37-93 recapped but its a hummer - Printable Version
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+--- Thread: 37-93 recapped but its a hummer (/showthread.php?tid=10953)
37-93 recapped but its a hummer - DanH6464 - 11-14-2014
I just completed a full recap of 37-93 to include filter caps and restuffed bakelite blocks. Powered up and stations are coming in strong (and no smoke lol) however I'm getting a pretty good hum across the band on station or not and it's not affected by volume. If I turn it up the hum doesn't get louder and when you turn the volume down...the volume goes down but the hum stays the same. The speaker is practially in shreds poorly repaired and radio has not been aligned. Any imput appreciated.
http://www.nostalgiaair.org/PagesByModel/228/M0013228.pdf
RE: 37-93 recapped but its a hummer - jerryhawthorne - 11-14-2014
Possible heater/cathode leakage in AF section either first audio or output tube? Have you tried subs? Hum bucking coil miss wired possibility if you removed wiring? Just some ideas.
Jerry
Edit: Cap #6 miss wired, note polarity with positive going to chassis ground. Other e-caps going to center tap with their negatives, not chassis ground.
RE: 37-93 recapped but its a hummer - DanH6464 - 11-14-2014
The polarity on the two 8's and the 10mfd appear to be correct...I swapped the output tube out with a spare (no change) don't have spare 1st audio...set of spares ordered but not arrived. Did'nt remove anything but did put it back to the orignal design from where they had bypassed the bakelite blocks and the filter caps
RE: 37-93 recapped but its a hummer - OZ4 - 11-15-2014
For another test you might try connecting a .1uf cap from the control grid of the output tube to ground. That should tell you where the hum is coming from. There shouldn't be high voltage on the grid side of C22 so a low voltage cap should OK. This will short the signal to ground.
If it's quiet then it's the 2nd det or before.
And if not then it's the output section and or power supply.
I just had one with a bad output tube that was shorted from the filament to the cathode. The AC hum could be seen on the grid side feeding back to the det. And it was a brand new tube.
RE: 37-93 recapped but its a hummer - jerryhawthorne - 11-15-2014
Chris has a good plan for a test. You could also pull the 1st audio/det tube and see if the hum disappears. If it doesn't, likely power supply or output tube.
Jerry
RE: 37-93 recapped but its a hummer - DanH6464 - 11-16-2014
I pulled the 6r7g....still humms like a crack addict....I'm liking the mallet idea lol
RE: 37-93 recapped but its a hummer - DanH6464 - 11-17-2014
Checking the power transformer I'm getting 310v on each side of the winding and 5v at rec tube...which sound right...I think.
RE: 37-93 recapped but its a hummer - jerryhawthorne - 11-17-2014
Dan, your down to the power supply filtering, hum bucking coil or output tube. You have changed the output tube, that doesn't leave much! Possible new defective e-cap?
Jerry
RE: 37-93 recapped but its a hummer - DanH6464 - 11-17-2014
gator clipped .01 cap to pin 5 of the 6f6g...it knocked nearly all of the volume/voice off of the speaker but the hum was not affected...so what does that tell us?
RE: 37-93 recapped but its a hummer - DanH6464 - 11-17-2014
I changed my existing output tube with a very dubious looking replacement glass 6f6 ...low hum still there and the radio didn't seem to perform as well.
RE: 37-93 recapped but its a hummer - DanH6464 - 11-17-2014
I have a set of tubes coming from tubes and more but still haven't arrived. Might just hit myself with the mallet instead lol
RE: 37-93 recapped but its a hummer - Mondial - 11-17-2014
The filtering of the power supply in that model is somewhat marginal. You might try adding an additional or larger value cap across C6A, the second filter after the field coil and see if it reduces the hum. As Jerry said, there is not much left to check but the power supply or speaker field.
RE: 37-93 recapped but its a hummer - Arran - 11-17-2014
Have you tried disconnecting one of the filter caps yet? Either the input one, before the field coil, or the output one after? The hum should get louder or change if either one is disconnected. I'm wondering if one of the filter caps may have a bad connection on the negative side, or that you may have confused common negative with the chassis, common negative is the center tap of the H.V winding on the power transformer. On part #6 the positive end of that capacitor must go to chassis, the chassis is more positive with respect to the center tap, parts #6A and #6B also have their negative end going to the center tap, but not the chassis.
Regards
Arran
RE: 37-93 recapped but its a hummer - DanH6464 - 11-18-2014
It's now singing!! Many many thanks to Jerry, Mondial and Arran...I know I'm the retarded son you never wanted lol. Bad wire connection to the rectifier north of the field coil as well as a bad new e cap. It's singing clear as a bell now. Now I get to figure out how to patch the hole in the top of the cabnet. Thanks again guys for all the help..you guys are a life saver for a newby retard.
Dan in LA
(Not Los Angeles or Lower Alabama)
RE: 37-93 recapped but its a hummer - jerryhawthorne - 11-18-2014
Glad you got it running Dan. Now the fun starts.
Jerry