Blown left channel...

[StivVid]

I recently sold my S.E.X. 3.0 with C4S upgrade to a friend.  A few days ago while he was standing in a room adjacent to where the amp was, he heard a loud popping sound and looked over to see sparks inside of one of the tubes.  He ran over, shut off the power, replaced the tube and found that the left channel is no longer working, and the left driver of his HD650's is blown.

He asked me to look it over and see if I can fix it for him.  I've run all the resistance checks for the kit that the instructions call for before the C4S upgrade.  All those check out.  Decided to insert the tubes and apply some voltage.  Tubes glow fine, but the LED's on the right side of the C4S board as you look at it laid on the amp's top no longer light up.  Checked the voltage on OA and OB.  OA reads 64.7, but OB is way off the scale and 305 volts.

Which component or components on that board should be replaced?  Should anything else in the amp's circuit be checked?  I noticed that the resistor attached to terminals 33 and 21 looks a bit discolored like it's radiated quite a bit of heat.

[Paul Birkeland]

How does the resistor measure between 33 and 21? 

You'll want to compare the DC resistance of the plate choke on each channel to be sure there isn't an issue there (even though this has nothing to do with the C4S voltage problem).  This is the DC resistance between the blue and red wires on each plate choke.  I would also measure the DC resistance of the output transformer primaries, where the red and white wires attach.  Just be sure that you still have consistent readings between channels.

There's a little blue resistor that goes to pin 9 on each tube, do they both measure/look OK?

How does the 680 Ohm resistor look that sits just behind the 12 pin tube socket?  If that resistor came loose or wasn't all the way soldered, you could get some fireworks.

Beyond that, you will want to replace both transistors on the C4S board if there aren't other problems present in the amplifier. 

[StivVid]

Ok.  If I'm doing this right...  resistance between the red and blue wires on the plate chokes read 826 ohms on the right side and 827 on the left.  Resistance between the terminals where the red and white wires attach to the plate chokes read 209 and 210 ohms.  The blue resistors attached to pin 9 on the tube sockets both measure 1254 ohms.  The 680 ohm resistor behind the tube socket looks good and measures 674 ohms.

So I'm looking at replacing both transistors on the left side of the C4S board.

Could a faulty tube have caused this, or am I likely missing some other problem?

[Paul Birkeland]

It's very rare, but a faulty tube can do some strange things.

There's a 249K resistor that connects between the 0.1uF coupling cap and the 22uF/450V power supply cap.  If that resistor is not well connected, the amp can go into thermal runaway until the tube self destructs.  This will appear as a DC voltage at terminal 18 that's positive and significant, I would expect that you'd need to see more than a couple of volts for things to get crazy.  Do note that your meter may see a few negative millivolts here, and that is normal.

[StivVid]

First of all...  Thank you so much for taking the time to help me.  You guys are all so gracious with your time and knowledge and I really appreciate it.

I'm getting 0 volts at terminal 18.  Is that correct?

[StivVid]

Just did the rest of the voltage checks.  I see that 18 is supposed to measure 0 volts.  That's good.  Terminal 16 is measuring 297 volts.  That's not good.

[Paul Birkeland]

Yes, that looks good.

[StivVid]

So... before the C4S upgrade, terminal 16 should measure 65v-95v.  After the upgrade that figure jumps up to around 300v?

[StivVid]

Is that because my transistors are fried?

[Paul Birkeland]

No, having 0V at terminal 18 is good.

Your C4S board is still not functioning on that side and replacement transistors are likely in order. 

It is often possible to put your meter on the continuity setting (so it beeps when the meter leads touch each other) and look for internal shorts between any two legs of each of the transistors.

-PB

[StivVid]

If I order these parts should I be back in business?

https://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=242918871&uq=636453153179142863
https://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=242919112&uq=636453153179142863

[Paul Birkeland]

Those links didn't seem to go anywhere.  You need a PN2907 and an MJE5731A.  Digikey stocks 5-10 varieties of PN2907; any of these will work.  Digikey stocks one MJE5731A (listed as MJE5731AG), it will also work just fine.

[StivVid]

Ok.  Thanks again for all your help.  Hopefully that'll do it.

[Paul Birkeland]

If not, you know where to find us!

[StivVid]

Well... the new transistors arrived today.  I've just finished replacing the transistors for the left channel of the C4S board with the new ones.  The LEDs on that side still aren't lighting and voltage at OB still measures high, albeit a little lower than before.  Now the measurement is 243v.  Any ideas?