hughes & kettner Tubeman Transformer Blow

[binito]

Dear,
I have a hughes & kettner Tubeman . This pedal has a transformer inside to convert 12 to 250V ( B+ Voltage of 12ax7)
This transformer broke down and I can not find a transformer 12-250 2VA to replace it.

I tested with a transformer 230-12 and placed backwards. That is, enter 12Vac in the secondary to take 230Vac by the primary. But it only takes out 130Vac and it's not enough.

This is the schematic:
https://plus.google.com/photos/10552...24657196258226

This is the original transformer:

[nosaj]

Have you verified there is actually 12Vac entering the transformer? What symtoms did the old transformer display. It seems likely if 12VAC is not entering the transformer at the rated voltage you would get less on the output. 12 vac in 130 out on a 250v transformer, seems like your input may be more like 6VAC. Do you have 15vdc at r61 and 62?
If so then check following.


Check D10, d13, d14, d15 to see if the are good.
r67, r58, c44 and c45, If those test good .
Cut track just after r58 and c45 junction You should have 300. If not something farther down could drag it down.

nosaj

[J M Fahey]

Agree.
A backwards 230-12 transformer MUST give you 230V if fed 12V on the secondary ... what else could it give you?

Of course, you either not feed it 12V or high voltage winding is severely overloaded.

After all, something burnt the original one.

EDIT: am I blind?
Your 230:12 transformer, connected backwards, must give you even higher voltage than the original one, which clearly says it has a >>> 15 <<< Volt secondary
So when fed 12V it must give some 200VAC at secondary.

[nosaj]

Agree.
A backwards 230-12 transformer MUST give you 230V if fed 12V on the secondary ... what else could it give you?

Of course, you either not feed it 12V or high voltage winding is severely overloaded.

After all, something burnt the original one.

EDIT: am I blind?
Your 230:12 transformer, connected backwards, must give you even higher voltage than the original one, which clearly says it has a >>> 15 <<< Volt secondary
So when fed 12V it must give some 200VAC at secondary.

Ha I missed that too... but is the original really burnt? Was it tested with voltage off the PCB? I would revisit the original transformer.

nosaj

[Kick]

Hello there.
I use the hughes & kettner Tube Factor, but the symptom seems the same. In my case the symptom is temporal that it happens 2 or 3 times after it's powered on for 10sec to 1min or some.
These are the symptoms
1. The sound's amplitude decreases roughly to half.
2. The voltage of the input of the trans drops to 5 or 6V. respectively the output drops to roughly 130V. ( At normal situation it's 13.5V and 240V)
3. It keeps +12V, -12V, +18V, and +12.6V at the other rails.

I have changed the power supply, the tube valve, and C30, C37,C38 capacitors. But it still happens.

This is the schematic

http://www.sugardas.lt/~igoramps/article10/tf_actor.pdf

Any help would be appreciated, thanks.

Kick

[olddawg]

What is the power supply to the transformer? The transformer in your picture says 15v to 350v. That will give you the turns ratio. The thing to remember is that it is EXTREMELY low current and probably designed right on the edge. Any excessive loading is going to pull down the secondary. Are you sure the original transformer is bad.

[Kick]

I have two identical hughes & kettner power supplies, PSA 0812. So it wouldn't be the cause.
I have not changed the original transformer. Sorry for the confusion, it is the case of Mr binito.

The thing is the symptoms are temporal, but as I am a newbee I don't know where I start.

Thanks

[nosaj]

I have two identical hughes & kettner power supplies, PSA 0812. So it wouldn't be the cause.
I have not changed the original transformer. Sorry for the confusion, it is the case of Mr binito.

The thing is the symptoms are temporal, but as I am a newbee I don't know where I start.

Thanks

Ok not having a gutshot you'll have to work with me. Page 5 I think the power supply schematic you will need to monitor pins 3 an 4 the 250v side of the step up transformer .
Right now we are trying to see where it is failing in circuit.

Old Dawg there are 2 transformers in this. The AC wallwart and the ac stepup transformer. The stepup is having voltage drawn off the Primary and the Secondary.

I would like to know if the 18v rail also drops voltage too.

nosaj

[J M Fahey]

The voltage of the input of the trans drops to 5 or 6V. respectively the output drops to roughly 130V. ( At normal situation it's 13.5V and 240V)

This is the big red flag and points to a problem in the 12VAC power supply ... do both do the same?

The supply ad states they have some kind of "overload protection", probably a PTC in series .
If supply is working on the edge of its capacity, ior the presumed series PTC is rated too tightly, maybe it surpasses trigger temperature and blocks primary current (so secondary drops a lot) and after cooling everything returns to normal.

I would LOVE to test that pedal with *another* 12V 1.2A transformer.
In fact, a slightly larger one, such as 12V 2A or better.

Just in case , insert a 1.5A or 2A fuse between transformer and pedal, so if we have a "mystery short" in the pedal, fuse blows and nothing catches fire.

[Kick]

I just now measured it.
At normal situation, The pin3 and 4 have 240V, and 230V at the P010, which is supposed to have 280V. And 18.5V at 18V. and 11.1V at 12.6V for the heater.
Though, some voltages are not enough, it still works.

When it drawn off, occurred roughly 25 mins after powered on, pins 3 and 4 had just 70V, and 70V at P010, and 19.8V at 18V. As it happened only 1 or 2 mins, I didn't have the time to probe other points. But when I measured the rest of the voltages, apart from the transformer, stayed reasonably the same.

Thanks for your help.

[nosaj]

I just now measured it.
At normal situation, The pin3 and 4 have 240V, and 230V at the P010, which is supposed to have 280V. And 18.5V at 18V. and 11.1V at 12.6V for the heater.
Though, some voltages are not enough, it still works.

When it drawn off, occurred roughly 25 mins after powered on, pins 3 and 4 had just 70V, and 70V at P010, and 19.8V at 18V. As it happened only 1 or 2 mins, I didn't have the time to probe other points. But when I measured the rest of the voltages, apart from the transformer, stayed reasonably the same.

Thanks for your help.

Ok repeat and see what pins 1 an 2 get.
I'm with juan on a beefier AC wallwart though.
nosaj

[Kick]

Hi Juan. Thanks for your reply.
Though it sounds logical, I think it's not the power supply as I have measured several times that the rails 12V, -12V, and 18V stays roughly the same. Which are powered directly from the power supply. So only the primary of the transformer seems drops.

Thanks for your kind suggestion.

[nosaj]

Hi Juan. Thanks for your reply.
Though it sounds logical, I think it's not the power supply as I have measured several times that the rails 12V, -12V, and 18V stays roughly the same. Which are powered directly from the power supply. So only the primary of the transformer seems drops.

Thanks for your kind suggestion.

So are you afraid of experimenting? Get another AC wallwart and lets find out instead of thinking. We're going the way of Edison try it if it works go with it.
nosaj

[Kick]

OK I will try. But unfortunately I don't have the bigger 12VAC supply at the moment.

I will try and let you know what it's like.

See you there. Thanks

Kick

[nosaj]

OK I will try. But unfortunately I don't have the bigger 12VAC supply at the moment.

I will try and let you know what it's like.

See you there. Thanks

Kick

Not sure of your location but the thrift store can be your friend. Look for 12VAC wallwart, unless you have a variac then you can dial in 12VAC an apply it.


nosaj