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Old 8th Nov 2021, 6:14 pm   #101
frsimen
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Default Re: Philips 543A problem detector stage

Your IF measurements over for the first IF transformer are showing a rather wide bandwidth. If the second IF transformer has the same sort of performance, the set will be 12dB down at +/-13.5kHz.

I don't know if that is a reasonable figure, it sounds wider than average. Many sets are 25-30dB down at +/-9kHz for comparison. Maybe a forum member with a Philips 543A can cast some more light on the matter.

A similar measurement through to the detector may help to establish if there is a problem.

Paula
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Old 8th Nov 2021, 10:17 pm   #102
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Default Re: Philips 543A problem detector stage

Applying a little more thought, I realise that I have overlooked the effect of AGC on the measured bandwidth. Unless the applied signal is extremely small, the AGC circuit will alter the gain to try to keep the output at a constant level. This will result in the bandwidth of the first IF transformer appearing to be much wider than it actually is.

I should have added an extra thing to do in the original test set up. Apply a short circuit across the AGC capacitor (C24 on the trader sheet) before making your measurements. If you do that, combined with the 10k damping resistor as before, the bandwidth of the 1st IF transformer can then be measured more accurately.

Paula
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Old 8th Nov 2021, 11:42 pm   #103
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Default Re: Philips 543A problem detector stage

It has fancy double tuned IF transformers. Why not just follow the alignment instructions in the Trader?

PS: Instability in these Philips sets is often due to failure of the wire connection to the valve metallisation.

Last edited by PJL; 8th Nov 2021 at 11:55 pm.
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Old 9th Nov 2021, 12:23 am   #104
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Default Re: Philips 543A problem detector stage

PPS: Please ignore the PS as it is a later set!
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Old 9th Nov 2021, 8:02 am   #105
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Default Re: Philips 543A problem detector stage

Well, I just re-read this whole thread.
Interesting.


A few basic questions.

Did you ever personally hear un-distorted AM audio, music and speech, on this set for reasonable lengths of time, before the fault occurred?
Or has the set always been like this?


Otherwise, do you know the history of the set?
For instance, did it come from a source where a repairer gave up on it as a problem?


Can you please configure the set so that it has your AM distortion, then record some audio from the speaker (only, no electrical connection to recorder) as a WAV file on say a Tablet or PC, or mobile phone or similar, then post it here as a dummy PDF, so as to give us a longer session time for our interpretation.



If you have strong ideas that "others" have played with this set before you, you may have to consider that they have removed components for test, or one end, then soldered them back incorrectly.
And there may be more than one instance, where you will have to check both ends of every component in an area.




Both Traders and Philips circuits show no mains earth connection.
Caution re safety and earth hum loops interfering with audio.
And I suggest you replace "RF Interference " cap, C64 Trader, C75 Philips, just "on spec".



I think a mistuned IF stage, where it is mistuned away from IF centre freq with a resultant wider bandwidth normally will not give AF distortion, just a lower RF signal presented to the detector, and possibly better audio "highs".
Mistuning may remove some tendency to oscillate.

Have you carried out a whole receiver tuneup, as per Philips and or Trader?
As others have said, the instructions are crucial for a set like this.



One thing that is missing from this thread is a check of overall AM sensitivity, and of AGC performance, with test figures for both.
I cannot find specific performance specifications in either Trader of Philips??
Can you please carry out industry standard tests, on AM, and report back.
Watch for hum loops, Sig Gen to Receiver.


More later.
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Old 9th Nov 2021, 11:10 am   #106
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Default Re: Philips 543A problem detector stage

Thank you Radio1950 and Paula for your comments. I'll carry out the tests you suggest.
I made a audio file from my phone (using Easy Voice Recorder) previously which I tried to send to this forum but it would not attach. How do I make a dummy pdf ?

I bought this set a few weeks ago but have no knowledge of it's history however it looks quite untouched apart from the replacement which I reported on post 94 about the replacement of the V2 anode feed resistor and it's associated decoupling C. The resistor is now the correct value of 2k2. I have also replaced the black tar capacitors.

I had to replace a couple of capacitors on the FM side which now works fine (no distortion).

Regarding A.M. Yes I have had it perform with no distortion for periods of time. This can sometimes be brought about simply by the slightest touch with the scope probe on the connections on the first IFT (see some of my previous comments in this thread).
I have lifted one end of many components around this IF and measured them with my LCR bridge all OK.
I have not resorted to a receiver tune up because of the fact that I have been able to restore correct operation as described above so I suspect it's something else and I want to be logical about this.
However I decided to do another experiment today just to prove that this is not an audio amplifier fault (although I knew in heart of hearts that it isn't) I fed the detected audio (disconnected from the Philips amplifier at C66 (Philips) / (C48 Trader) and fed it into the Gram input of my Ferranti 255. As expected the audio reproduced by the Ferranti amplifier produces the same low, distorted sound.

I'm not sure what you mean by 'Industry standard' tests so would appreciate some details. I will then certainly carry them out.

Thanks again
C
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Old 9th Nov 2021, 12:13 pm   #107
Christoffrad
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Default Re: Philips 543A problem detector stage

Here are some examples of the audio recorded via my phone then zipped. However, it is not so easy to hear the way the bass modulates the higher frequencies from this.

Chris
Attached Files
File Type: zip Audio of distorted sound 6.zip (375.6 KB, 46 views)
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Old 9th Nov 2021, 12:57 pm   #108
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Default Re: Philips 543A problem detector stage

If you have a wobbulator you could see what the overall IF response curve is like now or I guess you could map it out by hand but I am not sure what that will tell you. I would do the alignment then investigate if the distortion is still present.
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Old 9th Nov 2021, 3:47 pm   #109
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Default Re: Philips 543A problem detector stage

That does sound like an overload. Which station was that first one?
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Old 9th Nov 2021, 3:48 pm   #110
Christoffrad
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Default Re: Philips 543A problem detector stage

As I repeated the IF bandwidth test I realised that I had previously made incorrect connections so please ignore my post 99
I have now done the test according to the latest suggestion by Paula...
ie 10k connected across the 2nd IF primary (V3 load) and AGC line grounded
I killed the oscillator and open circuited the switch connection between front end and V2 grid then fed the signal generator into V2 grid via 0.1uf
here are the results
for an input 5mV emf, the signal at V3 anode = 1.4v p-p peaking at 470.4kHz
6dB voltage points found to be 463.5 and 477.3kHz = 13.8kHz BW (6dB)

for an input 50mV emf, the signal at V3 anode = 12.4v p-p peaking at 470.6kHz
6dB voltage points found to be 461.9 and 479.3kHz = 17.4kHz BW (6dB)

For an input 200mV emf, the signal at V3 anode = 45v p-p peaking at 470.45kHz
6dB voltage points found to be 460.7 and 480.2kHz = 19.5kHz BW (6dB)

measurements taken with x10 probe. Since the readings were taken off a scope screen there is obviously some potential tolerance on the voltage measurements.

This suggests to me that the IF tuning for IFT1 is OK

If the AGC line is made active by removing the grounding the output on V3 anode for 200mV input changes from 45v p-p to 32V. I would have expected a bigger change.
For a 5mV input there's no change when AGC is or is not grounded of course.

Last edited by Christoffrad; 9th Nov 2021 at 4:14 pm.
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Old 9th Nov 2021, 5:00 pm   #111
Christoffrad
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Default Re: Philips 543A problem detector stage

Replying to Peter_sol
The audio file was selection of stations including Caroline(first one), Gold, Absolute, and Talksport.

The audio from the set is actually very low requiring near full volume setting.
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Old 9th Nov 2021, 5:33 pm   #112
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Default Re: Philips 543A problem detector stage

That recording sounds horrible. As Peter_sol said, it sounds like overloading. With the AGC short removed, can you measure the screen grid voltage on the ECH81 and the EBF80 as you tune through a strong signal.

What you should find is that the screen voltage rises significantly on a strong signal compared with what it is on a weak one.

If it doesn't, it suggests either that there is no AGC voltage being generated (measure across the AGC capacitor - the voltage should become more negative on a strong signal) or there is a connection problem between the AGC feed and the control grid (g1) of the ECH81 or EBF80 valves.

The selectivity measurement looks a lot better than the one posted yesterday, so it's probably not an alignment issue in the first IF transformer.

Just seen your latest post. 32V is a high signal level and should give a very high output volume. I suspect now that your issue is one of the filter capacitors at the output of the detector. C38 or C39 (Trader numbers) should be checked. A low resistance leak there will upset the AGC and audio in one go.

Paula

Last edited by frsimen; 9th Nov 2021 at 5:40 pm. Reason: Added follow up about latest post.
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Old 9th Nov 2021, 5:51 pm   #113
Christoffrad
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Default Re: Philips 543A problem detector stage

Paula
Are you suggesting I do this using a fairly strong broadcasted signal or one fed into V2 grid from the generator as above?
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Old 9th Nov 2021, 6:10 pm   #114
frsimen
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Default Re: Philips 543A problem detector stage

You could use either. The signal generator will give you more control on the signal levels.
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Old 10th Nov 2021, 9:26 am   #115
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Default Re: Philips 543A problem detector stage

Is it possible that someone has attenuated the audio on AM and that's why it appears low? Obviously you may still have a fault causing distortion.
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Old 10th Nov 2021, 12:23 pm   #116
Christoffrad
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Default Re: Philips 543A problem detector stage

Paula, Since the closure of R Cambridgeshire, Radio5 is my strongest station. If I tune through this, there is no change in the screen voltages. However if I inject 5mV into the ext antenna socket the screen volts on both valves just moves slightly (by 2v) and increases significantly more as the injected level is further increased (eg increases by 28v on V2 scr and by 23v on V3 scr at 50mV emf input)

Peter - there has been no alteration to the circuit to attenuate the audio.

I suspect the distortion arrises because the detector receives such low signal.
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Old 10th Nov 2021, 2:39 pm   #117
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Default Re: Philips 543A problem detector stage

I have also done the AGC check using the loudspeaker voltage change method:

for a change across the loudspeaker from 1.25V to 4V p-p (10dB) the signal generator must change from 15mV to 170mV emf (21dB)
Similarly, for a LS change from 400mV to 1.25V (10dB) there is a change of generator input from 4.5mV to 16mV which is 8.6dB

And here's a test repeat:
for a change 200mV to 632mV (10dB)(loudspeaker) corresponds with 5.5mV to 17mV (generator) = 9.8dB
and 632mV to 2V (10dB)(LS) corresponds with 17mV to 45mV (generator) = 8.2dB
and 150mV to 474mV (10dB) (LS) corresponds with 640uV to 1.2mV (gen) = 5.5dB


So there does seem to be an AGC problem

Last edited by Christoffrad; 10th Nov 2021 at 3:02 pm.
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Old 10th Nov 2021, 4:22 pm   #118
Christoffrad
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Default Re: Philips 543A problem detector stage

Today ( following on from the tests above) I built a duplicate detector filter and AGC. I used new components and attached it to the detector IFT and disconnected the existing filter/AGC components.

The result is the same low distorted audio as before.

If I ever solve this case I will do justice to a good bottle of wine!

before I remove thse parts and restore the old circuitry, are there any measurements I should make?

C
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Old 10th Nov 2021, 4:45 pm   #119
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Default Re: Philips 543A problem detector stage

Tune in a weak station and attach a short wire to V2 grid. If volume improves you have an issue with the aerial coils. I still think that any testing is of limited value until you have done a full alignment. The alignment should pickup any tuned circuit faults and then you will know it is correctly setup and you can investigate any remaining issues. For example, a failed IFT secondary capacitor would still provide coupling but you would lose gain and alignment would see that as a failure to peak.

It's very unlikely to be AGC as you have already noted it is lacking sensitivity.
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Old 10th Nov 2021, 6:54 pm   #120
frsimen
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Default Re: Philips 543A problem detector stage

Curiosity got the better of me. I opened up my Grundig 2068 to see what happens when some fairly basic faults were applied. The circuit on AM is fairly similar to that of the Philips, so I would expect similar behaviour with one either set.

The AGC didn't make much difference when it was disabled, so that was crossed off the list.

The second try was to apply a short circuit capacitor in the secondary of the IF transformer that feeds the detector. The result? Horrendous distortion and much lower audio output. Not much AGC action either. So, that looks promising as the cause of your trouble The offending capacitor is C37 (trader reference).

If you follow PJL's alignment advice, you will find that detector tuned circuit won't align very well if C37 is faulty.

Paula
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