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 Post subject: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sat 03, 2018 4:02 pm 
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Joined: Nov Fri 23, 2007 6:42 pm
Posts: 196
Location: Noblesville, IN 46060
Good morning all,

I'm at the point of believing the power supply hum in this radio is just a design flaw but thought I'd socialize the problem before throwing in the towel on this AC/DC radio.

All the components were original except for an added .022 cap between B+ and ground which I eliminated. I did the usual recap, replaced out of spec resistors, checked lead dressing and aligned the radio. The radio plays well but the level of power supply hum is objectionable in my opinion. It is very audible when not tuned to a signal. The hum is coming from the B+ circuit for the plates of the twin 25L6 output tubes. This circuit comes off the 25Z6 rectifier and is filtered by a 30 uF filter cap. The B+ circuit for the remaining tubes runs through a filter choke and has a 40 uF filter cap. This B+ circuit has very minimal hum.

I've tried changing the 25Z6, both 25L6s, increasing capacitance on the first filter cap, checked lead dressings and reversed the hum bucking coil leads. The only thing that reduced the hum was significantly increasing the first filter cap and the reduction was very slight. The scehmatic for this radio is in Riders, Allied page 14-45.

I'm out of ideas and would appreciate any thoughts.

Cheers,

Nick


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sat 03, 2018 5:34 pm 
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Joined: May Wed 16, 2012 6:58 pm
Posts: 270
Location: Eastlake, Michigan
Have you checked the speaker field coil resistance? Try disconnecting either field coil lead and connect a PM speaker to the output transformer voice coil leads. Still have hum?


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sat 03, 2018 5:56 pm 
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Joined: Nov Fri 23, 2007 6:42 pm
Posts: 196
Location: Noblesville, IN 46060
Thanks for responding.

The field coil resistance matches the schematic. The OT is mounted on the speaker. I connected I test speaker with a P-P OT and still have approximately the same level of hum.


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sat 03, 2018 6:47 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4641
Location: Near Brandon, Iowa
You could try feeding (through a cap and resistor) the B+ voltage hum component at the first filter cap to the cathode resistor of the first audio amp stage, using the B+ ripple voltage to actively "buck" the output tube plate signal hum, which will have the same common hum source but will be 180 degrees phase-shifted.

In the past several months there was a thread here that discussed how to do this.

Another scheme would be to simply move the output transformer B+ connection from the anode of the first filter cap to the anode of the second. If this is done it may be helpful to boost the value of the second filter cap.


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sat 03, 2018 6:59 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 11825
Location: Mpls, Minnesota
Can you vary the hum with the volume control?

Dave


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sun 04, 2018 7:43 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 2581
Location: Lexington, KY USA
Schematic:

http://www.nostalgiaair.org/Resources/download.asp?FN=\M0000976.pdf

With a push pull output stage, the ripple current at the output plates should cancel. Could something be different between one 25L6 and the other one?

If the push pull circuit is working correctly, schemes to inject ripple into the audio amplifier out of phase should not work in this set.

You might try grounding each audio tube grid through a capacitor ( one at a time, but you might also try using two caps to shunt both output tube grids at once ) and see what that does to the hum.

Turn up a station a little and verify that both output tubes are contributing about the same power to the speaker. Again, use the capacitor to shunt first the grid of one tube, then the other.

Has someone changed the wiring to the hum-bucking coil on the speaker?

Ted


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sun 04, 2018 7:50 pm 
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Joined: Nov Fri 23, 2007 6:42 pm
Posts: 196
Location: Noblesville, IN 46060
Shunting the grid on the 25L6 that is fed by the 6Q7 with varying values of capacitance reduces the hum. .047 or greater provides significant reduction. Shunting the grid of the other 25L6 does not effect the hum. I've tried several other 25L6s with no effect on the hum. The output of each 25L6 looks identical on an oscope.

I've tried reversing the leads on the hum bucking coil and that does not have any effect on the hum.

Stumped.


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sun 04, 2018 8:11 pm 
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Joined: Nov Fri 23, 2007 6:42 pm
Posts: 196
Location: Noblesville, IN 46060
I made a mistake in my last post, I should have said that hunting the grid on the 25L6 that is fed by the 6J5 phase inverter with varying values of capacitance reduces the hum.


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sun 04, 2018 9:46 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 2581
Location: Lexington, KY USA
I wonder if a bigger capacitor shunting the grid would kill most of the hum? Sort of sounds as if the phase inverter is introducing the hum.

It might be useful to know if both output tubes are actually functioning. You could measure the voltage drop across each half of the output transformer and also the winding's resistances, then calculate the plate currents. The currents should be nearly the same.

When stumped, it doesn't hurt to verify all the resistor values and voltages on the tube pins. Sometimes this leads to an answer. Did someone install a 4.7meg resistor where a 4.7k is required? sort of problems, that can't be solved by someone in another state.

Just measure the resistors in-circuit. And connect the meter where the resistor is supposed to go in the circuit, to verify it's connected correctly. Most resistors in simple radios can be measured accurately in-circuit. You can ask here if in doubt about any particular resistor.

Ted


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Wed 07, 2018 6:20 am 
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Location: Ann Arbor, MI
Where does the 6J5 phase inverter get it's B+ from? Is it from the second B+ filter or the first? I would expect that it should come from the second filter, could something be mis-wired? Or could the 6J5 have a heater to cathode leakage?

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Tom


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Wed 07, 2018 7:06 am 
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Location: Victoria, Australia
I would make sure that the first filter caps are not on the wrong side of the field. Hum modulation can get in via the 6K8 or a tube leaking AC to a cathode.

The line cap is there for RF & should be on Neutral (Common) I would check that. It should be a line cap (X/Y), as it can end up on Active (Line).

Marc


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sat 10, 2018 7:36 pm 
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Joined: Nov Fri 23, 2007 6:42 pm
Posts: 196
Location: Noblesville, IN 46060
Just found time to scratch my head some more over this radio.

The 6J5 gets it's B+ from the second filter capacitor. I've tried several other 6J5s with the same result (I've tried substituting every tube in the radio). The first filter cap is in the proper location. I've been over the schematic several times and everything checks.

To be clear, the hum is audible at all times, not only when tuned to a station. The line cap is between the AC common and chassis ground. It isn't an X/Y type but then the original wouldn't have been either. Reversing the wall plug has no effect on the hum.

For now I have a .047 from grid to ground on the 25L6 fed by the 6J5. It makes the hum tolerable without significant reduction of output. I'm just about convinced this radio is working as designed.

Cheers,

Nick


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sat 10, 2018 10:27 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 2581
Location: Lexington, KY USA
NIck,

What you describe is not proper operation as originally designed!

What does shunting just the 6J5 grid to ground with a capacitor do for the hum? Haven't heard back what you found.

The symptoms you have might be caused by an open grid on the 6J5, or some serious mis-connection there.

It doesn't hurt to use an ohmmeter to verify resistances. 6J5 grid resistor values are suspect. No need to disconnect anything.

Ted


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sat 10, 2018 11:56 pm 
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Before this I would have fired up an Oscilloscope to see where the hum was getting in. One of the oddball ones I have seen is a cap destined to go to ground hooked to a heater and it is not that difficult to turn a PP amp into a multi-vibrator: That's where the scope is handy as the latter will be a square wave & a bad cap, or dud resistor (possibly) a saw tooth.

Induction is not out of the question especially with modern caps, as they have no "Outside foil". I often get issues in the 2nd detector area from that & have to add shielding or move something.

The one that seems to have been overlooked is the speaker. These have been known to develop coil leakage and some designs will see that leakage manifest as hum. Also the "Hum Buck coil" comes into this: Is it actually wired in? Isolate the voice coil & sub in a permag speaker.

Caps exposed to direct mains (utility power) should be approved line caps as they are not supposed to fail short C1 is prime example of one where someone could get zapped by a cap that leaks, or fails short. Not fitting the correct cap for the end use is just making an intrinsically unsafe device, worse, or just as bad as it already was, when it can be easily made a tad safer.

Marc


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sun 11, 2018 5:53 pm 
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Joined: Nov Fri 23, 2007 6:42 pm
Posts: 196
Location: Noblesville, IN 46060
Here's a follow up to last questions and suggestions.

Shunting the 6J5 to ground with several different cap values has no effect on hum.

Substituting a PM speaker with appropriate output transformer does not eliminate the hum. The hum bucking coil has continuity and I've tried reversing it's connections.

My oscope shows an audio modultated saw tooth signal at the plates of the 25L6s. This doesn't seem unreasonable since the plate circuit is directly off the 25Z6 and only has one filter cap. I tried moving the 25L6 B+ circuit to the second filter capacitor. This eliminated the saw tooth signal on their plates but the hum is still there.

All resistor values check out on my DVMM. As mentioned, I've swapped every tube in the set with multiple substitute tubes and tried various filter capacitor values.

Nick


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sun 11, 2018 7:33 pm 
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Location: Ann Arbor, MI
Look at the 6J5 tube socket and see if one of the heater pins and the cathode or cathode resistor are connected to ground with the same wire. If so, it is possible that that wire has a poor connection to ground such that there is a small AC voltage on it. This can happen if the ground connection is made through a rivet.

A signal on the cathode works the same as a signal on the grid. It is the difference between the cathode and grid that matters.

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Tom


Last edited by Tom Schulz on Feb Sun 11, 2018 7:36 pm, edited 1 time in total.

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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sun 11, 2018 7:34 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4790
Location: Gainesville, Florida
I didnt see the answer for easyrider question


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sun 11, 2018 10:59 pm 
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Joined: Nov Fri 23, 2007 6:42 pm
Posts: 196
Location: Noblesville, IN 46060
the 6J5 cathode resistor and heater ground are the same connection. I re-flowed the solder and attached a jumper to a good ground and the hum remained. I checked all the socket grounds, they are all riveted and soldered to the chassis.

To Easy Rider's question, the hum does not vary with the volume control.

Enough for today. I'll take another look next weekend. Maybe I'll have an epihany. Thanks to all for your suggestions.

Nick


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sun 11, 2018 11:42 pm 
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I think I would be checking if the component (likely cap) value somewhere is one decimal point too high. Saw tooth is significant that is often created by CFL's & deliberately with Neon tubes to form a relaxation oscillator, where the voltage rises to a critical level, at which point there is a discharge, it all goes back to zero & starts again.

A cap is a common cause as they can get to a point & flash over & resistors and, as suggested, bad jointing (bonding). The scope should surely give an indication of the point of origin. RF on the Plates of th OP is often evident but can be an indication of a de-coupling cap issue. If Paper Filter caps were replaced with Electrolytic, that would require the addition of a decoupling cap on B+ of around 0.1mfd to get rid of RF riding on B+.

You can feed the Plates of the OP with DC that is not as clean as they do not amplify.

Marc


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Tue 13, 2018 6:56 pm 
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Location: Ann Arbor, MI
If you remove the 6J5 is the hum still there? If the hum remains then I think that there must be too much ripple on it's B+ feed. Perhaps the filter cap on that feed is bad, even if it is new. If the hum goes away, try shorting the grid to the low end of the cathode resistor and see if that kills the hum.

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Tom


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