Author Topic: Grounding Issue?  (Read 2085 times)

revolverone

  • Full Member
  • ***
  • Posts: 159
    • View Profile
Grounding Issue?
« on: November 09, 2015, 11:35:35 AM »
Last summer I picked up a little Magnavox Consolette.  I begin working on it a couple of months ago off and on.  It had a VM changer in it and the changer was definitely not in the best condition.  I decided to replace it with a Micromatic Changer I pulled out of a junk console I got for $10 a Goodwill.  So I rewired the power adapter and cut out the plinth and the changer worked fine with it.  For the sound I bought a "Y" adapter to run the changer RCA out (stereo) to the input on the amp (mono). Then came the terrible vibrating hum when I started testing it.  I thought may it was the e-caps in it and replaced them all with new ones. No change.  So I plugged in my mp3 player through an mono adapter and it sounded fine.  Plugged the changer back in, and the same vibrating hum.  I think I may have a grounding issue with the audio.  If I touch any portion of the metal on the changer, especially the record arm it gets considerable louder.  Anyone have any idea as to what could be causing this and what the fix is?  I thought the grounding wire coming off the cartridge was grounded through the RCA cable thus to the amp chassis.

AstroSonic100

  • Full Member
  • ***
  • Posts: 222
    • View Profile
Re: Grounding Issue?
« Reply #1 on: November 09, 2015, 02:30:30 PM »
First step in curing the hum is to find out if your set has a power transformer? Or does it have output tubes such as 50C5 or 35C5?

Also what chassis number is the amp?
Ray

revolverone

  • Full Member
  • ***
  • Posts: 159
    • View Profile
Re: Grounding Issue?
« Reply #2 on: November 10, 2015, 08:28:07 AM »
It has two 50C5s and one 35c5.  I believe the AMP # is 155AA.

Larry H

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 2642
  • Little Rock, Arkansas
    • View Profile
Re: Grounding Issue?
« Reply #3 on: November 10, 2015, 04:12:52 PM »
50C5s and 35C5s in the same amp?  I've never heard of such.

50C5s are very prone to hum when they heat up.  If you've got replacements, sub them to see if that's the issue.
--Larry

AstroSonic100

  • Full Member
  • ***
  • Posts: 222
    • View Profile
Re: Grounding Issue?
« Reply #4 on: November 10, 2015, 04:16:26 PM »
Just as I thought.  You're AMP-155AA does not have a power transformer.  I have attached a jpeg of the schematic for the amp (sorry for the poor quality - pic swiped from eBay).  You probably also have another small chassis for the tone controls and volume which probably also has one or two tubes.  If so, that would have another chassis number stamped on it.

You will need to make a change for the power connection.  See the pdf file I have attached.  The .01 disc capacitor is located under the on-off switch assembly.  I also just uploaded the factory service manual for the W602 Micromatic changer which should apply to your model.

I ran into a similar problem when I used a Micromatic changer from a portable with an AC-DC portable.  I installed it in an AC console it caused the same type of hum.  I also had to make a change to the power connection.

Please make this modification to the changer before working on the rest of the set.

A word of caution.  Use extreme caution when working with these "AC/DC" sets.  According to the schematic of the amp, one side of the AC cord goes to one side of the .047 mfd cap.  The other side of that cap goes to the metal chassis.  If this is a paper cap, it will need to be replaced.  This cap isolates the B- bus from the metal chassis.  However, it is still possible to get a nasty shock or worse if you should accidentally touch the metal chassis with your hand and also touch an object such as test equipment that has a three prong line cord with a ground pin or touching a water faucet (I know that sounds far fetched but people have received a nasty shock by doing just that).

If you have a GFI (Ground Fault Interupter) outlet available you can plug the set into that while working on it.  That would detect any electrical leakage and shut down power to the outlet so you don't get shocked.

Once you have completed the modification to the changer and the set is put back together, it should be safe to use keeping the above safety issues in mind.


Ray

Larry H

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 2642
  • Little Rock, Arkansas
    • View Profile
Re: Grounding Issue?
« Reply #5 on: November 10, 2015, 04:22:01 PM »
Just as I thought.  You're AMP-155AA does not have a power transformer.  I have attached a jpeg of the schematic for the amp (sorry for the poor quality - pic swiped from eBay).  You probably also have another small chassis for the tone controls and volume which probably also has one or two tubes.  If so, that would have another chassis number stamped on it.

You will need to make a change for the power connection.  See the pdf file I have attached.  The .01 disc capacitor is located under the on-off switch assembly.  I also just uploaded the factory service manual for the W602 Micromatic changer which should apply to your model.

I ran into a similar problem when I used a Micromatic changer from a portable with an AC-DC portable.  I installed it in an AC console it caused the same type of hum.  I also had to make a change to the power connection.

Please make this modification to the changer before working on the rest of the set.

A word of caution.  Use extreme caution when working with these "AC/DC" sets.  According to the schematic of the amp, one side of the AC cord goes to one side of the .047 mfd cap.  The other side of that cap goes to the metal chassis.  If this is a paper cap, it will need to be replaced.  This cap isolates the B- bus from the metal chassis.  However, it is still possible to get a nasty shock or worse if you should accidentally touch the metal chassis with your hand and also touch an object such as test equipment that has a three prong line cord with a ground pin or touching a water faucet (I know that sounds far fetched but people have received a nasty shock by doing just that).

If you have a GFI (Ground Fault Interupter) outlet available you can plug the set into that while working on it.  That would detect any electrical leakage and shut down power to the outlet so you don't get shocked.

Once you have completed the modification to the changer and the set is put back together, it should be safe to use keeping the above safety issues in mind.
You talk about making a modification here, but don't go into specifics about what he should do.  Please be more specific.
--Larry

Larry H

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 2642
  • Little Rock, Arkansas
    • View Profile
Re: Grounding Issue?
« Reply #6 on: November 10, 2015, 04:25:33 PM »
Shane, take a clippie and attach one end to something on the underneath of the changer, and the other end to something on top of the metal chassis.  If the hum goes away, you will have to run a permanent ground wire.  I've seen more than one 70s vintage turntables that had ground wires to attach to a receiver.
--Larry

AstroSonic100

  • Full Member
  • ***
  • Posts: 222
    • View Profile
Re: Grounding Issue?
« Reply #7 on: November 10, 2015, 06:00:59 PM »
Shane - My original changer in the attached photo came from an AC/DC portable.  The current connections in the photo I made for operation in an AC only set. 

I have drawn in the modifications you need to make to your changer so it can be used with your AC/DC amp.

Steps:

1.  Remove turntable

2.  Remove cover to on-off switch assembly.

3.  Refer to the attached photo.

4.  Disconnect lead [shown with a red X] of disc cap from the terminal with the black only wire connected to it.

5.  Follow the black dotted line and connect this lead to the bare wire connected to the solder lug attached to the changer base.  You will notice a blob of solder right under the black arrow I drew.  This is where Magnavox originally had connected this cap when it was for AC/DC operation.

6.  Replace switch cover.

7.  Replace turntable.

The above modification isolates the metal base of the changer from one side of the AC line.  This should eliminate your hum problem.

Ray

revolverone

  • Full Member
  • ***
  • Posts: 159
    • View Profile
Re: Grounding Issue?
« Reply #8 on: November 11, 2015, 10:08:50 AM »
Thanks Ray!  I will try this modification tonight.

revolverone

  • Full Member
  • ***
  • Posts: 159
    • View Profile
Re: Grounding Issue?
« Reply #9 on: November 16, 2015, 09:52:05 AM »
Well I got delaying on trying this modification until last night.  Unfortunately, I'm still getting the same results.  I know I did it exactly as instructed. Any other suggestions?

AstroSonic100

  • Full Member
  • ***
  • Posts: 222
    • View Profile
Re: Grounding Issue?
« Reply #10 on: November 16, 2015, 06:14:46 PM »
Have you tried reversing the AC plug on the amp?  Sometimes this will eliminate some or almost all of the hum.

Can you let me know how the ac power to the Micromatic is connected the amp vs. how the original VM changer's power was connected to the amp? 

The micromatic's molex power connector uses three pins. 

One pin is for a red wire that goes directly to one side of the motor. 

The bottom pin on the molex connector goes to one side of the changer's on/off switch. 

The other side of the switch is connected to a black and yellow wire and also one side of the .01 disc cap.  The black wire goes to the other side of the motor.

The yellow wire goes to the middle pin on the Molex connector.  Power from this pin is used to turn an amp on/off.

You also need to check for a broken ground wire on the leads from the phono cartridge to the RCA jacks on the bottom of the changer.  This could be another source for hum.

Ray

revolverone

  • Full Member
  • ***
  • Posts: 159
    • View Profile
Re: Grounding Issue?
« Reply #11 on: November 28, 2015, 12:13:02 PM »
Ray, thanks for your tips.  I just got back to this project this morning.  I do have some more information.  I plugged a Magnavox Collaro changer into the amp just to see what would happen -- same results.  When I touch any metal part of the changer, I just this loud hum.  Something isn't grounding correctly in the amp is my thoughts.  Just fyi, with the Micromatic, I had to do some adapting -- the power connector and stereo to mono, But with the Collaro it's got the correct plug to go into the amp and it's also mono.  This to me eliminates the problem being in the changer.  I definitely feel like it's an amp problem.  What would be your next thoughts?

ts 1

  • Newbie
  • *
  • Posts: 10
    • View Profile
Re: Grounding Issue?
« Reply #12 on: March 30, 2016, 08:54:31 PM »
I Put a collaro changer from my transformerless hot chassis (AC DC?) console into my stereo master and had a far more pronounced and unbearable hum than when it was in its original unit. Larry's suggestion of a clippie ground wire worked for me. Thanks Larry thank you vintage HiFi. Very pleased now.