tuning facility on my 9082. That gives a total swing at 300MHz of 300kHz. Scale that down to 300kHz, that would represent a swing of 300HZ, very much less than was being measured on your 9081. Before fitting a replacement diode, try a 22pF capacitor in place of D3. The fine frequency control won't work but it will be useful to see if the rapidly changing reading on the counter stops. If it doesn't, there will be some more fault finding to do to sort that out. Paula