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Discussion in 'Amp Tech Center' started by jondanger, Sep 24, 2016.
Caps are correct. You have been measuring bias voltage with the power tubes out correct?
So I'm looking at that geofex power transformer testing procedure, and it says I should have more than 1M ohm from both sides of the primary to the chassis. I'm reading less than 2 ohms to ground from the black wire that goes from the PT to the power switch, and also from the white wire that goes from the PT to the ground switch. Am I in the uh-oh zone? Is my PT fried?
Those measurements were with the tubes in. Tubes out, I have -44v on the back of the bias pot.
Funny I was just reading through that myself. Do you do the test with the amp unplugged? If not there's continuity to ground through the breaker box.
Is it unplugged? Do you still have the Death Cap in the amp?
No, it was plugged in. I am a dummy. Whew!
I would be inclined to do the full power transformer test at this point per the Geofex procedure. As long as your comfortable with safety requirements of high voltage testing.
I just went through it, and everything seems to test fine.
I usually use the switch on the limiter to disconnect the amp from the mains, just got past me that time.
yet another busted amp thread where people are recommending circuit modifications BEFORE the amp is even diagnosed much less operable.
Well, I resisted going to get a 10k trimmer after work because I want to get this thing right before I start goofing around in it. So we're going to stay on the straight path until big cleans fill the air.
Hey, if you know what's wrong with the amp by allvmeans jump in here and show us how it's supposed to be done.
I'm looking at this phase inverter section, and it looks like one of these 68k resistors up by the top of the picture are supposed to tie to a 220k resistor, then a .01 cap. I'm not seeing that connection. Is there a tie in under the board? Just trying to trace this out and make sure things are in the right place.
Try measuring DC resistance between the connections. If they are tied together you should see a fraction of an ohm.
Check the bias adjust pot. I've seen them go get intermittant & cause symptoms similar to what you describe. Everything looks good with the tubes pulled but with the tubes in you get very odd readings at the output tubes, bias pot & filament.
The weird thing is that I can't even find that 220k resistor on the board. I'm taking care of a homework thing and then I'll look at this again.
As far as testing the bias pot, am I basically removing it from the circuit, checking resistance from outer lug to outer lug, then making sure that I'm getting an even taper by checking from the middle lug to the outer lugs while turning the pot?
I don't see a 220K in the photo either.
Measure bias pot from the wiper to either side of the lugs. Slowly turn and wiggle the wiper shaft and look for abrupt changes on your meter. Repeat on the other outer lug to wiper. You can either measure DC resistance (amp off of course) or DC volts with the limiter and amp on. If the pot is bad you still have more problems than the pot.
I will stick with what I said.
Hey let's focus on what's important, which is me getting $300 worth of free advice. [emoji12]
I'm confused by the difference between what is in the schematic from the 68k resistor that comes off the balance pot, to pin 7 of the phase inverter. The 220k resistor seems to be completely absent, and the .01 and .1 cap don't connect in the way it says they should. Any insights? Here is a photo to compare to the schematic.
Check out this photo of the phase inverter circuit from a MV Twin from this Premier Guitar article. Note: orientation is upside down from my photo
I spy a 220k resistor attached to a .01 cap directly to the left of that dog turd cap. Color me confused. Mine sure doesn't look molested. There is an unused eyelet where the resistor and cap would tie in. Hmm . . .
I'm not sure you're looking at the right schematic or layout for your amp. It looks right to me: