Blown 220uf filter capacitor? no voltage to terminals 1-10 :(

[billyperez84]

Im hoping someone can help me with this.  I built the crack, worked great, perfect. I decided to instal a pilot light. I clearly installed it in the wrong place.   I installed the positive side of the light to pin 9 of the preamp tube socket and I saw some smoke.  The 220uf filter cap located at terminal 21 doesn't read on my meter after I removed it from the amp. I also got a voltage reading of 500+ at this terminal after the amp failed. Ive ordered 3 more 200uf 250v caps from mouser.  My question is two fold. A) is the cap the problem?  My tubes light up but I get no voltage readings from terminals 1-10. B) is there anything else I can check for on the transformer to make sure I didn't damage it?    Thanks so much, I really need to get this thing back up and running!  If anyone knows how to properly install a pilot light please let me know.  Thanks in advance

[Paul Birkeland]

A) is the cap the problem?

No.

My tubes light up but I get no voltage readings from terminals 1-10. B) is there anything else I can check for on the transformer to make sure I didn't damage it? 

The AC voltages for the transformer are printed on the sticker.  You will want to put the black probe on "0" and the red probe on the terminal next to it with a labelled voltage, then you can check each voltage.  Since your tubes glow, the transformer is likely OK.

Thanks so much, I really need to get this thing back up and running!  If anyone knows how to properly install a pilot light please let me know.  Thanks in advance

It's still not clear how you installed the pilot light, as they have 2 terminals and we only know that you wired one terminal to A9.

-PB

[billyperez84]

Thanks for the response.  I soldered the neg terminal to 16 in order to try to take advantage of the ground. Probably a bad move.

[Paul Birkeland]

If you have a 6V pilot light, you should just use the 6.3V winding that's available.  The ground connection to this winding serves as a reference for the winding, and shouldn't normally carry much current.

All of this has absolutely nothing to do with the 220uF capacitor, however, as that is on the high voltage side.

-PB

[billyperez84]

Thanks, that explains how to connect the pilot light.  Still trying to figure out what happened to the amp though. I checked the transformer. Checks out fine however the voltage is a little higher than listed on the windings of the bottom row.  I have removed the 200uf capacitor that I suspect is dead. I am not getting voltage to the terminal strips along either side of the transformer. Keep in mind this amp worked perfectly as wired prior to the pilot light installation.Let me know if theres any more info I can provide as Im sure there is. Thanks

[Grainger49]

Billy,

Unloaded transformers float to a higher voltage than rated.  The rated voltage is loaded.

So what you are reading is totally normal.

[billyperez84]

Correction I am getting voltage to terminal 21. But nothing on 11-16. Im hoping that replacing that bad cap will do the trick.   Thanks Grainger, that makes sense.

[Grainger49]

Ok, 21 is the end of the diode string.  This is good news.

So if your diodes are good you will see 200V DC there, probably more since the circuit is unloaded.

[Paul Birkeland]

Correction I am getting voltage to terminal 21. But nothing on 11-16. Im hoping that replacing that bad cap will do the trick.   Thanks Grainger, that makes sense.

Again, this has nothing to do with the capacitor.  You could remove all 3 220uF caps and you'll still get DC voltage on those terminals.

The two 270 Ohm resistors and the black wires that follow them carry the DC voltage to the rest of the circuit, I would suggest that one of them is loose.  (And that your 220uF cap is OK)

-PB

[billyperez84]

So after taking this apart a bit more, it appears that the 270ohm 5w resistor that stretches from 21-15 is not registering on my meter and looks a bit burned on the bottom side.  Guessing this could be my problem and probably the source of the smoke that I saw on startup.

[billyperez84]

That did it, back up and running, thank you for the information it's been very helpful and educational. As far as the pilot light, Ill try connecting to the 6.3v winding.  What about grounding for that? Thanks in advance.

[billyperez84]

When I attach the pilot light to the 6.3v terminal it burns the 270ohm 5w resistor. Luckily I have a bunch of those...Any thoughts?

[Paul Birkeland]

When I attach the pilot light to the 6.3v terminal it burns the 270ohm 5w resistor. Luckily I have a bunch of those...Any thoughts?

Don't use the pilot light.

As far as the pilot light, Ill try connecting to the 6.3v winding.

 
That might well work.

If your wiring of the pilot light is causing excessive current draw in the high voltage power supply, I would recommend throwing in the towel on the pilot light before you cause serious damage to the amplifier.  To cook and blow one of those 270 Ohm resistors, you would have needed to draw 150+mA of current through the high voltage supply, which is several times what the circuit would normally draw.

[NightFlight]

I don't get the pilot light. An indicator the unit is powered?

Why not drop in a replacement power switch that has light built in to draw directly from the AC source?  I've thought of doing this, but I don't know if additional noise might be generated and get into the unregulated PS. Good luck.

[Nathan]

glowing tubes are my pilot light