[Jamie]

I'm still trying to solve the problem of my Philco 620. It has a terrible hum. The radio was previously repaired and I've corrected a few mistakes.

There are three electrolytic caps - (two 8mf and one 16mf). Right now, all three are grounded to the chassis, because that's how they were when I got it. I think this is wrong and it's the source of my hum.

When I look at the schematic, I see the 16mf (part no 52) grounded to chassis, but the two 8mf (part no 67) grounds go somewhere else. Am I correct? And if so, where do I run the ground from the two 8mf caps?

http://www.nostalgiaair.org/PagesByModel...013804.pdf

Thanks

[Ron Ramirez]

Yup, that's your problem. Both (67) electrolytic negatives go to B-, not chassis ground. This is clearly shown on your schematic - center tap of the high voltage secondary of the power transformer, connected to the top of resistor (70) and bypass capacitor (68 ).

[Jamie]

Thanks Ron. I spent some time working on it today. I got the electrolytics wired up according to schematic, but it only cured about 75% of the hum. There is still a hum, it is not tunable and it is not affected by the volume control. I only had about an hour to work on it today. It's no where near as bad as before, but it still needs to be fixed.

I'm going to follow the entire schematic tomorrow, starting from the electrolytic caps, since that seems to be where the previous changes were made. There is also a loud screeching sound at certain parts of the volume. If I jiggle the volume control around, I can sort of control it. Mostly it happens when it's turned up loud. I drenched the volume control with contact cleaner and it didn't help. The radio picks up very distant stations, and if you can ignore the hum, they come in clear. It'll be a real player if I can get it sorted out.

[morzh]

Also keep in mind that the #68 cap has its (+) - PLUS terminal to the chassis and NOT the (-) Minus. Many people make that mistake.

It also helps if you do not use the word "ground to somewhere else". You van ONLY ground to the Ground (or you can call it "chassis"). Otherwise it is not grounding. But of you call it "grounding" it kinda sets your mind to try to connect it to GND whereas it might not be connected there at all. Using right terminology disciplines the mind and eventually prevents you from connecting things to where they do not belong. Like to the Chassis all the time.

[Jamie]

Thanks Morzh! Are you sure it is #68? 68 is a .25mf cap and not an electrolytic (if I'm reading it right, and Lord knows I might not be!) .. Please explain further. How can I tell which end is positive by looking at this schematic? I see none of the normal indicators... just two equal lines. I apologize for using the wrong terminology. You are absolutely correct, I shouldn't call it ground.

[codefox1]

Yes, the chassis should actually be at a positive voltage with respect to the center tap of the high voltage winding, and there is a resistor and an electrolytic capacitor with it's + side connected to the chassis to make it so. The principal purpose is to produce a negative bias voltage source for the output amplifier circuit. If this part is missing, bad or mis-wired, you're going to have hum for sure.

[morzh]

It is 68, but you are right, it is 0.25uF and usually ytou see it at 10uF, so at this value you are better off using a plain non-pol cap so you do not have to watch polarity, but should you ever deal with other radios you will see that electrlytics are used there and the Chassis is positive.

[Jamie]

Originally there are three electrolytics on this radio, one 16mf can and a dual 8mf. Which one has positive going to chassis?

The 16mf can had one lead. I assumed it was positive and that it's chassis was negative, and soldered in the new one accordingly.
The dual 8mf can had two center leads and one off to the side. I assumed the dual center leads were the positives and the side was the negative. The news ones are installed accordingly.

Part of the previous repair was a wire jumped from that side lead on the 8mf can to the chassis. This wire has been removed. This solved 75% of the hum problem. Also, cap no 68 was not the original cap, someone had replaced it, the replacement was huge and it was totally melted.

Since I last posted, I've found another wiring alteration, corrected it.. and now the hum level is down to just barely there. The radio plays extremely well, and tunes in distant stations. To be honest, if I weren't a perfectionist, I'd call it done. But it still has a slight hum, and I am determined to fix it. If one of the caps is wired in backwards, please let me know which one.

[Ron Ramirez]

Which one has positive going to chassis?

None in the 620. Morzh may have been thinking of the 630 in his initial post in this thread (post #4), which is very similar to the 620 but uses a 10 uF, 50 volt electrolytic between B- and ground, with positive going to chassis ground. Note that he corrected himself in post #7. The 620 (your set) uses a 0.25 uF cap (which is a paper cap, no polarity) from B- to ground.

[Jamie]

Thanks Ron. I worked on the 620 for several hours today. I traced the schematic as best as I can and I can't find anymore errors. I used sandpaper on the tube shields, and double checked all the grounds. At this point, I think the slight hum is coming from a bad tube. The volume control has some issues too, and it could be coming from that. I'm calling it good for now. I'll get some tubes to swap, and find time to take the volume control apart at a later date. The radio plays well and the remaining hum is barely audible.

I tell you, I learned a lot from this radio. I feel a lot more confident reading schematics now. And I know I am capable of spotting and repairing old wiring errors.

Thanks to everyone who tried to help me. I really appreciate your time and effort. You guys are a life line to a newbie. If we should ever meet, dinner is on me.

[morzh]

So far both 8uF caps' minuses are connected to the rect minus which is the centertap of the transformer's secondary winding, pin #4, and the 16uF's minus is grounded (chassis).


As for your hum....some hum may be normal - may be your grounding, may be something else.
But a question arises: you did not say if the radio was fully re-capped or you do not know that? Changing electrlytics solves the main problem but those tubular and backelite caps can still be leaky so it pays to replace them as well.

[Arran]

I think that I have mentioned this in previous threads but getting the negative connections mixed up on the filter caps in a pre war radio is one of the most common errors people make. The point to remember is that the center tap of the high voltage winding is always the most negative point in the circuit, if it happens to be connected directly to the chassis then that means the chassis is also the most negative point, if not then assume that the chassis is more positive then the center tap of the power transformer.
They used to use this a lot in pre war sets, and sometimes the odd early post war set, as cleaver way of creating a C- voltage, sometimes called a fixed bias supply. However they do not so much create a C- voltage as make some element of a tube, usually the cathode, more positive with respect to the control grid, creating a small potential between the two. This originated with the earlier AC sets where they used directly heated cathode tubes like #45s but they carried on the practice long after there was no need to do so. Later on in the 1930s some sets dispensed with this idea and went with a cathode bias, they made the cathode more positive then the chassis by using a resistor and capacitor connected between the cathode and the chassis. This is what most normal sets used, then you have some whacky circuits employing things like bias cells, Canadian built Rogers and Marconi sets loved using those, probably one of the most worthless ideas they ever came up with.
Regards
Arran

[Jamie]

Morzh, I totally recapped the radio, including all the bakelites. Arran, thanks for that information. That stuff is good to know and I'll file it away for future use.

The only thing I found that puzzles me is this..there was a pigtail mounted under one of the bakelite caps. It was an inch long, cut off, and I could find nothing that looked like it should go there. I don't know for sure if it was original or not, but it was mounted under a grounded bakelite cap. I assume that Philco would've just used the grounded lug off the cap....so, with that in mind, I guessed that it was left over from the previous attempt at repair. Still, it's puzzling.. why put a pigtail there, when a grounded lug is sticking out right above it?