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Old 8th November 2001, 01:49 AM   #1
diyAudio Member
Join Date: Oct 2001
Location: Sydney, Australia
I am very new to amp design and construction, so I need some advice. I have an older amp that I made some modifications on and now the transformer overheats as soon as the amp is turned on. I believe that the transformer would meltdown if I did not intervene.

Here are the changes -

Removed power cord and installed a IEC Mains Filter AC Chassis socket.

Removed a 230 or 250 volt mains voltage selector swith and wired to 230 Volts on which I have been using it on.

Added a ferrite filter ring on the mains wiring inside the chassis.

Replaced a combination on-off and speaker A, B, A & B, and headphones selector switch with a simple toggle on - off switch. Removed the wiring for speakers B and head phones and wired "A" speakers directly to the output jacks.

A few notes on the transformer -

3 Primary wires -

White - which I assume is the neutral, or negative wire.

Brown - which appeared to be used for the 250 Volt windings

Orange - which appeared to be used for the 230 Volt windings
(As near as I could tell these wires were not in series or paralleled by the voltage selector switch, but used seperately to determie the input voltage.)

5 Secondary wires -

Red x 2 - each of which are wired into a diodeand then they both are wired on the positve side of the main power supply cap.

Blue x 2 - both of which are wired directly to the negative side of the power supply cap. One wire did have a small light bulb in line before the cap to light an indicator lamp on the front panel. I removed that lamp.(could that be the problem.)

1 Black - which soldered to the Star ground of the chassis.

Interestly the amp works fine and plays music as well as it ever did if not better since I eliminated quite a few switches and etc. by wiring the outputs direct instead of the signal going through a selector switch. But I have not been able leave the amp on for very long as within 5- 7 minutes of cold start it is way too hot to touch.

Any suggestions, I may try to sketch a wiring diagram and post it.

Surf, Sun & Sound

I just remembered that there was this little green thing that crossed the mains before the transformer, I did solder it back on between the positive and negative wires after the switch and the fuse but before the transformer. I could not tell that it was directional, sice with a continutiy tester I could get a reading either way. It is lime green about 1/4" by 3/8" with the following markings - ARCO 1.06 3300 / 20 630 V - .
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Old 8th November 2001, 07:43 AM   #2
diyAudio Member
Join Date: Oct 2001
Location: Sydney, Australia
Default A Neophyte Mistake

I found the problem, after some very serious headscratching. Remember the small lamp that was in line with one of the negative secondaries? By eliminating it, I essentially paralleled the secondary windings on the transformer. A very big no-no on a E-I transformer in which the windings were not matched well enough for paralleling. By putting a small lamp in-line, it works as a resistor and evens out the voltage differences between the secondary windings and allows both to soldered to the negative pole of the supply cap.

Put the small lamp back inline, and all is well.

A learning experience.
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