Orange RockerVerb 50 MkII - No Bias Voltage

[mikeboone]

My sons RV50 was red plating and naturally the first thing I checked was the bias voltage on pin 5 of the power tubes, and there's none. I have the power board schematic for it and did some basic trouble shooting to and narrowed it down to a bad filter cap or bad solder joint.

The bias supply if fed right off the 115VAC input from the wall. I have 151VDC at the cathode side of the rectifier diode but no DC voltage past that. I thought perhaps the filter cap was bad an loading the voltage down so I changed it out. No change. I then swapped the rectifier diode just to be sure nothing weird was going on with it. No change.

I attached that part of the schematic and a photo showing what I am finding.

[g1]

Are you sure the bias is coming off the primary side of the PT, usually they take it off the secondary.
Anyway, any DC to be measured would be negative and on the anode side of the diode. I have a feeling your meter is getting messed up by AC voltage.
Have you checked R4? What is the AC voltage reading on either side of it?

[52 Bill]

Are you sure the bias is coming off the primary side of the PT, usually they take it off the secondary.
Anyway, any DC to be measured would be negative and on the anode side of the diode. I have a feeling your meter is getting messed up by AC voltage.
Have you checked R4? What is the AC voltage reading on either side of it?

If he's getting 150 volts dc then R4 would be ok, but if R3 went open the voltage would not get to the tubes.

[mikeboone]

I suppose it is on the secondary. There is 150VAC on both sides of R4. But only the cathode side of the diode has 150VDC. There is no voltage, DC or AC, on the anode side. I would expect to see the negative DC on the anode, but theres nothing. Not sure what is going on. Both of my meters are Fluke (models 175 and 189) and read the same thing. The resistors (R3 and R4) are measuring 220k and 15k respectively.

All the voltage reading are with respect to ground.

I checked all points along the path for shorts to ground and found none.

This is probably something simple, i am just overlooking it.

[mikeboone]

I just needed to sleep on it. This amp has two (2), yes two (2), HT fuses. They are adjacent to each other on the back panel. I assumed (go ahead say it, I deserve it) that one was the line fuse. Not so. The line fuse is in the fuse block. I think at the end of the a bad power tube(s) caused the HT fuses to blow. I replaced one of them, obviously not the one on the leg facing the bias circuit, causing may spare set of tubes to red plate.

Spares are in it running just fine. Shunt method shows about 38mA per tube so we should be good to go.

[pdf64]

It may have been better for the take off point for the bias supply to be on the hot side of the fuse.
Pete

[mikeboone]

And I still have no explanation for the 150VAC and DC that I was seeing on the cathode of the bias rectifier diode. Its been nagging at me all day. Maybe somehow it was something that was back feeding from the other leg of the PT sec... ??? Dunno.

[g1]

Due to the open circuit, there was no current flow. So no voltage dropped across any components means the AC voltage is the same at various points, like on both sides of the 220K. The fact that the AC voltage was the same on both sides of the 220K was a clue that there was no current flow. Now with the circuit complete you will measure around 150VAC on the PT side of the 220K, but some lower AC voltage at the cathode of the diode.
As far as the DC voltage you measured, I thought it may be a meter glitch as some meters don't like to measure DC where there is AC present. Any DC there should have been negative, I think you were getting a positive measurement (which was coincidentally the same as the AC reading)?
Try measuring your AC wall outlet with the meter set for DC and see what it reads.

[g1]

Due to the open circuit, there was no current flow. So no voltage dropped across any components means the AC voltage is the same at various points, like on both sides of the 220K. The fact that the AC voltage was the same on both sides of the 220K was a clue that there was no current flow. Now with the circuit complete you will measure around 150VAC on the PT side of the 220K, but some lower AC voltage at the cathode of the diode.
As far as the DC voltage you measured, I thought it may be a meter glitch as some meters don't like to measure DC where there is AC present. Any DC there should have been negative, I think you were getting a positive measurement (which was coincidentally the same as the AC reading)?
Try measuring your AC wall outlet with the meter set for DC and see what it reads.