Philips 334A Problem Hum
 

I’m by no means new to restoration but this one has me scratching my head. A pronounced speaker hum which appears immediately on powering the set up. It should be a simple matter but so far it’s escaped me.
Some background:
- A 1933 AC mains model with indirectly heated cathodes throughout
- Hum appears before valves have even warmed up
- Is apparent and unchanged even with HT disconnected
- Hum unchanged with output valve removed
- Speaker is PM with no field coil
- Power supply fully restored and recapped
- Filaments are supplied with 4V AC centre tapped winding
Is it possible that the speaker is responding to an induced voltage in the output transformer ? It is mounted off chassis and some distance from the power transformer so it would seem to be fine.
Wondering of the CT filament supply could be the culprit. I haven’t tried it yet but will see if going the DC route will make a difference.
Any suggestions welcomed...thanks.

Re: Philips 334A Problem Hum
 

Check the integrity of the insulation all around. One side of the transformer might be non-intentionally referenced to ground by fraying / damage and a circuit formed where it shouldn’t be, even a tiny current.

Re: Philips 334A Problem Hum
 

If it still hums with the output valve removed then it is unlikely to be a problem with the output valve filament supply. Somehow you are getting an AC ripple onto the output transformer even with no output valve fitted. That is weird. Does it still hum with the rectifier removed? Cheers, Jerry

Re: Philips 334A Problem Hum
 

Yes, it is odd and remains with the rectifier out. Why I am also thinking it's some sort of leakage/induced current situation. I've tried removing the mains earth connection to the chassis too.

Re: Philips 334A Problem Hum
 

I don't know this set, but you sometimes get inductive coupling between the mains and output transformers.

Re: Philips 334A Problem Hum
 

Yes Paul is right. It's all a question of how the output transformer is situated in relation to the mains transformer......the layout of some sets is less than ideal! It isn't always just proximity but ideally the output and mains transformers should be at right-angles to each other to help cancel out any stray fields produced.

Some manufacturers overcame the problem by mounting the output transformer below the chassis.

I have several sets that show this phenomenon, most notably is a 1955 Baird AM/FM radio. There is a slight hum from the speaker as soon as it's switched on...it'll do it without any of the valves in place! When I was working on it many years ago I experimentally moved the output transformer around and could minimise it but it would have meant mounting the transformer in a very awkward place so I simply fitted it back as original. In practice, the hum is very low and not noticeable when listening at normal volume.

It was much less of a problem with fully screened 'drop-through' transformers.

Don't know what to suggest to you and it may well be normal. A certain amount of hum was always accepted as 'the norm' back in the day.

Re: Philips 334A Problem Hum
 

Does the transformer become warm quickly?- I'm wondering if it could have shorted turns in a way that is causing excess stray field.

Re: Philips 334A Problem Hum
 

Yes, it's looking like this stray coupling is the most likely cause. The hum level is really high...well above the normal levels one usually gets. The two transformers are separated by about 6 inches but that may well not be enough. I'll see what I can do to increase that and maybe also put the output one in some sort of shield. Given the 1930s vintage it's possible that the transformer designs were less than ideal too. Thanks for the help folks.

It runs cool and the input current is within spec so hopefully that isn't the case.

Re: Philips 334A Problem Hum
 

There's a Philips 334A playing here on youtube https://www.youtube.com/watch?v=_ykQBC5aluw doesn't sound as if there is much hum there, albeit the bass response of my laptop speakers is abysmal so I probably wouldn't hear it anyway. Cheers, Jerry

Re: Philips 334A Problem Hum
 

it is odd and remains with the rectifier out. Why I am also thinking it's some sort of leakage/induced current situation. I've tried removing the mains earth connection to the chassis too.[/QUOTE]

I do not have a circuit diagram for the 334a but if it uses a 1823 rectifier removing the valve would make no difference as the mains transformer would still be in circuit as the rectifier heaters are directly heated, you would have to break the ht + feed to output transformer,the mains transformer may be leaky between primary and rectifier heater windings , I am assuming the circuit is similar to 333a, if this is the case you may be putting AC on to the Electrolytics, A radio engineer had this problem 50 years ago with a McMichael, that finished up with the cap blowing the top of the cabinet off, I was standing beside him when it happened, it looked like he had been tarred and feathered
John

Re: Philips 334A Problem Hum
 

Have you investigated my suggestion in #2?

Re: Philips 334A Problem Hum
 

Not yet but thanks. Plan to take the chassis out of the cabinet this weekend to investigate the various ideas.

Re: Philips 334A Problem Hum
 

When I last tackled it I also suspected AC through to the smoothing caps but I checked and there's very little ripple. Plan to test transformer insulation resistance this weekend, as best I can at least. It uses the 1823 rectifier. Valve lineup is E446 mixer, E409 osc, E446 IF, E444 det and 1st AF, E443H output.

Re: Philips 334A Problem Hum
 

Here's the schematic for the 334A. It's the tropical version of the 336A with an extra SW band (95-200m), no LW and fitted with as multivoltage mains transformer. Bought it some 15 years ago but it say in my parent's attic till recently when finally I took it down and tackled it.

Re: Philips 334A Problem Hum
 

Somewhat unhelpfully, the schematic doesn't show the PSU.......

Re: Philips 334A Problem Hum
 

The complete schematic (from Radiomuseum) shows output valve as directly heated. Biassing done with resistors in HT return line. Don't know if this helps any.......?

Re: Philips 334A Problem Hum
 

I have had a similar problem in a different model Philips radio and found that a mu-metal plate made from a screening of an old oscilloscope reduced the hum to an almost inaudible level.
There was no space to move either the output or power transformer, and shows less than displaced transformers.

I hope that you can find a satisfactory solution,
with kind regards,
Desmond

Re: Philips 334A Problem Hum
 

The PSU is there, bottom left of the schematic on p2 once it's rotated for normal view.
I have the chassis out this morning to try a few approaches.

Re: Philips 334A Problem Hum
 

Good call Desmond. I'm going to try that with a small metal box I have from an old sig gen.
To your point I found this pic of another 334A radio showing what could be a shielded version AF transformer mounted on the speaker. Mine is up there too but its an ordinary open frame transformer.

Thanks, Paul

Re: Philips 334A Problem Hum
 

An update and feedback for everyone who kindly helped with suggestions. I removed the output transformer and extended the wiring from the output valve to give me some additional distance from the chassis. Then put it in an aluminium box to test for hum reduction. It dropped significantly - maybe by about half. I wasn't expecting it to disappear altogether and it hasn't disappointed ;D ! So I will be mounting it higher up in the cabinet and in the metal box. A good enough outcome and thanks again to all. I also checked the power transformer secondary and associated AC wiring isolation from ground and no apparent problems there.

Just very happy to have this wonderful old set going again. Amazed by how sensitive and selective it is for such an early design.

Cheers
Paul