The PHILCO Phorum
Post re-cap 38-4 hum. - Printable Version

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Post re-cap 38-4 hum. - hffenninger - 04-11-2016

New guy here, thank you for the add!
Philco 38-4
I just completely recapped this set, resistors were within tolerance, and I am very happy how it now receives. Impressive audio booms in, tone switch completely functional, sounds great! The only problems are that there is still a low hum and the low end of the band does not seem to receive as well as the upper end.
Thanks in advance for any suggestions.
Frank

RE: Post re-cap 38-4 hum. - klondike98 - 04-11-2016

Welcome to the Phorum!
Icon_wave

RE: Post re-cap 38-4 hum. - hffenninger - 04-11-2016

(04-11-2016, 08:46 AM)klondike98 Wrote:  Welcome to the Phorum!
Icon_wave

Thank you!

RE: Post re-cap 38-4 hum. - hffenninger - 04-11-2016

(04-11-2016, 12:58 AM)hffenninger Wrote:  New guy here, thank you for the add!
Philco 38-4
I just completely recapped this set, resistors were within tolerance, and I am very happy how it now receives. Impressive audio booms in, tone switch completely functional, sounds great! The only problems are that there is still a low hum and the low end of the band does not seem to receive as well as the upper end.
Thanks in advance for any suggestions.
Frank

Sorry for the newbie questions, but I saved 8 console radios from the dumpster and have acquired a great passion for these beautiful radios.  This is my first complete re-cap. I know I have so much to learn and appreciate the help. Could I add additional filter caps? Or, I see mention of isolating the hum by removing tubes, what would the procedure be for that?

[Image: https://scontent-lax3-1.xx.fbcdn.net/hphotos-xfl1/t31.0-8/12970904_1170543639631129_2797424576203600179_o.jpg]

RE: Post re-cap 38-4 hum. - Radioroslyn - 04-11-2016

May be some sort of RF interference generated by  a number of things like cellphone/laptop charger, led lamps, flat screen tv ect.. Anything with a switching power supply, they can make a lot of trash!!!!.

Terry

RE: Post re-cap 38-4 hum. - mikethedruid - 04-11-2016

First thing I would do is take a battery powered transistor radio, and listen to the AM band using it alone, the Philco OFF. See what interference you hear across the band with that, and where the stations come in well. See if that "hum" is there in the transistor set too. If it is, that would mean the problem comes from outside the radio. If there is no hum in the transistor set, then look to the radio. Where I live, for most of the evening half the AM band is swamped by a bad sodium vapor lamp on my neighbor's house. I know it is the cause because it always happens when that particular light is on, every time, and not when it is off. From 550KC to about 1000KC there is a horrible 60 cycle buzz. The light usually goes out about midnight, and after that I get great reception across the AM band. Good luck hunting down your culprit.

RE: Post re-cap 38-4 hum. - Kenneth F. Besso Jr. - 04-12-2016

DID you change out the electrolytics (capacitors) in the tin can,,,????

RE: Post re-cap 38-4 hum. - hffenninger - 04-12-2016

(04-12-2016, 06:28 PM)Kenneth F. Besso Jr. Wrote:  DID you change out the electrolytics (capacitors) in the tin can,,,????

Yes sir I did. And all four of the bakelite blocks as well.

RE: Post re-cap 38-4 hum. - morzh - 04-12-2016

Is the hum constant level or does it change across the dial?

Was the humbucker in the speaker ever touched? Is it present?

Were all tubes checked for shorts, especially HK ones?

Is the GND good throughout the chassis?

Last thing, a little hum might be normal.

Last thing, some

RE: Post re-cap 38-4 hum. - hffenninger - 04-12-2016

(04-12-2016, 10:02 PM)morzh Wrote:  Is the hum constant level or does it change across the dial?

Was the humbucker in the speaker ever touched? Is it present?

Were all tubes checked for shorts, especially HK ones?

Is the GND good throughout the chassis?

Last thing, a little hum might be normal.

Last thing, some

Thank you for the reply,
The hum is constant not affected by volume or tuning. All tubes TV7 tested good or better. I would love to know how to thoroughly check chassis grounds.

FF

RE: Post re-cap 38-4 hum. - morzh - 04-13-2016

Basically you see wherever things are grounded using lugs riveted to the chassis (if there are such) and make sure the resistance to the chassis is as low as possible (a typical Fluke DMM will show about 0.2 Ohm). Sometimes oxidation develops between the rivet and chassis and so the grounding suffers.
Long leads of filter caps might matter.
Power cord split inside the chassis and creating a large loop might matter.
Even poor connection between some tubes' pins and the respective sockets might matter.
Heaters hookup to the transformer winding if done improperly might matter. The proper way being, they run in parallel, the transformer leads coming in at the same tube's heater (closest to the transformer) and then proceeding in parallel 'till the last tube.
Sometimes I see the transformer having one lead going to one tube (say first in line) and then the other lead going to the last one. This creates a large loop and the current being sizeable then creates lots of EMI.

RE: Post re-cap 38-4 hum. - hffenninger - 04-13-2016

OK, thank you. I will look at those conditions. I notice that when I touch the grid cap of the rf tube, it gets quiet and reception is improved.

RE: Post re-cap 38-4 hum. - morzh - 04-13-2016

See if maybe the wire that goes to it goes close to something that can induce the hum.
Or try a shielded wire in its place, solder the shield to the chassis.