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Old 25th Sep 2021, 12:22 pm   #1
Christoffrad
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Default Philips 543A Low HT on FM

My Philips 543A has low audio and sensitivity and does not work on FM The HT on A.M. is low and on FM very low.

Component references below refer to the manufactures service sheet with the Trader sheet 1208 in perenthesis ( ).

I have checked the rectifier EZ80 as good even at 120mA with the Avo valve tester.
Smoothing caps C74 (C66), C1 (C62), C2 (C61) and C25 (C47) have been checked by temporary replacement. C59 (C54) and C60 (C55) have been replaced because V6 current was excessive (now OK).

The rectifier output across C74 (C66) was found to be 275v AM, 220v FM
HT across C2 (C61) is 250v AM, 75v FM (should be 285v and 190v respectively)
By removing the HT feed to V1 by isolating tag 22 of SW.F2. (inserting an insulator between the switch contacts) the HT on C2 rises from 75v to 235v

From the voltages given for the V1 anodes and the normal HT it can be calculated that V1a has an anode curent of 7mA and V1b 6mA which is equivalent to a 15k load on the HT supply. When this load is simulated the HT becomes 235v. If V1 is operated with the V1b 10k load resistor disconnected the fault remains, and also if the valve is removed altogether so the problem seems to lie amongst the few components of the V1a anode circuit.
Before I dive in amongst these cramped and seemingly unlikely suspects I thought I'd canvass opinion.
Has anyone had this problem?

BTW there seems to be some circuit differences in the way that Trader have interpreted the Philips service data around the AM/FM changeover switch. Philips seem to have it correctly drawn.

Chris
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Old 25th Sep 2021, 12:28 pm   #2
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Default Re: Philips 543A Low HT on FM

Oops Correction
If the load is simulated with the 15k resistor the HT becomes 190v as it should be.
(not 235v as stated above)
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Old 25th Sep 2021, 1:41 pm   #3
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Default Re: Philips 543A Low HT on FM

Sounds like shorting vanes on C6 tuning gang. Mind you, with C22 in series that would have to be s/c as well. Can you measure the actual anode voltage on V1a with the fault present? A useful check might be with power off and the SW.F2 switch insulated to measure the resistance between V1a anode feed supply and chassis. Then work your way down the anode feed resistor chain. There's always the possibility of a solder blob or stray wire shorting the anode pin to chassis. The other area to look at is that for some reason there only seems to be a grid 2 feed to V4 on FM so check if V4 is drawing excessive grid 2 current on FM. Jerry

Last edited by cathoderay57; 25th Sep 2021 at 2:10 pm.
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Old 25th Sep 2021, 1:58 pm   #4
Christoffrad
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Default Re: Philips 543A Low HT on FM

V1a anode measures 0.5v
That point also measures 4k7 ohms to earth
measurement is independent of position of the tuning C
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Old 25th Sep 2021, 2:47 pm   #5
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Default Re: Philips 543A Low HT on FM

Quote:
Originally Posted by Christoffrad View Post
BTW there seems to be some circuit differences in the way that Trader have interpreted the Philips service data around the AM/FM changeover switch.
Out of interest what are the differences in the Trader schematic?

Lawrence.
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Old 25th Sep 2021, 3:10 pm   #6
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Default Re: Philips 543A Low HT on FM

The offending component has been found! -- C28 (C7 on Trader sheet) 4n7
The common V1 Anode HT decoupling
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Old 25th Sep 2021, 3:30 pm   #7
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Default Re: Philips 543A Low HT on FM

Hi Lawrence
After further study It seems to be right.
The Philips data shows an accurate representation of the wafer switch SW.F2. for AM/FM both schematically and the physical layout in the form of a series of changeover switches.
The Trader sheet shows this switch as descrete switches so whilst it's electrically correct it's not quite so intuitive when you try to relate the physical switch operation to the circuit.
So the changeover switch that controls the HT feed shown on the extreme right of the Philips circuit is represented by Trader as though it were two SPST switches at opposite sides of their circuit diagram.
Perfectly fine electronically but not so easy to relate to in practice.

Chris
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Old 25th Sep 2021, 3:58 pm   #8
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Default Re: Philips 543A Low HT on FM

Yes but S29 and S15 are both shown on the same wafer, that in conjunction with the switch table shows what's what, I like the consistency of the Trader representation of switches, they are much easier to understand than in some of the manufactures info.

Lawrence.
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Old 25th Sep 2021, 4:16 pm   #9
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Default Re: Philips 543A Low HT on FM

Yes as a circuit diagram the Trader is more understandable, and the table defines the functions fine. However I was in the position of having to determine how I could fault find in a practical way by isolating certain functions and for that it was necessary to be able to translate the electronic task to the physical entity. So in this particular circumstance the Philips sheet won the day.
Now the FM works but there's some work to do on the A.M. and audio side which remains low on AM and tending to a bit of distortion especially on bass.

Regards
Chris
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Old 25th Sep 2021, 4:40 pm   #10
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Default Re: Philips 543A Low HT on FM

Was there any smoke ? I would have measured C7 first go.

Lawrence.
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Old 25th Sep 2021, 4:46 pm   #11
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Default Re: Philips 543A Low HT on FM

No smoke.
But I keep an eye on those electrolytics and feel the can for warmth!

Tomorrow I may embark on the A.M. and audio issues.
Finally there will be the cloth to consider. There's a hole in it. I have a contact in The Netherlands who keeps Philips genuine cloth but it can be pricey and I only paid 5.50 for the radio!

Chris
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Old 29th Sep 2021, 8:44 pm   #12
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Default Philips 543A problem detector stage

The next problem I'm facing with this set is a distorted output from the detector V3. V3 is a EBF80 double diode pentode which I have tested on my AVO valve tester as having a gm of 3 (spec is 2). The distortion is readily attributed to the fact that the valve is running into it's non linear operating characteristic because the anode current is only about 4.5mA. And the crushed output waveform can be clearly seen on the scope.
I have checked the anode load (2k2) which has just 10 volts across it (should be about 90v) The screen volts are about right at 73v. The AGC line is -0.9v (measured across C49 (Philips date sheet ) (C24 Trader sheet).
The voltage on V3 grid1 is -0.9v (should be -1.25v) which sugests this valve should be harder ON than it is. The Anode load and Screen grid resistors (2k2 and 82k) have been checked. Unfortunately non of my other sets uses this valve to be able to check by substitution.
Not surprisingly there is also distortion on FM where the valve is used as a IF amp.

Any ideas please?

Chris
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Old 29th Sep 2021, 10:11 pm   #13
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Default Re: Philips 543A problem detector stage

Have you checked R16/C41 ( makers manual), the HT decoupling components for the anode of the EBF80 pentode?
6.8k ohm and 6800pfd
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Old 30th Sep 2021, 12:39 am   #14
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Default Re: Philips 543A problem detector stage



What anode load on V3? Its anode load is a couple of IFT primaries. There's only the decoupling RC in series with them.

Typical Ia for the pentode is given as 5mA in the datasheet so 4.5mA doesn't seem way out of line.
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Old 30th Sep 2021, 9:44 am   #15
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Default Re: Philips 543A problem detector stage

I think they were having a bad hair day when those voltages were recorded....

Lawrence.
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Old 30th Sep 2021, 10:07 am   #16
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Default Re: Philips 543A problem detector stage

Hi Chris
The V3 anode load is shown as R16 (Philips data) plus the two IF primaries.
R16 is given as 6k8 (In this set Philips have fitted a 2k2)
The data sheet says Va should be 195v in AM mode and the HT rail should be 285v so Ia should be (285-195)/6k8 mA =13mA
My HT rail is a bit low at 230v and I measure 12 across the 2k2 = so 5.5mA
I attach two oscilloscope displays
First is V3 anode receiving AM signal when 1mV is injected into V2 grid ( 470kHz with mod 400Hz)

The second is the detected signal after the IFT ie junction C47/R39

The compression of positive swings can nbe seen.

Any ideas please


Chris
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Old 30th Sep 2021, 10:28 am   #17
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Default Re: Philips 543A problem detector stage

If I radiate a signal into the antenna this distortion seems to dissappear so maybe the above traces should be disregarded as due to my method of injecting into V2g.
Yet the distortion on live broadcast is very evident.
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Old 30th Sep 2021, 11:33 am   #18
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Default Re: Philips 543A problem detector stage

As both FM and AM are distorted, I would first check the audio stages through the PU input. What capacitors have you replaced so far?
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Old 30th Sep 2021, 11:47 am   #19
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Default Re: Philips 543A problem detector stage

Thanks PJL
If I feed audio into the gram input, it sounds OK.
I have replaced C63 (C55 on Trader sht), C60 (56), C62 (59), and C59 (54).
I plan to connect an external amp to verify the distorted detector output.
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Old 30th Sep 2021, 12:55 pm   #20
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Default Re: Philips 543A problem detector stage

I haven't got the circuit here but I'm pretty sure the IFT primaries in series connect to the anode of the EBF80. Have you checked either of the primaries for resistance? One of them could be O/C and it's just leakage feeding the valve anode (been there before...led me a merry dance until I realised what was happening). Not uncommon for the windings to suffer 'green spot' so check for primary resistance first....will probably only be 50 ohms or so but the circuit might give a guide or check the primary of one of the other IFTs to get an idea. There won't be too much difference between them.

One of the primaries being O/C would certainly account for the low anode volts.
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