PT question

[selloutrr]

I have a Risson LTA120 I picked up as a project. It's one of the early versions no reverb guessing it dates '74. I has different Iron then my other two and a slightly different signal flow.

I followed my other LTA120's the best that I could and now upon power up it blows fuses. I checked the polarity of the Filter caps (-) is to ground. What i'm finding strange is possibly the PT is wired wrong. Here is what I have going on.

The Power Transformer is model PX3618
one side has

two black wire (city power - in )
two Green wires ( Heater voltages )

the other side has

two Red wires ( red to red tests 833vAC ) I'm going to have to run KT88's this is going to kill 6550's
one Yellow with a red stripe center tap
one White with a blue stripe ?

the other LTA120 have the Yellow to Ground and the White running off to the PCB.

I've disconnected the PT from the amp no wiring shorts to ground all grounds go to the ground buss.

When I wire the PT I get continuity threw all the legs to ground and each other, but it works out of the amp.

It has to be something to do with the White/blue stripe wire.

Where am I futzing this up? A schematic sure would be nice, but no dice they where printed on drafting paper and over the years the pile shematics stuck together and descentigrated the early years.

[ToneMerc]

How the the bias fed? Is the wht/blue a bias tap possibly?

TM

[Structo]

Have you tried contacting Bob Risson?
Not sure if he would know but if anybody would he would.

[selloutrr]

That would make sense if you follow the circuit through the PCB it does go to the Bias pot about 6 components in.

Metering the AC current to ground through each leg

Yellow/Red stripe 13Vac
White/ Blue stripe 30Vac

(a) Red + (b) Red = 833Vac

(a) Red + Yellow/Red = 412Vac
(a) Red + White/Blue = 419Vac
(b) Red + Yellow/Red = 419Vac
(b) Red + White/Blue = 419Vac


I've replaced the four diodes in the rectifier and the additional 1N4007 in the bias chain with know good ones. The filter caps test good, polarity is correct. The PT is wired the same as my other LTA120's. I'm sure its a simple over sight.

The PT is wired into the amp with a Turret lug strip.

White / yellow / red / red

from this strip wires feed up the chassis to the PCB
one wire goes to the Ground bus on the PCB (yellow)
one wire goes to bias tap? (white)
the other wires (red) go to the SS rectifier point 1 and point 2

With the PT wires removed no continuity is shared between posts.
Only the Wire going to the Ground bus has continuity to ground.

With the PT wired to the turret Lugs It meters continuity across all four posts and ground. effectively shorting when it's powered up.

At one volt on the variac it has 6.5 amps. 7amps blows the fuse.