On 6 Mar, 20:39, wrote:
On 6 Mar, 18:55, wrote:
On 5 Mar, 18:04, Kurt wrote:
wrote:

I have a very old valve radio from a company called Lafayette.
It has 6 valves which are

problem, when the volume is increased the sound is distorted
completely,
it can only be heared ok when the volume is very low, and putting the
ear onto the speaker. Any ideas?

There are several possibilities, but the first 2 that cross my mind
a
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.

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