DIYbanter - View Single Post - Tried to recap a tube receiver, and failed.

bz

(Derwin) wrote in news:4bAYh.5322$Dq6.2769@edtnps82:

In article ,
l says...

On Sat, 28 Apr 2007 04:02:09 +0000, Homer J Simpson wrote:

"Derwin" wrote in message
news:6mzYh.5305$Dq6.3498@edtnps82...

I tried to re-cap a tube receiver (amplifier + tuner), and now when I
turn it
on it starts playing music for a few seconds after it warms up, then
the music
fades out and and hum takes over. I don't really know what I'm
doing, and the
wiring inside looks like a mess of spaghetti. I'm pretty sure I
replaced the
caps properly, but I used .068uf WIMA caps to replace .050uf
wax/paper caps,
and I'm wondering if the value difference, added up over 8 caps would
make a
difference?

First, you need to 'divide and conquer'. Divide the set in half and
determine which half holds the problem. Is it the receiver section or the
amplifier section?

Signal injection and signal tracing should let you determine which half the
problem is in.

If it is in the receiver, you will need to determine whether it is in the
front end (tuner), the oscillator, or the IF, or the detector.

If it is the amplifier you should again be able to narrow down the problem
to what section of the audio amplifier.

It is possible that you have an open grid leak resistor.
With leaky bypass capacitorys, one of them might have been 'doing double
duty'. When you replaced the caps, you may have thrown away a significant
portion of the grid leak.

Check for a grid that is going highly negative and cutting off the tube
(plate voltage going high).

Check for open resistors.

It is also possible that one of the new capacitors is defective.

Look in a tube manual and look at the curves for the tubes used in the
amp.. In an audio amplifier, you would expect class A operation, with the
tube biased into the middle of a linear portion of the curve unless you
have push/pull amplifier, in which case a pair of tubes would work against
each other in class AB or B.

Hope this helps.
.....

--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

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