unit hums loudly regardless of volume

[tempus fugit]

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

 

[Mike Berger]

You could be getting hum through the tonearm wires to the pickup,
but I'd bet on filter capacitors.

 

[CJT]

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).

 

[tempus fugit]

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

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.

 

[Asimov]

"tempus fugit" bravely wrote to "All" (27 Oct 05 11:17:42)
--- on the heady topic of "unit hums loudly regardless of volume"

tf> From: "tempus fugit" <[email protected]>
tf> Xref: core-easynews sci.electronics.repair:346437

tf> Hey all;

tf> I'm trying to repair an old tube portable record player. When it is
tf> on, it hums really loud, regardless of the volume.

Have you tried turning the ac plug around?

Which way the plug is inserted matters with these. Let the record
player warm up, then insert the plug one way or the other, and then
paint a mark for which was the quietest way. One way: lots of hum, the
other: quiet hiss.

Those old tube units used to get B+ directly from the powerline (or
sometimes a voltage doubler) and used a large value resistor bypassed
by a 0.01uF to RF ground the tone arm shielding to neutral. The power
for the filaments often came from a secondary winding in the motor
coil. Don't use this type of record player near the bathtub. Lethal!

Another possibility is a broken wire at the cartridge.

A*s*i*m*o*v

.... Just a little force field zap.

 

[CJT]

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.

 

[TimPerry]

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.

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.

possibly a selenium rectifier. back in the old days they hadn't figured out
to make high voltage silicon diodes yet.

the usual warnings about selenium: emits poison gas when it burns

replace with modern diodes is recommended.

 

[tempus fugit]

Thanks for the suggestions guys. I'll try some of these things out.


"tempus fugit" bravely wrote to "All" (27 Oct 05 11:17:42)
--- on the heady topic of "unit hums loudly regardless of volume"

tf> From: "tempus fugit" <[email protected]>
tf> Xref: core-easynews sci.electronics.repair:346437

tf> Hey all;

tf> I'm trying to repair an old tube portable record player. When it is
tf> on, it hums really loud, regardless of the volume.

Have you tried turning the ac plug around?

Which way the plug is inserted matters with these. Let the record
player warm up, then insert the plug one way or the other, and then
paint a mark for which was the quietest way. One way: lots of hum, the
other: quiet hiss.

Those old tube units used to get B+ directly from the powerline (or
sometimes a voltage doubler) and used a large value resistor bypassed
by a 0.01uF to RF ground the tone arm shielding to neutral. The power
for the filaments often came from a secondary winding in the motor
coil. Don't use this type of record player near the bathtub. Lethal!

Another possibility is a broken wire at the cartridge.

A*s*i*m*o*v

.... Just a little force field zap.

 

[mark]

Change the A.C. filter capacitor that should cure usually in the 10's of
microfarids at 150V

 

[Mike Berger]

That sounds more like a selenium rectifier -- if it were bad
you'd smell it!

 

[CJT]

Change the A.C. filter capacitor that should cure usually in the 10's of
microfarids at 150V

If the voltage on it measures 150V, then a 250V cap might be a better
choice for a replacement.

 

[tempus fugit]

Tried the turining around the plug trick, but it made no difference. I also
disconnected the phono plug that connects the tone arm to the amp unit,
which also made no difference.

Here is something curious though. I accidentally touched my scope while my
hand was on the record player chassis and got a fairly good shock. The scope
has a 3 prong AC plug (the record player doesn't). I should've measured the
voltage difference between the sope chassis and the record player chassis,
but I didn't think to. I'll have to do that. Perhaps he chassis is live. I
didn't think this would be the case though, since there is a cap with the
negative connected to the case, as well as a few other green wires.

Thanks


"tempus fugit" bravely wrote to "All" (27 Oct 05 11:17:42)
--- on the heady topic of "unit hums loudly regardless of volume"

tf> From: "tempus fugit" <[email protected]>
tf> Xref: core-easynews sci.electronics.repair:346437

tf> Hey all;

tf> I'm trying to repair an old tube portable record player. When it is
tf> on, it hums really loud, regardless of the volume.

Have you tried turning the ac plug around?

Which way the plug is inserted matters with these. Let the record
player warm up, then insert the plug one way or the other, and then
paint a mark for which was the quietest way. One way: lots of hum, the
other: quiet hiss.

Those old tube units used to get B+ directly from the powerline (or
sometimes a voltage doubler) and used a large value resistor bypassed
by a 0.01uF to RF ground the tone arm shielding to neutral. The power
for the filaments often came from a secondary winding in the motor
coil. Don't use this type of record player near the bathtub. Lethal!

Another possibility is a broken wire at the cartridge.

A*s*i*m*o*v

.... Just a little force field zap.

 

[CJT]

Tried the turining around the plug trick, but it made no difference. I also
disconnected the phono plug that connects the tone arm to the amp unit,
which also made no difference.

Here is something curious though. I accidentally touched my scope while my
hand was on the record player chassis and got a fairly good shock. The scope
has a 3 prong AC plug (the record player doesn't). I should've measured the
voltage difference between the sope chassis and the record player chassis,
but I didn't think to. I'll have to do that. Perhaps he chassis is live. I
didn't think this would be the case though, since there is a cap with the
negative connected to the case, as well as a few other green wires.

That capacitor might be leaking or (worse) shorted. It can be a lethal
failure (I read just this week about a minister in Waco who was
electrocuted during a baptism when he grabbed a microphone that was
"hot."). Be careful.

 

[CJT]

That capacitor might be leaking or (worse) shorted. It can be a lethal
failure (I read just this week about a minister in Waco who was
electrocuted during a baptism when he grabbed a microphone that was
"hot."). Be careful.

Here's a cite to that story:
http://www.cbsnews.com/stories/2005/10/31/national/main995829.shtml

 

[tempus fugit]

I tested the cap and it showed about a half meg of resistance. maybe
something is shorted somehwere else. Oh, and I checked the voltage between
the record player chassis and the scope chassis - 120VAC (I thought that's
what it felt like). Odd though, I would think with 120v on the chassis that
it would pop a fuse at the breaker box.

Thanks

 

[CJT]

I tested the cap and it showed about a half meg of resistance.

We're talking about a capacitor between the chassis and one side
of the line, right?

It should be infinite (assuming you tested it out of the circuit).

When in doubt, replace it (or just snip it out and don't replace it).
Google "death cap" if you'd like some further opinions, typically from
the world of guitar amps.

maybe

something is shorted somehwere else. Oh, and I checked the voltage between
the record player chassis and the scope chassis - 120VAC

Reversing the plug in the wall socket should change that (but don't
count on it).

(I thought that's

what it felt like). Odd though, I would think with 120v on the chassis that
it would pop a fuse at the breaker box.

Not until your hair catches fire.

You really need to be careful with this device. I can't stress that
enough. Death is not to be trifled with.

Thanks

 

[tempus fugit]

We're talking about a capacitor between the chassis and one side
of the line, right?

I havent reopened the chassis, but it seems to be connected between the
ground of the speaker and the chassis.

Reversing the plug in the wall socket should change that (but don't
count on it).

Reversing the plug did correct that. Is this a design shortcoming (if so,
there's gotta be a safer way to set that up) or should I be looking for a
fault somewhere?

You really need to be careful with this device. I can't stress that
enough. Death is not to be trifled with.

I am being very careful, not to worry, but I appreciate your concern and
helpful advice (including those on personal safety).

Thanks again

 

[CJT]

I havent reopened the chassis, but it seems to be connected between the
ground of the speaker and the chassis.

OK, that's something totally different.

Reversing the plug did correct that. Is this a design shortcoming (if so,
there's gotta be a safer way to set that up) or should I be looking for a
fault somewhere?

That's normal for a "hot chassis" device. But such a device won't have
a power transformer, and I thought you said this one did. Could you be
confusing an output transformer with a power transformer?

If you are (and I suspect you are) then you are such a newbie that you
had better stop now and enlist the help of somebody who knows what
they're doing before you kill yourself. No kidding. I don't want to
have anything more to do with this pursuit.

tempus may fugit, but not for the dead

You really need to be careful with this device. I can't stress that
enough. Death is not to be trifled with.

I am being very careful, not to worry, but I appreciate your concern and
helpful advice (including those on personal safety).

Thanks again

 

[Ken Weitzel]

Hi...

Sorry for the top posting; but the rest is getting a bit
convulted with a confusing mixture

Just wondering... has tempus or any of the rest of you other
old timers considered the possibility that it might be a dc
"choke" speaker?

Take care.

Ken

I havent reopened the chassis, but it seems to be connected between the
ground of the speaker and the chassis.

OK, that's something totally different.

Reversing the plug did correct that. Is this a design shortcoming (if so,
there's gotta be a safer way to set that up) or should I be looking for a
fault somewhere?

That's normal for a "hot chassis" device. But such a device won't have
a power transformer, and I thought you said this one did. Could you be
confusing an output transformer with a power transformer?

If you are (and I suspect you are) then you are such a newbie that you
had better stop now and enlist the help of somebody who knows what
they're doing before you kill yourself. No kidding. I don't want to
have anything more to do with this pursuit.

tempus may fugit, but not for the dead

 

[CJT]

Hi...

Sorry for the top posting; but the rest is getting a bit
convulted with a confusing mixture

Just wondering... has tempus or any of the rest of you other
old timers considered the possibility that it might be a dc
"choke" speaker?

That's an interesting possibility I hadn't thought of.
I haven't seen one of those in quite a few years. Did
they even make portables before PM speakers became the norm?