80s Super champ debuggery

[MikeH]

So I got this super champ; when it came to me it was chassis only, partially gutted, but all of the important things were there (OT, PT, etc). A lot of components were removed (seemingly randomly) but pretty much the entire bias section had been removed. God knows why.

I have all of the missing components replaced (I’m 99.999% sure). I don’t have all the signal tubes yet (curse you compaction!) but I do have some 6v6s lying around so I threw some in there to check how much current they’re pulling and it’s way too much. Even with the current limiter they pull like 60-70 mA. Without the limited it just keeps going up. I killed the power when it hit 100.

Checking the bias voltage, I see there IS no bias voltage. The PT taps for the bias voltage (BRN) are working, and I get AC right up to the diode after the 47r resistor but no DC on the other side of the diode. No voltage of any kind. I tried a different diode and no difference. I looked up the fender part designation for that diode and it appears to be a 1N4006. The specs also match up (1A 800v). I did not have a 1N4006 so I am using a 1N4007. It doesn’t seem to me like that would matter, but I don’t know. Does it?

Any idea what the problem would be or where to look next?

Here’s a good schematic: http://www.stratopastor.org.uk/strat..._schematic.jpg

[Enzo]

No, it doesn't matter. I stock only 1N4007, and use those for any 1N400x I might need. The first thing that diode sees is the 70uf 100v filter cap. SInce this is a negative supply, the POSITIVE end is GROUNDED. Any chance you installed it backwards? (You would not be alone in this) Or is that cap shorted? Or is the bias supply line grounded for any reason? You could lift the Anode end of the diode and see if there is voltage there now. I am looking to find if the problem is voltage not getting there or is it being loaded down.

[MikeH]

Thanks!

Diode and cap are oriented correctly- I double checked that about 20 times because, yeah been there.

Cap is not shorted. Resistance to ground after the diode is about 1.4k, which seems low?

[Enzo]

Look at your schematic. That cap has three resistors in series across it: 390, 390, and 680 ohms. Those add up to...tada... 1460 ohms. Normal.

Go to the cathode end of the diode. Put one one meter probe there. Now with the other, probe the far end of the 47 ohm, I hope you get 47 ohms more or less - the value isn't critical. 40-60 ohms, in other words not open. Now move that probe to ground. You should still measure that 47 ohms plus the transformer winding resistance. If you get an open reading, then either the transformer winding is open or there is a broken connection to ground from the transformer or possibly within the transformer.

Or go the other direction, ground one probe and with the other check for winding continuity at the 47 ohmer.

[MikeH]

Resistor measures 50 or so ohms. Resistor plus transistor windings is around 1.3-1.4 meg. I’m getting 80v AC across that winding. HV tap is 313V AC, B+ with no tubes is 430v DC.

The diode/continuity setting on my DM gives me no connection with the red probe on the cathode, black on anode, and .599 in the other direction. I don’t know what that number means, but does that jive? I’m sure it’s super unlikely to have a whole batch of mislabeled diodes.

[g1]

Resistor plus transistor windings is around 1.3-1.4 meg. I’m getting 80v AC across that winding.

Sounds like the low side of that winding is not grounded.

[Enzo]

Yes, either the ground connection to the transformer is open or the winding itself.

You are measuring 80vAC there because your meter is acting like an antenna. I bet if you plug only your hot probe into the hot slot on a wall outlet, you would measure substantial AC as well. There is an induced voltage, but due to a lack of continuity, it cannot provide even a small amount of current.

[MikeH]

Hmmm... well the low side is definitely grounded. So blown transformer.

Weird- I’ve never seen that. So there’s AC voltage there just through induction I assume, and there’s just not enough current to get through that diode?

Roughly what amount of resistance should I be seeing across those windings?

And where the hell can you source a new PT for one of these things? I’m guessing not easily.

[MikeH]

Ok- if I have any hope of sourcing a replacement I need to figure out the voltage of the bad secondary- There should be -55v after the diode which is a half-wave rectifier right? Could the source voltage be calculated by doing the opposite of an RMS calculation? Or does that only apply to full wave rectification? If anyone is willing to walk me through the math I’d greatly appreciate it.

Apologies; I only know enough of the math and science behind these things to be dangerous.

[Helmholtz]

My Super Champ measures 56VAC brn-brn and -55.6VDC at the first filter cap (after the diode).

[MikeH]

That works too! Thanks!

[g1]

If you have too much trouble locating a PT replacement, you could add a small transformer for the bias, or even source it (bias) from the existing circuitry.

[Enzo]

Let's make sure first. Disconnect both brown wires so they now stick up in the air. Measure resistance between them. Is it still open?

If so, the transformer is bad, but might be fixable. Since it needs to come out anyway, remove the transformer, and take the end bells off it. The transformer is wound with enamel wire, but the wires coming out are insulated. Inside you will find the external wires connected to the winding wires. See if one of the brown wires has broken its connection. Also see if one of the winding wires coming out to the external wires might have broken in a repairable way.

[Enzo]

It looks like the part number is 019146, verify. Is that the number stamped on the transformer?

[MikeH]

When I went to desolder the secondary I noticed the wire was broken off, but the insulation was melted to the terminal making it appear to be soldered. Transformer works fine now; I’m getting about -50v bias supply.

Thanks everyone! Glad I didn’t have to replace it.