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Old 26th Apr 2007, 1:02 pm   #1
Studio263
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Default FM tuning drift - any ideas?

This is driving me mad! The set in question is a B&O "mini deluxe" (1963) and in the first ten minutes of use it consistently needs about 500KHz of tuning correction to keep it on tune. After this it's OK for as long as it's on.
The set is pretty simple and conventional, it uses an ECC85 for the FM front end, followed by an ECH81, EF89, EBF89 and two Ge diodes for the detector. The FM front end is in a metal box and is capacitively tuned. There is no AFC, an oversight clearly!
I've replaced the usual suspect components on the main chassis and the set works very well otherwise.
The ususal "freezer spray" routine has failled as the ice that forms puts the tuning miles out by itself. I'm also reluctant to go mad and replace everything in the front end as I have heard that some of the capacitors have carefully chose temperatue coefficients.

Any ideas?
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Old 26th Apr 2007, 3:31 pm   #2
Kat Manton
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Default Re: FM tuning drift - any ideas?

Hi,

I generally expect valve VHF sets with no AFC to drift about a bit after switching on from cold, sometimes for as long as half an hour before settling down once the temperature has stabilised. If your set only drifts for the first ten minutes, I'd consider that to be pretty good.

Given it's capacitively tuned, if this is the usual air-spaced metal plate variable capacitor, I'd expect some drift as this warms up and the metal expands. Perhaps permeability tuning became common on VHF sets for this reason? I find permeability tuned sets drift less, but they still drift.

Admittedly, B&O might've managed to produce a drift-free valve VHF set. It'd take someone else with the same set to confirm or deny whether this is the expected behaviour for this set.

Regards, Kat
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Old 26th Apr 2007, 4:24 pm   #3
Ray Cooper
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Default Re: FM tuning drift - any ideas?

These earlier valve sets could drift about quite a bit, though 500kHz does sound somewhat excessive.

Just a few ideas to toss about:-

Do you have a selection of ECH81 valves available? If so, it might be worth swapping them about to find the specimen that gives least drift.

I see that you've been changing the 'usual suspects'. The only suspects that'll make much difference, since the set has no AGC, are capacitors round the local oscillator. Some manufacturers specified components having particular temperature coefficients to try and minimise frequency drift: if any of these have been replaced at any time with ones having different temperature coeffs, the drift can only increase.

Many moons ago, there used to be a gadget called a 'Tempatrimmer': looked like a trimmer, but it was effectively a fixed-value capacitor that could be adjusted to have different temperature coefficients. Does anybody remember these gizmos? I've no idea where you'd get one: commercial sets never used them, far too expensive: but if you could insert one in the proper place, and had a vast amount of patience at your disposal, you could tweak it to eliminate frequency drift almost completely.

As a final query, you say that the detector uses germanium diodes, but I note that one of the valves is an EBF89: what are the diode sections used for in your particular receiver?
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Old 26th Apr 2007, 4:28 pm   #4
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Default Re: FM tuning drift - any ideas?

The Oxley Tempatrimmer. Often specified for use in VHF VFOs (Variable Frequency Oscillators. As far as I know they haven't been available for years. In fact when restoring some commerical made amateur gear I've noticed that although specified these trimmers were never fitted, ordinary trimmers being used instead.
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Old 26th Apr 2007, 4:45 pm   #5
Mike Phelan
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Default Re: FM tuning drift - any ideas?

Quote:
The only suspects that'll make much difference, since the set has no AGC, are capacitors round the local oscillator
I am sure you meant AFC, Ray.
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Old 26th Apr 2007, 5:06 pm   #6
Studio263
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Default Re: FM tuning drift - any ideas?

Thanks for all your help so quickly!

in answer to some of the questions, the EBF89 diode sections are used as the detector and AGC diode for the AM bands for which the ECH81 is the frequency changer. They could not be used as an FM detector as they share a common cathode.
The ECH81 has been replaced for other reasons (very lazy oscillator that needed 240V mains to start, this is only a 220V set! (now converted), also low gain once working). Could a defective 1st IF stage (as is the function of the ECH81 on FM) cause tuning drift on FM?
I've always considered valve FM sets to be a bit drift-prone too but I can't imagine this being any less annoying when the set was new. Perhaps the Beomaster 900K (all transistor with AFC, available the next season) was designed to address these shortcommings?
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Old 26th Apr 2007, 10:38 pm   #7
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Default Re: FM tuning drift - any ideas?

Quote:
I am sure you meant AFC, Ray
Too true. They shouldn't put the 'F' and 'G' keys next to one another, and I shouldn't have fingers like bananas...
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Old 27th Apr 2007, 12:38 pm   #8
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Default Re: FM tuning drift - any ideas?

Kat raises an interesting point here about the method of tuning. I wonder if that does have an influence? Certainly the sets I have that are most stable use permeability tuning... If this B&O does use a capacitor is it somewhere that will get warm in 10 mins? To require 500 kHz of re-tuning I would have thought a significant expansion would be required and the tuning capacitor would have to be very hot indeed?

From memory many of the free-standing valve FM tuners used capacity tuning so it cannot be too bad? (although this may well have been combined with AFC)

I would be inclined to look at some of the components in the tuner myself. I’m sure whilst some sets may need a little while to settle this need not be the case. One of my first restorations was a Bush VHF54 - “ok” as sets go, but hardly the last word in circuit design either. Finding out of spec components throughout the rest of the set I dived inside the tuner head too (based around two EF80s – great to have something to use all those spares up!). It was all quite a fiddle and, with hindsight over-the-top, I decided I never wanted to go inside it again – and replaced everything – resistors, caps; the lot!

This was before the age of the digital camera so lots of sketches were made so that component position and lead routing could be easily replicated. Capacitors were selected with good stability characteristics, (and good resistors too) - my theory being that I could never hope to reproduce any original correcting / complementary temperature coefficients in place with the originals, but if the replacements were good enough then I may get away with it.

The result? A set that can sit for months unused - or receive a daily blast – that unless a duster has knocked the tuning dial (some chance!) will warm up bang on tune from the last time…and stay there.
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Old 27th Apr 2007, 3:37 pm   #9
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Default Re: FM tuning drift - any ideas?

Variable capacitance tuning can be perfectly stable, even without AFC. Philips, for example, produced some excellent sets using this method of tuning.

Hopefully another B&O owner will be along soon, to confirm whether or not your set's behaviour is normal for that model.

Have you tried a few different ECC85s yet? You may find one which reduces the drift to an acceptable level. The other thing I would check is the HT supply to the VHF front-end. It may be low, or varying with temperature as a feed resistor changes value. If the set is very compact, and the front-end is influenced by heat from the power supply or output stage, there may be little you can do to improve it.

Hope this helps,

Roy
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Old 28th Apr 2007, 6:15 pm   #10
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Default Re: FM tuning drift - any ideas?

Thanks again everyone for your help, I have the circuit diagram now so I feel more confident in delving inside the front-end box. I think I will try a "new" ECC85 first and then have a close look at the HT feed resistors, along with their ascociated decoupling capacitors. Failing that it's back to the Beomaster 900K, all transisitor and with AFC...
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Old 29th Apr 2007, 11:41 am   #11
Mick Mearman
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Default Re: FM tuning drift - any ideas?

Just a shot in the dark here but does tapping the tuner cause a drift? if so are there any spring metal earths on the shaft inside the tuner? I say this because they used to be a nightmare in early TV UHF tuners we used to unsolder them clean them and refit, they remained a problem until varicaps came in then we had problems with the 30 volt stabilizers drifting.
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Old 30th Apr 2007, 8:06 pm   #12
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Default Re: FM tuning drift -problem solved!

Well, I got it in the end!

With the aid of the service manual I quickly noticed that the HT voltage was rather low. I pounced on the selenium rectifier (a common cause) but a replacement made from four 1N4007's and a resistor did not produce the expected improvement in either voltage or stability.

It was then I spotted the real culprit, someone had fitted a cathode resistor of the wrong value in the output stage (should have been 390R, 330R was there, looking hot and bothered). This had lead to the output valve passing excessive bias current, sucking the lifeblood out of the HT supply and making the transformer and rectifier (located right next to the FM tuner unit) get very hot and bothered.

Things are back to normal now and it works properly again, many thanks to everyone that helped.
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Old 30th Apr 2007, 10:09 pm   #13
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Default Re: FM tuning drift - any ideas?

This has been an interesting thread. The matter of VHF tuning drift has arisen before so, for future reference, perhaps this thread could be archived ? or maybe just the first and last ( immediately prior to this ) postings could be archived ?

I've a Bush VHF70 which loves to drift for the first half hour or so ( but only by 200KHz or thereabouts ) and I've never found the cause as I've only ever faffed around with valves and capacitors in respect of this problem.
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Old 30th Apr 2007, 10:31 pm   #14
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Default Re: FM tuning drift - any ideas?

I have made it sticky for now, so we don't lose it!
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Old 6th Jun 2007, 11:28 am   #15
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Default Re: FM tuning drift - any ideas?

Hello all,

interesting -- I had a similar problem months ago with a Philips B2D33A: During the first 5 minutes it drifted, so that the station became very distorted (when it was clear at the beginning). I assumed oscillator freq drifting, and I was right. It moved about 150 kHz downwards, which was enough for that effect. I measured the frequency with a spectrum analyzer, but I also did the same with an auxiliary quartzstable detunable oscillator.

I also controlled the HT stability, but in my case, it was stable. The exchange of the ECC85 was not successful either.

At the end, I decided to exchange a condenser for the oscillation against one with a high negative temp coefficient. I played with some combinations. Then I tested it again, the drift only was 50 kHz, less enough to accept it.

I know, it is not possible to get zero drift. But I don't believe, that the measured amount of 150 kHz was not the target of the design. I also know, that there are measures to compensate as well it is possible. So, maybe one of the components which were for compensation (mostly caps) had lost its compensating properties a bit. It's hard to find out, which it was.

Dieter
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Old 30th Aug 2007, 10:37 pm   #16
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Default Re: FM tuning drift - any ideas?

Hi!

Well done to Member Studio263 on catching that one - even my 1963-edition Quarrington "Radio And Television" doesn't pick up on excessively-low HT as a cause of VHF tuning drift!

As well as the lowered HT, the output valve would be running excessively hot as well, and the extra heat from this would affect the temperature of the VHF-front-end components to a greater extent thean the original set'd designer may have allowed for! For this reason whenever I've worked on any set using a B8A/B9A based tetrode/pentode or triode-pentode, I've always added extra decoupling for the O/P valve G2/previous anode - 10K 2W from HT to G2/triode anode decoupled with 8-10uF to earth won't affect volume too much but it does provide a noticeable hum reduction and can greatly help O/P valve life!

Chris Williams
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Old 24th Nov 2007, 9:46 pm   #17
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Post Re: FM tuning drift - any ideas?

Reading on from this about the valve getting too hot reminded me of the thread about the black screening cans reducing the heat by conduction as a black body radiator.

Pure guesswork on my part and apologies if it has already been mentioned in this thread.

Worth a try!

Geof
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Old 3rd Aug 2008, 3:34 pm   #18
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Default Re: FM tuning drift - any ideas?

My Sobell AGF707 from 1957 suffers badly from FM drift.....most significantly for first 30 minutes or so it really is so annoying.....it can drift off frequency almost entirely at times.....
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Old 13th Oct 2008, 8:38 am   #19
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Default Re: FM tuning drift - any ideas?

I bought a Champion Mid fifties Tuner recently. looks like a radio but just a tuner .Im using it through pickup of my Hmv . As far as I can see it doesnt drift . Its not had any new components as fara as i know from new either ! but has been re aligned . I dont think it was ever used
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Old 30th Aug 2009, 9:27 pm   #20
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Default Re: FM tuning drift - any ideas?

What a useful thread !

I have the exact same type of set on the bench here today.
It has the exact same fault - Tuning drift within the first couple of minutes.
But for the exact opposite reason in that the output valve cathode resistor has gone high in value.
It's the original resistor, solder joints were untouched etc. but for a 390 ohm
resistor, the measured value of 590 ohms (out of circuit) is way too high.
A new resistor of correct value cured the thing.
Obviously a weak point in these sets.

Martin
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