power transformer problems

[billybillybilly]

i have a princeton reverb . problem is that there is 5VAC on the exterior of the transformer (on the copper/aluminum straps) . ive done continuity tests to see if something is shorted to ground or anywhere else and cant find a thing . all solder points are good . the amp was not blowing fuses or anything to suggest excessive current draw . the amp actually sounded really good . the PT is a hammond 290AX . i noticed that the PT was getting real hot sooner and sooner . ive since removed the transformer and tested it out of the amp and the 5VAC is still present . there is a bigger problem . this is the 3rd one to do this ... one of em actually ended up with 100VAC on the exterior of the transformer . so my question is , could Hammond actually be having an issue with these ? or more than likely , have i done something wrong ?
any ideas would be greatly appreciated !

[J M Fahey]

What do you mean by "I have 5VAC (or 100VAC) on the exterior of my transformer"????
What do you measure that with?
On what scale?
Where do you apply measurement probes?

[billybillybilly]

i mean that if i touch the ground probe of my digital voltage meter to the chassis (or any earth ground) , and the other probe to the outside of the transformer (not one of the leads coming from the primary or secondary of the transformer, but the actual bell or iron pieces) i get a reading of 5 volts of alternating current .

[J M Fahey]

Is the transformer bolted to the chassis?
Isn´t its frame or structure actually grounded by the screws?
With amp unplugged for safety, please measure continuity or resistance between the iron stack/end bells/chassis.

[billybillybilly]

yes it is bolted to the chassis . but taking a good look at the transformer , the bolts are isolated from the transformers body by nylon washers (odd ?) . it only makes physical contact to the chassis through the bolts . so , the transformer is only connected to ground via the center tap of the high voltage secondary and the virtual center tap on the heaters . there is no continuity between the stack/bells and the chassis (bad ?) . thanks for helping me with this .

[Enzo]

COnnect the black meter probe to chassis, set for AC volts. Now grip the point of the red probe between your fingers and hold it up in front of your face. How many volts do you read? Here in my shop, I just got 5.32VAC. Not sure what my personal best might be, but I'll accept 5v. A little induced voltage in the copper stap doesn;t sound bad to me.

Having your meter indicate 5v on the outside of your transformer is not a problem it is a symptom. WHat is the actual problem you need to fix?

[billybillybilly]

what brought me to this point was a seemingly excessive heat issue . even at idle , the transformer was getting to hot to touch (after 15min or so) . even with no load it would heat up , took longer but still got hot . inside the amp , there were no signs of excessive current draw (blown fuses, burnt components , blistered wire , etc) . what lead me to check for voltage on the frame of the transformer was a thread on here where someone was having similar overheating issues and found an AC voltage on the frame of their tranny and they had dubbed it faulty . they had a hammond 290 series similar to mine . aside from the to hot to hold issue , there were no other issues .

for what its worth , im only putting out about 1/2 a volt today ... might be low on electrolytes .

[Jazz P Bass]

what brought me to this point was a seemingly excessive heat issue . even at idle , the transformer was getting to hot to touch (after 15min or so) . even with no load it would heat up , took longer but still got hot .

If the transformer in question got hot without a load (ie: wires disconnected from the amp) on the secondary wires, then it is bad.

The larger issue is "why" did you go through three of them.

[billybillybilly]

The larger issue is "why" did you go through three of them.

thats what i cant seem to figure out ! the circuit is a - bf princeton reverb aa1164 .
could the transformer body being isolated from the earth ground have caused it ? perhaps causing normally induced voltages to burn through some insulation ?

[Enzo]

Leave the insulating washers in place on the transformer bolts.

GO over to RG's Geofex site and look up his simple transformer tester. Disconnect every wire of your transformer and apply the test. That should let you know if you have a shorted turn or somehting .

It is normal for power transformers to get hot. I can;t say if yours is excessive or normal.

[billybillybilly]

Leave the insulating washers in place on the transformer bolts.

GO over to RG's Geofex site and look up his simple transformer tester. Disconnect every wire of your transformer and apply the test. That should let you know if you have a shorted turn or somehting .

It is normal for power transformers to get hot. I can;t say if yours is excessive or normal.

yea i figured since they are manufactured with the washers in place , theyre probably ment to be there .

i know theres discussion on the rules of thumb as far as 'how hot is too hot' and im not looking to start that debate .

ill throw together that transformer tester and update .

[billybillybilly]

ok im back (sorry for the delay) with my new 'shorted winding tester'. thanks for the advice ! this thing is super handy !

it appears that only 2 of the 3 transformers are actually bad . both have shorts in the 6v tap and one also has a shorted 5v tap and primary ...
coincidentally , i had recently replaced the light socket (typical fender style). the first didnt hold the bulb tight and would work intermitantly
, and the second seemed fine . ive used a weber chassis and the sockets dont seem to fit quite right .

i didnt have any issues until i replaced the first light socket ... im thinking the problem is in the socket . ive wanted to start using a different style status light anyway .

once again , thanks for the help !