How to troubleshoot this?

[JustMike]

I picked up a Marshall 4203 Artist head that is dead. It's a hybrid amp that has a solid state front end into a 30w EL34 output section. I've determined that the PT primary is shorted. I guess it's worth a $100 investment in a new PT, but what if something else is wrong with the amp? I don't want it to turn into a money pit.
 I can assume it's going to need a cap job.
 If the OT reads minimal resistance on it's windings, I think I can assume that it's OK.
 The solid state preamp may be a PITA to troubleshoot and I'm concerned about that because if lightning or a power surge took out the PT, who knows what it did to the rest of the circuit. I think I'd feel OK if I can troubleshoot the preamp with a +-15VDC PSU.
 Any tips for troubleshooting this as is?

[shooter]

pitch it, build a simple tube amp
otherwise; surf up "troubleshooting complex circuit boards"

[thetragichero]

these sell for about 400 bucks on reverb. preamp about as complex as a distortion pedal, so should be easy to troubleshoot (especially if you have a scope). doesn't look like any specialized/obscure ics so if one is toast common 4558/tl072 types should just sub right in

[JustMike]

I'm in the process of replacing the Power transformer and just noticed that pin 7 is tied to ground. It looks to be original factory wiring. The new PT doesn't have any secondary center taps (like the original). I'm thinking that I should make an artificial CT for the heaters, but I'm also assuming that this method that I know nothing of was good enough for Marshall, so why mess with it?
 What say you?

[thetragichero]

the original wiring has the heaters like fender did with the champ with one side going to ground. yes, if it were my amp i would remove the connection to ground on the 6.3v and wire in an artificial center tap.

[sluckey]

but I'm also assuming that this method that I know nothing of was good enough for Marshall, so why mess with it?

Why indeed. Let it be.

[JustMike]

Got the transformer wired in, changed the dual 50 & 33 filter caps, tested voltages-all nominal. Plugged in some tubes and applied power. I'm getting filaments and all voltages are spot on. But there's a faint hum in my bench speaker even though the amp is on standby. There are a couple of 47u caps on the +-15v preamp rails. Coincedently, this node/secondary is always hot and not affected by the standby switch.  (thinking out loud here - But how would the hum be getting thru the output stage and to the speaker?

[sluckey]

Faint hum lets you know the amp is on. Could be coupling between the PT and OT.

[JustMike]

Faint hum lets you know the amp is on. Could be coupling between the PT and OT.

 I'm assuming then that this could be the fault of the new PT, since I don't think this level of hum would be acceptable from the factory. I also assume a quick test would be to power it up with the power tubes pulled and if there's hum it pretty much has to be xfmr coupling?
 

[JustMike]

 As a follow up, I rotated the OT 90 deg. which moved it about 2" farther away from the PT. Little if any difference. I then bolted a 3-1/2"x 4" piece of steel (from an old 2RU blank panel) to the chassis in between the xfmrs and it made little difference.

[sluckey]

If possible, unbolt the PT from the chassis but leave the wires connected. Try to support the PT so it does not touch the chassis. Still got faint hum?