Ampex 351 output transformer trouble

[kpearsall]

Here's a fun one for you guys  :

Working on an Ampex 351 with an intense crackling happening. Seems loudest between pins 3 & 8 on 12au7 output tube. Worked it out to an apparent open connection between the center tap and blue primary wire on the output transformer. Trying to really verify this before money is spent on a new transformer. If I disconnect the blue wire from circuit though, I'm still measuring voltage on it. Same voltage as on the brown primary and CT. If I disconnect the brown primary and leave the blue one connected, Then the red CT measures 125VDC instead of 440 as it does with the brown primary connected. I'll try to attach a schematic.

Any guidance greatly appreciated.

 

[kpearsall]

OK, as per usual I figured it out as soon as I asked for help 

Apparently it was a rather shoddy connection between the blue primary wire, and the spade connector at the end of it. Cleaned, reflowed, and voila!

I'm gonna start asking for help at the beginning of projects instead of at the end. Very nice sounding preamp by the way. Condenser mics seem a bit hot for it though. Hard to get them to clean up without a pad.

 

[pucho812]

makes me sad everytime I hear a 350/251 hacked into a preamp vs it's designed use as tape machine electronics. Good job finding the problem and fixing it.

 

[Mike Cleaver]

As someone who used, repaired and loved these units when used with the matched tape transports, I too am sad to see the two pieces parted but it's better to see them get some use rather than being pitched into a dumpster as thousands were when digital appeared on the scene.

 

[kpearsall]

Thanks Pucho and Mike. Unfortunately, I have 4 of these right now, two of which are completely gutted. The other two WERE modded when I bought them but not the full electrical audio version. That's why I still follow the original version schematic. In fact, if we pop the tubes back in, and desolder the two leads to the phantom circuit, it would work the same as original. And if I ever can get the tape deck portion, that's exactly what I'd do!