Smoking 5F1 - advice needed

[leevc5]

I am getting smoke after the amp warms up at the positive side of the 16uf cap.  I have checked the wiring and it conforms to the layout.  Where should I look for the potential cause?  Should I start with replacing the cap and see if that solves the smoking?

[sluckey]

Here's what I'd do and in this order...

Feel the cap to see if it's hot. Measure the dc voltage at that point. Measure the ac voltage at that point. Change the rectifier tube. Change the cap.

Could be as simple as a bad cap. If the smoke is indeed coming from the cap it will need replacing even if you find another problem that caused the cap to smoke.

[hesamadman]

What Sluckey said is what I would do. But I'm curious, are you sure you have the properly rated voltage cap?

[leevc5]

Thanks to both of you, I have already swapped out the rectifier.  I'll swap out the cap this evening.  Yes, the voltage is correct I ordered it as part of the Hoffman kit parts list.  I will verify the voltage before putting in the new cap.
Best regards, Lee

[Paul1453]

You can sometimes sniff out hot smoking parts with your nose, and not burn your fingers touching a red hot part. 

That resistor connected to that cap may be under wattage rated, maybe?
If that resistor is actually the part getting hot and smoking, upgrading to a 5W resistor might solve the problem.

But the advice given by sluckey is what you should follow 1st.  ;^)

[HotBluePlates]

... If that resistor is actually the part getting hot and smoking, upgrading to a 5W resistor might solve the problem. ...

No, if the resistor is smoking, there is excessive current draw somewhere which should be fixed.

[Paul1453]

... If that resistor is actually the part getting hot and smoking, upgrading to a 5W resistor might solve the problem. ...

No, if the resistor is smoking, there is excessive current draw somewhere which should be fixed.

OK, but if that resistor was a 1W instead of the 3W shown maybe that could be the smoky part?

If a 3W is smoking, excess current draw is likely indicated.  correct?

[HotBluePlates]

Check out the 5F1 layout, which shows a 45v drop across the 10kΩ. That drop results in ~0.2w dissipated by the resistor.

Yes, the voltage drop cannot be guaranteed to be the same in Lee's amp. But Fender would have used a 1w carbon comp resistor in the original amp, which was appropriately uprated to keep the resistor cool.

I'm pretty sure Hoffman indicated a 3w resistor here because it is the first step up from the 1/2w resistors he stocks.

I think the chances are good there is a short or partial short just after the 10kΩ resistor due to wiring error. I've not seen caps smoke (but perhaps I'm uninitiated), but resistors often do. If there was a short to ground right after the 10kΩ, it would be trying to dissipate 11.5w which would be sure to char it.

[labb]

First thing I would check would be if the e-cap is oriented correctly.

[Paul1453]

Thanks for the explanation HBP.

That calculating stuff is where my basic repair tech training was weak and usually unneeded to identify and replace a faulty part.   

[HotBluePlates]

First thing I would check would be if the e-cap is oriented correctly.

Yes, everything should be verified. Usually, electrolytic caps explode when a reverse-polarity voltage is applied.