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Old 16th Sep 2007, 1:41 pm   #1
squinancy
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Default Iffy reception in Philips 450A

Not being rooted in proper professional traning I am seeking an explanation for an annoying phenomenon in one of my Philips sets. This Philips 450A is a wartime model with an AZ1, ECH4, ECH4, EBL1 line-up. What could explain that, especially in shortwave but to some extent in medium wave as well, stations do not "burst in" as they should during tuning but need to be approached at the presumed dial position from left to right and right to left several times slowly waiting till a particular reception position from grainy crackles becomes stronger and establishes itself. After that the station becomes stable and the qulatity of the sound ok. Local fine-tuning then becomes possible with slight adjustments of the knob. However, if one turns the knob well away from the station it will again become elusive, hard-to-get, dropped as it were, until fished back again. Also, the first sounds of the station are grainy, non-sharp and cracky as well as weak. A few (one or two) stations with very strong reception signals are exceptions to this. The set does not have week reception as such, but the stations need to be detected in this way before they can be consolidated. They hide behind a screen.

The original valves have been changed though not to new/old as the ECH4s seem to have prohibitive price tags. The 32 - 32 electrolytic has been replaced with a healthy 47 - 47. The aerial is also decent. I thought it was insufficinet voltage somewhere but cannot detect low voltage. As before, I am willing to learn.
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Old 16th Sep 2007, 1:58 pm   #2
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Default Re: Iffy reception in Philips 450A

Sounds like it could be an AGC problem. Have you checked/changed the capacitors and high value resistors connected to the AGC rail?
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Old 16th Sep 2007, 2:25 pm   #3
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Default Re: Iffy reception in Philips 450A

Not the resistors. I will see if I have suitable values and will go ahead as suggested. Thanks.
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Old 16th Sep 2007, 3:16 pm   #4
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Default Re: Iffy reception in Philips 450A

If it was me, I would hook a meter to the AGC line and see what happens when a station is tuned in. It's not necessary to know what voltage should be on the AGC line at any one time...it will vary from station to station in any case. I'm just wondering about grid current in one of the controlled valves. The AGC should always be negative with respect to chassis and even with no signal, should be slightly negative.

I had a set (Philips 470A) that had problems on the AGC line because the AGC rectifier diode in the PEN4DD was leaky after about five minutes running and gradually got worse. The AGC line eventually went positive and the mixer and IF valves became very unhappy.....!


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Old 16th Sep 2007, 9:05 pm   #5
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Default Re: Iffy reception in Philips 450A

Not unlike my experience with this Philips, as there is definite deterioration with more running time. Will need to take a good hard look at the issues that Rich is mentioning, perhaps investing the necessary funds into unused valves in addition to changing the resistors.
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Old 17th Sep 2007, 11:42 am   #6
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Default Re: Iffy reception in Philips 450A

This is more of a question than a suggestion.
Is it possible to temporarily substitute a germanium signal diode for the AGC?
It's a good thread this one!
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Old 17th Sep 2007, 1:11 pm   #7
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Default Re: Iffy reception in Philips 450A

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Is it possible to temporarily substitute a germanium signal diode for the AGC?
That is exactly what I did! You have to identify which diode connects to the AGC line and then disconnect the connections at the valveholder. You then simply substitute the germanium diode for the valve. In my 470A, the set ran perfectly with this arrangement until I was able to replace the PEN4DD and return the circuit to original.


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Old 17th Sep 2007, 6:01 pm   #8
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Default Re: Iffy reception in Philips 450A

This is learning in the fast lane for me.
I haven't got a germanium signal diode at the moment but will start looking, especially as this symptom might repeat itself in other comparable sets. Valuable thread indeed, worth archiving. Thanks as ever.
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Old 18th Sep 2007, 12:00 pm   #9
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Default Re: Iffy reception in Philips 450A

If the behaviour is exactly the same when you tried other valves it is most probably something else so don't go out and buy replacements yet. The simplest and probably best method of testing a valve is leaving it in circuit and measuring the pin voltages - you are then testing it as it is going to be used.

Unfortunately I don't have a circuit for this. A faulty AGC capacitor normally leaves the set on full gain and is unlikely to cause the symptoms you have described. Similarly Richards leaking AGC diode caused the signal to deteriorate as the valve warmed up so it's unlikely to be that.

I would start with Grahams suggestion of checking the high value resistors normally found in the AGC circuit. Following that you'll need some guidance from someone with a circuit diagram to check the valve voltages!
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Old 18th Sep 2007, 4:45 pm   #10
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Default Re: Iffy reception in Philips 450A

Richard Newman:
Quote:
If it was me, I would hook a meter to the AGC line and see what happens when a station is tuned in.
Have you tried this, yet? It seems the obvious first step. If the AGC line is going positive at any time, it will wreak mayhem: the RF valves will start to go into grid current, their input impedance will drop dramatically and damp their tuned circuits to the point where they might just as well not be there.

Another phenomenon that can cause spurious results is grid-emission. This can happen with very old valves - emissive material gets evaporated off the cathode, and deposits itself on any nearby cooler objects. The control grid is the prime objective. As the grid warms up in normal usage, it starts to behave as a little cathode all by itself, and the grid voltage thereupon goes walkabout. Valve replacement is the only cure if this happens.
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Old 18th Sep 2007, 5:37 pm   #11
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Default Re: Iffy reception in Philips 450A

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Originally Posted by squinancy View Post
I haven't got a germanium signal diode at the moment but will start looking, especially as this symptom might repeat itself in other comparable sets.
Any scrap AM transistor radio will contain a suitable diode. You can also use the base-emitter or base-collector of a germanium transistor, or use a Schottky silicon diode from a scrap switch mode power supply: http://en.wikipedia.org/wiki/Schottky_diode

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Old 18th Sep 2007, 7:13 pm   #12
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Default Re: Iffy reception in Philips 450A

I think it very unlikely to be the detector given the description of the fault so I would not worry about the diode substitution for the moment...AGC voltage and resistors first
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Old 18th Sep 2007, 10:58 pm   #13
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Default Re: Iffy reception in Philips 450A

Many thanks for all this. Very educational and eye opening. High value resistances and the AGC valve the main suspects. Unfortunately with the coming of autumn my teaching job is increasingly encroaching on my ability to spend enough quality time with the radios, which is my activity of choice. Plus I am going to a conference soon. But all the suggestions in connection with the Philips and the other issues will be followed up conscientiously.
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