Fender Princeton Reverb II troubleshoot

[donegone]

Amp keeps blowing fuses. Tested bridge rectifier circuit. Disconnected everything from the filter cap and worked backwards to the power transformer. Disconnected bias supply and fill supply. Keeps blowing fuses. Disconnected the rectifier tap and it doesn't blow. I've replaced the power transformer and filter cap with new and it still occurs. Something is loading the circuit to ground and I can't figure out what. Suggestions? Ive included a PDF of the schematic and an image of what I have connected when the fuse blows.

[Don Moose]

Disconnected the rectifier tap and it doesn't blow.

Is that the RED (HV, B+) tap or the BRN (5VAC to tube rect if present) tap?

[Enzo]

Check resistance to ground from pin 3 of each power tube. Any shorted or low resistance? Disconnect the center tap of the OT from B+. ANy help? Make sure the OT and the choke are not shorted winding to frame.

[donegone]

I had already suspected the choke but I disconnected it and the fuse still blew. The amp stays on for a little longer having completely removed the choke, but it still blows fuses. Ive already eliminated the OT by completely disconnecting it and all the B+ supply past the filter caps. Its just throwing me for a loop because I have removed everything from the circuit besides the rectifier and the filter cap. I felt I already ruled out the filter caps by replacing them but it seems as soon as I connect them to the b+ supply, fuses start blowing.

[Enzo]

ANy chance a filter cap is wried backwrds? Or defective?

Start with the basics. COnnect the HV winding to the rectifiers ONLY. NO filter caps at all, not even the ones you ruled out. Still blow? If not, add in one filter stage. Nothing more, just one cap. Now?

ANd did you check each rectifier diode for forward drop? Not ohms meter, but diode test for drop voltage. And more important for any reverse leakage your meter can dtect.

[donegone]

I found the problem. I was ordered the wrong power transformer. Was blowing fuses cause to high of voltage. I now have the correct xformer installed and the original problem of blowing fuses was caused by the filter cap. I had assumed that the correct transformer was ordered and the fuses blowing were still cause of the original problem. So you can see why I frustrated (but my stupid mistake for not measuring voltages first!). The amp is now working but I now have a much less aggrivating problem. The "Lead Level" pot does not sound to be working. When I read it on a scope I only see a VERY small difference in the waveform when moving from 0ohms to 100Kohms. From what I have read this "lead level" works in conjunction with the master and they both act as a master level (?). Ive connected the pot straight to ground, omitting the octocoupler connection completely, and the scope reads those pots as too master levels basically. I dont know much about the optocoupler package accept one side is a led and the other a photo transistor. I have tested the optocoupler but according to what ive read it is reading fine. Thank you for your assistance. I appreciate your help enzo, and your patience.

[Enzo]

OK< so the control works if you ground it. That means the "problem" is with the LDR circuit.

When the LED is on the photocell inside goes to low resistance. COnect your ohm meter to the bottom end of that pot and chassis, then turn the LED off and on and see if the reading changes.

There should be about -6.2v on one end of the LED. The other end runs from the LDR through a 270 ohm resistor (check it), to the tip contact of the red jack (check continuity). Then the cutout contact of the jack (make sure the tip cutout contacts close electrically) completes the circuit to ground through the pull switch on the 11540 volume control. (check continuity to the pull switch and check the switch itself.)

[donegone]

I was only getting roughly -1vdc. I lifted the zener and I had -20vdc. Checked the diode and it was shorted. This means that the LED was not working allowing the photo transistor to conduct to ground?

[Enzo]

Yes, the LED inside was niot lighting, so it couldn;t shine on the photocell and drop its resistance, so the volume control would never have an effect. replace that zener.