[Winky Dink]

Has someone got half a minute to assess the hum on this set before I put the chassis back into the cabinet hopefully for the last time.

The hum comes on as the set warms up and is only slightly increased with the volume control. It isn't especially noticeable when the audio is on. The only thing I've done to address the issue is to switch the 6A7, but with no change. Here's link to a 30-second video clip.

https://picasaweb.google.com/coldrb/Phil...directlink

My question is whether it's worthwhile to try to kill the hum, or should I leave well-enough alone?

[gregb]

I had a listen to the video and sorry but I don't hear any hum.

Gregb

[Warren]

I didn't hear anything either... must be really low or something you can only hear if your right there.

[Winky Dink]

Thanks, guys. If you heard the audio, then I'm misinterpreting what I called "hum." Now that I know it's normal, I can declare victory and reinstall the chassis.

[Ron Ramirez]

I heard the hum in the video. Did you replace the electrolytics, and are they installed correctly? Not all negatives go to chassis; some may go to B- instead. Check your schematic to be sure.

[morzh]

Yes there is a very pronounced hum, I was using earphones.
Also it does not seem to depend on tuning, it is constant level across the scale.

What Ron said, plus

If the speaker is using a humbucker, is it wired correctly?

And, not sure that applies but just in case : have you seen if the hum depends on the antenna used, on teh position of teh radio relative to the wall, on the loads in your house turned on/off?

Also, do other radio exhibit the same hum or just this one does?

[PAradiogeek]

I also heard the hum. I would definitely want to eliminate or at least lessen it. I would find it annoying.

Ron and Morzh's suggestions are probably your best bets.

Good luck,
Jon

[TA Forbes]

Agree about the electrolytics. Make sure that #s 58, 59, and the can that houses #s 51, 73, and 76 have been replaced.

Also check power resistor #64.

It is always a good idea to replace all paper and bakelite-block caps and check/replace resistors out of spec. Personally, I replace resistors greater than 10% out of spec.

[Winky Dink]

Prior to applying power, I replaced all electrolytics, paper caps (tubular and Bakelite blocks), and any questionable resistors; checked every coil including the speaker for continuity and appropriate resistance; and tested all tubes for transconductance and emissions.

I believe I have wired everything correctly, but (you may find this hard to believe) I have actually made mistakes in the past. I can read a schematic pretty well, but when I get befuddled I resort to sketches.

[Image: https://lh5.googleusercontent.com/-r8rVl...520645.JPG]

The BC resistor and condensers 58, 59 were particularly troubling because the condensers has been replaced with a couple of cardboard electrolytics under the chassis, so the physical wiring was changed from the original. I also wasn't sure which end of the resistor was +, -, B, or C. I rebuilt new cans for 58 & 59, and rebuilt the 3-condenser can.

This is a before/after comparison. All replacement components are reproductions.

[Image: https://lh6.googleusercontent.com/-ihp5d...520645.jpg]

The power resistor has three terminals. On this picture, numbering the terminals from left to right as 1, 2, and 3; can you give me a designation for each terminal? I think I connected the middle one as C-. As for C58 and C59: I think the negatives are connected to the BC resistor and the positives to opposite ends of the field coil. The Philco schematic doesn't indicate (+) and (-) on the electrolytics.

I have to pull the chassis again, then start verifying my connections and check that I don't have any reversed polarities on the electrolytics.

Thanks for the help. It'll take me a few days to check out everything you've mentioned.

[TA Forbes]

Correct, - side of 58 and 59 go to the end (lead "3") of resistor 64, .09uFd cap 63, and to c.t. of the HV winding of 77. The - side of the can containing 51, 73, and 76 is ground.

Re check solder connection quality for all B+ circuitry, also re check any filter caps that go to ground. A poor solder joint will introduce hum in your set here.

You do nice chassis resto work!!!

Also like your diagramming. I do the same, but not as "lifelike" as your drawings are.

[Winky Dink]

Will check all of that. Thanks.

[Winky Dink]

I made a checklist for all of the previous suggestions. But first I removed a capacitor which I put in because I thought had been added in a later run--but it was not in Service Bulletin 234. The volume of the hum dropped at least 50 percent.

I went through each item on my checklist except for verifying the polarity of C58 and C59. I didn't want to open the rebuilt cans, so I disconnected those capacitors and temporarily wired in new ones of the same values, being certain that the polarity was correct. This made no change in the hum, so I assumed that the C58/59 cans had been wired correctly.

Then I wired in new capacitors which were twice the values of the original C58/59, and there was no discernable any hum at all.

I think it's ready to go back in the cabinet, but while it's still on the bench, I'd like to ask about one more issue which I bypassed when I was first checked chassis voltages several months ago. These were my readings with 124VAC line voltage.

[Image: https://lh5.googleusercontent.com/-D9Z_y...520645.JPG]

I asked about this briefly on another forum, didn't resolve the issue, but proceeded with the restoration because the radio seemed to be working well. I just rechecked the 85 plate, and it's still 44V. This shows the voltage peripheral to the plate, but it's beyond my understanding why the 267V drops to 44V instead of the expected 82V.

[Image: https://lh5.googleusercontent.com/-qBwpK...520645.jpg]

So, is there any reason I shouldn't put higher-value caps in C58 and C59?

Should I be concerned with the low 85 plate voltage, or should I leave it alone?

[Steve Davis]

You don't want to go up too much on 59 or you will stress the rectifier. But go ahead and increase 58 some.

Have you replaced all the capacitors? Capacitors 50, 51, and 62 could cause a low voltage on the plat of the 85 tube. Also check the value of resisters 48 and 49.

Steve

[Winky Dink]

Thanks. I will experiment with various C58/C59 values to find the lowest combination that kills the hum. Also will recheck the caps and resistors you mention. All caps have been replaced except micas, and all resistors more than 10% out of spec were replaced. I've checked C50,51,62 and R48,49 numerous times, but I may try replacing them one at a time in case something is failing under load.

[TA Forbes]

It may be that at some point in the 645's history it had a component failure that "stressed" the power transformer. An increase of the filter capacitor values is a valid way to fix this.

Keep an eye on the power transformer. Make sure it is not getting to hot.