[OZ4]

I used a cap out of an old switcher wall wart power supply and put it in an AA5 and it got rid of the hum real nice. I think it was a 160uf at 200v.

[Jake Blake]

Not an expert here.
But I would like to suggest you check a few voltages before modifying the filter component, ( not that I'm beyond modification).
I'd check the plate voltage of the 50L6 first, . . . pin 3.  I see on the voltage list it's supposed to be 76 volts, but I wonder what it gets up to in these appliances that were originally designed for 110 volts a.c. It may be seeing as much as 125 volts from the a.c. line on mild weather days, these days.

This is something I intend to pursue with my 39-6 when I get to that point.  Somehow, I assume over-voltage would be of higher consequence with a unit having a power transformer, but the performance may be more-directly affected in these AC-DC units.    

Jake

[terrylett]

Well the thing is I swapped out multiples of ALL the tubes, replaced all the resistors and capacitors so even if I found a unacceptable voltage what would cause it?

Also what would fix it?

I put in another 20 mf electrolytic with + end on pin 4 of 50L6 and - end on B- and I still get a low hum.

I have another speaker/transformer assembly from a different Hippo project and it is an exact match.

I suppose I could install it to see if that helps...

[morzh]

I do not think changing speaker could possibly help.

Some very low level of hum is present on many DC-DC sets.
There was once, about 2 years ago, one chassis I did that had pretty pronounced hum, and I could never locate the source; I ended up lowering the output tube's input resistance where it was being picked up and it too care of most of it.

Sometimes it is a wire that runs where it shouldn't, like nearby the power path while being a part of some high impedance input.
A tracer would probably help locating the point where it starts.

[terrylett]

I know about some sets having a hum but I have 2 other Hippos that don't have this issue.

As I said before you can even hear the hum when tuned to a station.

I don't know what you mean by a "tracer" - your knowledge dwarfs mine.

I purposely routed the replacement filter caps away from the AC line cord and those caps are brand new out of the plastic 20 mf Sprague Atoms.

I can poke around some more or put this Hippo away as a back burner project (after all I only spent $10.50 on it) and move on to my Crosley 516 Tombstone and my latest acquisition a Philco 42-350.

Thanks for all the input everyone.

[morzh]

A tracer is a simple circuit that has a probe and the speaker for the output.
You poke places along the signal path and listen to it at the probed point. This way you could possibly pinpoint the place where distortion/hum originates or where the signal stops propagating.

Like this one.

https://www.ebay.com/itm/Philco-Model-70...SwlddciZLU

[morzh]

Do you have a Field Coil speaker or a PM?
If it is the FC speaker, the schematic shows no bucking coil. I wonder.....
If you plug 500 Ohm 10W (for the test could be 5W) resistor instead the field coil, and use a PM speaker - would the hum disappear?

[terrylett]

Interesting about the tracer; hopefully I will be able to afford one soon.

The speaker is an FC. I have a 10W 20 ohm resistor on hand. It is the only one I have in my resistor stash that is over 2W.

I could use it if you thought it would not damage anything else and I have 3 PM type speakers sitting around.

How would I go about wiring everything for the test?

Since I was talking about swapping in the exact same FC speaker assembly (speaker - field coil - transformer) that I had from an old hippo I was going to disconnect the 2 red, 1 orange, 1 blue and a green wire on the other side that goes to the tuner, would it be easier to do that first or try the PM test?

[morzh]

20 Ohm is too little; while not likely to damage anything, the filtering quality will be compromised as the RC time constant would be 25 times less so the hum might actually increase.

Here's what you could do: leave the field coil connected but only disconnect the speaker's voice coil from the output xfmr and connect a PM speaker to it.
Or: same about the field coil, but disconnect the primary of the output transformer and use another output transformer together with a PM speaker (if you have one from Zenith portables or whatever similar).

Either will do. The first one won;t require you to find another output transformer.

This way you do not need a resistor as the Field coil stays in place.

[terrylett]

OK I want to thank everyone for some great troubleshooting and suggestions.

Morzh enlightened me on the use of a tracer, some possible cap replacements and a temp PM speaker test and Radioroslyn gave me some direction with the temp electrolytic on 50L6 and the fact that I might be staring at an output trans issue.

I decided that the almost pristine "totem pole" speaker - coil - trans assembly was something I wanted to do so I did it.

The result seems to have helped a decent amount. You need to be close to the speaker to hear a light low pitch hum and when tuned to a station it is only evident when there are pauses in conversation and from a distance across the room you cannot hear any hum.

I think if I do anything else it would be revisiting solder points for blob drips that I can solder suck and maybe swap in a different cap or two.

At this point I think I'm satisfied (again, hey for $10.50...).

Thanks everyone.

Over and out.

[Arran]

I would say that 20 uf is awfully low for a filter cap in an Ac/Dc radio, regardless of whether it has a filed coil speaker or not, I would have expected at least a 30 uf or a 40 as an input cap, and that or higher as an output one, 50 uf/50 uf are quite common in those double section toilet roll style condensers. I can recall replacing filter caps in Ac/Dc radios with 22 uf, 160 vdc jobs, and the sets always had some noticeable hum, such that I replaced them a few year later when I found some of a larger value. In any event the 20/20 unit may have been a, incorrect replacement that someone else stuck in there.
Regards
Arran

[morzh]

Arran

My Hippo, I think 48-460 and uses a PM speaker. This uses 30uF for the first cap and 20uF for the second with 220 ohms in between.

This one uses 2x20uF but then it uses the Field coil which is 560 Ohms plus inductance.

I think the reason is this radio uses the cheap speaker without the humbucker. The noise heard might be due to this and there is nothing much one could do.

However one might try to fashion a humbucker coil around the Field coil and connect it in series with the voice coil.

But first we need to do the PM speaker test to establish for a fact that this is the case.