Egnator Rebel 20 head short in power supply

[Randall]

A young guy new to tube amps brings me a new to him Egnator Rebel 20 with both fuses blown,a dead 6V6, and a dead short. With no tubes in it, my light bulb limiter glows bright. I start by disconnecting the HV secondary at QC 104 and 105. Light bulb goes out, preamp tubes light up, and the front LED comes on. I have almost 600vav on the red secondary wires, so at least it isn't the PT. Right away I find D101 tests good in circuit, but D102 measures 35ohms in circuit both ways. OK, we can assume D102 is shorted.

My question before I take this compact and tightly wired little head apart, and it won't be easy, is can someone vouch for and explain why a shorted diode in this spot would cause a dead short? And I know I should check the filter caps while I am in there, but is there anything else someone might think of to do, because I don't want to do this more than once if I can help it?

https://el34world.com/charts/Schemat...r_rebel_20.pdf

[Delta362]

What is the resistance value of R116? It's essentially in parallel with D102 and may give you a false reading.

Try lifting QC101 and QC2 and then power on with the LBL. I'd also consider pulling the preamp tubes to isolate the additional power supply draws..

C139 has note 8, but I don't know what note 8 says. Perhaps that cap is shorted?

[Helmholtz]

..can someone vouch for and explain why a shorted diode in this spot would cause a dead short? And I know I should check the filter caps while I am in there, but is there anything else someone might think of to do, because I don't want to do this more than once if I can help it?

When D102 is shorted, it passes AC. As C127 has very low AC impedance, the result is a shorted lower half of the HV winding.
As C127 had to pass AC current it's probably damaged.

BTW, the red connection dots at the HV winding are an error. Any shorted winding on a transformer would make the primary look like a short.

[Randall]

R116 is 330K and measures thus in circuit after the cap settles down.

For some odd reason, there is nothing connected to QC101. The wire from the standby switch is connected to QC109, which is right next to it and at the same point. No matter, lifting it is the same as an open standby switch, no?

With QC104 and 105 reconnected, QC102 lifted, all tubes out , and standby switch open, and no HT fuse F1, I still get a bright bulb.

There is no idication about the numbered red areas on the drawing that I can see, but I did measure to ground on both sides of node resistors R118, R122 and R123, and found nothing that would indicate any shorted caps. I realize caps can test OK with no load, and still fail in circuit under pressure, but since we are looking for a dead short or close to it, can we lean towards no shorted filter caps given the very high readings to ground and slow charging rate?

I am still looking for an explaination of how a shorted diode in this position might cause my symptom?

[Helmholtz]

I am still looking for an explaination of how a shorted diode in this position might cause my symptom?

I suggest to read my post above. Follow the path of the AC current before the standby switch.

[Randall]

Thank you Helmholtz, I missed your explaination the first time. I will replace C127. I always learn from you and appreciate your skills. We are lucky to have you here.

[Enzo]

What exactly do you mean by dead short? To me that means zero ohms. I get a sense you use the term just to mean the bright bulb. And all the bulb means is excess current flowing

If D102 shorts, that leaves D101 directly across the two red wires, the OT secondary. A diode across those wires results in every half cycle, the diode shorts the winding. That will draw huge current and light the bulb bright..

D102 is already bad, but when it shorts it puts serious stress on D101, so replace BOTH.

[Jon Snell]

C127 will be fine as I expect all of the current flowed through the faulty diode and D101. Replace the pair and all should be good.
1N4007 will be fine here.

[Helmholtz]

I think both Enzo and Jon Snell are right.

As long as D101 is good, the large positive current through D101 and shorted D102 is the main reason for blowing the fuse.
It is likely that the path through D101 kept C127 charged positive and thus prevented ecap reverse current. So C127 might be good.

My earlier explanation was wrong, sorry. (It only would have made sense with an open D101.)

[Randall]

Turns our D102 was indeed the problem and the cure. I replaced both diodes as suggested as well as C127 out of caution since I had one on hand and it was accessible. I didn't want to have to pry this board out again! It came in with a dead 6V6, so I wonder if that caused stress in rhe PS, leading to this diode to fail.

Anyway, it is working again, and thanks as always to the wonderful fellows on this site. You are always very helpful and generous with your knowledge.

[g1]

C139 has note 8, but I don't know what note 8 says.

BTW, the red connection dots at the HV winding are an error. Any shorted winding on a transformer would make the primary look like a short.

Including here a newer schematic with component values. All those caps that were in the red boxes on the other schematic have been omitted. I think that other schematic was not for production. They still didn't correct the error of the red dots on the HV winding though.

[Randall]

g1 I think you omitted the newer schematic. I'd like to see, I have another issue with the amp.

[The Dude]

His download works for me, or do you mean it's not the one you're looking for?

[g1]

g1 I think you omitted the newer schematic. I'd like to see, I have another issue with the amp.

Do you not see an attachment in post #11 ?

[Randall]

I don't know why, but I see no attachment on this end.