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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Tue 10, 2017 11:32 pm 
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I put the power resistor back on the candohm. It is drawing current. As you suspected, it didi not fix the hum.

The two legs of the push pull interstage transformer are each putting 7.7 vac on the 71A grids. The waveform on the grids is a square wave, of 2 microseconds per cycle. That works out to 500 Khz.

Most strange!


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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Tue 10, 2017 11:45 pm 
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Seriously? 7.7 volts AC of 500 kHz? That's not what I expected you to find. But I guess that goes to show why a scope is sometimes invaluable.

If you really do have that much "500 kHz" on the grid wires coming out of the transformer, then you've got a serious RF oscillation going on in this radio, which might possibly create the hum. Is capacitor 23 (.001 uF) in place, wired from plate of detector to chassis ground? It's purpose is to prevent this kind of thing. If so, try adding another capacitor of similar size from the plate of the 1st audio tube to chassis ground. This should definitely reduce or completely eliminate the 500 kHz oscillation. You could also try a similar capacitor connecting between the two plates of the output stage.

Is the frequency of the oscillation affected by the setting of the tuning dial? Since this is a TRF receiver, I would expect it to oscillate more or less at the dial frequency. One might also expect the oscillation to be stronger or weaker at different dial settings (for example, stronger at one end of the dial than the other).

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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Fri 13, 2017 12:51 am 
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Ignore the earlier report of a square wave oscillation at 500 Khz. That was some king of internal oscillation in the scope, (it is a good scope, Tektronix) and has never had problems before.

I did put a grounded .001 uF cap on plates of AF preamp and amp, made no difference. Also on plate of detector which lowered the pitch of the hum dramatically but not the amplitude.


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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Fri 13, 2017 1:43 am 
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The .001 uF cap would only have made sense if there was an RF oscillation, which is what your previous scope reading seemed to suggest. Now that you've found that was just a scope problem, we are back to looking for 60 Hz. A .001 uF cap will have little or no effect on 60 Hz signals.

You mentioned that a .001 uF on the plate of the detector "lowered the pitch" but not the amplitude. What do you mean by "lowered the pitch"? Surely you do not mean that the frequency went down. Perhaps you mean that the mix of harmonics changed? (for example, it was quite "buzzy" before adding the cap, and now more of just a "hum"?)

Or did the frequency actually change? In which case, this has nothing to do with 60 Hz hum from the AC line, and is instead caused by a low frequency audio oscillation.

Is your scope working now, or did you have to set it aside as non-working at this point? If you put your AC voltmeter across the two output stage grids (across the two outer connections of the secondary of the audio interstage transformer) do you see an AC voltage up in the range of a volt or more? A few millivolts doesn't matter, but an AC signal at the volt level or above will be amplified into a fair amount of hum at that point.

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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Sat 14, 2017 2:47 pm 
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Measuring between the 2 legs of the push pull transformer reads 35v AC!! No wonder there is hum. Also measures about 1 v DC.

Cap on the plate of the detector merely reduces the hi frequency harmonics. Sort of like the old 2 position tone controls. (Treble cut controls)

PS: on that square wave previously reported, I should have known better since this radio cannot produce a square wave, but the time base on the 'scope can! I think it was a grounding problem.


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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Sat 14, 2017 3:35 pm 
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Some other voltages: Power supply points F G and D all read about 9 volts AC. I have replaced the filter caps. No difference.

Plate on the 26 first AF tube reads 41v AC! No wonder there is loud hum!


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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Sat 14, 2017 6:26 pm 
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I'm not sure if we can trust your AC meter readings, since some of the points you are measuring have both AC and DC present. Many AC meters read incorrectly if there is a DC component present. So for example, the reading on the plate of the 1st audio tube may not be right. However, if by the "push-pull" transformer you mean the secondary of the audio interstage transformer (and not the output transformer), there you should have minimal DC component, so the reading is likely correct.

If you're using a modern digital multimeter, one way to make the AC readings more reliable in the presence of a DC component is to put a large nonpolar capacitor (around 1 or 2 uF) in series with one of your two voltmeter probes. In other words, pass the voltage you are trying to measure through a large capacitor. That blocks the DC. In this case, we need a large capacitor since we are looking for quite low frequency (60 or 120 Hz) AC.

In any case, it does sound more and more like your power supply filtering is simply quite poor. Seems like you're getting substantial hum everywhere you look now.

Can you describe precisely how the grounded connections are made? Specifically, can you tell me how the following are connected:

- centertap of HV winding of power transformer
- negative side of each of the five filter caps (all labeled as 33 on the schematic)
- centertap of the B filament winding on the power transformer

For each of these, we need to more that merely that it is "grounded." Where is it connected, and how is it connected (screw, solder, etc.)

Also, can you describe (or provide pictures) for how the following are connected:

- all sections of Candohm resistor 35 (or the resistors you've used to replace it)
- hum balance control 30
- centertap of filament winding C on the power transformer

My apology if you've already mentioned this, but have you subbed the 80 tube for a known good one?

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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Sat 14, 2017 7:39 pm 
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voltage readings on output of push pull interstage audio transformer with a 2 uF nonpolarized cap to ground:

Both legs read -1.9v DC to ground and 66 VAC to ground (slightly different than last time)

Will get back to you with descriptions of the grounds


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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Sat 14, 2017 8:51 pm 
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Swap out 80 tube: no difference

Refer to this attached picture of the power supply and candohm:

Neg leads of all 5 filter caps are soldered to lugs which are bolted to the chassis

All 5 lugs of candohm are soldered including the green power resistor

CT of push pull interstage transformer is soldered to a lug which is riveted to chassis. Wire very thin and flimsy.

Connections to R30 are soldered and control looks to be in good shape.

I do not know pin out of power transformer, so am unable to describe the CT ground of B winding, C winding and HV winding.


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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Sat 14, 2017 9:10 pm 
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view of power supply. Am trying to shrink the picture size.


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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Sat 14, 2017 9:36 pm 
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While you work on reducing the size of the picture to post it, see if you can figure out where the CT of the HV winding is going. This is absolutely key. A wrong connection here could cause all kinds of problems.

For chassis connections, measure resistance between various points and make sure none has a resistance of more than 1 ohm. Mechanical (not soldered) connections to chassis sometimes go bad over the years, with higher resistance that causes problems in power supply filtering.

By the way, if you don't have a good program for image processing, a free one that is fairly simple to understand and works very well is Irfanview. Search on Google for free download sites. Used very widely -- you can trust it to be OK software for your machine. If you install is, what you need is in the "Resize/Resample" action under the "Image" pull-down. You'll succeed on the very first try. Max size for posting here is 800 x 800 pixels.

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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Sun 15, 2017 6:52 pm 
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figured out the CT leads on the power transformer for B, C, and HV, using an ohmmeter. All are grounded to the chassis with no more than 0.4 ohm resistance, or less. I don't think a picture would add anything. Although it is close up you really can't see the connections in detail

Also, all caps that are supposed to be grounded to the chassis have 0.4 ohms or less from the negative lead to the chassis. I then went around and touched all the negative leads with a known good ground, made no difference as you would expect.

I am stumped.


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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Sun 15, 2017 7:35 pm 
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The CT of winding C should not be grounded. It's not clear that grounding it would cause massive hum, but it would cause excessive current in the output tubes, which certainly would increase the hum level. Can you get into some detail regarding how your resistor 35 is wired?

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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Sun 15, 2017 8:00 pm 
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my mistake. Pin 4 of the candohm is grounded. Pin 5 of the candohm, which is the CT of winding C, has 1024 ohms to ground as it should.


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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Sun 15, 2017 8:26 pm 
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I wonder if your filter choke has a shorted turn that is rendering it ineffective in its role as part of the filter?

Are you feeling confident enough in your scope to use it to look at the AC ripple at the rectifier filament, and also at points F, G, D, and E? We need to see the ripple waveform with a calibrated scale so we know the ripple amplitude. It will also be useful for looking at the hum in the signal path of the amplifier.

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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Mon 16, 2017 3:54 pm 
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Yes, will try this with a better ground. I will also check cold resistance of the choke. Any idea what it should be? I know low but not zero.

I have one of these if you think it might help:

https://anatekinstruments.com/products/ ... artest_kit


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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Mon 16, 2017 5:13 pm 
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I have one of those too. I've never used it on a big filter choke, however, so I'm not sure how high the Q is supposed to be, and how many lights on the tester will light up for good vs. bad.

A cold resistance reading might not be able to show a resistance change associated with a shorted turn. If it's just one turn out of thousands, you'll never see the difference in a resistance reading, even if you knew the correct resistance. Cold resistance is probably in the hundreds to a few thousand ohm range.

Perhaps the best way to see if this is the problem is to place a rather large capacitor (say 20 uF or even larger) in the position right after the choke and see if it improves things significantly.

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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Mon 16, 2017 10:54 pm 
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Choke reads 400 ohms DC cold. Ring tester says low Q (1 yellow light)


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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Mon 16, 2017 11:03 pm 
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Not sure what to make of that, although I wouldn't be surprised if that is the correct Q for a choke like that. Try adding a big cap after the choke and see what it does to your hum. At least 20 uF.

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 Post subject: Re: Philco 86 Hum (BIG one)
PostPosted: Oct Mon 16, 2017 11:24 pm 
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OK, scope is behaving itself.

All frequencies are 1 kHz --exactly 1 cycle per ms. Wave form is a peaked sine wave, not quite a triangle wave. I am suspicious of even number frequencies but believe it is correct because the wave cycle varies with sweep speeds.

Amplitude is 10v on the filament, and 5v on points DEFG.

I put a 40 uF electrolytic across the input capacitor of the pi filter and it cut the hum amplitude in half or less and it is not heating up. (I was concerned about explosion)

Is the solution to change all caps to 40 electrolytic?


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