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Restored Philco 46-420 code 121 has slight low pitch hum. - Printable Version

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Restored Philco 46-420 code 121 has slight low pitch hum. - terrylett - 03-17-2019

Hello gurus. I recently picked up a Hippo on ebay for $10.50 probably because the case was in multiple pieces. I had a spare case in fine condition so I jumped on the sale. I recapped and decided to put new resistors in as well. After I cleaned everything up and powered it on it works fine. It gets multiple stations, the tuner cap moves freely and the volume control is not scratchy at all.

The only issue I have is that there is a slight 60 cycle hum after the tubes heat up. The hum doesn't get louder when the volume is increased but it is a little irritating. You can hear it in the background when you are on a station too.

I thought maybe I had some leakage in a tube (concentrating first on the 50L6) but I swapped out 13 50L6s, over 20 35Z5s and about 8-10 of the 7C6s, 7B7s, 7C7s and 7A8s. I would have trouble accepting that I have a tube issue.

All the replaced cap values are what is in the schematic and parts list except a couple of .05 mf were replaced with .047. I think also a couple of replaced caps were 400V instead of the original 200V caps. The electrolytics are brand new 20mf and my solder points are clean and shiny (I made sure my connections were good - especially to B-).

What I am wondering now is if it would be worth replacing any cap or caps with a lower value as I read in the past that that can reduce or eliminate hum.

Any suggestions?

Also would it be a hum reduction to install a Y2 safety cap? I guess on the neutral line.

Thanks in advance.

RE: Restored Philco 46-420 code 121 has slight low pitch hum. - Radioroslyn - 03-17-2019

Try connecting a 20 or 30mfd 160v cap, the + side to pin 4 of the 50L6 and the - to B-.

RE: Restored Philco 46-420 code 121 has slight low pitch hum. - terrylett - 03-17-2019

OK so are we talking about an electrolytic capacitor? 20 or 30 mfd sounds like a third electrolytic.

The 2 existing 20 mfd electrolytics are currently connected to pins 4 and 8 (+ end) and the - end to B+.

I will give it a try but I want to make sure I'm going to do your suggestion correctly.

Thanks.

RE: Restored Philco 46-420 code 121 has slight low pitch hum. - Radioroslyn - 03-17-2019

>The 2 existing 20 mfd electrolytics are currently connected to pins 4 and 8 (+ end) and the - end to B+.
Both - ends of C15 should be connected to B-. If not you get a bunch of hum and the b+ volt will be low. Remove the cap w/the b+ connected to the - and chuck it replace w/new.

RE: Restored Philco 46-420 code 121 has slight low pitch hum. - terrylett - 03-17-2019

Both caps - ends are already connected to B-.

They are also brand new.

So forgive my ignorance but if you are suggesting that I reroute (or replace) one of the 20mfd electrolytics which one would I do that to?

The one connected to pin 4 of the 35Z5 or pin 8 of the 35Z5?

RE: Restored Philco 46-420 code 121 has slight low pitch hum. - terrylett - 03-17-2019

Oh man.

This is what happens when your health ain't top notch.

I see where I earlier stated that both - ends of the filter caps were connected to B+.

They are not. They are connected to B-.

Sorry about the run around.

RE: Restored Philco 46-420 code 121 has slight low pitch hum. - Radioroslyn - 03-17-2019

Sorry I meant the 50L6 not the 35Z5.

RE: Restored Philco 46-420 code 121 has slight low pitch hum. - terrylett - 03-17-2019

So the original 2-section electrolytic cap had the red and orange (+ leads) going to pin 4 and pin 8 of the rectifier 35Z5 and the common black lead going to B-.

All I did was dump the original and install 2 20 mfd 160V electrolytics and run one + lead to pin 4 of 35Z5 and the other + lead to pin 8 of 35Z5 and ran both - leads together to B-.

That's the way the radio was wired originally.

RE: Restored Philco 46-420 code 121 has slight low pitch hum. - Radioroslyn - 03-18-2019

Yes that's correct. What I'm purposing to do is connect an additional 20 or 30mfd across the second 20mfd cap you replaced making it's value 40 or 50mfd depending on what value you add to the one that is already there. This will give us an indication whether it's a power supply issue or something else.

RE: Restored Philco 46-420 code 121 has slight low pitch hum. - terrylett - 03-18-2019

OK I understand the parallel caps adding to a higher value but when you say the 2nd cap, I really don't remember which of the 20 mfd caps I installed first - the one that goes from pin 4 to B- or the one that goes from pin 8 to B-.

Should I just pick one of those 2 and put another 20 mfd across it?

RE: Restored Philco 46-420 code 121 has slight low pitch hum. - morzh - 03-18-2019

Is C13 in place, new and is 0.2uF as shown? (the L1 is not that important and could even be simply shorted out).
Also is X-cap (should be X-rated, though this won't affect the hum, just safety measure) C15 there and also new (the value is not overly important but should be at least 10nF)

RE: Restored Philco 46-420 code 121 has slight low pitch hum. - terrylett - 03-18-2019

ALL caps are in place and are the exact value of the ones they replaced (except for the 2 .047 mfd caps that replaced a couple of .5 mfd caps).

I haven't done any safety cap installations yet whether X or Y type so they are not a factor right now.

Thanks.

RE: Restored Philco 46-420 code 121 has slight low pitch hum. - Radioroslyn - 03-18-2019

If you do as I suggested in post #2 the additional cap will be across the 2nd filter cap.

RE: Restored Philco 46-420 code 121 has slight low pitch hum. - terrylett - 03-18-2019

I don't want to beat a dead horse but in post you said:

"Try connecting a 20 or 30mfd 160v cap, the + side to pin 4 of the 50L6 and the - to B-."

There was never an electrolytic on pin 4 of 50L6. There was (and still is) an electrolytic on pin 4 of 35Z5 so I'm going to assume that's what you meant.

In a later post you said "I meant 50L6 not 35Z5" so that was confusing me.

I'll go ahead and solder in another 20 mfd across the pin 4 of 35Z5 and test it.

Thanks.

RE: Restored Philco 46-420 code 121 has slight low pitch hum. - Radioroslyn - 03-18-2019

>pin 4 of 35Z5 so I'm going to assume that's what you meant.

It wasn't what I meant. On the 35Z5 it has no internal connection on pin 4 and the schematic doesn't show any connection there. With that being said I have no idea what is connected there ( it's used as a tie point).

>There was never an electrolytic on pin 4 of 50L6.

This is true. But there is a wire that connects to the second filter that IS connected there (pin4 of the 50L6). I chose that point as it was easier to describe than trying to describe the different between where the two + sides of the filter are connected.

GL