I have a Grunow 500 (Riders 4 Grunow page 1) The radio after replacement of all caps plays great HOWEVER I get a loud hum which is non tuneable and I think it is the Hum Bucking coil. I have read about doing an ohm meter check on the Hum Bucking coil which I do not see illustrated in the schmathic to begin with and I really don't like to take the speaker apart to find these leads to check to see if there is a short. I have read on other forums that changing the value of the electrolic cap going to the power transformer of the speaker to a higher value from 4mfds which it is now to 33 mfds might help. But this might put a strain on the rectifier tube. Is their a better way to work on this problem? I could replace the speaker but I like to keep the set as original as possible. Thanks!

Is it really a Grunow 500 (Chassis 5A)? The Riders schematic shows that this set has no humbucking coil,
and that the only 4 uF electrolytic in the set is the AC bypass for the audio output tube. The two B+ filter electrolytics are both 8 uF.

Is the hum present even with the volume control set at minimum?

CV,

There is another 4uf cap at the junction of a 100K resistor and the 1meg volume control to filter the bias on the 75 control grid. He might want to check the polarity of it as it's plus goes to chassis.

Clifton

:Is it really a Grunow 500 (Chassis 5A)? The Riders schematic shows that this set has no humbucking coil,
:and that the only 4 uF electrolytic in the set is the AC bypass for the audio output tube. The two B+ filter electrolytics are both 8 uF.
:
:Is the hum present even with the volume control set at minimum?
:

Yep, I missed that one. I superficially scanned the schematic because I wasn't certain that it pertained to the actual set under discussion. That cap is the grid bias (B-) smoothing cap. If it were missing or reverse-connected it would certainly add to the ambient hum, since the ripple would be impressed directly on the audio amp's grid, even at zero volume setting.

:Yep, I missed that one. I superficially scanned the schematic because I wasn't certain that it pertained to the actual set under discussion. That cap is the grid bias (B-) smoothing cap. If it were missing or reverse-connected it would certainly add to the ambient hum, since the ripple would be impressed directly on the audio amp's grid, even at zero volume setting.
:
OK CV
Let me see if I got it right You are saying that the 8 mfd cap going to the speakers power transformer might be the problem. Am I right?? The cap if it is that you are talking about IS THERE. I will check to see if the polarity is correct. Would an increase in value of that cap help or will this cause other problems. Thanks again

Nope, the cap to look at is the 4 mfd 25 V one that is connected to the low side of the volume control.

:CV,
:
:There is another 4uf cap at the junction of a 100K resistor and the 1meg volume control to filter the bias on the 75 control grid. He might want to check the polarity of it as it's plus goes to chassis.
:
:Clifton
:
:
::Is it really a Grunow 500 (Chassis 5A)? The Riders schematic shows that this set has no humbucking coil,
::and that the only 4 uF electrolytic in the set is the AC bypass for the audio output tube. The two B+ filter electrolytics are both 8 uF.
::
::Is the hum present even with the volume control set at minimum?
::
:Yes it is the 500 (5A chassis) Sorry I made a mistake on the 2 electrolic B plus filters value they are 8 mfd. not 4. Also the hum is present when the volume is way down almost to the point turning the set off. And that's what I taught, that in the schemathic there is no hum buckinbg coil. Which is really a surprise due the very large field coil on that very heavy speaker. Then how was this issue dealth by the3 designers of this radio, to supress this noise?? Thanks
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