UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Standard Beam Approach

jamesinnewcastl · 10th Mar 2023, 10:09 am

Hi All,

Finally, testing on the main receiver has started. I'm not a radio person so a lot of what is happening is a first for me! Attached is a picture of the rig I'm using.

The AGC circuit means I have a sort of floating earth and I realise that the 3-pack 2uF capacitor in the set that is used in the AGC should have a common positive rather than the common negative which I assumed when I restuffed it! Still not sure I had to break into the nice job I made of it and change to non-electrolytics (imported from America), just to be on the safe side. Repaint, new label...

After more reading I realise that the Local Oscillator needs to be operating all the time so putting just the VR107 in the set I turned it on and hoped. No burning, no smoke, no oscillation.

Lesson 1. Make sure you have added that last wire...

Lesson 2. You won't find any oscillation if you keep putting the scope probe on points that have a decoupling capacitor. Apparently too, the main anode doesn't oscillate even if the lower grids are oscillating. Eventually, after grindingly checking every single component and all the connections, I find a point that is oscillating! Hurrah! Learned a little about Hartley Oscillators though.

Lesson 3. The LO is going at 22.2MHz, but the IF is supposed to be 7MHz! Another couple of hours working out the resonant frequency and it looks like I am missing lots of pFs. I don't know about you but I find that thinking away from the kit, usually in the bathroom, sharpens the mind. Bing! The receiver works on 28MHz (ish) and the LO was at 22.2MHz. Subtracting one from the other and volia, about 7MHz! So I didn't have the problem I thought I had. I have a feeling that the kit will work perfectly and I will be the one being constantly corrected!

The next step is to add in and test the RF front end and put in a signal. Really looking forward to that!

Cheers
James

10th Mar 2023, 10:09 am	#66
jamesinnewcastl Heptode Join Date: Jul 2009 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 723	Re: Standard Beam Approach - Rebuild update Hi All, Finally, testing on the main receiver has started. I'm not a radio person so a lot of what is happening is a first for me! Attached is a picture of the rig I'm using. The AGC circuit means I have a sort of floating earth and I realise that the 3-pack 2uF capacitor in the set that is used in the AGC should have a common positive rather than the common negative which I assumed when I restuffed it! Still not sure I had to break into the nice job I made of it and change to non-electrolytics (imported from America), just to be on the safe side. Repaint, new label... After more reading I realise that the Local Oscillator needs to be operating all the time so putting just the VR107 in the set I turned it on and hoped. No burning, no smoke, no oscillation. Lesson 1. Make sure you have added that last wire... Lesson 2. You won't find any oscillation if you keep putting the scope probe on points that have a decoupling capacitor. Apparently too, the main anode doesn't oscillate even if the lower grids are oscillating. Eventually, after grindingly checking every single component and all the connections, I find a point that is oscillating! Hurrah! Learned a little about Hartley Oscillators though. Lesson 3. The LO is going at 22.2MHz, but the IF is supposed to be 7MHz! Another couple of hours working out the resonant frequency and it looks like I am missing lots of pFs. I don't know about you but I find that thinking away from the kit, usually in the bathroom, sharpens the mind. Bing! The receiver works on 28MHz (ish) and the LO was at 22.2MHz. Subtracting one from the other and volia, about 7MHz! So I didn't have the problem I thought I had. I have a feeling that the kit will work perfectly and I will be the one being constantly corrected! The next step is to add in and test the RF front end and put in a signal. Really looking forward to that! Cheers James Attached Thumbnails Last edited by jamesinnewcastl; 10th Mar 2023 at 10:24 am.