Repairing an old Grundig tube radio

[grupt]

I have a Grundig tube radio - 4070U - which I love dearly. I bought it
almost five years ago and had it restored by someone who knew what he
was doing. I no longer live in the same area and can't benefit from
his services again.

It has two problems. The right channel cuts out intermittently. The
sound can be restored in two ways: if I turn the volume way up, or
(sometimes) if I give it a rap on the case. In both situations, the
sound is restored with a small pop. In addition, the sound also
returns if I do nothing, i.e., if I wait a few minutes. But then of
course it disappears again.

The other problem is that the sound in the left channel is extremely
weak. It is possible to hear it if the right channel has cut out, so
something is arriving at the speaker, but not much. This one is not an
intermittent problem.

The restoration in 2001 included replacement of capacitors. Not sure
if it included replacement of tubes.

Any suggestions received with gratitude.

 

[Arfa Daily]

I have a Grundig tube radio - 4070U - which I love dearly. I bought it
almost five years ago and had it restored by someone who knew what he
was doing. I no longer live in the same area and can't benefit from
his services again.

It has two problems. The right channel cuts out intermittently. The
sound can be restored in two ways: if I turn the volume way up, or
(sometimes) if I give it a rap on the case. In both situations, the
sound is restored with a small pop. In addition, the sound also
returns if I do nothing, i.e., if I wait a few minutes. But then of
course it disappears again.

The other problem is that the sound in the left channel is extremely
weak. It is possible to hear it if the right channel has cut out, so
something is arriving at the speaker, but not much. This one is not an
intermittent problem.

The restoration in 2001 included replacement of capacitors. Not sure
if it included replacement of tubes.

Any suggestions received with gratitude.

The intermittent right channel will likely be something very simple, such as
a bad joint or dirty tube base, but the low level left channel is likely to
be something more challenging, and if you don't have any experience working
on this sort of kit, is probably best left to someone who does. Just one
quick thought though. Some of these old radios had a ' GRAM ' input on the
back for a record deck, and a selector switch, sometimes part of the
wavechange switch. If this were the case on yours, could be bad switch
contacts, particularly if you don't change band very often. Could even still
be this if it doesn't have a gram input. You might try giving the bandchange
switch a vigorous scrub back and forth if its a rotary type, or a good "
working " if it's the old Grundig piano keys.

As far as going inside yourself, the high voltages involved can be
dangerous, and I would hate for you to get a serious electric shock. If you
have any old mom and pop TV shops in your area, there's bound to be someone
there who is still tube-savvy, and could sort these problems for you.
Failing this, bit more of a long shot, but there are still lots of tube amps
in use by musicians. If you have a local musical instrument store that takes
in repairs, they will likely know someone experienced in tube equipment
repairs, that may be able to help.

Arfa

 

[Matti Kaki]

I have a Grundig tube radio - 4070U - which I love dearly. I bought it
almost five years ago and had it restored by someone who knew what he
was doing. I no longer live in the same area and can't benefit from
his services again.

It has two problems. The right channel cuts out intermittently. The
sound can be restored in two ways: if I turn the volume way up, or
(sometimes) if I give it a rap on the case. In both situations, the
sound is restored with a small pop. In addition, the sound also
returns if I do nothing, i.e., if I wait a few minutes. But then of
course it disappears again.

The other problem is that the sound in the left channel is extremely
weak. It is possible to hear it if the right channel has cut out, so
something is arriving at the speaker, but not much. This one is not an
intermittent problem.

The restoration in 2001 included replacement of capacitors. Not sure
if it included replacement of tubes.

Any suggestions received with gratitude.

Try to shake the tubes when the problem is on. It this helps
the tube sockets/pins need cleaning. Removing tubes half an
inch and placing again back should help for a while. Do this
few times to clean the pins and sockets.

I use pencileraser to clean tube pins. Do _not_ use files or
any kind of hard cleaning method like sanding.

You can drop a small amount of LPS-1 spray to each pin and
rinse the extra fluid off and then assemble the tube by
moving them up and down some times to clean the socket.

 

[carneyke]

Matti,
Why do you oppose cleaning the tube pins with sandpaper / emory cloth ?
Just curious as I've done that before as well as using a very small
round file to clean the pins of the tube socket.

 

[Yukio YANO]

I have a Grundig tube radio - 4070U - which I love dearly. I bought it
almost five years ago and had it restored by someone who knew what he
was doing. I no longer live in the same area and can't benefit from
his services again.

It has two problems. The right channel cuts out intermittently. The
sound can be restored in two ways: if I turn the volume way up, or
(sometimes) if I give it a rap on the case. In both situations, the
sound is restored with a small pop. In addition, the sound also
returns if I do nothing, i.e., if I wait a few minutes. But then of
course it disappears again.

The other problem is that the sound in the left channel is extremely
weak. It is possible to hear it if the right channel has cut out, so
something is arriving at the speaker, but not much. This one is not an
intermittent problem.

The restoration in 2001 included replacement of capacitors. Not sure
if it included replacement of tubes.

Any suggestions received with gratitude.

Start by swapping Speakers R/L - L/R ! even if it means you must
unsolder to swap speaker leads.

Yukio YANO

 

[Matti Kaki]

Matti,
Why do you oppose cleaning the tube pins with sandpaper / emory cloth ?
Just curious as I've done that before as well as using a very small
round file to clean the pins of the tube socket.

Because these tools are scratching surfaces. Especially if you
have silver plated pins or socket these are gone if you use file
or sanding for removing oxidation. All the tiny scratches can
oxidate and the cleaning is harder and harder in the future.

The pencileraser (if you are able to find one) is very good when
cleaning all kind of connectors. There is one exception. If the
equipment has sensitive electronics this pen can produce some
high voltage which can destroy something. Never happened to me.

Other nice tool cor cleaning connectors is glass fiber pen. Its is
very sensitive method but you must be aware of tiny glass fiber
"threads" which easily cut off. They can penetrate your skin.

If you want use emery paper please use 1200 which is very very fine.
Or you can use "used" 600 paper. Cleaning the sockets is harder.
I use alcohol and tiny carved woodstick.

 

[carneyke]

I do remember the old static problem, supposedly caused by rubbing edge
connectors with the eraser. Never did see any damage (no recalls). I
pulled a few fiberglass pen fibers out my finger too and gave up on
using them. I've worked in the A/V and Instrument repair business for
over 30 years, sounds like we have similar experience. Take Care, Kevin