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Dillen
ModeratorI use a “glass-hair”/fiber brush.
But if there is oxidation, you can clearly see it as black flakes.Martin
Dillen
ModeratorWelcome back, Peter!
Martin
Dillen
ModeratorDifficult to say what could be the reason, but if you start working or diagnosing keep in mind that Beomaster 900 has positive ground.
Martin
Dillen
ModeratorTypical sign of aging capacitors. Grab a service kit and replace the lot.
Martin
Dillen
ModeratorDoes your volume control have one or two ICs?
Martin
Dillen
ModeratorThanks Mark.
Martin
Dillen
ModeratorWhat happened to my (long) reply to this thread from apprx. four days ago?
Martin
Dillen
ModeratorThe 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
Dillen
ModeratorVery nice indeed.
For shipping considerations, it would also be good to know where you ship from.Martin
Dillen
ModeratorDepends on the type of Beovox 3702.
Kits are readily available:
https://www.beoparts-shop.com/?s=3702&post_type=productMartin
Dillen
ModeratorIt’s a known problem with this drive type.
Glue in a small piece of carton or plastic card (apprx 0,5 mm thickness) here:You can give the exposed motor bearing a tiny drop of sinter oil.
The belt for the motor must NOT be tight!Martin
Dillen
ModeratorIt’s not uncommon to see the Beocords rubber feet missing as they harden with age.
The outer parts break off, while the inner parts fall inwards where they will often
get stuck between the Beocords bottom plate and the drawer, causing a braking effect.
Look in from the front with a torch light.Martin
Dillen
ModeratorDifficult to diagnose from a distance, but I have seen aftermarket cables with
wrong pin configurations.Martin
Dillen
ModeratorCheck continuity in the cables.
Martin
7 April 2024 at 09:24 in reply to: Beogram 1600 arm going back to rest position after 2 seconds #54151Dillen
ModeratorWhich cartridge are you using?
And what is the tracking force set to?Martin
Dillen
ModeratorSolenoid leads touching the platter underside?
Martin
Dillen
ModeratorIt’s cast metal.
It can come out if you heat it carefully, not melting the plastic, only softening the glue.Martin
Dillen
ModeratorNot sure what you mean?
VTVM usually refers to a Vacuum Tube Volt Meter (F.e. Bang & Olufsen RV9/RV11 etc.,
but today most digital meters have a suitable high impedance so will work fine for most readings of this type.Martin
Dillen
ModeratorDont’ use readouts of old and bad components for reference!
Set the new trimmer in its center position, fit it and refer to the servicemanual for
the corresponding measuring points and correct settings.Martin
Dillen
ModeratorDid you try what I suggested?
Martin
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