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Post by stanley on Dec 19, 2022 18:12:38 GMT 12
Hi,
I have a really, really tricky one. I have a columbus model 44 that hums and its quite loud for filter hum. I have recapped the radio and replaced the out of spec resistors. It is there no matter what the volume, band or tone settings are. I think I've ruled out everything, so this is where we are at.
Hum is present with -
- All tubes pulled except 6v6 and 5y3
- Coupling cap between the 6v6 and 6sj7 disconnected and tubes all pulled as above
- HV disconnected from everything except the 6v6
- Wiring is correct according to the schematic
- Heaters running on external regulated DC supply
- Filter choke disconnected and bypassed
It draws ~35mA on the HV with all the tubes inserted. The radio is fitted with a permanent magnet speaker The hum appears to be amplified by the 6v6, its also present on the cathode.
Any ideas?
Thanks
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Steve
Society Members
vintageradio.co.nz
Posts: 727
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Post by Steve on Dec 20, 2022 12:21:56 GMT 12
I have no ideas unfortunately, you've tried everything and more than I probably would have - the only suggestion I can make is lead dress, but I can't imagine hum as bad as you say unless the incoming phase is somehow wrapped around everything...
If it helps I can pull mine out of the cabinet and send some chassis photos for comparison - I've done nothing to mine (I haven't even had it out of the box before)...
Cheers, Steve
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Post by Richard on Dec 20, 2022 13:34:45 GMT 12
35mA current draw on the B+ with all the tubes inserted is very low? The 6V6 should draw that sort of current alone. I'm stating the obvious, but have you tried changing the 6V6? or re-checked the wiring around the 6V6 and the output transformer etc?
I just checked the 6V6 datasheet: "zero signal" max plate current is 45mA at a plate voltage of 250V
are you sure the 6V6 is actually drawing current? what is the voltage across the 300 ohm cathode resistor? it should be around 12 volts
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Post by stanley on Dec 22, 2022 18:13:33 GMT 12
Thanks Richard and Steve.
There is ~10v voltage drop across the 300 ohm resistor when running on the dim bulb. The 40mA was with the 6v6 being the only tube inserted.
I had another bit of a brainwave, I disconnected the B+ to the radio after the capacitors and put a load resistor of 4.7k across it at 180V output (reduced voltage). The hum is still present. I put a known good 5Y3 tube in and the (large) ripple reduced a little bit but not completely. The hum is at 100Hz but is not very even. I'm wondering if the power transformer is a bit sick?
Any thoughts?
After doing some more thinking.. The uneven portion is the 5V heater being superimposed on top. 20V ripple is still quite a lot. I wonder if the filter choke has some shorts in it. The cabinet did have large piles of wax on the wood...
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Post by Peter Walsham on Dec 22, 2022 19:41:04 GMT 12
The filter used in this radio is a tuned (resonant) one, and capacitor values need to be right for the filter to tune to 100Hz. What values of electrolytic filter capacitors have you fitted (although I don't think the value of the electrolytics would be too critical) and has the capacitor connected to the bottom of the filter choke been replaced with a 0.5uF? The value of the 0.5uF capacitor would be the critical one, as it forms a series tuned circuit (tuned to 100Hz) with the bottom winding of the choke. Is the filter choke connected correctly? I was going to suggest that you check both halves of the power transformer secondary for voltage, but your comment about the hum being 100Hz seems to suggest that the power transformer is OK. If the choke does have shorted turns, there's no way it'll be able to tune to 100Hz, so that may be the cause of the hum problem.
A 'brute force' 'work-around' might help with the hum, if the choke does have shorted turns. What I would try is to connect a diode between the output of the filter choke/2nd filter capacitor and the B+ rail of the radio, and connect a large value (say 47uF) capacitor to the cathode of the diode & earth and see what happens to the hum level. Alternatively, remove the choke altogether, and fit two series connected 470 Ohm 10 watt wire wound resistors between the rectifier output & the B+ rail. Connect a 22uF electrolytic capacitor as the 1st filter capacitor, and a 47uF electrolytic capacitor (as the second filter capacitor) to the junction of the two resistors, and another 47uF electrolytic capacitor (as the third filter capacitor) to the B+ rail. I'll defy the radio to have any hum after that!
Peter
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Post by Richard on Dec 23, 2022 11:44:41 GMT 12
"I wonder if the filter choke has some shorts in it. The cabinet did have large piles of wax on the wood..." I would suspect the choke and it sounds like it's been getting hot. Peters idea to replace with resistors should prove. 
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Post by stanley on Dec 23, 2022 13:32:28 GMT 12
Thank you Peter, I've never seen a tuned circuit in a power supply before.
I had a look in my books and didn't find much information on tuned circuits in power supplies either.
But it makes sense, I graphed the impedance of the components and it makes perfect sense.
I measured the choke as 22uh and 2 uH which is far to low. So the choke came apart, it had a few breaks, areas with arcing and wasn't in overly good condition. It has ~1000 turns on one side and ~3000 on the other. I calculated the required inductance should be around 5H, which would be about right for the size of the core and quantity of wire.
When calculated, the only critical parts were the 0.5uf cap and 5H choke, the rest wasn't so critical.
I am considering rewinding the choke out of curiosity to see if the calculated inductance was correct but need to wait for my wire to arrive.
The choke was getting hot but I wasn't sure if it was the transformer or choke as they sit on top of each other.
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6A8G.
Society Members
Hard working kiwi soundmixer.
Posts: 69 
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Post by 6A8G. on Apr 23, 2023 14:03:42 GMT 12
Hi Stanley, did you manage to sort this? I was wondering if at some time in the speaker's past, someone did some work on it which altered the wiring to the hum bucker coil? Very unlikely but not unheard of.
John:)
Bother!
Sorry I see it's a PM speaker.
Pse disregard:(
Shamed.
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