Hi guys
I have a Washburn PB30 bass amp with a shorted power transformer, some guy brought it a few day ago, asking me to fix it.
The primary winding (120VDC) seems to be ok but the secondary is shorted. Its supposed to be V-0-V (symmetrical).
I used an 18-0-18 volt transformer that I bought a few years ago for a console but it is way to big for this amp. Anyway, it started the amp right away.
It has a pair of TIP41C / TIP42C power transistors and no additional regulator. Its a quite simple amp design.
I have no clue what the original voltage on this transformer should be, since I cannot find the schematics on the internet.
But I'm quite sure its more that 18VDC.
I know a guy who makes transformers for a living and he would help me out.
So, any ideas?
I have a Washburn PB30 bass amp with a shorted power transformer, some guy brought it a few day ago, asking me to fix it.
The primary winding (120VDC) seems to be ok but the secondary is shorted. Its supposed to be V-0-V (symmetrical).
I used an 18-0-18 volt transformer that I bought a few years ago for a console but it is way to big for this amp. Anyway, it started the amp right away.
It has a pair of TIP41C / TIP42C power transistors and no additional regulator. Its a quite simple amp design.
I have no clue what the original voltage on this transformer should be, since I cannot find the schematics on the internet.
But I'm quite sure its more that 18VDC.
I know a guy who makes transformers for a living and he would help me out.
So, any ideas?
Usually you can estimate transformer voltage ratio still being shorted. Get a filament lamp some times bigger than the power rating of the transformer and same voltage as line one. Wire them in series, but lamp will bright close ss it would be directly to line. With a ddm, measure the voltage at traffo's primary, and then secondary. If the short affected a little part of the winding, you can estimate the voltage ratio and thus, the original voltage output.
In the other hand, if the trafo has its primary open, try to remove carefully the tape that cover the wire. Perhaps there is thermal fuse inside, you can short or bypass it and measure secondary voltage. In both cases, use always a series lamp and extreme caution when manipulating elements directly wired to mains. Try to keep one hand at pocket during test or where you can't close the circuit from line or neutral to ground.
Any shorted turns make all of the transformer shorted.
No such a thing as "primary good, secondary shorted" or "secondary partly shorted"
Any shorted turns around a magnetic core collapse any and all magnetic field(s) in it.
Magnetically, all turns around a core are in parallel.
But I am not sure yet how was it determined that transformer is shorted?
As of the 30W amplifier, a 18VAC +18VAC power transformer looks about right.
What is the speaker impedance?
Proper supply for 25-30W into 8 ohm amplifier, compatible with a TIP41/42 pair.
No such a thing as "primary good, secondary shorted" or "secondary partly shorted"
Any shorted turns around a magnetic core collapse any and all magnetic field(s) in it.
Magnetically, all turns around a core are in parallel.
But I am not sure yet how was it determined that transformer is shorted?
As of the 30W amplifier, a 18VAC +18VAC power transformer looks about right.
What is the speaker impedance?
Proper supply for 25-30W into 8 ohm amplifier, compatible with a TIP41/42 pair.
"But I'm quite sure its more that 18VDC."... I meant to write 18 VAC. After rectification it should become approx. 25.4VDC.
Speaker is 8 ohms.
The primary winding measures 16.6 ohms, secondary measures 0 ohms. Fact is that the fuse blows as soon as I plug it to the wall.
What you are saying about the transformer windings and stuff is interesting. In theory... But have you ever actually done this? I mean, This type of amp has an inexpensive trafo: 2 separate windings stuffed inside an E-shaped metal core. And then most probably was baked. There is no way to "carefully remove" anything on this thing...
Speaker is 8 ohms.
The primary winding measures 16.6 ohms, secondary measures 0 ohms. Fact is that the fuse blows as soon as I plug it to the wall.
What you are saying about the transformer windings and stuff is interesting. In theory... But have you ever actually done this? I mean, This type of amp has an inexpensive trafo: 2 separate windings stuffed inside an E-shaped metal core. And then most probably was baked. There is no way to "carefully remove" anything on this thing...
Own a small amplifier factory, starting in 1969
So far have made over 14.000 amplifiers, designed and wound all their transformers, so yes, I have actually done this, many times 🙂
If you have a 18+18VAC transformer, use it, it's about perfect for a 30W 8 ohm amplifier.
Your 2 x 22.5VAC might be a little too much, maybe it's fine.
Check the main filter caps value/capacitance and specially rated voltage.
I expect 30-35V caps there but please confirm.
You can not go above rated voltage.
Back in the day I sold amplifiers to some Venezuelan Musicians, including Vytas Brenner, when he played Progressive Music (late 70s early 80s).
Nowadays we have lots of Venezuelans in Buenos Aires, including four of my Doctors, go figure.
Very well trained and hardworking people, they are getting the best jobs because of that 👍🏻
