Testing power trans

[davoux]

I just got a seymour duncan convertible and it has no heater at any tubes. I want to test the transformer to see if there is I would assume about 6.3v. There looks to be about a 6 pin connector from the transformer to the PCB board. Can I disconnect the connector and turn the power on to take a voltage reading without harming the transformer? 2nd question, any best guess on the color code for the heater? TNX

[Enzo]

Transformers don't care. You could take it out of the amp completely and power it up on your bench if you felt like it. I don't recall on the SD, but standard 6v would be green.

OK, I just looked it up. Apparently the green wires are the 5VAC for the recto tube on this amp. 6VAC for heaters is Brown.

An open heater winding would be exceedingly rare. Are there any other voltages present? An open primary is far more likely if the tranny were bad.

But most likely would be open fuses or opens in the wiring/connectors/traces.

[davoux]

Thanks

Thank you Enzo. I just got this thing in the mail and I think it took some bruises along the way. The seller is not real interested in helping me out other than filing an insurance claim which means the Post Office would take it and make it landfill. I don't really want that to happen but I don't want to take on too much liability to save it so I wanted to check the big stuff. I also don't want to go too far into it and risk voiding the insurance.

When I got it the tubes were out but not overly well packed. I plugged them in and turned on the power. There was some light sparkly (not heavy arcing) that went on in the 5U4BG rectifier for about 1 second and then that quit. Nothing else lit at all. I had a spare 5U4GB and tried replacement - no luck. The only thing that works is the fan and the main fuse. I kind of slid the chassis out a little so I could see if there were any obvious loose wires or blown parts or PCB mounted fuses. I see they have 4 diodes in there so I assume they are to rectify the heater as I assume the 5U4GB rectifies the B+.

There were balls of foam all through the chassis. It came from the cover over the plug in modules intended to keep them in place but it was deteriorating badly. I tried testing the conductivity of the foam because probably some got under the PCB also and could possibly short out anything. Aren't there usually seriesed resistors in the heater circuit to prevent a fillament dead short from taking out the trans or diodes though.

Long story, but if the large $ trans is ok, I may just eat the other damage and fix it rather than letting it go to scrap.

[Amp Kat]

You might also check the modules and make sure your not loosing some contact there as those preamp tubes do have to have heater voltage. Allthough they are in a string of rectified voltage form the power tubes. Here's a site you may enjoy on these amps
http://www.diyguitarist.com/GuitarAmps/Convert.htm

[davoux]

Question heater circuit

Thanks for your response. On the heater circuit though how could the modules affect the whole circuit? As I see it, the heater is a parallel circuit where if 1 or 2 tubes were not making contact, you would still light up the rest of the tubes. In this system it has 12 tubes and not a one lights. All I can see is that the problem must be in or close to the transformer. I checked for loose connections - found none. For the rest I don't want to pull the PBC because I don't want to screw up an insurance claim or sending it back to the seller. To me the most obvious are the diodes but I don't think I can test them without pulling the PBC and lifting a leg on each diode.

I did check out that site and it is great. Lots of great info - a real resource. My only problem was I couldn't download the power schematic it seems to be damaged so it won't download.

[Enzo]

Schematic Heaven has a copy, I jusy downloaded it to see what it looked like compared to my files. It seems complete. It is a different drawing from the two different version drawings I have on file, but it also includes the incorrect power tube screen grid switch wiring that is on my copies. Note: I refer to different versions, but I mean of the drawing. The circuit appears the same in each.

[davoux]

Enzo

Thanks Enzo, as always you are a Prince.

[Amp Kat]

I couldn't open that power schemo either. The only way to really test the windings is to pull the feed wires and see if you get 6.3 there. I have one in my shop so I'll see if I can find a way to get to them without the board off. The board is off on this one and it is a pain. You have to be real careful putting those connectors back on as they break off extemely easy when you reconnect them.