HRO M 1.7-4.0Mz Coil problem

[GW3OQK Andrew]

Jon, I am quite willing to make my original notes public as a PDF file. If you or someone else could host it on a web site without potential readers having to sign in I'd send it to you. That is, just have a clickable link to where you can read the file.

The inductor values do not need adjusting.
73
Andrew

PS I have found the notes published on this thread
https://www.vintage-radio.net/forum/...d.php?t=111382
https://www.vintage-radio.net/forum/...d.php?t=111382

[Jon_G4MDC]

Hello Andrew, thanks for the notes which have got me thinking on this again.
Nice HRO and PSU!

The ceramic filter mod you added on 2nd IF is another thing I have always meant to try but never got around to. You have me thinking about getting back to that too.

[Jon_G4MDC]

Here is the tinkering about I have been doing.

I measured the capacity vs rotation of a spare HRO Capacitor gang and constructed this to play around with how linear it might be made.
(Seems to open OK in Libre Office Calc as well as Excel)

As I have no idea of the value of the LO coil in a 1.7-4.0 MHz Coil Pack I have just picked a number out of the air. Does anyone have the measured value perhaps?

[GW3OQK Andrew]

Hello Jon
Yes, the HRO capacitor is remarkably straight line-frequency. I'm sorry many other receivers didn't use them but stayed with what appears more like straight line wavelength. I don't highly recommend the ceramic filter mod for it is too narrow for speech.

Given that you provided capacitor values in your spread sheet I just used the electrobunker on line calculator I mentioned for 1.7 to 4.0 MHz, and find that 65 micro Henries produced a good looking linear scale result with a padder of 258 pF.

Good luck, Andrew

[Jon_G4MDC]

That's interesting Andrew. I had hoped that I could find some way to fit the CFM455K with decent termination and enough isolation it would achieve it's stopband spec. (I think that's the SSB one - going by memory only).

I will play with your suggestion of 65uH- it seems a bit high to me but let's see.

[GW3OQK Andrew]

Jon, the figure is too high as you guessed. Referring to my original pencil notes I worked out the coils in the 1.7 to 4 MHz pack tune 1.69 to 4.01.
RF section are 31.2 uH.
The osc section coil is 24.9 uH, tuning 2146 to 4466.
There is no padder for the RF coils and 880 pF in the osc section.

Yes, it would be great to fit one of those ceramic filters. I wonder if one could be fitted in the mixer anode circuit, input from anode to HT and output to the IF transformer primary. The common and case of the filter would be at HT. My ceramic filter was very high impedance wheras the Murata is low as you implied.
73, Andrew

[Jon_G4MDC]

Thanks Andrew, I'm getting a bit uneasy that we're straying off somewhat from the OP's thread here.

I have only just found the fixed capacitor in the oscillator section and that is giving me problems. It measures about 850pF but it's marking is nothing like that.

Anyway my Excel calculation sheet doesn't have it since it was unknown to me at the time.

I'm thinking it might be good to have a thread all about HRO Coils. I searched to see what has gone before but I could only find bits here and there. It should be a sticky! If we don't work out what they all were soon, the evidence will have disappeared.

Here is my oscillator coil (A gen cov only 1.7-4.0 Coil Pack bought NOS in Box in Goldhawk Road, 1990s sometime). For the LO I see 36 Turns in total and the cathode tap at 6+7/8 turns.

The matter of the ceramic filter also deserves a discussion. I'm still thinking on that one. At IF2 you might have enough preceeding gain that a grid resistor might terminate the filter (about 500R IIRC) and the loss of gain would go un-noticed. The IFT would need to be tapped to make a 500R feed - maybe by a capacitive divider or such. It could be fun to play with. Again is this a different thread to start?

[GW3OQK Andrew]

Jon, if the original topic about HRO coil problem is now finished, and Ribbonmics has dropped out, start a new thread about fitting a ceramic filter. I bet somebody has done it.
73, Andrew