My Second Radialva Groom Restore

[Radio Fixer]

Maybe he means this one: US Antique Radio Forum

Not dark, least wise was OK when I used to go there

G

[Radio Fixer]

Neat winder David but I could never make it: too much metal bashing and lathe parts for me. Fortunately, not being a Ham I have never had need to wind wave coils. For radios hand traversed ('scramble') winding has always been amply good enough. This sometimes gets the claim levelled at it that turns from one layer may drop down a layer or two and be subjected to more voltage. That is compared to layer winding with paper interleave (often needed with thin wires to keep the new layer 'level'). However modern wire (and I would never use old) has insulation of up to 1000V so scramble winding doesnt have a problem.

Gary

[Geaney]

As I'm getting to the end of the mechanical stuff I started thinking about the electronics side of things. One area that concerns me is alignment. The doc says the IF frequency is 470KHz. While I have the IFT's out I thought I'd hook up the signal gen to their inputs, hang a scope on the outputs and tweak the input freq to see where they resonate. One resonates at just about 400KHz and the other one at about 417KHz.

I'd certainly appreciate any comments on whether this is serious - the fact that they both resonate at different frequencies and that they're about 15% low versus the spec.

I doubt I can do anything with the coils or the slugs but I could replace the mica caps and see if that helps. I don't know if this is worth doing or not hence I need the benefit of the team's experience again.

Cheers, John

[Radio Fixer]

Well John you dont give enough info on how you are doing this.

My method of seeing that the IF's resonate at around the right frequency, out of the chassis, is to feed one tuned circuit across the generator output via a resistor of say 47 to 100K (the higher the better depending how much O/P you can get from the gen and how sensitive the scope is). So the tuned cct has one side earthed and the other to the gen feed via the K resistor. Now I put a scope, with a X10 probe, across the tuned circuit and tune the gen like you did. At about the right IF freq, signal should appear on the scope and peak as you go thru resonance. Keep all leads short with good solid connections.

The X10 probe is essential as it is normally only 10pF in shunt with the tuned circuit, a X1 will be a lot more and put the circuit much more off tune.

Of course you want an accurate way of measuring freq and I assume you have a counter? This can be connected via a T to the gen or as I do using a coaxial changeover switch (between the feed to the IF and the counter). You certainly dont want to place the counter across the tuned circuit as that just adds more capacitance.

So you should get a fairly good idea of where the IF tuned circuit resonates. The IF cap is likely to be about 100pF so with the 10pF of the scope the freq will be slightly lower but as freq is inversly proportional to square root of C it wont be too much out.

Just for interest you can see if shorting the other winding out makes much difference... not done it lately but it shouldnt.

If you want, and have the freq counter, you can measure Q by measuring the freq at resonance and then tuning the gen frequency either side until the waveform amplitude drops by 3dB (to 70%). Note these two frequencies f low (fL) and f high (fH). Q is then f at resonance / (fH - fL). As an example say F resonance is 460 kHz and fH = 465 and fL = 455 then Q is 460 / 10 = 45.

Taking a quick look back at notes, I dont normally get as high a Q as that and 25 to 30 is about the norm. But this is not the same as a Q meter which I dont have and I take it as an indication only rather than an accurate measurement.

Another bit of info from these notes (1 chassis only but I have measured plenty) I got f resonances within 1% and 3% worse case and didnt try tuning them. Close enough to try in the chassis for me.

Hope this helps Gary

[pwdrive]

Hello John, I would second what Gary said as to ascertaining the resonant frequency of the IF transformers.
If after doing those checks you find that they are out of spec and the slugs are unadjustable at the end of the day then one option pending lack of good replacements is to convert the IF's to capacitor tuned instead of slug tuned, that would depend if there is room to fit compression trimmers and a means of making access to adjust them, I have done this in the past with some success but bear in mind the capacitor trimmer screw will more than likely be at HT potential on the primary side and the Q at resonance might not be up to originall spec if the settings of the original slugs were tweeked a long way off.

Lawrence.

[Geaney]

Thanks Guys, OK, I set it up again as outlined by Gary and to my joy, one of the IFT's resonates at pretty much exactly 470KHz. The other one was still off and unlike the good one, each side of the dodgy one resonates at a different frequency.

So, following the guidance from Lawrence I decided to see first of all if I could find fixed caps that would get me closer to the right spot. After much faffing around I found that one side would resonate at 468KHz with a 110pF and the other side would also resonate at 468KHz with 105pF (100+5 in parallel). It's not perfect by any means but I think I'll leave it like that for now and revisit once I get to the set going and can assess it's sensitivity, selectivity, etc. I'd be looking at fractions of pF to get it any closer.

I had the camcorder going while I was messing with it so I'll post a clip and you can see a newbie in action

Major thanks guys. As usual this forum proves to be the best!

Cheers, John

[Radio Fixer]

Why cant you adjust the slugs John?

I have never failed on one yet. If they are stuck, never use force on them they just break, use a solvent first. Some say Lighter Fuel but tiny drops of Acetone will work. Then rock the slug back and forth with a good fitting trimmer tool. Keep at it, it can take time and patience, gently does it.

Once you have one out you can often get the other out even if its slot is broken. Try a tool (you normally have to make something special to be long enough) from the other side as slugs were mostly slotted both ends. If both slug tops are broken then try a solvent for the wax or crud that was used and then a sharp scriber, grind the end to a needle point. Now dig this into an edge and twist the slug, its worked for me.

Damaged slugs can be used. File the end square, with a very fine file and then cut a slot with a junior hacksaw. It has to be very shallow as the material will break. Just make it deep enough that a tool (again make something special if needed) will engage and rotate it in a well cleaned out thread. Once tuned then drop a piece of wax down the hole and melt it with a soldering iron bit. It may need to be a special yet again, that is filed up to be long and thin to get down the hole.

Cheers Gary

[Geaney]

Why cant you adjust the slugs John?

Hi Gary, what puts me off in this case if the service sheet says very clearly that the alignment is done at the factory and then "glued" in place so no alignment is possible afterwards.

I have one or two IF transformers from a parted out set so I'll experiment on those to see if I can get them to adjust. Thanks for the encouragement!

Cheers, John

[pwdrive]

Why cant you adjust the slugs John?

Hi Gary, what puts me off in this case if the service sheet says very clearly that the alignment is done at the factory and then "glued" in place so no alignment is possible afterwards.

I have one or two IF transformers from a parted out set so I'll experiment on those to see if I can get them to adjust. Thanks for the encouragement!

Cheers, John

Hi John, from what you have just said it would sound like there were no alignment instructions for the IF transformers, is this so?

Also remember that when the transformers are all back in circuit a certain amount of stray capacitance will be introduced by the wiring, valve bases, interelectrode valve capacitances etc all these will effect the resonant frequency.
I would be tempted to temporarilary connect the transformers in circuit and get the reciever up and working, you can get a better idea then as to what's what, if the cores do have to be adjusted then as Gary says they normally come round by using various techniques to free them up if needed. If they do need adjusting and they are definately unmovable then it's either find a set of transformers that do or convert to capacity tuning with some small adjustable trimmers.
Just throwing some ideas up in the air.

By the way, just to discount one possibility, were any vintage domestic sets stagger tuned on AM, anyone know?

Lawrence.

[Radio Fixer]

True Lawrence circuit strays may have an effect but its probably quite small and the 10pF of the scope probe is compensating and probably exceeding that for a compact radio (think I saw a pic of the chassis one time and thats how I remember it). It seems John is adding capcitance so all his frequencies must be initially high. Also we have the inverse sq root factor for freq from capacitance.

John is sparse on info and I'm a little confused: are the cap values he mentions, 110 and 105 pF, the total or additional to whats inside the can? If additional that seems huge to me, I would expect it to be the total.

I have not come across stagger tuning on any small AM radio (on FM sometimes). Some exotic sets with two IF stages may have done it but probably still rare I believe.

What causes IFs to be off tune? Normally its fiddling as the main culprit. Mica caps dont normally move much nor does the coil winding but I did read long ago that it could be down to the permeability of the slug changing. I take it the IFs have been in the warm and dry for sometime?

Think John is on the right track to experiment with some 'junk' IFs and then have a careful go with the ones from the set ... if some solvent and judicious treatment wont move the slugs then he will have to do as you suggest and try in the set and see what happens. If he wants to be cautious he can note the position of the slugs and the number of turns to remove them assuming that he can and wants to take them right out?

Gary