AMI AMP MODEL B

I am working on a model b ami jukebox. when i plugged the amp in the
5u4 tube started to glow brightly. i removed it and the other tubes on
the bench. plugged it back in without the tubes and the input
transformer started getting hot. any ideas on whats wrong. thanks norm

em pinball wrote:
I am working on a model b ami jukebox. when i plugged the amp in the
5u4 tube started to glow brightly. i removed it and the other tubes on
the bench. plugged it back in without the tubes and the input
transformer started getting hot. any ideas on whats wrong. thanks norm

Perhaps it has a short - either inside the transformer, or on the output
circuit (choke, filter cap, etc.).

You can use the light bulb test (put a 25 to 75W incandescent only light
bulb in series with one side of the power going to the amp - hot side is
best) and if the lamp glows normally then there is a short, it should be
somewhat dim compared to if it was directly across the AC.

Here is a web page on the subject:

enewbie.com/light_bulb_current_limiter.html

Not rocket science, I've been using the same idea for over thirty years...

John :-#)#

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John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
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"Old pinballers never die, they just flip out."

em pinball wrote:
I am working on a model b ami jukebox. when i plugged the amp in the
5u4 tube started to glow brightly. i removed it and the other tubes on
the bench. plugged it back in without the tubes and the input
transformer started getting hot. any ideas on whats wrong. thanks norm

Bad clipping on that link, go here for the light bulb test:

http://tubenewbie.com/light_bulb_current_limiter.html

John :-#)#

--
(Please post followups or tech enquiries to the newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

em pinball wrote:
I am working on a model b ami jukebox. when i plugged the amp in the
5u4 tube started to glow brightly. i removed it and the other tubes on
the bench. plugged it back in without the tubes and the input
transformer started getting hot. any ideas on whats wrong. thanks norm

Tube amps in old jukeboxes are not hard to figure out. Here's a few
basic pointers:

1) Get the schematic. Vern Tisdale has one at
http://www.verntisdale.com/schem/ami-b.jpg

2) With NO tubes in the thing at all, the transformer should NOT get hot
and should NOT draw any current. If it is, your power transformer is
bad.

3) Put all the RIGHT tubes in there. Don't run it with just the 5U4,
as these amps depend on a load to prevent the voltages from going way
too high.

4) Make sure you've got the RIGHT fuse in it. If it blows, it's because
something is wrong.

Once the above is ok, then you'll find you have bad capacitors in the
amp. All the wax/paper types and electrolytics need to be replaced.
Antique Electronic Supply is a good source.

Any tubes that are milky white inside are bad.

BTW, there's high voltages (400V or so) floating around in there so be
careful...

On Apr 12, 9:45*am, em pinball wrote:
I am working on a model b ami jukebox. when i plugged the amp in the
5u4 tube started to glow brightly. i removed it and the other tubes on
the bench. plugged it back in without the tubes and the input
transformer started getting hot. any ideas on whats wrong. thanks norm



I had this on a G 80, but it was intermittent. The sound would "plop"
and the 5U4G valve heaters would glow brightly as this happened
It got worse and worse over time.

The power transformer had gone bad, part of the secondary was arcing
intermittently to
the transformer frame (which is at ground potential in relation to the
secondary). Replacing (or rewinding) the transformer was the only cure



Would need opinions from others on here who have more experience with
these transformers and their reliability,
but if it is practical,/ it might be more viable to have the thing re-
wound using modern enamelled
winding wire* (see below), with a modern safe bobbin used or buy a
modern equivalent than put another 55+
year old tranny that may fail in the same way any time and take out
other parts as well



*Recall in interesting article from an Australian manufacturer ROLA
many years back regarding problems like this.

With speaker output transformers/inductors/coils in valve circuitry
(and anything else with high voltage) the windings would often get
corroded spots on the enamel from electrolysis and then fail. This
had something to do with inconsistency in thickness of the enamel
coating on the winding wire (that also was its insulator) resulting in
breakdown from arcing to the frame of the transformer.

Methods were tried such as heat drying out the transformer then
sealing it with enamel, and then running the winding wire through a
bath of mercury and checking its resistance then re coating it in bad
areas, but didn't fully solve the problem.

In the end, someone there solved the problem with the invention of the
"Isocore" transformer, in which the entire transformer frame was
connected to the HT voltage (to make less difference to the potential
voltages) dried out, then potted in some form of insulating compound,
then encased in a second metal case that was mounted to the chassis.
The frame was live, but double insulated with an earthed shield over
the assembly for safety.

Don't know how well this performed in relation to heat, but doubt that
there would be much from an output transformer.