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

frsimen · 29th Mar 2023, 9:49 pm

Hello Mark,

Thanks firstly to Cruisin Marine for the link to the datasheet for the MV1634.

Wire ended varicap diodes are getting a bit thin on the ground these days. Cricklewood Electronics stock two which are fairly similar, but not identical to the original D3. The BB109G and the BB139. They are quite expensive, though.

The capacitance needed is quite high, so there is another approach that you could try to see if it works. The 1N400x diodes will behave as varicap diodes quite successfully. The capacitance is in the right region too, so if you have a 1N4004, 5, 6 or 7, you could try one of those. Another option is to use a regular LED, although I've not measured the capacitance of one, they have been used successfully in VFOs. It neither work, then you'll need the proper varicap diode.

There is one other capacitor which could cause jittery tuning, C29, which is used to smooth out the supply to the VCXO. Worth changing if you haven't already done so.

If it were mine, I'd try the cheap 1N400x or LED approach first, before splashing out on the more expensive parts.

Paula

29th Mar 2023, 9:49 pm	#279
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: Help needed with - Racal Dana 9081 Hello Mark, Thanks firstly to Cruisin Marine for the link to the datasheet for the MV1634. Wire ended varicap diodes are getting a bit thin on the ground these days. Cricklewood Electronics stock two which are fairly similar, but not identical to the original D3. The BB109G and the BB139. They are quite expensive, though. The capacitance needed is quite high, so there is another approach that you could try to see if it works. The 1N400x diodes will behave as varicap diodes quite successfully. The capacitance is in the right region too, so if you have a 1N4004, 5, 6 or 7, you could try one of those. Another option is to use a regular LED, although I've not measured the capacitance of one, they have been used successfully in VFOs. It neither work, then you'll need the proper varicap diode. There is one other capacitor which could cause jittery tuning, C29, which is used to smooth out the supply to the VCXO. Worth changing if you haven't already done so. If it were mine, I'd try the cheap 1N400x or LED approach first, before splashing out on the more expensive parts. Paula