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Old 10th Nov 2004, 6:57 pm   #1
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Default Philips 470A instability

Hi Guys.

Well the resoration of this set has gone well so far. Yesterday I posted a request for help obtaining a PEN4DD in the appropriate section but this morning my normal supplier came up trumps by ringing me to say he had found a NOS one 'in a dusty forgotton corner of his warehouse'! So I picked it up this morning and fitted it. However the set now comes on in an unstable mode. It sounds like IF instability which seems to disappear after a few minutes. This has only happened since fitting the new PEN4DD. Has anybody else come across this with PEN4DD's? I've tried decoupling while the fault is on (like extra IF filtering) but nothing stops it. Alignment is spot on. Prodding around under the chassis (moving wiring etc) again no difference. If I fit the old valve, it works fine except for the volume reduction after a few minutes due to the duff diode.

Very puzzling as the only interaction the output valve has is with the two internal diodes.

Any ideas?

Rich.
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Old 10th Nov 2004, 10:14 pm   #2
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Default Re: Philips 470A instability

Richard, Check very carefully that the metallising is earthed properly on the IF amplifier valve. Very often, there is a bad contact at the base/glass junction, which can be fixed by winding some 5A fuse wire around the base at this point and soldering it (carefully) to the lead which earths the metallising. Other than decoupling, I have known high value (well out of tolerance) screen grid resistors cause instability on Philips sets of this vintage.

Best of luck!

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Old 10th Nov 2004, 11:27 pm   #3
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Default Re: Philips 470A instability

Hi Rich

If you are still in trouble after trying Quantum and Leon's suggestions, I have a couple of thoughts.

1) Maybe the IF filter capacitors after the detector are letting RF (IF) into the output stage and need replacing. C16 & C20 (both 800pF) Trader Sheet 358.

2) Maybe the new output valve is a bit keen to go into RF oscillation. Try increasing the grid stopper from 10k to 22k.

Last edited by Paul Stenning; 6th Mar 2005 at 12:14 pm.
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Old 10th Nov 2004, 11:52 pm   #4
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Default Re: Philips 470A instability

Hi guys.

Thanks for your ideas....both checked before posting my plea for help !! Just for info, there is no cap between anode of the output valve and earth and the cap across the transformer primary was replaced along with all the others with a good quality 1500v type. The metalizing on the IF amp valve is good.

Anyway having played around for some time, I think I know what is happening . It is definately to do with the new output valve . It appears to oscillate at HF when first powered up. I confirmed this by connecting a 'scope to the anode. It would seem that this oscillation is being coupled into the I.F by the detector diode. It's acting like a BFO :! If I disconnect the detector at the valveholder pin and use a germanium diode instead, it all works.

Now I think know what the trouble is I have to get another replacement valve...it does have a feint blue glow inside when viewed in darkness so perhaps this is the cause but I have yet to see an output valve that doesn't glow slightly in this way. However I have noticed that the problem seems to be improving...last time I switched on after the set was off for over an hour, the instability lasted for only a few seconds so I'm going to leave the set running all day for a few days and see what happens .

Rich.
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Old 10th Nov 2004, 11:58 pm   #5
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Default Re: Philips 470A instability

Hi ukcol.

Thats a good idea about the grid stopper...had'nt thought of that. All resistors are reasonably within spec...pretty good for 66 years!

This is the first time I've worked on a set using a double diode output pentode...seems that they can suffer from odd problems.

Rich.
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Old 11th Nov 2004, 2:03 pm   #6
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Default Re: Philips 470A instability

Well Colin,

Increasing the value of the grid stopper didn't work. However having played around again today, I have improved the situation by fitting an ANODE stopper. I fitted a 100 ohm in series with the primary of the output transformer and decoupled it with a .001uF. It helped a lot but I am not going to leave the set like this. I have put the circuit back to original and it now awaits another PEN4DD. I'm sure the instability is due to some problem with the valve. It's probably why my supplier found the valve in a dusty pile...left there probably as a returned faulty item many years ago.

By the way Colin, the I F filters C16 and C20 are shown on the circuit as .00008uF. I make that 80pF not 800pF. Is that right? I think 800pF would be shown as .0008uF.

The capacitors also have 80 marked on them. They are ok anyway having proved that by temporary subs.

Thanks for the suggestions from you all. Sometimes these things are very frustrating but it's all part of the fun and interest.

Best regards,

Rich.
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Old 11th Nov 2004, 5:15 pm   #7
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Default Re: Philips 470A instability

Hi Rich

You are right of course, 0.00008 is 80pF not 800pF.

In mitigation I must say that 80pF seems a rather low value to filter a 128kHz IF with a 50k for R10; mind you 800pF is a bit large.
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Old 12th Nov 2004, 9:05 am   #8
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Default Re: Philips 470A instability

Yes. They do seem a bit low. During my investigations, I tried up to 390pF as filtering but you start to lose treble if you go above about 150pF so the manufacturers obviously knew what they were doing.

Anyway the circuit is all back to standard now and it ran all day yesterday without any problems. The PEN4DD seems to be improving with use so I'll see how it goes.


Rich.
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Old 12th Nov 2004, 5:19 pm   #9
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Default Re: Philips 470A instability

You've got to have sympathy for the poor old PEN4DD; all those years sitting on a dusty shelf and now having to actually do some work.
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