Fenman II MW oscillator coil

[Radio_Dave]

Hi,

I've just replaced a broken MW oscillator coil on my Fenman II.

The replacement part is from an identical chassis but it's obviously not set up for use in it's new position.

Please could someone tell me what effect changing this coil will have to the MW reception?

Thanks
David

[paulsherwin]

In simple terms, the oscillator coil adjustment will determine where stations will appear on the tuning scale. If you adjust the core you should be able to tune stations in and out.

Once the coil is adjusted so that stations are in the right place, you will need to adjust the RF coil for best reception.

What exactly is the problem with your set?

Paul

[Radio_Dave]

Unfortunately, the tuning scale is in the cabinet, and I haven't finished that yet, so I can't check the positions of the stations.

I have, however, noticed a that the magic eye is lying to me! Full signal strength is not where the eye indicates it is. Is this to do with MW oscillator coil?

Thanks
David

[paulsherwin]

Originally Posted by Radio_Dave

I have, however, noticed a that the magic eye is lying to me! Full signal strength is not where the eye indicates it is. Is this to do with MW oscillator coil?

The magic eye just indicates the AGC voltage, which is derived from the signal rectifier diode, i.e. it indicates the signal strength. The symptom sounds very odd and, on the face of it, impossible.

I suppose it's possible that the RF alignment is now so far out that it's difficult to determine the peak signal by ear. If I were you I'd start by doing an RF alignment as per the service data. You shouldn't need to touch the IF transformers unless they have been disturbed.

Paul

[Radio_Dave]

Erm... any tips on how to get an iron dust core, without a slot, out of a MW oscillator coil

[brymac]

Yup - tackle it from the other end, where you will find it does have a slot!! Bryan

[Mike Phelan]

Warm it up a bit with your iron first, David, then apply Bryan's fix.

[Radio_Dave]

Sorry for the delay, but I had to drill the dust cores out and find replacements!

I've done the RF alignment

However, the stations are both louder and clearer if I tune, slightly, away from where the magic eye shows me maximum strength

Any ideas?

Thanks
David

[brymac]

David - have you checked the value of the resistor on the magic eye bracket?

Bryan

[paulsherwin]

Does the Fenman II use a separate diode for detection and delayed AGC? (I don't have a circuit to hand.) If so, the relevant valve may be faulty. That's all I can think of. The resistors around the magic eye will affect it's performance, but not where it peaks

Paul

[brymac]

David/Paul - yes that's right! Duh! I was clutching at the proverbial straw (or resistor!!) Bryan

[Radio_Dave]

Originally Posted by paulsherwin

Does the Fenman II use a separate diode for detection and delayed AGC?

I think it does? it's a EBC41. I'll swap it for another and report back

[Radio_Dave]

Swapping the EBC41 has made no difference. Should I have a go at re-aligning the IFT's?

David

[paulsherwin]

No, don't do anything without working out what's going on. I wouldn't expect IF alignment to cause this problem though it may need doing anyway.

You should be able to measure the AGC voltage coming out of the EBC41 using a DMM. Does it vary as the magic eye varies, or is there some sort of discrepancy?

Paul

[Radio_Dave]

Found the problem! There was a broken 100pf capacitor (C49 on the 'Trader' FWIW). I've replaced it and the magic eye is working as it should .

Now that the radio is back in it's cabinet I can see that my RF alignment wasn't too successful. BBC Five Live is spot on at 433 metres but Virgin 1215 is at 257 and not 247 metres.

I'm going to blame this on my antiquated (rubbish) signal generator. Unfortunately, I'm sure that I'm not going to get it any better with the equipment I'm using, so why can't I use Virgin 1215 and BBC Five Live as the two signal points to calibrate the scale?

Thanks
David

[Station X]

If you're going to use off air signals to align the RF sections of a radio it's advisable to choose stations at the extreme ends of the dial. This should result in better tracking and dial calibration. The weaker the stations the better.

[paulsherwin]

There's absolutely no reason why you can't use off air signals to do an RF alignment, particularly for setting up the oscillator coil. The frequencies will be spot on, much more accurate than most valve era signal generators, and you can vary the signal by changing the length of the aerial wire in use

Good stations are Virgin on 1215, though it can be subject to fading, and R5 on 698. Many places have a local station right at the HF end which is ideal.

IF alignment is a different matter, though it *can* be done without a generator.

Paul

[Station X]

Be careful about using Virgin 1215 as a calibration point. It also transmits on nearby frequencies such as 1197, 1224 and 1233 in some parts of the country.

[Mike Phelan]

David
I find it is worth getting an inexpensive frequency counter just to hook up to the sig genny when you are aligning anything - probably just peace of mind.

To summarize:
Do the tracking first; align pointer so travel is what it should be with gang closed and open, then osc coil at LF end of scale, trimmer at high end.
Repeat until tracking is right.
Then align aerial coil for max gain, coil at LF end, trimmer at high end, and repeat until no more improvement.
You will have to look at the individual manual (not got it handy) to see if you do MW before LW.

[Radio_Dave]

Sorted! I used Virgin 1215 on 247 Meters, to align that end, and now all the stations are in their correct positions.


Thanks
David