Loud feeback HUM?! ah!!!! Restoring a record cutter.

[HardlyHumanFX]

Hello Trolls!

My name is Jason. I'm new here. I just acquired a couple Recordios and set about restoring Model 6B10 circa 1939. I found the schematics and components lists and soldered in new capacitors. I'm certain that was done correctly. HOWEVER! When I plug in any input into the microphone jack I installed, I get a hum that sounds a bit like feedback even though it's very similar to 60hz buzzing sound. It goes away (mostly) if I use a ground loop isolator.

There was a metal shield around the old microphone jack that was connected to the chassis. I did NOT put a new one in yet. Is it necessary?

If I tap the chassis with my screwdriver, I can hear a ring come out of the speaker. Is this indicative of a bad microphone amplifier tube (6SJ7GT)??

I have also noted that this thing isn't grounded.

So, I ordered a new microphone amplifier tube (6SJ7GT) and a grounded appliance socket/cord. Arrives Wednesday.

Any further suggestions would really be appreciated!

Attached photo and schematics.

Jason Anderson, 29
Orange, CA

[markrob]

Hi,

You are probably jumping the gun ordering up a replacement tube and grounded power cord.

What are you using for a mic? If its the one that comes with the cutter, it is probably a crystal mic and could very well be bad.
How are the selector switches set ?
After you plug in the mic, does the level of the hum go up and down as you rotate the volume control?
Does the radio play?
Have you measured the voltages shown on the chart in the Sam's Photofact?

Mark

[HardlyHumanFX]

Hi,

You are probably jumping the gun ordering up a replacement tube and grounded power cord.

What are you using for a mic? If its the one that comes with the cutter, it is probably a crystal mic and could very well be bad.
How are the selector switches set ?
After you plug in the mic, does the level of the hum go up and down as you rotate the volume control?
Does the radio play?
Have you measured the voltages shown on the chart in the Sam's Photofact?

Mark

Hi Mark!

When the unit is on and microphone is selected and nothing is plugged in, it's fairly quiet even if you turn up the volume. Once you plug in the original microphone (which I replaced the cord on) it loudly hums, uncontrollably loud if you turn the volume up at all from minimum. Even with the hum, however, you CAN hear that it is picking up my voice on the microphone.

If I plug in a ground loop isolator and connect a 3.5 jack to my laptop, it plays whatever I throw at it with no hum or distortion at all unless you go way up on the volume, like the very end of the knob selector.

The selector switches seem fairly clean. They are push-button type, reset by pushing any of them half way down. They're in pretty good shape. Phonograph/Public Address/Record Microphone/Record Radio

The radio plays well.

I have not yet measured the voltages. I am a newbie! I will check that out on the chart!

[HardlyHumanFX]

Hi,

You are probably jumping the gun ordering up a replacement tube and grounded power cord.

What are you using for a mic? If its the one that comes with the cutter, it is probably a crystal mic and could very well be bad.
How are the selector switches set ?
After you plug in the mic, does the level of the hum go up and down as you rotate the volume control?
Does the radio play?
Have you measured the voltages shown on the chart in the Sam's Photofact?

Mark

I should also mention the microphone worked if you held the original wiring just right. After I replaced the machine's capacitors/microphone's wire/microphone jack it stopped working right. That's why I feel like I screwed something up.

[markrob]

Hi,

So it seems like you are in good shape. Its probably a grounding/shielding issue. What did you replace the old jack with? How is it tied to the chassis ground? If I understand correctly, if you plug in a line level source into the mic input you have no hum issues. Is this correct?

Mark

[HardlyHumanFX]

Hi,

So it seems like you are in good shape. Its probably a grounding/shielding issue. What did you replace the old jack with? How is it tied to the chassis ground? If I understand correctly, if you plug in a line level source into the mic input you have no hum issues. Is this correct?

Mark

Hi Mark!

Last night I turned it on and there is no hum if there is nothing plugged in and minimal hum if I plug in a 1/8 line directly to my phone. That disappears if I use a ground loop isolator before I connect to the line.

If, however, I connect the microphone, it blares and buzzes at any volume level. So, I guess the microphone is bad. It was really hard to take it apart to replace the wiring so I guess I damaged the guts. And the cable is probably cheap/unshielded.

I never replaced the metal shield around the original jack. The original jack was 2 prongs. My 1/4" jack is too large for the metal shield to fit without touching it. It's a standard 1/4" jack. I just drilled out the brown material that held the 2 prongs and screwed the jack tight. No part of the jack itself is grounded, just has the two signal wires soldered to its leads.

Thanks for helping me! Let me know what you think. Also, I ordered some blanks. I've run out of space on my lone pvc/vinyl blank to experiment with. I plan to run different tones through and see what range my cartridge has. It's original and seems pretty damn good.

Jason