AC/DC radios (DAC90/90a etc) Mains Hum

[coopzone]

I know this is an old chestnut but no matter what you do, at best it seems that these radios all suffer from more hum. I think it's down to 1/2 wave rectifiers.

I have in the past managed to get the hum to an acceptable level, but it never goes away.

So I have a thought, they are AC/DC sets, so why not run them from DC? Has anyone tried this? How did you get on?

I know rectified mains (full wave ) will be about 320v no load. So it's a little high! I was thinking of a dropper resistor or maybe a zener diode?

Ideas anyone?

Derek

[Leon Crampin]

So, how do you generate your DC supply? Half-wave rectification merely replicates what's in the set. A full wave bridge will render the chassis live regardless of input supply polarity - dangerous on the Bush sets because the chassis is accessible through the holes in the bottom by touching ther knob fixing screws. If you use a transformer, you're back to a conventional AC only set.

Leave the Bush as it is - a cheap set. You can reduce the hum a bit by shunting the reservoir capacitor with another 22 uF component fixed behind the tag panel. As the UY41 is operating well below its full current capability, increasing the reservoir capacitor to about 54 uF should not be detrimental.

Leon.

[Heatercathodeshort]

I'm really puzzled here Derek. I have just livened up a couple of DAC90A receivers. One is later than the usual run of the mill but I can honestly say there is no mains hum even when very close to the speaker.
The UL41 is the prime cause of hum, much more than the main smoothing block that I very rarely find faulty.
Not so easy to run on DC but this should never be necessary. Just my thoughts. John.

[G6Tanuki]

Neither my Bush VHF81 nor my Eddystone 840A have noticeably intrusive hum.

Healthy UL41 / UL84 and smoothing capacitor along with proper 'dress' of the wiring is important. There is also a preferred order of connection for series heater valves to minimise the risk of hum pickup
I have seen quite a few radios whose designer was clearly oblivious to the guidance.

[McMurdo]

I have lots of ac/dc sets and none make any more hum than other types, I suggest either a problem in the set or some external hum-like interference (which is unavoidable these days)

[Techman]

Originally Posted by Heatercathodeshort

I can honestly say there is no mains hum even when very close to the speaker.

Mine's the same.

My DAC90A has only had the grid coupling capacitor replaced, all the other wax capacitors are original and still in place, as are the original electrolytics. The mains filter capacitor was 'blown', so although still in place under its clip, it's been electrically disconnected and not replaced with any other modern equivalent. There's no mains hum and the set performs perfectly with lively reception of all AM stations that are still receivable on medium and long wave, all coming through loud and clear - or as clear as would be expected on one of these with unmodified original tone correction.

So if you've got excessive hum then I would think that you must have got something wrong going on somewhere.

[Helder Crespo]

Originally Posted by Techman

Mine's the same.

My DAC90A has only had the grid coupling capacitor replaced, all the other wax capacitors are original and still in place, as are the original electrolytics. The mains filter capacitor was 'blown', so although still in place under its clip, it's been electrically disconnected and not replaced with any other modern equivalent. There's no mains hum and the set performs perfectly with lively reception of all AM stations that are still receivable on medium and long wave, all coming through loud and clear - or as clear as would be expected on one of these with unmodified original tone correction.

So if you've got excessive hum then I would think that you must have got something wrong going on somewhere.

They don't come much more original than that. I am quite happy with the hum on my DAC90s but both have had the kalee20 mod done, which does include a large 4.7uF cap between the rectifier and the UL41 cathode. I also increased the 16uF to 32uF, which provided an additional slight improvement.
- Helder

[Graham G3ZVT]

I wonder how much hum is due to the speaker field winding?

Would reversing the connections possibly "ding" the hum induced elsewhere?

I don't regard the slight hum you get with these sets a particular problem; modulation hum however is an issue at this location.

[agardiner]

I have restored a great number of DAC90A sets, and none have ever had much hum at all. Certainly nothing that would be noticeable at normal listening levels. As John said (#3) the UL41 is a common cause of problems including hum. Worth trying a replacement.

[Nickthedentist]

Yes, no discernible hum on my DAC90As or Ekco U243 either. They’re better than many of my AC only sets to be honest.

But the DAC90As have had new UL41s in some cases and all have had the pin 4 mod.

If there’s ever any hum, it’s from an extrinsic source, notably LED light bulbs.

[Herald1360]

Originally Posted by rambo1152

I wonder how much hum is due to the speaker field winding?

Would reversing the connections possibly "ding" the hum induced elsewhere?

I don't regard the slight hum you get with these sets a particular problem; modulation hum however is an issue at this location.

Speaker field winding

Not on any of the sets mentioned.

[Graham G3ZVT]

Thanks Chris, the hot weather's messing with my head

[Tim]

I did try this once in order to locate the source of hum in an AC/DC Defiant I was asked to look at. Since the hum was 50 hz, I couldn’t be sure if it was picking up hum from the surroundings or actually from inside the set. I had my “ big” AEI 500 volt 250mA DC supply then, so I was able to run the whole set from it. I did manage to locate and minimise most of the hum in that case. Of course doing this on the bench is different to a permanent arrangement. The “disadvantages” outlined above would have been matters to consider if your mains actually were DC.

[coopzone]

So what is the kalee20 mod ?

"they don't come much more original than that. I am quite happy with the hum on my DAC90s but both have had the kalee20 mod done, which does include a large 4.7uF cap between the rectifier and the UL41 cathode. I also increased the 16uF to 32uF, which provided an additional slight improvement.
- Helder"

[Helder Crespo]

Originally Posted by coopzone

So what is the kalee20 mod ?

https://www.vintage-radio.net/forum/...ad.php?t=35784

... and it's also important to undo the connections to pin 4 (used as tag) as mentioned above.

[Helder Crespo]

And here for a short but nice analysis of the results:
https://www.vintage-radio.net/forum/...ad.php?t=84048

[G6Tanuki]

With radios using an output transformer that has that tapped primary hum bucking arrangement, paradoxically increasing the value of the smoothing capacitor downstream of the HT resistor, or fitting a different value of resistor, can increase the hum.

I have also seen radios whose tapped - primary output transformer has been miswired, with rather hummy results.

[Station X]

This thread prompted me to turn on my Bush DAC90A and I can report that there's no hum whatsoever. This set is still running on its original smoothers and hasn't been modified in any way. Of course the waxies have been replaced.

My 40 year NAD3020 on the other hand has discernible hum but, as with Dansettes, it gets drowned out when you play music through it. I must get round to "servicing" it sometime.

[Station X]

One post moved to a new thread here:-

https://www.vintage-radio.net/forum/...d.php?t=193570

[ms660]

Originally Posted by Station X

This thread prompted me to turn on my Bush DAC90A and I can report that there's no hum whatsoever. This set is still running on its original smoothers and hasn't been modified in any way. Of course the waxies have been replaced.

Graham, out of interest (and also as a reference for others) could you post the DC voltage across the reservoir capacitor and the peak to peak ripple voltage across it, along with the AC input voltage to the receiver at the time of the test ?

Lawrence.