Smoking S.E.X. amp after capacitor upgrade! Help!!!

[audiblesoundwave]

Hi everyone,

I am in trouble this time!  I upgraded the 1000uf 35v capacitors to 1000uf 50v capacitors and the left channel side (terminals 21 thru 40) started to smoking.  I checked many time that the capacitors are installed correctly (the strip side vs the non strip side).  The right channel side is fine, no smoke, and seems to be working!  No smoke as least!

The first thing I noticed is the 620 ohm 2w resistor was 'brown' out!  I replaced it with a new one; however, the left side still has smoke after power on.  I put the old 1000uf capacitor back in but no help!  Still smoking after power on.

What could caused the resistor to burn out?  What else should I check? 

Any help is greatly appreciated!

Thanks,
Milton

 

[Paul Joppa]

Of course the first thing to do is to repeat the resistance checks before applying power.

Presumably, some other change in wiring occurred during the change of capacitor - perhaps another component was damaged, or another solder connection was broken, or a wire was bent to where it touched something it shouldn't.

The one clue we have at this point is that the 620 ohm 2 watt resistor was overheated. That suggests the 1000uF capacitor was installed correctly - if not, the resistor would be OK and the cap would have blown out!

Since the circuit worked before, we can speculate that only one new problem occurred. The only way to get an excess current through the 620 ohm 2W resistor is if it goes through the tube. That would happen if the tube lost bias, so check the resistance from the grid (pin B1 on the tube socket) to the negative end of the new capacitor and the 620 ohm resistor - I think that's terminal 32. It should read 250K ohms. A bad connection of the 249K resistor at T12 could have happened; inspect Terminal 34 at the other end and the 220 ohm grid stopper from terminal 34 to B1.

If that does not turn up anything, remove the tubes before testing it again, to confirm that the current is going through the tube. Then insert the A tube in the B socket (leave A empty) to see if a known good tube works properly on the B side. THat will give us more information. You can monitor the voltage across the new capacitor (and the 620 ohm 2 W resistor) with a voltmeter while it warms up; that voltage should go to 17 volts as the tube heats up. If it goes much higher then there is a problem - shut it down before you get more smoke. Always use clip leads to test voltages during startup so you ave a free hand to switch the power off if there's a problem. Your other hand is in your pocket, right?

 

[audiblesoundwave]

Thanks Paul for your reply!

Here is the update!  I checked the wiring again and I don't see anything wired wrong.  I must have done something else that I must have forgotten.  For the resistance, I checked and they seem to be correct as expected with a very small differences.

I inserted the known good tube in the B-side which has the problem.  No tube in the A-side.  I powered it up again and quickly did some voltage measurement.  The results are not right. 

I clipped the 1000uf 50v capacitor for the testing.  One time, it got so hot!  Was it over heated?  No good anymore?


Terminal
31        - 147v  (expected should be around 366v)
32        - 78v (expected should be around 17v)
34        - 76v (expected should be around 17v)
36        - 140v (expected should be around 350v)
B1        - 74v (expected should be around 17v)

Could it be both tubes are bad?  Could it be the 47uf capacitor?

Thanks for your time in advance!

Milton

 

[Paul Joppa]

Taking a long shot here, maybe the parafeed capacitor is shorted. Hitting the cap with the soldering iron could do that. That would dump current through the plate choke, shorted capacitor, output transformer to ground. Unsolder one end of the cap and see if that looks better.

The 100uF cap is destroyed; 75 volts is way too much for a 50v cap.

I'm amazed the fuse hasn't blown. And you're lucky the cap didn't explode - I should have mentioned the safety glasses, shouldn't I? (Well, often they just barf their guts out ...)

The fact that B1 is very negative says it's not a bias failure, and the good tube means it's not the tube, so that's what's left assuming the connections are all correct.

 

[audiblesoundwave]

Thanks again Paul!

Well, I took out the 1.0uf capacitor, 0.1uf capacitor, 47uf capacitor, and the 620 ohm resistor and re-solder them back in.  That seem to fix the problem.  The resistance and voltage measurement are within the expected values.

I do not really know what the cause was!  It could have been some wire touched and caused the problem.  Anyhow, I am glad the amp is back together and making good music!  I hope this is it for this problem.  Wish me luck!

Good night,
Milton