The AM Forum

Hi again folks,

Any experts on superregens out there? On my Twoer, I did the selectivity mod, which involves the substitution of a 7587 nuvistor detector for the original. But it's not working. There is the usual "rush box" noise, so apparently the circuit is oscillating, but no trace of any signals at all, not even birdies, not even my own transmitter 2 feet away, in band or out. I have double checked the connections and everything seems OK.

I knew it was risky to try such an old mod (from a 1973 issue of 73 magazine) but it seemed pretty simple, and it was highly praised in later issues of 73. I'm not a technician, but what seems odd to me about the circuit is that the output from the RF amp is coupled to the grid of the nuvistor through a new 5 pf cap, but there is also a legacy 7.5 pf cap (C24) from the RF amp output to ground. Doesn't that 7.5 pf cap provide an RF path to ground that attenuates signals before they get to the new detector?

Below are a photo of my modded detector (6BS8 socket in the center, nuvistor socket to the left) and the schematic of the mod. For the full article, entitled More Selectivity for the Twoer, see page 4 at
http://qsl.net/nz5n/Twoer%20Mods%20from%2073%20magazine.pdf. For a copy of the Twoer manual, including the schematic, see http://qsl.net/nz5n/Heathkit%20Twoer%20HW-30.pdf.

I have a rudimentary frequency counter on my MFJ 259B but could not get a reading on any VHF frequency.  Also, I am not hearing the receiver anywhere on my FT-847.  The old detector re-transmitted any signal it received all over the VHF frequencies and also generated lots of birdies.

Voltages on the nuvistor are 135v on the anode, 0 on the grid and 0 on the cathode.  Voltage on the screen grid varies as the regeneration control is turned.  I start hearing the "rush box" noise when screen voltage is increased to about 15v.  The "rush box" stops and the speaker goes quiet when the voltmeter probe is touched to the screen, grid, or cathode.  It did not do that with the original circuit.  Also, in addition to the "rush" sound, I'm hearing a high pitch whistle sound in the speaker, it never used to do that.

There are a couple of items in the 73 article that were unclear and required me to make a few assumptions.  The schematic of the mod shows 2 separate variable capacitors on the input, one marked "3-12" and the other C28.  C28 is the tuning control from the original circuit.  The article does not mention the addition of another variable cap, so I assumed that nothing else was needed and that the second variable cap in the schematic was an error.  Or was it?  The article says to remove R10 and C26 but it makes no mention of C27, the 22 pf cap originally attached to the stator of C28.  I assumed C27 also needed to be removed.  Finally, the article says that the 10 pf grid capacitor should be connected from pin 4 of the nuvistor socket "to the loop," not sure if this meant that the capacitor needed to be physically attached to the hairpin loop itself or just to the stator of C28 (to which the loop is attached), which is much closer to the nuvistor socket.  I assumed it was better to attach to the stator instead or running a long lead to the loop itself.

Any thoughts or ideas?  It will be hard to put the original circuit back in if this mod won't work!

73, Bill NZ5N

Thanks and 73, Bill NZ5N

The 3-12 is a 3 pf to 12 pf trimmer-type capacitor.

Is it a second variable capacitor, something different from tuning capacitor C28?  If yes, wonder why it was not mentioned in the article.

These are typical trimmer-type capacitors:

(http://www.legacypartsemporium.com/items/trimcaps.jpg)


For starters, it's a 73 Mag article. Most articles probably were never proofed for accuracy, completeness, or detail.

Or paid for even :o

Yes, 73 had some major flaws, but this mod was highly praised in two later editions of the magazine, so at least some people got it to work and were happy with it.  But it sounds like I may have made a big mistake by attempting this mod, may have turned my working Twoer into a door stop.

I have posted a high resolution photo of the mod, with tags identifying the main components, at:
http://qsl.net/nz5n/Twoer%20Mod%202.png

If anything looks wrong, any advice would be greatly appreciated.

Thanks and 73, Bill

So where's the 3-12 pf trimmer cap?

Well, that's the $6 million question.  There's no trimmer in the original circuit.  The text of the article says nothing about adding one.  Yet the schematic clearly shows 2 variable capacitors, one labeled C28 (the original tuning capacitor) and the other labeled "3-12".  Is this an error in the schematic or does the mod require the addition of a trimmer, even though not mentioned in the article?

The parts list calls C28 a "3 uuf variable tuning capacitor", does not say if 3 uuf is the upper or lower limit.

If a trimmer needs to be added, where should it be placed?  It could easily fit between the C28 stator and a chassis ground near the nuvistor socket.  If it needs to connect directly to the loop, a lengthy lead will be needed.

73, Bill

It is working now.  It did need a second variable capacitor.  I added a trimmer across C28  and it worked immediately.  Should have tried that as soon as I noted the discrepancy between the article and the schematic.

Thanks and 73,
Bill