Philips BSF 752A

Hello Radio Friends!

I have restored a Philips BSF 752A and am quite pleased with the result. However there is a problem, the HUM. This hum is starting immediately when switching on the Rx and I suppose it has nothing to do with rectifiers or smoothing capacitors. The hum is present even when rectifiers or/and power tubes (EL84) are pulled out. So, is it possible that the hum is inductively coupled to the loudspeaker transformers since these are very near the main transformer and if so, would it help to change to positions of them?
This equipment is made in Finland and very much alike the BX640A made in Netherlands which schematics I’ve used. I’ve also checked the earth points, which seems to be in order. It would be interesting to see what the experts say.
An image of the power stage can be seen from the following picture:
Attachments:

• Philips BX640A Schema (51 KB)

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Hei Lars-G. Lundelin

Noin äkikseen epäilen etteivät elkojen C1, C1a ja C2 kuorien kontaktoinnit maahan ole kunnossa. Pitkään käyttämättömänä olleeseen - varsinkin kosteissa tiloissa säilytettyyn - radion alumiinit kuten elkot muuttuvat johtamattomiksi al-oksiideiksi.  Esim C1 ja C1a jäävät kellumaan ollen toistensa kanssa sarjassa. Lataus/purkaus-tilanteessa toinen tai toinen on oikosulussa joten tilanne on hallitsematon. Näyttäisi siltä että B6:n muuntajan käämi S38 kautta kulkevat sykkivät latausvirrat indusoituvat toision kaijuttimiin niinkuin tuleekin tapahtua. Voi olla että ++ ja +-alueella on pahasti vuotavia virtarosvoja jotka yksin tai em syiden mukana ollen johtavat tämän sykkivän DC-jännitteen syöttämää virtaa maahan.

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Hello Lars,

It would be also great for other members to see some pictures of your BSF752A.
So could you please enter this "new model" in radiomuseum.org and upload the
pictures.
If you need help, please don't hesitate to contact me by Email

Terveisin

Tomppa

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Hi Lars_G,

Pls have look to attached picture. In the text there is instruction to is similar kind symptom you described.
(sorry the text is in Finnish)

regards
Ake Attachments:

• Philips_hum_sml (57 KB)

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Hi Tomppa!

Yes, you are quite right concerning the missing pictures, and I’m working on it. The reason why I’ve not yet uploaded my collection is that I do not own a proper camera. But I promise that these will be exhibited within a few months.

With kind regards

Lars

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Hi Eero!
Thank you for your suggestions concerning my hum problem. As you mention, the chassis is really covered with aluminium oxide and it's possible the earthing is not sufficient. The smoothing capacitors are renewed (all three)and earthed with copper wire to a screw in chassis, but going to recheck again.

Kiitokset neuvoista.

terveisin
Lars

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Hi Ake

Thanks a lot for the information. Very intersting, this seems to be a probable cause
to the problem.
By the way, the Ge-transistors arrived and they are already on place and I can now enjoy the stereo sound with both channels working.

regards

Lars

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Hello again Åke!

Your advice concerning the hum in my Philips BSF 752A cured the problem and the Rx is now almost free of background humming. A quite simple operation by exchanging the loudspeaker tapping on both windings (primary and secondary) on the treble transformer did the trick.
Thank you once again.

Terveisin

Lars-G.

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Hi Lars_G,
It's good to hear that both "problems" (Ge and hum)has been solved. It was just a coincidence that I saw the BSF 752A service documets when placed one other Philis document to my Philips folder.
I wish nice sounds from the both appliance !

terveisin
Ake

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