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Old 30th Sep 2021, 4:10 pm   #21
Herald1360
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Default Re: Philips 543A problem detector stage

The positive going half of the modulated IF looks compressed, but the AF signal on it doesn't look clipped or seriously distorted. Is the IF asymmetry not simply caused by the asymmetric half wave detector loading?

R16 C41 decouple the anode supply rather than form part of the anode load as such. Depends on whether dc or signal conditions are considered. Lawrence's comment might well be relevant, particularly if an ordinary vs valve voltmeter was used to take the voltage readings, when a lower than expected Va would be seen.

It's also suspicious to say the least that at 10mA anode current and 195V anode voltage Philips would be operating the EBF80 pentode at well over its rated Pa max! The 5mA figure in Philips's own valve datasheet for operation as RF/IF amplifier is much more likely to be where they were intending.
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Old 30th Sep 2021, 4:27 pm   #22
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Default Re: Philips 543A problem detector stage

I used the GRAM input of my Pye 76 as an audio amplifier instead of the audio section of this Philips, taking the detected audio out (via a 0.1uF) from the junction of R39, R17, C76 where it would feed into V5 via switches etc. I can confirm that the audio reproduced by the Pye is also similarly distorted. So this would seem to confirm that the audio stages of the Philips are not suspect and that it is a problem prior.
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Old 30th Sep 2021, 4:47 pm   #23
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Default Re: Philips 543A problem detector stage

Sideband
The answer about the IFT primaries in the V3 anode circuit is that they measure 8ohms together in series.
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Old 30th Sep 2021, 5:38 pm   #24
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Default Re: Philips 543A problem detector stage

I would not expect AM and FM to suffer the same distortion cause unless it is in the audio section. What types of capacitors are in this set as it's quite 'modern' so they may not be paper. To get FM working properly you do need to replace C49 (trader) and C50 if it is paper.
Not sure what your siggen is but if it's a basic valve model it would be worth scoping the output of the generator as they are not precision instruments.
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Old 30th Sep 2021, 6:34 pm   #25
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Default Re: Philips 543A problem detector stage

Hi PJL
This set does use paper capacitors quite extensively (the black pitch ones) C50 (Trader) (=C55Philips) is stated as ceramic in the Philips data.
I also wouldn't have expected a common AF fault between AM and FM other than in the AF amp and yet, as described above, when I use my Pye as the amplifier the sound is still distorted (I didn't check on FM). I note that V2 and V3 are used as IF amps for FM so there is some common AM/FM usage though I agree, everything would normally point to this being an audio amp problem. I have changed several Cs in the audio (described previously). BTW the character of this distortion is notable for the bass tending to modulate higher frequencies.

Broadcasted AM audio as it appears after the detector looks like the thumbnail below

Thanks again for all responses
Chris
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Old 30th Sep 2021, 7:26 pm   #26
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Default Re: Philips 543A problem detector stage

Hi Christoffrad

I wonder if you have checked/replaced the AGC line decoupler (Trader C24). If this has gone low in value, the low frequency modulation components will not be properly "stripped off" the AGC line, which may cause the odd distortion you are experiencing. Just 'scoping the AGC line might be revealing.
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Old 30th Sep 2021, 9:38 pm   #27
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Default Re: Philips 543A problem detector stage

just switched the radio on to make the AGC check suggested above and, before I touch a thing, eveything is fine..... loads of gain, no distortion, OK there is still an issue with the bass/tone control to sort but the audio is distortion free and I can see the improvement clearly on the scope of the detected filtered audio.

What the hell! ... I hate intermittent faults though (if that's what it turns out to be)

Well, tomorrow's another day, we shall see then if all is still OK.

Chris
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Old 1st Oct 2021, 1:54 pm   #28
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Default Re: Philips 543A problem detector stage

A possible problem....check if any of the chassis earth points are via tags riveted to the chassis. It is possible for such tags to become slightly high resistance over the years....don't be fooled by resistance measurements (unless it's several ohms). If the fault returns (which probably won't be until you have the set in it's cabinet again......), gently probe and press the rivet connections. It may show up a dodgy earth. I've had this before and it leads you a merry dance. If there is a dodgy tag you can usually fix it by soldering but you'll need a very hot iron. Failing that, drill the rivet out, clean the metal and fix with a nut, screw and star washer.

I had this problem in an IF stage that was going unstable.
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Old 1st Oct 2021, 2:07 pm   #29
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Default Re: Philips 543A problem detector stage

Unfortunately when I applied power today the problem was there again (there had been no physical contact since it was working fine last night!
I thought I may have found the problem when I discovered that the 10M grid leak resistor on V5 was o/c but replacing it does not fix the problem. I have also tried replacing the series coupling cap C57 (C52 Trader) but no luck.
I'll take a look at those rivets.
C
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Old 2nd Oct 2021, 3:31 pm   #30
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Default Philips 543A audio distortion

One of two repaining problems here is audio distortion which is present even using the GRAM input. There is plenty of gain but the audio HF is being modulated by the base frequencies.
I have replaced usual suspects C59 (Philips service sheet),C60,C63,70 and verified by temporary substitution C57,C64,C71 I have checked the capacitance of HT electrolitics and cathode bypass C65 by measurement also resistors R28 R44 and R37. HTs are OK @C74 250v, @C1 238v, @C2 250v, @V5b anode 76v, @V6anode 235v (all in FM mode) V5b grid -0.4v, V6g1 0V (ie -5.9V relative to Cath) Also checked Vol pot to ground.
There's loads of volume in FM mode although MW is still very low (that's a further problem but one thing at a time!)
Now I have run out of ideas.

Thumbnail shows the Philips version of this audio stage (I hope it's readable)

Chris
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Old 2nd Oct 2021, 3:33 pm   #31
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Default Re: Philips 543A audio distortion

Sorry Forgot the upload
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Old 2nd Oct 2021, 3:59 pm   #32
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Default Re: Philips 543A audio distortion

As a matter of elimination, have you tried another speaker? Doesn't have to be an exact impedance match for testing.
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Old 2nd Oct 2021, 4:35 pm   #33
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Default Re: Philips 543A audio distortion

Forgot to mention that. Yes tried that this morning. Thanks for your posting though.
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Old 2nd Oct 2021, 5:05 pm   #34
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Default Re: Philips 543A audio distortion

For FM you must replace C56 (10uF) discriminator capacitor and +ve goes to chassis. I also suggest you focus on AM first as it is simpler.
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Old 2nd Oct 2021, 5:44 pm   #35
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Default Re: Philips 543A audio distortion

Thank you PJL C56 was measuring 30nF instead or 10uF
That makes FM sound much better though there is still something not quite right.

I've noticed that as the tone switch clicks over between the boost and no-boost position there's not any real noticeable difference ... could be a clue?
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Old 2nd Oct 2021, 6:32 pm   #36
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Default Re: Philips 543A audio distortion

Hi Christoffrad

It used to be said that there was the right way, the wrong way and the Philips way !

I only have the Trader service sheet, which shows the fiendish negative feedback based tone control. I haven't had time or inclination to understand it and don't pretend to fully.

Using Trader designations: The Bass boost is engendered by the use of S23 and C51 (8200 pf) on the tap of the volume control (presumably this is mostly effective at low volume settings), and S26 in the negative feedback network which will be effective at all volumes.

Its also noteworthy that S25, S28 and S29 change the tone/feedback settings in FM mode only. I would expect this to increase the negative feedback and therefore quality on FM, maybe at the expense of gain, so your comment about plenty of volume on FM is perhaps slightly at odds, unless its a case of AM being weak in volume.

The setup has an earth tapped secondary for the output transformer. Are you sure this transformer is doing it's stuff, and there is audio on the feedback end of the winding as well as the loudspeaker end ?

With any feedback network, any hanky-panky might result in supersonic oscillation which would give odd effects. Your oscilloscope will soon reveal this.

Beyond that, I can only suggest checking any capacitors you haven't already, and cleaning any switch contacts.
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Old 2nd Oct 2021, 7:11 pm   #37
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Default Re: Philips 543A audio distortion

You should not trust your capacitor meter alone as it will not show the correct value if the capacitor is leaky.
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Old 2nd Oct 2021, 7:16 pm   #38
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Default Re: Philips 543A audio distortion

Thanks Mr 1936. I have just disconnected the feedback winding and my scope shows that it is working OK (now re-connected)
According to the Philips data the tone switch S23 (Trader) is ganged with S26 (Trader).
I'd like to understand this circuit a bit better. eg should the bass change noticeably as the switch operates?
Having changed the discriminator cap (C49 trader) the FM level is reduced as well as curing the FM distortion.
The low AM MW audio coupled with distortion will be my challenge once I'm satisfied with the audio side.
Actually I found that if I radiate a strong modulated signal into the ferrite antenna the audio is quite clean so the AM issue may be an alignment issue between oscillator and front end resonance.

Chris
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Old 2nd Oct 2021, 7:21 pm   #39
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Default Re: Philips 543A audio distortion

Thanks PJL. I use a PEAK atlas LCR device but recognise that leakage can give false readings.

Keep the suggestions coming!

C
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Old 8th Oct 2021, 1:25 pm   #40
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Default Re: Philips 543A problem detector stage

Try to inject a signal directly into the grid of V6 via an isolating 0.1uf capacitor, similar to what you did with the gram input. It won't be loud but you should be able to hear it. If it's distorted, focus your efforts there, or the output transformer. If it's clean, work backwards towards v5b until it becomes distorted. Careful about the voltages, but I'm sure you know this.

Last edited by Gabe001; 8th Oct 2021 at 1:43 pm.
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