AB763 Showman Replacement PT Info?

[signalpaths]

Hi Guys,

I haven't been on here in a while, finishing up my EE Degree this year is all consuming. But I've got a short break before summer classes start so I want to get a few projects done i've been putting off.

I've got this AB763 Fender Showman with a bad PT. There is a Orange wire that is burned and I'm not sure what it is. It is grounded to the chasis with the red and yellow mains center tap. I Got my new showman/twin MOJO Transformer, I think its a heybour. All the wires are the same as the original except there is no orange wire. There is however a green and yellow wire in place of it. I believe this is the filament center tap. Am I safe in assuming that the orange wire is the filament center tap? Is there any way to test to find out what the orange wire is? Do any of you fender experts know off hand what the orange wire is? Any ideas on what may have caused the wire to melt? Any help would be much appreciated.

Thanks Eric

[Amp Kat]

Can't see any other wire going to chassis except the heater CT. Take your meter and read ohms from the CT to each half of the other heater wire and it should measure half of what the across the other two wires. Other words if you measure say 20 ohms from the green to green (heater wires) and then orange to each side of the green should be half. If it's not it could have fried or is shorted. You can al;so take AC voltage measurements and get the same % just in volts instead of ohms. Disconnected speakers while at high power levels will cause that and tube shorts from old tubes or bad tubes.

[signalpaths]

Thanks for the response Amp Cat I will check the R like you say. My only fear is like you also say it may be cooked open.

[52 Bill]

I think that the orange wire is connected to a internal electrostatic shield.

What is the date on the transformer. That orange wire didn't appear until sometime in the late 60's/early 70's (I think), around the same time the filament center tap went away. Are there 2- 100 ohm resistors on the filament lines to ground?

[Amp Kat]

You may be right Bill and as the AB763 schemo I just saw had the Heater CT internally connected. All the other models and later have the 100 ohmers on them.

[signalpaths]

Thanks Guys I will report back tonight when I get home.


Eric

[signalpaths]

The amp has no 100 ohm resistors to ground on the filaments. when I tested for resistance btwn the filaments and orange wire I got 0.3ohms. I got the same reading between the 2 filament wires too. I'm guessing that means the filaments are fried open. So i guess since there's no 100 ohm resistors that the orange wire is the filament center tap? Anyone have any idea what could cause the center tap to fry like this?

Thanks Eric

[Amp Kat]

That reading on the filament supply seems about right and I don't think they are open. The orange wire is an electrstatic shield for the transformer and should be grounded. You can't see the CT wire for the heaters because they are internally grounded. Try disconnecting the heater wires from the pilot light if you think something is bringing it down or measuring the AC volts right at the pilot light for around 7 volts AC. If you get your heater voltage, your ok.