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repairing an amplifier

Discussion in 'General Electronics Discussion' started by konkas, Jul 8, 2016.

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  1. konkas


    May 31, 2016
    Hi all
    I'm trying to find the fault of an old amplifier (model from 1973).
    (attached schematic)
    The fuse (1 A slow) in the red circle blows all the time.

    Could it be the big 2200 uF capacitors that are broken and shortcircuits?

    Any other guesses?

    K TelefrangLABFschema_sk.jpg
  2. duke37


    Jan 9, 2011
    It is the mains input fuse so the fault could be anywhere after this.
    I would use a lamp limiter (40W) to run the amp without a fuse blowing all the time.
    There are 1.25A fuses to the amp supply. Remove these to isolate the fault within the power supply.
    If the capacitors have rounded ends from internal pressure, then these should be changed.

    You will need to break the connections at some points to isolate the fault.
    It may be easiest to disconnect the transformer secondary and check that it is OK.
    Then disconnect the bridge rectifier outputs to see if you get about 30V on positive and negative output.
    Then disconnect the fat capacitors to see if these voltages are maintained.

    Logical progression is the way to go.
    hevans1944 likes this.
  3. konkas


    May 31, 2016
    Thanks a lot!
    I will use this!

  4. konkas


    May 31, 2016
    Hi again!
    I carefully touched the rectifier diodes and two of them are broken.

    Can this really cause the mains fuse to blow?

    In the picture below the diodes are seen just above the fat capacitors.
    Btw, the black and green cables from the transformer are obviously going to the rectifier bridge but what are the two red cables for? It's not obvious for me in the schematic above.

  5. AnalogKid


    Jun 10, 2015
    According to the schematic the transformer has a center-tapped secondary. This might in fact be two independent secondaries. If so, the two red wires might be the junction of the two secondaries to form the center-tap.

    Yes, a diode can fail pseudo-short after heating up enough to fracture the case.

    To blow a primary-side fuse, the secondary must be pumping out a lot of current. As above, suspects are the transformer itself and the big filter caps, plus the output stage power transistors. I suspect the caps and transistors first because an internally shorted secondary would not take out the rectifiers, but a downstream short would. If both dead rectifiers are for the same power supply output (+ or - 33 V), that is a clue as to where to look first.

  6. duke37


    Jan 9, 2011
    There has been a lot of heat around the diodes as the board is cooked.
    The diodes should be replaced with high current diodes or a high current bridge.

    I mentioned a light limiter, you could make one by connecing some twin flex across a blown fuse and then connectiong this flex to a lamp/globe/bulb holder and fitting with a 40W light.
    Test before the diodes are replaced, the lamp should not light.
    Replace the diodes, the lamp should light for a fraction of a second and then die out. Voltages can then be checked, they will be low but reasonable. Some sound should be available but not high volume. If you try to get some, the light will brighten.
  7. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir konkas. . . . . . .

    It looks like it's not being a HIGH power amp . . .in its only using a 3055/2955 output pair.
    SO . .if it didn't take out either of their 1,25 amp . . . . +33 and -33 supply line fuses, they were not heavily involved.

    You just might be lucky in only having to replace the 4 full wave bridge rectifiers, with some upgraded 1N5406 diodes, for both a voltage and current cushion in the future.

    AS a precaution . . . . as to how long those (4) 2200 ufd caps might have taken raw AC during that little diode cracking escapade, plus 43 years of aging behind them, might as well replace them also.

    I am reading the toroidal power transformer as having the two yellow AC line wires feeding into one terminal of a DPST power switch and the other into the fuseholder.
    The BLACK and GREEN being the ~30VAC windings with the PAIR of RED wires being their center tap.

    I also concur on the "soft start up" via a series lamp . . . . initially.

    Is this a Telefrang Lab F unit . . . . .as I found ZERO info . . .but your schema is adequate.


    73's de Edd
    Last edited by a moderator: Jul 12, 2016
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