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Rockin' Andy 19th January 2007 08:19 PM

Excessive current draw in Fender Deville
Hi Everyone! My name is drew (but the name was taken - and I didn't want to be drew5465, so I'm "rockin' andy":)) and I have a small music store in Kirksville, MO, where I sell stuff and do mostly *basic* repairs. I have a Fender Deville that's giving me fits, and I was wondering if you guys could help. It's kind of a long story, but I want to make sure I give you all the particulars. Ok, here's the situation:
I had a customer bring in a Deville that needed a new input jack and a bias adjustment. The bias control acted a little screwy when I adjusted it (78mv, slight turn, 78mv, slight turn, 77, slight turn, 60mv!), but we got it where we wanted it (62mv - a little cold, but he liked it). The only other abnormality was that I pressed a wee bit harder than I probably should have on the wires going back to the transformers (they're on the non-visible side of the chassis) to get the preamp board out to replace the jack. Oh yea - I also switched the two preamp tubes and the power output tubes (but not the PI). Anyway, the guy has it on for 15 min after he gets it home, and it's sitting there, on, volume at 3 but he's not playing through it, and there's this loud hum and then it blows a fuse.
I followed a troubleshooting flowchart in a book by this cat named mitchel, and it said pull all the tubes, if fuse doesn't blow, replace preamp tubes, if fuse doesn't blow, replace output tubes, if fuse blows then (or, in my case, if current needle jumps way up on BK var PS), then it's probably the output tubes. Well, it all went down that way, but I replaced the output tubes with known goods and it still (after being on for about 5 seconds) starts to draw a huge amount of current and the output tubes start glowing waaay too red. BTW - either tube in either socket will make it do the same thing, but w/ both tubes out it won't. Anyway, I remember reading that a bad output transformer can also produce symptoms like this, so I disconnect the center tap of the OT (it was the easiest to get to - btw - it's hooked to b+ (484v), and the amp idles at 600mv and the tubes look their normal color - i.e, all is well.
Fender cust support (I'm not authorized, so they won't let me talk to the real brains) says yes, it's definitely the OT, and I should check everything in the output section (grid resistors, etc) b4 I put in the new OT unless I just like replacing parts over and over. Another tech I talked to said, yea, it's probably the OT, but you don't have to check everything - just replace, power up slow, and, if it doesn't start drawing excessive current, just confirm that the pin voltages are all correct and I'm good to go.
OK, here's my questions (sorry that took so long!)
1) This mitchell cat seems to know a lot, and he seems convinced (well, his book does) that if I pull the power tubes and the prob goes away, then its the PTs or an arc on the socket (I don't see an arc) - is he just wrong? Or, am I missing something?
2) is pulling the center tap on the OT and having the short disappear a decisive indication that I should order an OT, or should I disconnect the connections to the primary and secondary of the OT and check the resistance? I haven't done this yet merely bc I have no idea what to expect - what would be the *normal* resistance across the leads of the OT?
3) If it is the OT, can I determine whether or not I caused the problem? I mentioned that I was pushing a *little* hard on the wires going back to the transformers - could this have done it? Or, is it more likely that it went on it's own? (it's a '97). I did swap the tubes, and they were pretty weak (I'm not sure if they're shorted - the short light comes on on my tube tester, but it comes on for everything:) The good/bad needle works, though, it it says that one of the original output tubes that I swapped was in the bad zone
4) whether or not it's my fault or not (I'm paying for it either way - 15 mins out is just too short not to take the fall for it), what can I do to make sure the new OT (if that's the problem) doesn't go kablooey when I put it in?
Sorry - I know that's a LOT to ask!:) Any help you can provide would mean a great deal to me!

Rockin' Andy 19th January 2007 08:51 PM

the resistances on the OT primary are 113, 59.2. 54.4. The secondary (the resistance at the speaker jack) is 1.3. Ummm - the secondary is shorted, isn't it (it should be 8 ohms, right?).

Tom Bavis 20th January 2007 12:50 AM

It's the bias ... the tubes don't have the correct bias voltage to set the normal operating current. That flakey bias pot may have opened up - measure the voltage at the grid pins, and on the pot itself - it should be negative 30-40V I think. If there's voltage to the pot, not at the tube, it will need cleaning or replacing. And WHY can't they wire the pot so the tubes go COLD if it opens... easy enough to do...

Rockin' Andy 20th January 2007 11:27 AM

Wow, that's a theory I haven't heard yet! Gotta go to my daughter's quiz-bowl tourney today, but I'll definitly scope it out when I get home - thanx!

Rockin' Andy 20th January 2007 09:41 PM

Dude, you are definitely the man! I can't believe I didn't think to check the bias again, especially after it was acting kookey. Anyway, yea, it was out of control, and I gave it a 1/16 of a turn and then it went from 150ma down to 15ma. I sprayed cailube in it and it worked for a while, but as soon as the cailube dried it started acting silly again, so I'll just order another - but I do know that the pot was definitely the problem. Many, many thanx!!!

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