help troubleshooting a fender princeton

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seavote

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May 31, 2006
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http://www.prowessamplifiers.com/schematics/fender/princeton_aa964_schem.pdf

http://deeringamps.com/workbench/princeton_layout.html

hi ,im a novice i know a bit but im confused about where to go to get my brothers blackface princeton up and running again.about 18 -20 years ago he had an overdrive switch put in and the rectifier tube bypassed with a bridge diode. the tech installed a monster transformer for the B+ voltage and added another transformer. from what i could make out the transformer secondaries put out 34V. he used this to supply the 4 tube heaters (in series??) and to make a -34V regulated  for bias supply to the power tubes. no, i dont know why he thought it was a good idea,maybe he was experimenting and threw in whatever transformers he had on hand. after about a month the amp burst into flames and was ressurected about a month ago for me to repair. we decided to go back to stock. i removed the 2 transformers, got whatever components i needed to put the -34V bias supply back to stock,replaced all electrolytics,replaced two burned out resistors,replaced the power tubes and had the 12ax7,7025 and GZ34 tested.all tested good.i checked for continuity where ever i could through out the entire circuit.
i had a transformer from a dynaco sca35. it had the correct hi voltage secondary and a 6.3V heater supply but no 5V supply for the rectifier tube heater. i got around this by using a bridge rectifier(rebuilt,not the one the "mod" tech did) when i plugged the amp in i got  the correct AC Voltages up until the exit from the transformer. after that low MVs on the B+ and a high voltage( i think about 40Vs but dont recall)on the heater supply(with no tubes).
so i went and bought a hammond 290ax. a direct replacement transformer. took out the bridge rectifier, installed the rectifier tube. powered the amp up and now im getting only voltages in the mv range from all 3 legs of the secondary.
on the layout (see link above)diagram the transformer has a green and yellow ground wire that the hammond transformer does not. the red and yellow ground wire from the transformer is grounded on one of the transformer screws coming through the amp chasis.
another note: i replaced the filter caps canister with 4 individual caps, twisted there negative legs together and grounded them at the same screw.

im not sure where to go from here. hopefully im missing something very obvious(again im a novice)
id find it hard to believe the new transformer is at fault. is there a way to test it with no load without burning it out??
please any suggestions are appreciated.
thanks and  sorry for rambling,i just wanted to give you forum members as much info as possible. i'll shut up now and hopefully one of you can take it from here.
 
Can you verify the operation of the transformer out of circuit?  Pull the GZ34 rectifier and measure AC volts from the Red/Yellow wire to either Red wire (should be 325VAC).  Measure between the chassis and a Red wire (should also be 325VAC if the grounding is OK).  Check you have 5VAC across pins 2 and 8 of the GZ34.

Do the heaters in the GZ34 warm up if it's in circuit?

Here's the wiring colour coding of the 290AX if it helps any:

p-t290ax_wiring.gif


on the layout (see link above)diagram the transformer has a green and yellow ground wire that the hammond transformer does not.

I would bet the Green/Yellow wire is the centretap for the 6.3VAC heater supply.  The Hammond doesn't have a centre-tapped heater supply so you just leave it out.  If you find hum is present after the amp is up and running again you can try adding a pseudo-centretap to the 6.3VAC winding by using two 100Ohm 1W resistors in series across the 6.3VAC winding and grounding the junction between the two resistors.

At first glance all it looks like you'd need to do is simply sub the transformer into the circuit and ignore the missing Green/Yellow wire.  The colour coding of the wires is the same as the original.
 
Ask Hammond by Email (or check on their site) for the exact wiring diagram & wire colour designations for that exact transformer, they sell it as a Fender Princeton replacement so they should tell you exactly how to wire it up, if you don't have that info already, make sure the PT is wired 100% correctly.

After the above only turn the amp on with rectifer tube fitted & check voltages AC & DC, then check heaters.

Curtis has already posted up very good advice whilst I was writing this reply Good Luck ;D
 
"At first glance all it looks like you'd need to do is simply sub the transformer into the circuit and ignore the missing Green/Yellow wire.  The colour coding of the wires is the same as the original."
thats what i did. im getting voltages in the low mV range on all legs of the secondaries,with and with out the rectifier tube installed.
all i can think to do is make sure the solder joint at the on/off switch and fuse compartment is good
 
DOH!!!
problem solved. while i had "checked" the fuse, i never tested the fuse. it looked fine but was not working. so i learned a lesson i will always remember
 

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