Need to fix old valve radio

[b]

[email protected] ha escrito:
post here.
some very knowledgeable folk there. what they cant tell you ain't
worth knowing ;-)

http://www.vintage-radio.net/forum/

good luck
B.

 

[John E.]

The radio must be Italian? At least some of the markings on a cap were
Italiano...

http://www.geocities.com/[email protected]/

Geocities just stopped allowing access to the photos. I saw a few then a
message appeared saying that your "data transfer" limit has been exceeded.

 

[[email protected]]

The radio must be Italian? At least some of the markings on a cap were
Italiano...


Geocities just stopped allowing access to the photos. I saw a few then a
message appeared saying that your "data transfer" limit has been exceeded.

It is odd isn't it ? And the electrolitic has Russian writing???
(Maybe it was at the repair shop once or twice, just a guess)

Also, the front panel has the writing "Air King" which was once
a radio manufacturing company, so is it Lafayette or Air King,
very misterious indeed, or perhap Air King was the overseas
arm of Lafayette ???

John, I have put a smaller version of the pictures on the following
site, hopefully it will not run into these download limitations
problems.

http://www.geocities.com/[email protected]

 

[[email protected]]

I went to "nostalgiaair.com to see if the model number worked. I
couldn't find anything close, perhaps you can go to the site and see
if anything rings a bell.

Ok, with a digital multimeter, we should be able to get going. Let's
assume that the radio did not get fried due to a doubling of the input
voltage. We will check the B+ voltage for some sanity, and then go
from there. There should be a large chassis-mounted electrolytic
capacitor rated 30 uf or larger, at 200V or higher. The can should be
grounded to the chassis, and there should be two non-grounded lugs
connected to various components.

Check the DC voltage on each of the two lugs. I would expect each of
the voltages to be at least 150V DC, maybe as high as 350V, one should
be a few volts higher than the other. Is the speaker a permanenet
magnet speaker, or is the place where the magnet normally is located
actually a coil of wire with two wires leading to it (in addition to
the 2 wires that go to the speaker cone)?

Bob, can please explain were do I measure, from the pin of the
canned capacitor to where? If both pins of the capacitor are
almost at the same level, why is it a 200V rating?

I will check the speaker for you. It is mounted on a transformer, I
need
to get another picture for you. BTW, John has noticed one capacitor
with Italian markings, and I noticed that the canned capacitor has
Russian writing. This may suggest the originals were replaced. If so,
this could be a recurring problem.

If you don't see at least 150V on the electrolytic, check pin 8 of the
6x5. That is the output of the rectifier circuit. It should be
connected to one of the electrolytic terminals.

Is the sound clear at low volumes? How does it distort as you turn
the volume up, does it just get fuzzy, or does the volume go up and
get fuxzzy at the same time.

The sound becomes also louder.

 

[[email protected]]

...and lastly, the radio has not been used since the last time I
tested
it about 10 years ago. Then it worked fine, no I tested it again and
suddenly there is this problem. It is entirely possible that it was
knocked (I hope not dropped) while in storage, but there is nothing
visibly broken, and all the valves do glow.
BTW, I tried removing the valves one by one and seeing if one valve
did not "make a difference", but this approach did not work, any valve
removed results in no sound at all.

Schematic now available, at
http://www.geocities.com/[email protected]

 

[Radiosrfun]

Schematic now available, at
http://www.geocities.com/[email protected]

I thought I was following this thread but I'm wondering if I lost something
along the way.... the pictures/schematics shown in said website, are they
for the radio (Lafayette brand?) that this post was originally about? Was
this radio made by Air King/sold under Lafayette? Or am I presuming the
radio to have been a "Lafayette" due to the name of the poster?

By the way, thanks for the info... Though "I" didn't need it, I collect any
decent info I find - just in case.

 

[David]

I looked at some of the pictures. My guess is that you basically need to
measure every resistor and capacitor in this set. You most likely have
several resistors that have gone WAY up in value and several capacitors with
excessive leakage resistance. Do not be surprised if you have to replace
every one of those wax coated capacitors and many of the resistors as well.
A usual suspect for your symptom is the resistor that feeds the plate of the
audio amplifier (6SQ7) which should be in the 100K to 470K ohm range.

David

 

[Ken Weitzel]

I looked at some of the pictures. My guess is that you basically need to
measure every resistor and capacitor in this set. You most likely have
several resistors that have gone WAY up in value and several capacitors
with excessive leakage resistance. Do not be surprised if you have to
replace every one of those wax coated capacitors and many of the
resistors as well. A usual suspect for your symptom is the resistor that
feeds the plate of the audio amplifier (6SQ7) which should be in the
100K to 470K ohm range.

Hi...

Don't have any idea how valid it was, but back in the days when the
dinosaurs were walking around we used to gently squeeze the wax caps
with a long nose... if they were soft, they were replaced on general
principle.

Take care.

Ken

 

[hr(bob) [email protected]]

Schematic now available, athttp://www.geocities.com/[email protected]

Great for the schematic. Now we can get going. The case of the
electrolytic is the "ground" The two terminals are for two separate
sections of the electrolytic that share the same case and a common
ground.

Looking at the schematic, they are probably two of the three
capacitors shown as C14a, b, and c, at the output of the 6x5
rectifier, pin8, shown as . You will have to trace the DC voltage
developed at the pin8 of the 6x5 through resistors R13 and R12. The
voltage rating on the capacitors is 350 V, so I would expect the
voltage developed at pin8 is at least 250Volts DC. See what you
measure on pin 8 and then on the other two capacitor points, and then
we'll go from there. If you want, you can e-mail me directly, or else
we can do it this way, whichever you prefer. I have a meeting to go
to for the next several hours, will be back later on.

Bob Hofmann

 

[hr(bob) [email protected]]

Schematic now available, athttp://www.geocities.com/[email protected]

shown as K on the schematic.

 

[[email protected]]

How's the speaker? Could be blown.

Distortion could be caused by bias failure. Leaky capacitors (probably
called condensors when the radio was built) are a big problem with tube
(valve) musical instrument amplifiers.

There are several possibilities, but the first 2 that cross my mind
are:
speaker voice coil jammed against pole magnet, due to corrosion or
debris
bad connection, might be due to corrosion of a friction contact.
The other thing is that most of those caps look like waxed or oiled
paper types, which have a bad reliability record IME.

There are other things that can cause the distortion too, such as a
break in the output transformer, or a bias failure.

If you dont have a scope, there is a simpler way to check for signal
distortion at points along the signal path. Get yourself a very low
power audio amplifier, under 1 watt, hook it to a speaker and use this
to probe various audio signal points. To protect the amp you'll need
to put some components on the input:

X--||----/\/\----+----+--- amp signal input
| |
__ --
\/ /\
| --
radio chassis | |
-----------------+----+---- amp ground

Now, as long as output power is low, your ears will survive ok.

I'd start by checking voltages around the place, such as on the supply
lines, or capacitor + tags.

You can check the speaker by hooking up an external one in parallel.
Dont disconnect the original for the test, if a system like this has
no load even for a fraction of a second it can do itself considerable
damage.

Lytics can fail, but IME are quite far down on the list of most likely
failures.


NT

 

[[email protected]]

There are several possibilities, but the first 2 that cross my mind
are:
speaker voice coil jammed against pole magnet, due to corrosion or
debris
bad connection, might be due to corrosion of a friction contact.
The other thing is that most of those caps look like waxed or oiled
paper types, which have a bad reliability record IME.

There are other things that can cause the distortion too, such as a
break in the output transformer, or a bias failure.

If you dont have a scope, there is a simpler way to check for signal
distortion at points along the signal path. Get yourself a very low
power audio amplifier, under 1 watt, hook it to a speaker and use this
to probe various audio signal points. To protect the amp you'll need
to put some components on the input:

X--||----/\/\----+----+--- amp signal input
| |
__ --
\/ /\
| --
radio chassis | |
-----------------+----+---- amp ground

Now, as long as output power is low, your ears will survive ok.

I'd start by checking voltages around the place, such as on the supply
lines, or capacitor + tags.

You can check the speaker by hooking up an external one in parallel.
Dont disconnect the original for the test, if a system like this has
no load even for a fraction of a second it can do itself considerable
damage.

Lytics can fail, but IME are quite far down on the list of most likely
failures.

NT

I don't have such an amplifier. I have a DMM.
Regarding the speaker, if it is jammed then I wouldn't
here anything, or would I?
My inclination is leaking condenser/s, all the components inside
look very shining (a wet look) so I think a lot of condenser liquid
has vaporised inside. I hope it is not a health hazard (did they use
pcb's in such typical radios), but this would explain a reduction
in DC level and increase in DC ripple. If anyone knows exactly
what the DC level should be and what the AC ripple level should
be, I think this will be extremely useful. Also, there is this
mysterious
problem of the schematic mains voltages, which reads 110-125V
whereas this radio was always used with 230V

 

[hr(bob) [email protected]]

I don't have such an amplifier. I have a DMM.
Regarding the speaker, if it is jammed then I wouldn't
here anything, or would I?
My inclination is leaking condenser/s, all the components inside
look very shining (a wet look) so I think a lot of condenser liquid
has vaporised inside. I hope it is not a health hazard (did they use
pcb's in such typical radios), but this would explain a reduction
in DC level and increase in DC ripple. If anyone knows exactly
what the DC level should be and what the AC ripple level should
be, I think this will be extremely useful. Also, there is this
mysterious
problem of the schematic mains voltages, which reads 110-125V
whereas this radio was always used with 230V- Hide quoted text -

- Show quoted text -

I wouldn't worry about the 115 -230 issue as if it wasn't ok, you
would have fried all the tubes and pilot lamps by now.

What is the DC voltage on the C14-a, b, c capacitors? Measure from
the capacitor terminal to chassis ground.

Bob Hofmann

 

[Ancient_Hacker]

The problem is likely to be a bad resistor or capacitor, somewhere in
the last two audio stages (6SQ7 and 6V6). The plate of the 6SQ7
should be somewhere around 50 volts. The cathode to grid voltage of
the 6V6 should be around 20 volts (negative side on the grid!).
Anything wildly different than that may be a problem.

I would replace all the wax capacitors in that area just on general
principles. They're probably at least a bit leaky and will not be
getting any better over time.

Also check the resistors, they tend to go up in value.

And oh, it could be the speaker rubbing-- try moving the speaker cone
by hand to see if it rubs or is stuck.

 

[quietguy]

Personally, in this sort of situation I reckon the shotgun approach is the way
to go - just replace all the crappy caps, and all resistors over 50k - saves a
lot of time and in many cases fixes the problem in a short time. If the fault
is still there there is not a lot more to check

David (a different one)

 

[Homer J Simpson]

Schematic now available, at
http://www.geocities.com/[email protected]

Check the main filter (16/16/20 uF) voltages. Expect to see around 350 VDC
there.

 

[Ken G.]

That is a typical radio here in the USA .
The problem it has is most likely bad paper capacitors and filter
capacitors . Replace those and spray the tube pins and volume control
with cleaner and it should work fine . That radio is not worth much .
Its an ugly style . More of a fun project than anyhing else .

 

[[email protected]]

Personally, in this sort of situation I reckon the shotgun approach is the way
to go - just replace all the crappy caps, and all resistors over 50k - saves a
lot of time and in many cases fixes the problem in a short time. If the fault
is still there there is not a lot more to check

David (a different one)

an inappropriate idea.


NT

 

[[email protected]]

I don't have such an amplifier.

They are so common I bet you have. An old pair of pc speakers, old
walkman, pocket radio, unused cdrom, anything with low volume audio
output. If not, try the $ store for something with one in, or any
junked audio kit. Or make one, its fairly simple.

I have a DMM.
Regarding the speaker, if it is jammed then I wouldn't
here anything, or would I?

When they jam, they jam at one point. The voice coil & cone is not
perfectly rigid, so a very tiny amount of movement is still possible.

My inclination is leaking condenser/s, all the components inside
look very shining (a wet look) so I think a lot of condenser liquid
has vaporised inside. I hope it is not a health hazard (did they use
pcb's in such typical radios),

If this is so you may have an issue, as PCBs were the chemical of
choice. A good clean out may be called for. You cant steam clean wax
coated caps of course.

but this would explain a reduction
in DC level and increase in DC ripple. If anyone knows exactly
what the DC level should be and what the AC ripple level should
be, I think this will be extremely useful.

Unlikely, but if you give us the voltages we can tell you if theyre
reasonable or out of whack. You can also look up valve data for
electrode voltages, but bear in mind valves werent always used in the
way the mfr published.

Also, there is this
mysterious
problem of the schematic mains voltages, which reads 110-125V
whereas this radio was always used with 230V

if it worked on 240 then there's no further issue. Thats assuming the
mains lead you removed wasnt a resistance dropper.

As always with these things, theres little point guessing, you need to
test it out bit by bit. Vale rads are easier to do checks on because
the valves all become o/c when no heater power is applied, so most
components can be reliably checked in circuit.

An analogue meter for this kind of work has the advantage that
variations and poor connections are clearly visible, which is not so
with a dmm. A DMM should do it though, in most cases.


NT

 

[quietguy]

You obviousely have never had to work as a TV or radio tech where time is a cost to
your customer

David