Author Topic: weak left channel -  (Read 101 times)

Tedrick

  • Newbie
  • *
  • Posts: 17
    • View Profile
weak left channel -
« on: January 29, 2018, 07:00:52 PM »
Hi Folks -
so on my project piece, I have replaced the caps on the output board and on the control board my right channel sounds fine but my left channel is noticeably weaker --

I'm not sure where to start looking for this issue -- are there measurements I can take on the output transistors? would it be on the balance circuit?

Thanks for any insight on this -
Ted

electra225

  • Hero Member
  • *****
  • Posts: 1744
    • View Profile
Re: weak left channel -
« Reply #1 on: January 29, 2018, 08:02:31 PM »
What are you working on?
If it ain't broke, call me.  I can break it....

Tedrick

  • Newbie
  • *
  • Posts: 17
    • View Profile
Re: weak left channel -
« Reply #2 on: January 29, 2018, 08:15:35 PM »
this is a 1977 Chassis: R-34305-CC bicentennial console -
I did some more probing and pressing the output board firmly into the connector seems to have fixed the issue -
now I'm getting both sides normally --

what is the correct way to measure the left and right channels on the output board? Would I measure the pins on the output transistors or is there a series of measurement points that I should look at and compare left and right?

Thanks!
Ted

Tedrick

  • Newbie
  • *
  • Posts: 17
    • View Profile
Re: weak left channel -
« Reply #3 on: January 30, 2018, 08:08:04 PM »
hmm.... not sure if the issue is a connection issue or if something is going on with the balance pot -
I'm going to rig up a simple signal tracer and try to identify where I loose the channel --

I'm trying to learn how to use my digital oscilloscope - I'm not sure how to set up the scope for this type of signal tracing -- any tips would be very much appreciated --

Ted

Tedrick

  • Newbie
  • *
  • Posts: 17
    • View Profile
Re: weak left channel -
« Reply #4 on: February 01, 2018, 09:49:08 PM »
continued trouble shooting tonight -

initially thought it was the molex connection so I made a direct connection between the pins and the speaker wires -

This did not solve the problem so I rigged up a temp power cord and got the receiver on my bench -

I was able to feed a signal into the aux in pins and trace the signal on the control board using a simple signal tracer --

I feel like I'm getting closer.... it just felt good to get the problem to reproduce itself separate from the connectors --

Ted

Tedrick

  • Newbie
  • *
  • Posts: 17
    • View Profile
Re: weak left channel -
« Reply #5 on: February 04, 2018, 01:44:12 PM »
the issue I'm seeing is both channels will come in for just a moment when I touch the balance pot - so it's almost like a ground thing?

looking at resistors --

is it accurate to measure for resistance across the resistor while it is in circuit?

HiFiFun

  • Full Member
  • ***
  • Posts: 123
    • View Profile
Re: weak left channel -
« Reply #6 on: February 04, 2018, 02:47:55 PM »
looking at resistors --

is it accurate to measure for resistance across the resistor while it is in circuit?

Here is a good discussion of your question:
http://www.antiqueradios.com/forums/viewtopic.php?f=6&t=181178

Visual inspection is always first order.
HFF

electra225

  • Hero Member
  • *****
  • Posts: 1744
    • View Profile
Re: weak left channel -
« Reply #7 on: February 05, 2018, 03:52:42 PM »
If monkeying with the balance pot changes performance, even for an instant, you might take a closer look at that pot just to eliminate it from suspicion.  Magnavox instruments are known for suffering from dirty control pots.  Clean the balance pot carefully, then measure resistance, with the power off, to see if there is a dead spot in the control.  Good luck.
If it ain't broke, call me.  I can break it....

ed from Baltimore

  • Newbie
  • *
  • Posts: 36
    • View Profile
Re: weak left channel -
« Reply #8 on: February 12, 2018, 05:51:15 AM »
          Probably the reason old Magnavoxs suffer from noisy pots is the same reason that old Magnavoxs compress the carpet under the feet (especially on a French Provincial, the equivalent of stiletto heels) down to an 1/8 inch mat. The original  owners (who probably buy their teenage kids their own phono so they keep their mitts off the good hi-fi and out of the living room) usually touch nothing for 40 years except to raise the volume control once every 10 years as their hearing dulls. The turntable knob is probably frozen to "33" and the speaker switch is frozen onto "Internal" and the FM dial and tuning capacitor frozen to the local easy listening music station. 
    The Tape Monitor buttons on so many component receivers have never been pushed once since purchased new and lots of times, one or both channels will eventually disappear completely until the button is pushed in and about about ten times. The switches may be rated for 10,000 cycles but not for fifty years of absolute zero cycles. 
    Plugs and connectors are great when removing chassis' to service, and Magnavoxs always seem to have their full share compared to some not-so-good makes, but there's a down side. Its ironic that equipment built with such high quality from parts to quality assurance inspections to low heat buildup layout, and------- lots of plugs, edge connectors and sockets to ease servicing ---------that's seldom needed.
    But it can sure make the seasoned knob twister and twiddler look like a magician as he gets a cranky newly purchased console singing like a new stereo in front of the amazed new owners. 

electra225

  • Hero Member
  • *****
  • Posts: 1744
    • View Profile
Re: weak left channel -
« Reply #9 on: February 12, 2018, 01:45:47 PM »
Magnavox instruments, 60 years + old, suffer from dirty and noisy pots because the pots were not very good to begin with.  They were never designed nor intended for us to still be using them and demanding competent performance all these many years after they were built.
If it ain't broke, call me.  I can break it....

Tedrick

  • Newbie
  • *
  • Posts: 17
    • View Profile
Re: weak left channel -
« Reply #10 on: February 17, 2018, 09:15:15 AM »
Thanks guys -
yes - I am thinking I damaged the pot when cleaning it with deoxit -
first of all, I probably over did it with the deoxit and then in the process of back and forthing vigorously I damaged the swiper wiper inside the pot --

So.... I'd like to defeat the pot -

I have heard about this in other projects - taking a resistor and connecting the resistor from the left channel to the center leg  and again a same value resistor from the right channel going into the pot to the center leg ground --

I'm foggy with the part about inserting the resistor into the L and R circuit - do I lift the pot leg out of the pcb and solder the resistor there? Or do I just go from L/R leg to the center ground leg?

Thanks!
T