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Old 5th Oct 2007, 11:51 pm   #1
yestertech
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Default "whistly" LW on Philips 206A

I've recently acquired this set and after basic power repairs I was perplexed by the lack of signals. - There are several versions, mine is the one with the side contact valves. The chassis is stamped 206A -473 so I am assuming this is the IF for this set. On checking the IF transformer cores, several turns were needed to restore the gain ! One core was broken, and this was replaced and duly set up
The result is now quite good performance from a short wire aerial on MW, but the LW is a mess. There is bad whistling as the stations are tuned, and they seem quite weak. I have checked tuning gang earthing and the screening to the red painted valves( often a problem on Philips octal based valves, like EF39 ) , but these are all OK
The 'alignment' instructions for LW consist of removing turns from 2 capacitors. I am always wary of touching these...
Anyone else have one of these and experienced anything similar or care to suggest which area to target ?
Andy
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Old 6th Oct 2007, 12:05 am   #2
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Default Re: "whistly" LW on Philips 206A

Your IF is 470kHz. Do all the coils do something or do some of them do nowt. If one does nothing, look carefully!

Then, line up IF and RF as per the sheet and see what happens. Is the wavechange switch clean.

Cheers,

Steve P.
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Old 6th Oct 2007, 9:54 am   #3
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Default Re: "whistly" LW on Philips 206A

Maybe the IF stage is self oscillating due to a faulty decoupling capacitor(s)
Just a thought.
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Old 6th Oct 2007, 1:19 pm   #4
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Default Re: "whistly" LW on Philips 206A

Hi!

You could well have IF instability in your set. Try adding a 2k2 1/2W resistor decoupled by a 0u1 450V paper/polyester cap to earth in the HT feed to the IF stage, if no help, then put a sensitive DVM across the cathode resistors of the FC, IF and DDT stages, then short each grid to chassis or the AGC line in turn, looking for any change in the DVM reading. Any change indicates the stage concerned is unstable or running into grid current, either of which can cause whistles!

I haven't a circuit to hand to quote ref nos from I'm afraid, but also watch for:-

a) Corrosion on LW aerial/oscillator coil ends and/or wavechange switch contacts,

b) Defective LW oscillator padding capacitor, may be a trimmer in this age of set,

c) Oscillator grid leak (typically 47k with 100p series grid capacitor) connected between FC triode grid/cathode altered in value or high,

d) High ESR electrolytics in power-pack, shunt (very carefully!) a 0u1 450V across HT+ line and chassis while whistles are prevalent to see if they clear,

e) O/C tone correction capacitor across primary of output transformer,

f) O/P valve running into parasitic oscillation, try a 10K 1/2W in the G1 lead and a 100R 1/2W in rhe G2 lead, *cutting the leadout wire connected to the valveholder-tag* as short as possible in each case!

g) Misalignment - if the LW tuned circuits aren't properly aligned, you will tend to get whistles from the Oscillator frequency *minus the IF* breaking through from stray couplings in the FC valve and other components, whilst if the IFs are out, the IF transformers won't reject the unwanted frequencies produced by the FC process correctly! (A triode-hexode multiplicative mixer will produce in its anode circuit the IF, Osc + IF, Osc - IF, the signal frequency itself and the osc. frequency, it's the job of the IFTs to reject all the unwanted other frequencies!)

Let us know how you get on by the way!

Chris Williams

Last edited by Chris55000; 6th Oct 2007 at 1:34 pm.
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Old 6th Oct 2007, 1:30 pm   #5
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Default Re: "whistly" LW on Philips 206A

Hi!

PS - I forgot to mention the Philips Wire Trimmers, the official dealer method of dealing with these was to order new ones from Philips then unwind them to bring in the circuits a turn at a time.

Obviously you'll never get these now, so obtain a small 10-65pF trimmer cap, temporarily connect it across the circuit you are adjusting, then if you find you can't bring the alignment in, remove 1/2 turn from the trimmer *very carefully* then readjust, repeating the process until you can get no further improvement.

Once you've done this, measure the added trimmer very carefully with a Peak or an accurate LCR meter/bridge, then fit a small polystyrene (Suflex-type) or preferably mica capacitor of the *exact measured value* across the trimmer concerned. Don't use a disc ceramic as these have an unknown temp. coefficent and you may end up with tuning-drift and yet more whistles as the circuits wander off-tune when the set warms up!

Chris Williams

Last edited by Chris55000; 6th Oct 2007 at 1:38 pm.
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Old 6th Oct 2007, 3:05 pm   #6
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Default Re: "whistly" LW on Philips 206A

Off topic, but I have always used small value disc ceramics from Maplin which claim a zero temp. coefficient and to be designed for tuned circuit applications...Peter
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Old 6th Oct 2007, 4:37 pm   #7
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Default Re: "whistly" LW on Philips 206A

Quote:
Originally Posted by Chris55000 View Post
Hi!

You could well have IF instability in your set. Try adding a 2k2 1/2W resistor decoupled by a 0u1 450V paper/polyester cap to earth in the HT feed to the IF stage, if no help, then put a sensitive DVM across the cathode resistors of the FC, IF and DDT stages, then short each grid to chassis or the AGC line in turn, looking for any change in the DVM reading. Any change indicates the stage concerned is unstable or running into grid current, either of which can cause whistles!


Chris Williams


Hi Chris

I would like to say, that in my opinion, although this method can indicate instability it is not foolproof.

This is because in a perfectly healthy set the FC and IF valves will show a change in cathode current when the signal grid is grounded as a result of normal AGC action.
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Old 6th Oct 2007, 6:47 pm   #8
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Default Re: "whistly" LW on Philips 206A

Alot of odd whistling problems I get here are to do with my PC network...It seems to effect different radios in different ways...Peter
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Old 6th Oct 2007, 8:20 pm   #9
Chris55000
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Default Re: "whistly" LW on Philips 206A

Hi!

Agreed, but I did actually suggest shorting the grid to chassis *OR* the AGC if used, so as not to cause misleading effects through altering the grid-bias on a healthy stage!

Chris Williams
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Old 6th Oct 2007, 9:34 pm   #10
Sean Williams
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Default Re: "whistly" LW on Philips 206A

Could of course check the metalising on the valves, see if there is a grounding fault.....
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Old 14th Oct 2007, 1:16 pm   #11
yestertech
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Default Re: "whistly" LW on Philips 206A

My sincere thanks to everyone who contributed to this thread. This was one weird set !
I needed to replace some of the power txf wiring and decided also that the original smoothing cap wasnt looking so good.
Having done all the remedial work, I power the set up and "presto" LW seems OK now.
It's weaker on LW R4 than MW R4 but I think that's fairly general on a short aerial these days ??
I'm still puzzled as to why the MW beehive trimmer ( C30 trader ) appears not to have any discernible effect, although, that said, MW seems pretty lively.
The set is now reassembled and has been through the 'wash cycle' and spruced up somewhat!
See pic below
Andy
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