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Heathkit GR-681 TV schematic?

Discussion in 'Electronic Basics' started by Phil Symonds, Mar 1, 2004.

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  1. Phil Symonds

    Phil Symonds Guest

    I have a curbside find...chassis of a Heathkit GR-681 TV. Pretty well
    stripped, but this on had the transformers, and lots of caps, resistors and
    other small goodies. Also had the HV supply (this is surplus as far as I'm
    concerned, read on).

    My interest is in figuring out if I've got stuff that could be used to make
    a homebrew guitar amp (low output is fine), but I don't have any info on the
    transformers. I'm seeking possibly a schematic and even better info about
    what's what on the transformers. I've got five pieces of iron that I'm
    trying to figure out. Any help would be appreciated. Thanks.

    Power transformer
    94-143 (hard to read)
    394-7242 (Foster, 42nd wk, 1972)
    lots of wires (10 in all)

    Filament transformer
    549-7249 (Midwest Coil & Transformer, 49th wk 1972)

    Vertical output transformer (any possible audio use?)
    549-7304 (Midwest, 4th wk 1973)
    six wires on the secondary

    Audio Transformer
    60-7301 (Mfg? 1st wk 1973)
    Terminals #1 , #2 on once side
    Terminals in this order on the other side #3 #5 #4

    60-7240 (mfg? 40th wk 1972)
    What rating?
  2. Jamie

    Jamie Guest

    One man's junk is another man's gold!
  3. Phil Symonds

    Phil Symonds Guest

    Only if I can figure out what I've got!
  4. Phil Symonds

    Phil Symonds Guest

    update: p/n is probably 54-243; Herm from Foster emailed me today, so maybe
    I'll get lucky :~} Still looking for help on the rest.
  5. Mike Berger

    Mike Berger Guest

    Can't you just use a voltmeter to identify the voltages on the power
    and filament transformers? The colors should indicate center taps.
  6. Phil Symonds

    Phil Symonds Guest

    I've got a bit electronics experience, but feel out of my depth here. I've
    been dabbling in this for about 2 years, so there are many things others
    take for granted that I'm clueless about. These trannys are all pulled from
    the chassis, so there are no hints except for the PT and the VOT which both
    had the yellow wire soldered to the chassis. I'm not clear what the yellow
    to chassis tells me. You say the colors should indicate center taps. Which
    (typical) colors? What are the indicators?

    Also, I think you are assuming that I've got AC running on the primary. I'm
    not sure which wires are the primary, so I don't know how to hook them up.
    If I did, I could bolt the trannys to a board and start figuring it out.
    For example, how do I identify the primary on the PT when all the wires come
    out of one hole? Should I remove the cover and look inside? What are the
    chances that I'll get the cover back on OK? How do I figure out the rating
    of the choke? How do I figure out the secondary taps on the audio OT? I
    think you get the drift. I need some pointers or even some reading about
    this. Willing to help?

  7. Mike Berger

    Mike Berger Guest

    Power and filament transformers almost always run from the AC
    line, so let's assume you can use your standard AC voltage. On
    a typical power transformer, the black leads are primary -- sometimes
    you'll find a centertap, often black with a colored stripe, so you can
    use the transformer for 117 or 234 volts. Though it's less common in
    consumer electronics, there may even be taps for high and low line
    voltages (ie: 110 v. and 130 v. primary taps).

    Filament is almost always green, and a centertap is usually green with
    a yellow stripe.

    Everything else should be secondaries -- look for matching color pairs,
    with a striped wire of the same color being a probable centertap.
  8. Phil Symonds

    Phil Symonds Guest

    Thanks for the lesson. I just needed someone to tell me that what I thought
    was right. Fortunately for me, someone at Foster Transformer emailed me
    with the PT specs and colors, so I don't have to fish for them. I'll bolt
    the filament xfrmr to a board and power it up. It will be simple to measure
    it. I'm not sure what to do about the Audio OT and the Vertical OT, but
    maybe if I get the filament xfrmr running, I can use it as a low voltage
    source on the secondaries to see what comes out on the primary. From there
    I might be able to figure the ratio and therefore the ohms. Then there's
    the choke. Essentially the same as a big resistor?

    BTW, the filament is Yellow + Yellow/Green and the primary is either
    Black+Black/Red or Black+Black/White!

  9. Henry007

    Henry007 Guest

    Just see what the windings are connected to. If the output transformer has
    wires which go to the speaker, note which ones they are. Often, there is a
    singel primary winding, and a tapped secondary (ie com-4ohm-8ohm) For a
    guitar amp, its not too crucial, just note which side is connected to the B+
    and which goes to the speaker. Measure the speaker resistance as well, and
    note what output tube it uses. Also note what rectifier tube the set uses
    (5U4 by any chance?)

    Vertical output transformer may be useful, dunno.

    Power tranny will definitely be useful. Also try and save any metal can caps
    that don't look swollen or have a burst base, they will probably be ok when
    reformed. Also any ceramic sockets or tagstrips are worth saving.

    Some darn good guitar amps can be made with those bits, thats for sure.
  10. Phil Symonds

    Phil Symonds Guest

    This is a nice thought. Unfortunately the opportunity wasn't present. This
    chassis was pretty well stripped and nothing connected when I got it. The
    OT is puny and it's clear which is the primary (two terminals) and secondary
    (3 terminals), but it would be real helpful to know what the combinations
    I'll be harvesting caps, resistors, terminal strips, and wire. No sockets
    present. The power tranny is borderline for a 2 tube amp with a bridge
    Thanks anyway.
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