300B not lighting up [solved]

[danox574]

I've gone and done it now.

My left channel started making a humming sound and I'm stopped getting audio out of it - noted that the 300B
tube was not glowing at all.  I had actually left it on for a couple hours on accident, so it was pretty warm.  I'll re-test if the tube comes on while it's cold.

I did, however, decide to 'quickly' swap the 300Bs between the two channel, setting aside the silicone rings, to see if the problem moved with the channel.  This was the stupid part - I plugged in the 300B into the right channel rotated 90 degrees left.  The large pins at the top left of the socket (facing the amp) plugged quite easily into the smaller, lower left hole, I didn't get any tactile feedback that I had botched plugging it in.  I noticed it wasn't glowing and I was staring at it and spotted the orientation problem from the top.

Now, the right channel makes no audio.  The smaller tubes glow as normal, the left channel doesn't glow, and the right channel does glow but has no audio.  Neither tube produces audio on the right channel, but only one was plugged in incorrectly, so I assume the tube survived the ordeal.  There is nothing visibly damaged as far as components on either side.

So, I need to troubleshoot no power on the 300B on the left channel and determine what I might have done to the right channel by inserting the 300B rotated 90 degrees.

Tonight I should be able to get on the bench and measure voltage across the board, and if there's anything else you need me to inspect or measure, please let me know, I'll get all the information I can tonight and your feedback would be appreciated if you can help.

Thank you bottlehead wizards,
-Dan

[Paul Birkeland]

I'd start with voltages.  If you have a 300B that doesn't glow in either 4 pin sockets, and the other 300B glows in both sockets, then you have a dead 300B.

[Paul Joppa]

I'm not sure how to do this, but you'll want to be sure that the sockets still make good contact with the tube pins.

You can use your meter to measure the DC resistance of the filaments (fat pins) - it will be obvious if a filament has failed!

[danox574]

To be clear, both tubes glow when plugged into the right channel, neither tube glows when plugged into the left channel.  So I don't think I killed a filament.  I will measure them both anyway.

I have always had trouble with the 300B left socket since I got the amp - recently, the 300B (sitting on the silicone ring) was making intermittent connection with the tube.  A solder reflow didn't help, and I noticed from the bottom of the socket that one of the smaller pins was not making good contact because the pin was extending out the bottom of the socket a bit - lower than the other three - I compressed it with a set of needle nose pliers and pushed it back up into the socket.

I think I'll order a set of replacement sockets for it and swap them both out.

How nice it would be if this was simply a connection problem.

Nothing produces audio from the right channel though - but tonight, I'll be retesting everything and getting a full set of voltages for you guys and anything else you'd like to see.  I'll look for any visible socket problem when I'm in there.

[danox574]

Ha! Stuffing the big pin in the little hole expanded the connector on the right channel - after compressing it from the back with a set of needle nose, I have connection on the small pin again and audio is flowing on the right channel.

It's just a matter of finding out why my 300B isn't heating on the left.  Will do the voltages tonight.

Great call!

-Dan

[danox574]

For my left channel problem, the 4 LEDs on the C4S board are not right - A Side comes on after 10-15 seconds, and the B Side is out.

OK, so here are the measurements:
Terminal 11 - 182.1V
Terminal 12 - 0V
Terminal 13 - 0V
Terminal 14 - 0V
Terminal 15 - 194V
Terminal 16 - 0V
Terminal 17 - 181.7V
Terminal 18 - 0V
Terminal 19 - .006V
Terminal 20 - 0V

Terminal 27 - 0V
Terminal 28 - 0V
Terminal 29 - -7.48V
Terminal 30 - -7.48V
Terminal 31 - .929V
Terminal 32 - 0V

Terminal 39 - 0V
Terminal 40 - 218.8V
Terminal 41 - 0V
Terminal 42 - 0V
Terminal 43 - 218.8V
Terminal 44 - 0V

Terminal 45 - 0V
Terminal 46 - 218.8V

I actually appear to be experiencing the same problem I had with this right after building, which you can read about here:  http://bottlehead.com/smf/index.php?topic=6835.msg66595#msg66595

Paul suggested I might have had a backwards shottky diode.

This was my resolution last time, from the other thread

Actually, after I posted these photos I went back and reflowed the solder points on all four UF4007 rectifiers and BINGO, up and running.  Everything works, nothing appears to be damaged.  That was Terminal 6 and 7 on the PT-7 and terminal 39 and 40.  Don't have power amps yet so just ran the outputs through a Stereomour input and all was well.

So, it mysteriously went away last time, but based on the voltages I posted, plus the symptoms with the B LEDs, it appears that the original problem is back.  I don't think it's heat related because I powered up the amp cold now and I have the same issue.

[Doc B.]

Well, if reflowing those joints worked last time, I'm just sayin'...

[danox574]

Well, I agree but it doesn't seem to be the solution this time unfortunately.

I am measuring appropriate capacitence from 28 to 31, indicating the caps are good and my jumper from 29-30 is intact.

Schottky diodes have correct orientation and have been re-flowed.  But even last time, that re-flow solution seemed a bit dubious to me as the connections were shiny and I have never had an issue with solder on these tabs across several kits, I was quite surprised when it started working.

Symptoms remain the same (voltages and B side not lighting).  One of the two big caps from 28 to 29 is bulging on the top a bit, but I seem to recall that being the case when I got it out of the bag (and one on the other side is too). 

[Grainger49]

It sounds like when you retouch the solder joints something is getting moved to make contact again.  I would try the chop stick test.  That is taking a non conductive piece of wood and moving all terminals and solder joints around while listening.  You will probably fine one or two that cut in and out.

[danox574]

The tube has been very dead, continuously, since I found that way a couple days ago.  Through all of it's moving back and forth, testing, moving, shifting, reflow, I'm not getting anything now.

I will try your method, but I don't expect to hear audio cutting in and out if the tube is intermittently getting power - I'd have to get it into a position where it's lighting up the 300B long enough to actually get audio to start to come through.  I can poke around but I suspect it's going to be difficult to spot if I'm making a connection with pressure.

I'm hoping that the voltages can point me to a specific problem area where I can work - that's a lot of terminals and area to check - do the voltages tell you anything about the potential problem spot?

[danox574]

I'm pretty stumped.  I worked around every terminal I can reach - reflowed the filament socket ports since I can't poke them when powered up.  Spent several seconds on each joint, trying to see if the B LEDs would flicker or I would get any indication that voltage was getting to the 300B, but nothing.

I'm going to reflow on the PT-7 next and check the diodes again, because my POS IN on the PSU board is basically zero.  The LM1085 is not getting voltage to create the 10V from.  The 6BQ5 is lighting up so I'm sure I'm getting 6.3V out of the transformer but pin 29 is reading -7.5V instead of +6.8V, so my instinct is to find a problem somewhere in those two caps and 1N5820s.

If I'm off base please let me know.

[danox574]

Solved.

Gave the 1N5820s a good twist while reflowing the solder - one of them must have been floating inside the terminal and not making contact, or barely making contact.  Really had to get in there and make sure they moved when reflowing due to the extremely stiff leads.

[Grainger49]

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi244.photobucket.com%2Falbums%2Fgg7%2FGrainger49%2FSmiles%2Fwelldone.gif&hash=332f2549437c6c0b1e58c7565889dca0e76246e4)

Now time to listen to some music!