Re: unit hums loudly regardless of volume

tempus fugit wrote:

Thanks for your reply.

Sorry, here's some more info:

There is a power transformer.
Only 1 diode that I could see.
Measured voltages at cap in question was ~150v.


If the cap had DC on it, the diode is probably ok.  The capacitor(s)
could be dried out (although I think you said you checked them, so
that's not too likely).  From there, you move toward shielding type
problems as already described elsewhere in the thread.

Only 2 tubes - 12AX7 and can't remember the power tube offhand.

If one of the tubes had an internal heater-to-cathode short (or for
that matter, a short at the tube socket), that could cause a lot of hum.
Such shorts are not uncommon.

At any rate, now you have plenty of ideas to consider.

There was also some small device which I assume was a transformer that hadd
100mA marked on it. It looked like a trasformer with cooling fins.


"CJT" <abujlehc@xxxxxxxxxxx> wrote in message
news:436100B7.1090008@xxxxxxxxxxxxxx

tempus fugit wrote:


Hey all;

I'm trying to repair an old tube portable record player. When it is on, 

it

hums really loud, regardless of the volume. I assumed that this must be 

a

problem with filter caps, but they test OK for ESR and shorts. 1 of the
problems is that there are 3 or 4 in a can, so I don't know what the


values

are supposed to be.

Is there anything else that may cause this symptom? I suppose I could
parallel some caps across the existing ones, but I don't even know what


the

values are supposed to be.

Thanks




One or more diodes in the power supply (assuming it uses them) is
shorted.  If it has a rectifier tube instead of semiconductor diodes,
then either the tube is shorted or, if it has its own filament supply
(e.g. 5U4), then that supply could be shorted inside or outside of the
power transformer (assuming it has a transformer), although there's a
pretty good chance the tube would be glowing bright red if that were
the case.  If it's all tube and has no power transformer, then you've
got a potentially lethal hot chassis and you'd better be d*mned careful.
Check for shorted line bypass capacitors if it's this last case.

Or it could be one or more capacitors, as earlier suggested, or a break
in some shielding.

As you can surmise, additional information about the circuit involved
could narrow the possibilities (and possibly suggest others).

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