CurtPalme.com Home Theater Forum

[Ohmess]

Hi All --- I know there are a bunch of electronic gurus here; hopefully someone will take pity on a rookie and jump in on this. I need help with a transformer question that is likely fairly basic, but is beyond me.

I bought a non-operative Furman PS-8R power sequencer that I had hoped to use to enable a remote power up of my amplifiers, two of which do not have remote power up options. I isolated the problem to the main transformer that powers the logic board and further determined that there was a lack of continuity on the primary side of the transformer. I had hoped that the problem was on the connections to the windings, but alas it is in the windings somewhere. Thus, I don't think the transformer can be repaired.

Here's my problem. The transformer was a specialty unit custom manufactured for Furman. Primary is 120V AC, two 19v DC secondaries. I found a supplier who may have stock (they didn't say for sure), but they have a $100 minimum order policy plus shipping and handling. I paid $25 for the unit and could probably buy another one for $100.

I'd like to fix the unit I have (that way I don't have to admit it was a stupid decision to buy one that was broken), but my lack of basic electronics knowledge is holding me up. I think I would need a transformer that outputs a secondary voltage close to 19v and modify it. I'm not sure what "close" means (better to go 20v and step down?) and how exactly to go about modifying the output voltage. Also, I don't know how to determine the power rating (and cannot measure the transformer I have).

Suggestions?

[macgyver655]

Need more info. Do you have a service manual? How do you know the output is 19v dc? If its A/C input, its A/C output unless there is some sort of circuitry built into the transformer.

[Curt Palme]

A lot of times the transformers can be repaired. Post a pix of it, and I'll advise.

A lot of times there's a little thermal fuse in the middle of the transformer, to protect against overtemperature. Everything from tiny AC adaptor to larger transformers have that. The wires of the thermal fuse usually come out to the surface of the transformer under the paper wrapping. Undo the wrapping, bypass the fuse, it will work again. THe thermal fuses always blow for no reason at all, and as long as there's a primary fuse on the unit in line with the AC cord, there's no fear of fire.

[Curt Palme]

A lot of times the transformers can be repaired. Post a pix of it, and I'll advise.

A lot of times there's a little thermal fuse in the middle of the transformer, to protect against overtemperature. Everything from tiny AC adaptor to larger transformers have that. The wires of the thermal fuse usually come out to the surface of the transformer under the paper wrapping. Undo the wrapping, bypass the fuse, it will work again. THe thermal fuses always blow for no reason at all, and as long as there's a primary fuse on the unit in line with the AC cord, there's no fear of fire.

[Tom.W]

Quote,

OK, few steps to get there:) i am going to talk you through a visual way of checking the internal thermal fuse in the power transformer

1. Disconnect unit from the mains
2. First check for the obvious, an overlooked blown fuse. Check them with your multimeter set to the x1 ohms range.

It should read less than 1 ohm!! Sometimes fuses can look ok, but actually be faulty. Replace any blown fuse if you find one with the same type and rating. Fire it up again if you find and replace any blown fuse, Otherwise....

1. Identify the mains power transformer. It is the biggest and heaviest component in the unit.
2. Inspect the transformer, see if you can spot 2 rows of thick tin plated copper square shaped (about 1x1mm) mounting pegs/terminals that connect the rest of the amp to the transformers internals via single core enamal coated copper wires generally wirewrapped about these same square wire pegs. The primary circuit(mains side) is the thinnest copper wires.
3. If you cant get into a position to see these terminations, you may have to remove the transformer to get a better look. Don't be afraid to clip any cable ties that have been used to tidy up any excess wiring to the transformer
4. Look for a pair of tinned copper wires(not enameled copper) connecting to these same wirewrap posts. The same posts will also have a rather thin ennameled copper wire connected to it.
5. This is the thermal fuse that has probably failed.
6. Check this for continuity with your ohms meter to read zero ohms.

If it is open circuit(reads nothing)... then this is why your amp has stopped. You will need to jumper this internal thermal fuse and the amp will work again. Once you have identified which external connections of the transformer are wired to the internal fuse, you can do it from the outside of the transformer on the connection pegs. The thermal fuse is there to satisfy an American mains standard, that we dont need with the MEN system that is used, so no harm comes from jumpering it

If you are unlucky, the transformer will have the wiring going straight into it, and you may have to actually carefully cut your way into it to find the thermal fuse and then jumper it. The thermal is always mounted on the outside of the windings, but may be under a few layers of tape, plastic, and sometimes a copper band. these are a bit harder to fix, and often you may have to remove the transformer to get access to it. If you think its going to be a difficult one, stop before you start and see if you can find an electrically savvy mate to help you.

End quote.

[Ohmess]

Thanks for the tips guys. Based on the confident statement it can probably be repaired, I worked with the connections on the primary side of the transformer. I resoldered the connections and managed to get the transformer to output 19v (AC, not DC as the Furman guy had told me).

Only one of the delayed switches works, so I have a problem on the board somewhere. The lone aluminum electrolytic capacitor seems to have thrown up on the board; I'll replace that and see where I'm at. Thanks gang.