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Dillen

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Viewing 20 posts - 121 through 140 (of 718 total)
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  • in reply to: Beogram motor bearings issue #56641
    Dillen
    Moderator

    Most likely not the set with the collared bearing.
    As mentioned in the bearings sets descriptions, you will have to look at the motor to see
    which set is needed.

    Martin

    in reply to: Beocord 1200, no recording or playback #56640
    Dillen
    Moderator

    Sure you have the tapes playing side against the tapeheads?
    The tape could be twisted so it has the backside against the heads.

    Martin

    in reply to: Beogram motor bearings issue #56630
    Dillen
    Moderator

    Remember lubricating should be with an oil suitable for use with sinter-bronze. It needs to have the
    right additives. Lubrication is done with the bearings submersed in the oil, while vacuum is pulled to get
    the air out of the pores in the bearing metal. Then leave them submersed without vacuum for 24 hours to allow
    oil to suck in.
    If not oiled correctly the bearings will only run fine for a short time (hours or days).
    Factory new bearings, pre-oiled and ready to fit is available from Danish Sound Parts.
    I would recommend that as it also includes new reservoir felts and takes wear to the old bearings out of the equation.

    Martin

    in reply to: Beogram motor bearings issue #56627
    Dillen
    Moderator

    A photo or two showing how far you’ve come would help us suggest the next step.

    Martin

    in reply to: CD3300 removing boards #56598
    Dillen
    Moderator

    Dismounting the servo circuit board from the bottom of the disc drive you must work carefully.
    You will have to release the ribbon cable running to the laser block from its socket on the pcb.
    This can be tricky – particularly because the cable will almost certainly break at the sharp corner where it’s secured, if pulled.

    Martin

    in reply to: Help Identify Tonearm #56530
    Dillen
    Moderator

    Correction: It cannot be from a 606 due to the different pivot system.

    Martin

    in reply to: Help Identify Tonearm #56524
    Dillen
    Moderator

    It could be from a GR 605 (1959) or 606 (1960).

    605_1

    IMG_9921

    Martin

    in reply to: BM 4400 Mexico #56467
    Dillen
    Moderator

    Put a scope on an output.
    Check for oscillation.

    Martin

    in reply to: What are you working on now? #56370
    Dillen
    Moderator

    Work1

    Martin

    in reply to: BM 4400 Mexico #56367
    Dillen
    Moderator

    The modern grey stuff often sold for use with computer CPUs is useless. Some of it dries hard over a week or two.
    Silicone pads – some are fairly good, others are junk. I don’t like them, and I throw them away on sight.
    The original white heatsink compound, that was also used originally, works best. It will last for decades.

    Martin

    in reply to: BM 4400 Mexico #56358
    Dillen
    Moderator

    A drifting idle can be caused by bad thermal contact to the heatsink.
    Did you use the proper white heatsink paste?

    Martin

    in reply to: Beogram 1700 – starts at wrong speed – 66 RPM #55970
    Dillen
    Moderator

    A worn and slipping belt can allow the motor to accellerate to easily.
    The motor pulley simply slips on the belt at startup.

    This, coupled with electronics that will accept to lock in at twice the correct speed, causes this.
    The cure is to replace the belt.

    Correct belt here.

    Martin

    in reply to: Beomaster 1900 ( it just stays in standby mode) #55930
    Dillen
    Moderator

    I use a “glass-hair”/fiber brush.
    But if there is oxidation, you can clearly see it as black flakes.

    Martin

    in reply to: Old timers Beovox 3800.Mid range drivers. #55760
    Dillen
    Moderator

    Welcome back, Peter!

    Martin

    in reply to: Beomaster 900 newbie aux input buzzing question #55724
    Dillen
    Moderator

    Difficult to say what could be the reason, but if you start working or diagnosing keep in mind that Beomaster 900 has positive ground.

    Martin

    in reply to: Beomaster 1900 humming/hiss #55615
    Dillen
    Moderator

    Typical sign of aging capacitors. Grab a service kit and replace the lot.

    Martin

    in reply to: Beomaster 1900 – Volume control issue #55419
    Dillen
    Moderator

    Does your volume control have one or two ICs?

    Martin

    in reply to: Welcome #55314
    Dillen
    Moderator

    Thanks Mark.

    Martin

    in reply to: Welcome #54937
    Dillen
    Moderator

    What happened to my (long) reply to this thread from apprx. four days ago?

    Beomaster 6500 & 7000 improvement of pre-amp circuit

    Martin

    Dillen
    Moderator

    The insulation capacitors are there for a reason.
    I would NEVER remove them.
    Actually, they are signs of a very high quality circuit design.

    So what are they, and why are they here?
    They are small ceramic capacitors placed in strategic places through the signal path, from the signal path to ground.
    They are called insulation caps because they insulate the individual signal path stages HF-wise.
    They are here because you cannot put a load on f.e. a power supply, without seeing some kind of reaction on its power output, and
    because most OpAmps have a very high slew-rate.

    If an OpAmp switches fast from off to on (or vice versa), the current it “accellerates” will have to come from somewhere.
    In other words, a sudden spike commanded on an OpAmp output will see a corresponding (reverse) spike on its power supply rail(s).
    This supply will often be supplying other circuits in the signalpath as well, and these circuits will also see the spike in their supply causing them to instantly produce a spike on their output.
    Actually, the OpAmp itself will almost always also cause a spike, as will most semiconductors (and some capacitors).

    The spike on the power supply rail will of course also affect the very OpAmp that caused it (with a brief delay), and
    the (very!) brief power surge will cause a new spike on its output.
    The result is a very high frequency “ringing” (fading burst of spikes) until the new current flow situation has settled (throughout
    the signal path!).

    As the spike reaches stages with higher and higher amplification and higher currents involved, the corresonding
    spikes on the power supply rails will be larger – again feeding back to the earlier stages.
    If the power supplies are not suitably decoupled, due to f.e. tired/sluggish filter capacitors, this could in severe cases end in
    self-oscillation and a burned amplifier.

    Small caps (100nF or so) are often placed directly on OpAmps (and other circuits handling audio signals) power supply pins.
    This helps quite a bit when viewed from the power supply side.
    But any spike already produced will go on to the next stage, be that tonecontrol, volume or amplification.
    If nothing is done, the spike will continue to the next stage and cause the next OpAmp to (attempt) to
    reproduce this “signal” which, in case of an amplification stage, would only make matters worse.
    The spike will grow through the amplifier, and the more it grows the heavier spikes form on the power supply rail and the
    more ringing will be introduced to the signal.

    This signal could eventually end at the tweeter (as speakers crossovers usually don’t cut off HF), and it can contain quite an amount of energy.
    A Zobel-network on the amplifier output would take some of it, but even this small circuit could be put to hard work.
    Bad power supply filtering caps (the small 10uF) in f.e. Beomaster 5500 can cause ringing throughout the preamp and output stages,
    and the small Zobel network resistor can actually burn from the resulting HF energy contained in the signal.
    The output stage will also run warm.

    Putting an insulation cap (a few pF) on the signal path to ground will drastically improve matters as it will be seen as a
    dead short for the very high frequency contained in the bursts.
    A capacitance this small will have no influence on anything within the audible frequency range.
    Quite opposite in fact, as it will remove spikes and bursts that are not part of the original signal.

    In some cases, – typically early in the signal path, the OpAmps (or whatever circuit handles the signal) power are fed through
    a low-pass filter, often seen as something like 47-100 Ohms in series with the power supply/ies and 100uF to ground.
    This is done for all stages individually, of course, – or it would be pointless.
    The JAMO Promix 200 which is at first glance simple but, by far, one of the best sounding discoteque mixers, uses this in dozens of places.
    Beogram CD5500/6500/7000 does too in the analogue stages.

    I strongly recommend leaving insulation caps well alone.
    It is very high quality circuit design.

    Martin

Viewing 20 posts - 121 through 140 (of 718 total)