Philips 584 (1934), hum problem.

[Sideband]

Hi Guys. A spot of advice from those members more versed in older models please!

I'm currently restoring one of these for a work collegue and it's all going very well at the moment. Last night was The Great Switch On and it all came to life after some 30 years of silence. Paper and electrolytic capacitors have been rebuilt with new components and apart from
alignment and reassembly, it's as good as finished.

HOWEVER
I've noticed a fairly high hum level. Not to the point where the sound is gargling or anything like that but a fairly loud hum in the background. My question really to anyone is 'What sort of hum level can I expect from this set'? Lets elliminate smoothing, heater/cathode leaks and incorrect wiring (as far as I can be certain). I know that the PM24M is OK (subbed) and the AF amp is OK (albeit somewhat microphonic). The directly heated output valve probably doesn't help although my 274 which uses the same valve doesn't hum as much. It is definately power supply related but I don't seem to be able to improve it much.


Any thoughts from the team?


Rich.

[Radio_Dave]

I don't know the model, but if it's AC only with a 'proper' isolating transformer then the 'trick' of the day was to swap the polarity of the mains.

It might be worth checking chassis/earth connections too. I notice many pre war radio's tend to have bolted down chassis tags which may not be making good connections anymore. Are the screened cables ok/are they still there?

David

[Sideband]

Hi Dave. Yes it has a mains transformer. I agree with checking screened leads as well. In fact I had to change the screened leads to the top caps as the old stuff had gone hard and was falling to bits.

Hadn't thought about changing the mains polarity. The mains input is an area that still needs attention as the original connector has been removed and I have to find an alternative.

I think I can also eliminate interaction between the mains TX and output TX as the chassis is still out on the bench. However one thought has just occurred to me. There is screening foil along the bottom of the cabinet which will obviously help when the chassis is back in place. Maybe I have some mains pick-up from the bench so I'll check that point when I get home tonight.


Keep the ideas coming!

Rich.

[Ed_Dinning]

Hi Richard, ensure that the vol control cover is earthed.
In bad cases it is sometimes necessary to remove the earth from the heater line and add a 100R pot across it with the wiper earthed. Adjust pot for min hum. It's known as a "humdinger".

Ed

[Sideband]

Hi Ed. Good point about the volume control cover. The hum doesn't vary when I touch the chassis but still worth checking. The set doesn't have a humdinger and I had thoughts along those lines. Not sure if this will work with the directly heated output valve though.....it already has a bias resistor and cap in the heater circuit. I'll have a look around there though.


Rich.

[Keith]

Richard,

I've had a similar problem with an M23 with a PM24M output valve. This set does have a humdinger with which a hum minimum can be obtained. I noticed, however, that substituting different output valves gives very different hum levels. Possibly more gain on some, I suppose, but I'm not convinced. Worth trying.

Keith

[Sideband]

Circuit is very similar to the one on Trader sheet 26 for the 588. There it shows an 800 ohm resistor connecting from the centre tap of the heater winding down to earth. Because of this, I dont think a humdinger could be fitted as it's the bias for the output valve.

If you pull out the AF amp valve (must remember that there is HT on the top cap....ouch) the hum remains so it's in the output stage or power supply. Could be the reccy I suppose. Haven't subbed that yet as there is a nice healthy 260v on the cathode. Tomorrow I'll spend some time trying to isolate it.


Rich.

[Sideband]

OK. Further tests today show that the problem is the SP4 AF amp. It's going into grid current. I first noticed that the anode volts were dropping as the set got warm. They started off at about 145 and dropped right down to about 30 volts after about half an hour. I thought it was the 320K resistor feeding the anode but it proved OK. I then measured the grid volts and they were up to 5 volts positive (WRT earth) by which time the volume had dropped considerably and the hum level had gone up. I didn't have an SP4 and the only pentode I had with a B5 base was an S4V. Same pinning so I thought it was worth a try even though it's classed as an HF pentode. Well it works, with no sign of grid current and the set has run all afternoon with this valve in it. The gain is lower and so now I seek a replacement SP4.

The hum level is acceptable now and remains the same so I'm happy there are no other problems.

Thanks for your ideas on this.


Rich.

[ronbryan]

Richard

Whilst I expect that the problem with grid current is down to the SP4 triode, it might be worth checking that the IF (or is it RF?) filter capacitor is working correctly and removing the hf part of the signal from the audio.

If the hf is not removed effectively, the signal will cause the af valve to run into grid current, and cause distortion, as I found with a Zenith H-615 set I repared for Howard a few months ago.

Ron

[Sideband]

Hi Ron.

There is a 100pF and a 200pF for I.F bypass. I tend to ignore mica caps as I have very rarely found them faulty. However they are worth a check just in case and they are fairly easy to get to. Never thought of that.

I must say though that the S4V is still working well as I write this and Talk Sport is loud and clear. Anode volts have remained steady at 165v since I switched on about 3 hours ago.


Rich.