LO Squegging?

[N4LTA]

I am testing a reproduction of the 1942 regenerative IF superhet today and am having trouble with the LO.

The output looks like it is squegging  - I get a 3.3 Mhz signal that looks like it has 14 Mhz mixed with it.  The literature says to reduce feedback to stop the squegging. I have removed about 4 of the 9 turns on the tickler winding, but all that is happening is the oscillator output has droppd to 2 volts from 15 volts.

The output waveform looks like a mixer output.

The circuit is very simple using a 6J5 triode with a two winding plug in coil. The coild is tapped for a bansspread cap and the main tune is across the whole coil.

Any ideas.

Pat
N4LTA

[Tom WA3KLR]

Try lowering the B+ voltage.

[WQ9E]

Pat,

Does the "squegging" frequency follow the tuning of the main oscillator?  It sounds like the 4'th harmonic may be quite intense.

Rodger WQ9E

[KM1H]

Try another tube and also reverse the coil secondary leads for starters.

Im looking at the circuit now and it sure is basic. I assume that C-13 and RFC are installed and correct?

Carl
KM1H

[N4LTA]

Yes - I used a 2.5 Mh choke and the B+ is 105 Volts from the regulator. I tried a second tube also. The bypass cap is now a good .01uF ceramic - it was a .005 as specified - I changed it to make sure that wasn't the problem

Yes - I think it is a 4 th harmonic problem. All the leads are short #16 tinned wire and the frequency is much more stable than I expected. Stable with the 4 th harmonic or whatever it is?

I have not reversed the tickler yet.

It is about to give me a headache. It is so simple - there is not much that can be wrong?

I am having thoughts of sticking in a 6AG7 pentode oscillator as giving up. I am build these with a friend who has never built a superhet receiver and I agreed to build twins and help him get his going.

[KM1H]

Metal or glass 6J5? Is pin 1 grounded?

I meant the oscillator link turns.

You might try a 6C5.

Off to bed, I need my beauty rest.

Carl
KM1H

[N4LTA]

FYI - This is what I am getting off the plate with a 47 pf blocking cap to the scope probe. The Bandset cap is move from full mesh to open for the two photos.

The tube is metal and pin 1 is grounded. The BFO uses a 6C5 and works fine. It was suggested that I add a series 27 ohm resistor in the grid. The grid lead is longer than I like . I layed the radio out per the handbook though.








Pat
N4LTA

[KM1H]

That looks like a classic case of parasitics Pat. Plus a bit of 2nd harmonic.

The 27 Ohm resistor is something to try. If that doesnt work Id wind a mixer coil without the regen winding and remove the pot, etc from the circuit. Get the radio to work as a normal receiver first before getting fancy.

The old books go thru taming parasitics and feedback in mixers.  Look at a well known stable mixer for ideas. I think the S-40 series has a 6SA7.

Im doing tower work today so wont be replying much until dark or rain.

Carl
KM1H

[N4LTA]

I havent got to the mixer yet. It is the last thing to build. I wanted to getthe LO working first. A 6C5 is a bit better. The 27 ohm made no difference. Lowering my finger into the lo coil and holding it very lightly eliminates the parasitic, so it seems that the whole assembly has a  self resonance at the parasitic frequency.
The parasitic is in the LO - not the mixer stage. I need to clean up the LO before I start with the mixer.

I wonder how many of these were build and the builder never knew there was a problem.

I am thinking of re doing the oscillator to a more modern 6AG7 one?

I had decided to leave out the regen cap in the IF already. I was going to try it as a straight superhet first  in the mixer stage and then add it and see if it improved the receiver.

Be careful on the tower. I need to get my Log Periodic beam up and my 6 meter long boom down. It's hard to find tower help these days. I fell 16 feet late last year and have decided to hang up my belt.

Pat

[WU2D]

A Few ideas...

Try replacing moving your bypass cap to a different spot closer to the plug in coil. Improve your ground at the bypass.

Perhaps the bandspread tap IS producing the second frequency. In other words the bandspread tap winding is getting feedback and is resonating at the higher frequency reflexively with the bandspread cap as a SECOND TANK along with the main or full winding and main cap.

Remove the tap temporarily.

If this stops the issue, insert some resistance in the tap.

73's Mike Wu2D



 

[N4LTA]

Pulled it all out and used a tapped  coil hartley circuit - same caps - rewound a new coil.

First time results below.




I hate it when these mysteries happen. I tried just about everything including removing the Bandspread tap to clean it up?

Thanks everyone for the tips and assistance.

Pat
N4LTA

[KM1H]

Now try the original again. I never cared for that 20's design Armstrong circuit myself; I dont think it likes hi gain tubes. Try it with a 27

Carl
KM1H

[N4LTA]

I have a couple of unused Arturus 227 blue glass tubes.

I think I'll hold of on that project though.

Thanks,,

pat
n4lta