"Asimov" wrote in message
...
"Arfa Daily" bravely wrote to "All" (19 Jan 06 10:36:03)
--- on the heady topic of " Marshall 4140 Amp question"

AD From: "Arfa Daily"
AD Xref: core-easynews sci.electronics.repair:355768

AD In contrast, if the output stage is drawing anything like enough
AD current to damage a transformer - and, IME, these will happily drip
AD wax for at least 5 minutes without failure - then the HT fuse should
AD ideally blow, or the surge limiter / smoothing resistor smoke, glow,
AD and go open.
AD It's probably just a matter of opinion and experience, which you
AD obviously have. I'm just making sure that those who might be reading,
AD and have less experience than you and I, understand the possible
AD dangers and cosequences d:~}

AD Arfa


A typical primary uses wire about the diameter of a hair. It really
can not drip wax for long. In some cases they are damaged merely by
high voltage spikes. The insulation breaks down. This is especially
true for older transformers because if the enamel got hot over time it
tends to oxidize (turns to carbon) and the damage is cumulative.

The thing about audio transformers is that they are to an extent
responsible for the tonal character of the amplifier. So if you
replace it with something equivalent it might never sound exactly the
same. It is a matter of the stray capacitance and feedback in how it
was wound. So you understand my being more protective of an audio
transformer than of an electro.

Now we know one of the electros is bad. It is possible the voltage
stress would have uncovered the weak one right away in a shower of
sparks, just as you said. However, an esr meter might be better.

A*s*i*m*o*v

.... I remember the 6SN7...

So do I ...

Are we talking output transformers or power transformers here ? If output
transformers, then I would agree that the design of the tranny is critical
to the audio performance of the amp. However, I cannot accept that for a
high power group amp, the primary is wound with wire as thin as a hair when
a) it has a resistance as low as a few ohms per winding half, and b) has to
stand peak currents approaching an amp. Even allowing for age, the windings
have to get very very hot, and stay so for a very considerable length of
time, in order to carbonise the winding insulation coating. When was the
last time you managed to burn this coating off successfully with just a
soldering iron tip at the better part of 300 degrees C ?

The situation is much the same with power transformers, which do not,
however affect the sound providing they can supply the required voltages at
the required currents. I can't remember in 30 odd years of repairing these
things, ever having to order a replacement power tranny, ( perhaps they're
more robust this side of the pond because they have to stand twice the
voltage ;-) ) and only one or two output trannies which had failed as a
result of clear problems such as a defective output tube which had
originally blown the HT fuse, which had then been bridged by silver paper to
finish the gig ...

I cannot agree that it would have been better to have found the defective
cap by letting it blow up in the OP's face as a result of overvolting. When
I was first in this business back in the early seventies, and working for a
large TV rental chain, there was a lad at another branch who suffered a
serious skin burn and partial loss of sight in one eye as a result of an
electrolytic can exploding in his face, so you see my reasons for advising
extreme caution with these things, and treating them with considered
respect. ESR meters are actually not very good with old can type
electrolytics, as their impedance was quite high in the first place, so any
readings you get are likely to be misleading if you are used to the readings
you get on modern caps, but yes, for all other cases, I would agree that an
ESR meter is an essential workshop tool for today's servicing.

Broadly, I am in agreement with you. We've both been around long enough to
know how to go about servicing this kit safely, and it sounds as if the OP
has too. There are many though, who will be reading this thread, who are not
experienced in this type of repair, and might one day decide to pull out
that old amp in the garage that uncle Willy used to play when he was in
Willy Wang and the CrapKickers, and run it up, and I'd hate for them to get
sprayed in exploding cap.

Arfa