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Old 18th Dec 2010, 7:52 pm   #1
Tractorfan
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Default "Fuzzy" FM

Hi Guys,
I recently posted my KB radiogram in the success stories section as it works a lot better than it used to and I'm really pleased with it. I replaced the ECC81 in the FM front end with an ECC85 after rewiring the heater connections. I realigned the set by ear as I've no test gear apart from a multimeter. It now pulls in stations very well even on its internal aerial which is quite an achievement in a house with very thick walls.
The only snag now is that the sound has a "fuzzy" distortion and no amount of tweaking will improve it. It's as if it is ever so slightly off tune. If the local oscillator was on the "wrong" side on the received frequency, would that cause it? If so, then I should be able screw the brass slug through its coil to shift its frequency to the "right" side. Yes?
Am I barking up the right tree? Or just barking?
Cheers & Merry Christmas, Pete
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Old 18th Dec 2010, 8:09 pm   #2
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Default Re: "Fuzzy" FM

There is an electrolytic cap in the FM discriminator circuit that can go funny. Replace but do remember it's +VE to deck.

Try realigning it. The IF is usually 10.7 Mhz.

Cheers,

Steve P.
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Old 18th Dec 2010, 8:10 pm   #3
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Default Re: "Fuzzy" FM

In my experience, aligning a 10.7 MHz IF strip and its accompanying detector by ear is difficult - if you want good results. It sounds as though the FM discriminator is a bit off-centre. There is a method of setting up a discriminator by using only a multimeter, but a sweeping generator + 'scope is the best choice.

OTOH, does the fuzzyness appear on all stations? You might simply have inadequate signal strength.

Al. [Skywave]
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Old 18th Dec 2010, 8:24 pm   #4
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Default Re: "Fuzzy" FM

Hi,
Thanks for your quick responses. Yes, the fuzziness is on all stations whether it's on a good aerial or the internal one. I'm not sure where the discriminator is, is it based around the EB91? There's an EBC91 as well but I think the two diodes in that are for AM detection and AGC.
Cheers, Pete
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Old 18th Dec 2010, 10:34 pm   #5
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Default Re: "Fuzzy" FM

Hi Pete. You cannot align a discriminator properly by ear. With experience you can get close but really the minimum you need is a signal generator and a multimeter (an analogue one is better). Usually one of the cores of the discriminator coil is tuned for minimum output and you need the meter to see this (this is the balance point so that the discriminator is balanced about the detected signal)....but you need to feed 10.7 Mhz in to the I.F strip at the recommended points in order to measure the minimum signal.

You could try a new EB91 in case there is a problem with one of the diodes and the discriminator won't balance anyway but ultimately you should aim at getting the FM aligned properly.


SB
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Old 18th Dec 2010, 10:49 pm   #6
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Default Re: "Fuzzy" FM

Try the electrolytic before touching the FM alignment. This capacitor is an important part of a ratio detector, although the value is not too critical. In my Bush VHF80 it had gone very low in value (presumably dried out by valve heat) so 'correct' tuning gave worst sound.
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Old 19th Dec 2010, 6:28 pm   #7
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Default Re: "Fuzzy" FM

Hi,
I whipped the chassis out of the cabinet (yet again. It almost does it on its own these days ) and had a look for the cap in question. I found a 0.1uF that I'd put in when I re-capped it, but I don't remember taking an electrolytic one out. Anyway I shoved a 2.2uF elec in with pos to chassis and it sounds much better. The only signal gen I've got is a pre-historic Cossor Ganging Oscillator that doesn't work. It's so old it won't go anywhere near 10.7MHz anyway.
Dave: the alignment was miles out before I started so I've got nowt to lose by doing it by ear'ole.
Sideband: I appreciate your comment about not being able to do it by the aforementioned ear'oles. As you say, I can get it close but not bang on.
Would a "wobbulator" help? I think I've got a circuit for a simple one somewhere in an old PW.
I've a few EB91s so I'll try those tomorrow.
Cheers & Merry Christmas, Pete
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Old 20th Dec 2010, 12:01 pm   #8
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Default Re: "Fuzzy" FM

Although the value is not critical 0.1uF would definitely have been too low. Somewhere around 4.7-10uF is about right for most valve-based ratio detectors. Too low and you get distortion (particularly bass notes), too high and it is difficult to tune because it responds too slowly to changes in signal level.
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Old 20th Dec 2010, 4:17 pm   #9
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Question Re: "Fuzzy" FM

Quote:
Originally Posted by Tractorfan View Post

The only signal gen I've got is a pre-historic Cossor Ganging Oscillator that doesn't work. . . .

Would a "wobbulator" help? I think I've got a circuit for a simple one somewhere in an old PW.

Pete
Isn't a 'ganging oscillator' and a 'wobbulator' the same thing?

Al. [Skywave]
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Old 20th Dec 2010, 8:27 pm   #10
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Default Re: "Fuzzy" FM

You can align a ratio detector without a sig gen. The IF needs to be correctly aligned, and you need two voltmeters. Put one across the electrolytic, the other at the output. Tune to a strong signal. Adjust the ratio detector secondary for peak voltage across the electrolytic. Adjust the primary so the output is half the first figure. This assumes a typical unbalanced ratio detector. If balanced (i.e. the electrolytic does not connect to chassis) then tune for zero volts on the second meter instead of half.

Not perfect, but close enough.
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Old 20th Dec 2010, 8:29 pm   #11
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Default Re: "Fuzzy" FM

Hi again,
This has been a very interesting learning curve for me as I knew nothing at all about re-capping before I joined this forum, and now I'm learning a lot about FM discriminators. It's great! Plus my radiogram is working better for its age than it has for years . Even though it's maybe not 100% yet.
I replaced the elec cap in the discriminator for a bigger one. It's now 10uF (I fitted a 2.2uF when I took out that 0.1uF) and things are getting better all the time! I also tried a different EB91 which only made a slight improvement. Still, I'm very pleased with progress so far.
Al: I'm sure you are right, but I haven't a clue myself. As I said, it's positively ancient with B7 metallised valves and a dial marked in "cycles per second", I very much doubt it'll get anywhere 10.7MHz .
Cheers & Merry Christmas
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Old 20th Dec 2010, 9:40 pm   #12
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Default Re: "Fuzzy" FM

Hi,
Thanks Dave for that advice. I'll have a bash.
I've just had another look at that ganging oscillator. It's a Cossor 343 and does go well over several MHz. Just got to get it working . I've even got the paperwork for it somewhere.
Cheers & Merry Christmas, Pete
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Old 20th Dec 2010, 10:35 pm   #13
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Default Re: "Fuzzy" FM

For a simple answer to the 'wobbulator'/sweep generator problem, a cheap solution could be a simple one transistor oscillator - based on an old 10.7MHz IF transformer, perhaps.

For the sweep, remove the varicap diodes from a scrap UHF tuner, and either apply the sawtooth from the oscilloscope's timebase or use AC to drive both the scope and the oscillator. The maximum rating of the diodes is nominally 28V, so a 6V transformer - ~17V pk- pk (variable for sweep width) superimposed on 0 - 9V from the transistor supply, for centre frequency adjustment, would work.

Obviously, further discussion would be best in another thread elsewhere, but I'm throwing the idea out here as it is relevant to what has been said in this thread.

Perhaps somebody has already done something similar?
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Old 8th May 2011, 8:52 pm   #14
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Default Re: "Fuzzy" FM

I chose to pull forward this older thread because I have a fault that seems to be very similar.
FM sounds as if the set was tuned just off center.
The set has ECC85 / ECH81 / EF89 / EBF89 / (2x OA79) / ECL82
New electrolytic cap in the discriminator and I tried adjusting the pri/sec discriminator coils but
the distortion remains.
It's not a lot, just enough to be annoying.

Tape input is perfectly clean on the scope, so the output stage is fine.
On AM it whistles more than usual, I get more whistles than stations (if any).
Any ideas ?

Martin
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Old 8th May 2011, 9:35 pm   #15
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Default Re: "Fuzzy" FM

Are all the metal shields there and are there any problems with magnetic earthing paint or earthing. Caps round the IF stages, especially de-coupling ones. Then check the alignment.

Cheers,

Steve P.
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Old 9th May 2011, 10:28 am   #16
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Default Re: "Fuzzy" FM

Yes, it sounds like IF instability.
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Old 9th May 2011, 6:04 pm   #17
Dillen_de
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Default Re: "Fuzzy" FM

Thanks for your replies.
Yes, all metal shields are in and there is no shielding paint anywhere (and shouldn't be either).
I will check the caps again and then the IF alignment.

Martin
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Old 10th May 2011, 3:05 pm   #18
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Default Re: "Fuzzy" FM

I had something similar with a Bush VHF61. I thought I had replaced all the Hunts caps, but then I found another one lurking. It is a while ago now so I cannot remember which it was but replacing it certainly helped. I have never got the discriminator quite right using the method on the Trader sheet (and taking into account that the Trader sheet is the "wrong way round") but it works well enough now to be used most days.

Paul
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Old 12th May 2011, 9:21 am   #19
Dillen_de
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Default Re: "Fuzzy" FM

OK, so I've changed capacitors.
A handful of caps that looked a bit like the Hunts Mouldseal types (but wasn't) was replaced.
No change.

A couple of other caps, including the discriminator cap, was also replaced with the
same result.

I then did an IF realignment as per Paul Stennings excellent instructions, it worked great and
brought a bit more sensitivity and with that a few more stations but the distortion remains.

I then tried swapping the full set of valves, no change.

Tape input is clean and nice.
AM likewise.
FM is distorted on all stations and at all volumes.
Tuning seems fine with the best sound reproduction at the well defined peak of the signal, also confirmed by the magic eye.
HT voltages are generally a little low so I think, I will concentrate on the (metal) rectifier next.
If that does not cure the poor thing, I think the two discriminator diodes will be next. I suppose that eventual differencies
in leakage etc. would play a role and those OA 79 could easily have passed their sell-by date.

Please chime in with any and all ideas, you may sit with.

Martin
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