Phil Allison wrote:
"Cydrome Leader"
Phil Allison


** Had an early 1960s guitar amp in this week, a "Maestro
Viscount"
made by
Gibson, aka the GA-16T. All the valves were new, JJ types: 2
x ECC803, 2x
6V6S and a 5Y3.

The accompanying note contained no fault report and asked
only for
two modest modifications:

1. Replace and rewire speaker wire with heavier wire.

2. Rewire to shorten power feed cable from tranny.

The speaker wires were soldered at both ends and just long
enough
to let the
chassis rest on a bench. The AC cable referred to was
internal and
went from
the chassis to a 230V to 115 V step-down fitted in the
factory 50
years ago.
The two wire cable had been wound neatly like a rope and
rested on
the bottom of the case next to the step-down.

The 240V lead was modern, 3 core and there was a wire linking
the
frame of
the step-down to the chassis.

I connected the amp to my Variac and gradually powered it up
to
about 00V - just as I expected it let out an almighty hum,
mostly
50Hz, through
the speaker. The hum mostly disappeared if the volume pot was
turned
down.

Of course bad electros were the cause of the trouble, but the
situation was
not so simple since as someone had been there before me,
decades
ago and substituted two pigtail electros for ones inside a
triple
electro that was
mounted off the chassis on a clamp.

It soon became clear that the third electro in the triple had
now
expired -
but that should cause only 100Hz hum, not 50Hz.

Then came the *shocker*, one of the pigtail electros was
grounded
to the AC
heater supply instead of the chassis - this imposed 3.15
volts AC
at 50Hz on
a DC rail ( the screen supply) that should have had only a
trace of
100Hz ripple.

Removing the dead triple electro, adding a new pigtail one
and
rewiring soon
put things straight leaving only a minor amount of 100Hz buzz
at
full volume.

There was one pigtail electro hanging onto the 5Y3 socket,
grounded
to a lug
on the frame. Soon as I shifted the ground point to the
common
chassis ground, even that hum vanished.

With the amp now working nicely, I modified the speaker and
internal AC leads as requested in the customer's note.

Apparently the customer believed the AC lead tied like a rope
was
causing the hum.

Oh dear..........

It was still an honest repair, so everyone wins. I'm sure it
took
less time to swap the wires than to explain it was something
else.


** Sure - I did what the note asked to avoid an argument and
since
doing it caused no harm, why not ?

Saw this post just now and it reminds me of an incident that
occurred more than 20 years ago. A repair tech friend had grown
rather tired of listening to customers telling him what needed to
be done with their gadgets. Someone brought in a VCR and asked to
have its R/P head replaced. The friend asked about the symptoms
and immediately deduced that the culprit was not the R/P head,
but refrained from making any comment. When the customer came to
collect the VCR later that day, the conversation went like this
(as related to me by another friend who happened to be in the
shop):

Did you replace the head?
Yep.
Is it OK now?
Nope.
Why?
Because that's not the problem.
Then why did you replace the head?
Because you told me to.