On Mar 6, 2:39 pm, 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
6v6 gt
6x5 gt
6sq7 gt
6sk7 gt
6sa7 gt

This radio was working fine but recently has developed the following
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?

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
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. 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