La2a fuse problem

[Chryst]

Hey guys,

I've had a perfectly functioning Drip Opto 4 La2a for a couple of years. Last week suddenly the fuse was blown. Replacing it did not solve the problem. Something is definitely drawing too much power because the terminal blocks were all black at the spot where the AC is entering the unit. I've replaced the terminal blocks and looked for shortages, but I've found none.

Would somebody know a good strategy to troubleshoot this one? Could it be bad tubes? I can't see any other spots on the board that have burn marks.

Any help is greatly appreciated. Thank you!

 

[PRR]

> Could it be bad tubes?

Unlikely a tube-fault would blacken the power terminals; but why don't you take the tubes out and see?

After that: disconnect (and mark!!) (and sleeve) all the PT secondary wires, try again.

A Lamp Limiter is wiser than blowing a crate of fuses. 60 W incandescent lamp should be fine.

 

[CJ]

you can also put an ohm meter across the pwr xfmr, with the cord unplugged of course, measure the pri, sec and 6.3 winds and post what you get,

 

[Chryst]

Thanks for the responses!

The lamp limiter is a great tip! I will build one very soon. Nice!

I replaced the GE 6005 (6AQ5) with a new one just to be sure. But it doesn't make any difference. I'm in the studio this afternoon and will try to measure the winds.

It seems strange to me that after years of trouble free service suddenly the transformer would die on me. If anybody has any other suggestions, please reply. Thanks for all the help!

 

[mjrippe]

If the transformer is OK, work your way through the power supply.  Rectifiers, caps, regulators (if used), etc.  A shorted component in the PSU will certainly pull some current.

 

[PRR]

> It seems strange to me that after years of trouble free service suddenly the transformer would die on me.

Not strange. That's how they die.

Transformers DO die. All insulation breaks-down under electric stress. The rate of break-down can be statistically estimated. The death-time of an individual transformer will be randomly more or less than the nominal lifetime. The designer can aim for 1 year, 10 years, 1,000 years. A "1 year" goal will have some parts dying in a month which is bad for business. A "1,000 year" design costs more than we want to pay. Lifetimes of 10 to 100 years seem to be the usual commercial goal. It is usually figured for constant use at maximum heat; a "10-year" part may live 50 years in studio use.

"Suddenly" is also part of the story. They are not complicated things. Either it works "trouble free" or it doesn't. They don't, like my grandmother, "linger" in a nursing-home for a decade with a good heart and bad joints. Unlike grandma, the transformer is connected to a BIG power source. You can pull many KiloWatts out of a wall-socket. When a 50W tranny gets a little sick, and conducts more than it should, the 30,000 Watt (short-term) jolt from the Power Company turns a small break-down into a total break-down.

However. Hang around DIY long enough and you see cases where a PT was suspected "bad" but turned out good. Capacitors fail short much more often. Rectifier specs are tough and many units are under-rectified. Silicon typically fails short. In a fat power supply, the PT will burn the Silicon open; in tube gear the low-current windings can't pop the diodes and the PT cooks. You *really* have to remove all loads and check. First the easy loads (tubes) because a shorted heater will "look" like a shorted heater winding. But then pull ALL the secondary leads off and see if it still acts shorted.

 

[Chryst]

Thanks a lot. This makes a lot of sense.

I've measured resistance of the PT.

500V tap 373 ohm
6.3V tap 0,3 ohm
250V tap  40,8 ohm

I've measured the rectifiers with my DMM and they seem fine. Mains is 220-240V by the way.

Thanks for your kind help. While this situation is rather frustrating, I'm very happy to be learning from you guys. Great!

 

[Chryst]

Does anybody know if these resistances are in the right ballpark? Thanks guys!

 

[crochambeau]

Those resistances would make sense to me, were I to come across them under these circumstances. Sense enough to feel comfortable with disconnecting the transformer from circuit and moving on to measuring it unloaded.

I trust there is no continuity between the mains legs and chassis?

 

[Chryst]

Thanks! No continuity. But when I'm powering up the unit with only the primaries in, the fuse also blows. Could this point to a faulty transformer? I also saw that the legs of one of the tubes had a slightly different colour indicating that it maybe went too hot sometime. Just to be sure, I swapped out the tube, but no luck.

Would I need to order a new transformer. Or is there something I overlooked? Thanks a lot for your help!

 

[keefaz]

Thanks! No continuity. But when I'm powering up the unit with only the primaries in, the fuse also blows.

No other component connected (power switch etc), just the primaries and fuse to mains?

 

[Chryst]

Yes, the power switch is connected. I think I don't really understand how to keep the fuse in, but take the switch out without rewiring. I checked the switch of course. Thanks a lot!

 

[Chryst]

Ok, I've connected the tranny directly to the mains with a fuse (0.5 A Slow Blow) in between and it pops immediately when I connect the power. This keeps pointing to a broken transformer right?

 

[keefaz]

Could you post more informations about the transformer, specs, wiring shematics etc..

 

[s2udio]

You have disconnected all the secondaries......? if not do so.
Also 0.5 seems a little small try a 2A slo.... if it still blows , yup new TX
You could also check for shorts to earth, or in laymans terms shorts to the TX Laminations
You will need to isolate it from the case to do this.
Put on probe from you're cont test to the core(earth point) and go around all the open ends.
Of course the TX is disconnected from the supply when doing this 

 

[Chryst]

Thanks for your help!

I've disconnected all the secondaries and fixed them so they couldn't do any damage. I will buy some 2A fuses and test for continuity. This is the tranny by the way:

Hammond 369jX
(http://nl.mouser.com/ProductDetail/Hammond-Manufacturing/369JX/?qs=dhCoiiGcT1LQ0MdlMI72/w==)

 

[keefaz]

So for 240VAC mains, you connected together 0V brn and 120V blk transformer primaries and connected 120V blue and 0V Wht transformer primaries to mains ?

[edit] well if it worked for a couple of years, I doubt of a miswiring problem

 

[Chryst]

That's what I did with a fuse wired in between. It wasn't so much an expectation of a wiring mistake, but Keefaz asked me to take the switch out of the occasion. Thanks!

 

[Andrewfrommontreal]

I have had the same exact story occur to me with a Drip LA2A. I don’t know much about it’s insides as it was built by Greg himself. Any thoughts of what could be the issue? I had my unit for less than a year and was probably on for a total of 40 to 50 days.

 

[rockinrob86]

I have had the same exact story occur to me with a Drip LA2A. I don’t know much about it’s insides as it was built by Greg himself. Any thoughts of what could be the issue? I had my unit for less than a year and was probably on for a total of 40 to 50 days.

You have to follow the troubleshooting advice or pay someone to do it. 

A fuse doesn't go bad (normally).  Something causes it to blow, and sometimes (but not that often without cause) it is the power transformer.

http://www.geofex.com/ampdbug/fuseblow.htm

Also, the main fuse is typically only to prevent fires.  It is not designed to save components in the unit, although sometimes they can do that (and not talking about units with fuses on the secondaries of the PT, that's what those are for!)