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Post by pollyshero on Jul 30, 2007 23:26:17 GMT -5
Greetings All - I'm back, and loaded with questions... Some of you are familiar with my sad situation: guitarnuts2.proboards45.com/index.cgi?board=Amps&action=display&thread=1135125976I've finally scrounged the courage to crack this thing open. After many years of foolish tinkering, poking, and head-scratching I've decided to proceed as cautiously as possible. I'm truly interested in learning how these things work and how to fix 'em when they don't. I'm using R.G. Keen's Tube Amp Debugging Page as a guide: www.geofex.comAs of now, things have changed a bit - at last report, all the preamp tubes and the rectifier tube would light, but the power tubes would not. Currently, for some reason, now none of the tubes light... Following the Keen page hierarchy; My amp has no sound at all, the AC power indicator glows, and the rectifier tube has been replaced with a known good one - to no effect. I am now preparing to test the power transformer - which begs my first (probably in a looooonnngg series of) questions. WHICH IS THE PRIMARY WINDING? If you click on the pic when it opens in Photobucket it'll enlarge and be easier to read. s70.photobucket.com/albums/i110/theudog/TranSchem.jpgI'm thinking Blk/Wht - for no other reason than "black & white" rolls off the tongue much easier than "brown & violet". Blu/Gray are out because they're center-taps. Red/Red is the high-voltage, Grn-wht/Grn-Wht are the filament leads, and Grn/Viol are for the plates. Or... One last one - off to the right, on the high-voltage supply, is the notation "+490V" - Would this be the B+ voltage? And if so, that's VAC, correct? Thanks in advance PH (EDIT: modified link to point directly to the drawing (partial schematic) instead of all of Photobucket's balogna.) |
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Post by sumgai on Jul 31, 2007 2:17:52 GMT -5
PH, Geez, when you "dally", you don't mess around, do ya? ;D April of 200 6, indeed! So, hoping that I've slipped this message in before you've started messing around, let's look as some answers........ There's a fuse in the filament circuit (the Green/White leads), have you checked that for Quality of Operation?   The Primary winding is always the one that goes to (or more properly speaking, comes from) the power source, or in most cases, the wall plug and cord assembly. However, the transformer used in your unit is made for export as well as domestic - it has two primary windings.  They are wired in either series (for higher input voltages) or in parallel (as shown, for domestic use). For that reason, the colors of the wires are a bit complex. Black and White is as good as anything to call the Primary colors. And strictly speaking, Blue and Gray aren't center-taps, though they are taps, just of a different nature - they are for the non-120/240 countries. Instructions for hooking them up were probably provided in the manual, particularly when it was exported. The Secondary windings are colored like so: Red/Red are the high voltage winding colors, as you guessed. Ditto for the Green/White leads to the filaments. The Green/Violet pair is NOT for the plates - this winding provides the negative bias for the grids of the power amplifier stage tubes. Also the Green/Yellow pair provides the low-voltage power to the solid-state portion of the amp. Yes that's the B+ allright, and it's well within the maximum allowed voltage for 6L6's and EL34's. Would you like to throw a life-line, or is "VAC" your final answer?  OHHH, so sorry, Mr. Hero, the correct answer is Nope.  With the exception of the filaments (aka the heaters), all voltages on a tube are DC. (And even then, some manufacturers will put in the extra effort to put DC on the filaments, to make the tubes a bit quieter.) You're welcome in retrospect. ;D HTH sumgai |
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Post by pollyshero on Jul 31, 2007 19:21:04 GMT -5
Many thanks sum.
I've spent the time since April 06 reading & fretting - As I said, I've wasted a lot of time being non-productive (electronically speaking). I've been doing homework - but even back in my skool-daze, I was never much good at it: I've always been a tactile learner, so now it's come down to "do it, or don't".
Your input is highly appreciated and is very useful to me. Look out fellas, I'M GOIN' IN. Thanks again!
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Post by pollyshero on Jul 31, 2007 23:17:02 GMT -5
~Sigh~
Fuse in the filament circuit is OK, but (no surprise) I'm stumped... Every test showed resistances/voltages within acceptable ranges except as below:
Mr. Keen's instructions refer to the "rectifier heater winding" - I can't find a seperate winding for just the rectifier tube - its "heater" is fed by the same 6.3VAC "Fil1/Fil2" winding as all the other tubes. May I assume there are transformers out there with such a separate winding? If so, and since mine doesn't seem to have one, may I disregard steps 3.4 (resistance section) and 4.9 (voltage section)?
And the one thing that's REALLY bugging me - Mr. Keen states in section 3.2 (resistance) to "Measure the resistance to the chassis from both ends of the primary winding. It should be more than 1M ohm." I get zero response from my meter - and there's no continuity either. I performed the same test on a tranny I knew was working The amp it's in pisses off my neighbors) with the same result, so I am somewhat confused, since every other test yielded results within parameters.
Thoughts? Is this thing a paperweight?
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Post by sumgai on Aug 1, 2007 4:12:04 GMT -5
PH, Hey, you forgot to ask one of us to hold yer beer! ;D Yes, that's a valid assumption. There are several different kinds of rectifier tubes, and over the years, they've all been used by most amp makers, at one time or another. Some require separate windings for their filaments, some can share the standard winding used for the rest of the tube complement. There aren't many differences between the two designs, for our purposes and intentions. Feel free to skip the noted test steps. A "non-response" indicates a completely open circuit. (Test your meter by shorting the probes - if you get anything, it's responding correctly.) Another word for circuit is "loop", and many, if not most, meters have the indicator "OL" (open loop) when the meter sees no connection between the two probes. But for your purposes, it suffices that since "open loop" qualifies as being much greater than 1MΩ, I wouldn't worry about the transformer itself. At least, not yet............  Nah, it's just that it's not easy to do remote diagnosis, but we'll eventually get it, don't worry. My next step would be to concentrate on the primary side of the transformer. In your first post, you did say that the only thing working now is the mains pilot light, thus......... STOP! You have pulled the power cord out of the wall socket, right? Good boy, I knew we could depend on you to be safe! Disconnect the primary side's header from anything else where it may connect, be it the power switch, or the printed circuit board, I don't care - just get the transformer isolated from everything else. When that's done, use your meter to measure the resistance across each of the two primary windings, color-coded as shown in your link. While R.G. says "less than 1KΩ", it's really on the order of 3 to 8Ω for a standard power transformer's primary winding. I've seen it even less, on certain units, but that's a good starting point. (Don't worry about the taps, the exact point along the winding is unknown - but do make sure they aren't shorted to any of the other leads.) It's a rule of thumb that if the primary leads show anything less than ∞ to ground, then the winding is partially shorted to the iron core. That doesn't always blow a fuse, nor even stink the place up quickly, but it will still cause problems, so better check for that condition while you're in there. If all the primary winding measurements turn up OK, then we'll repeat the process on the secondary side, starting with the filaments. I'm sure you can you handle that, given what I've said above. The main difference will be, each of the secondary windings will be much higher resistance across them, but the same admonition applies to the lead-to-ground reading. Report back when you've got all of this done.  HTH sumgai |
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Post by pollyshero on Aug 1, 2007 19:49:19 GMT -5
REPORTING: Removed tranny - completely isolated from the rest of the circuit. Resolved the "No reading" issue above by learning that "0.F" is a reading on my new digial multimeter (overflow).
Ohms
BLK/WHT 2.8
BRN/VIOL 3.0
GRN/VIOL 12.5
GRN-WHT/GRN-WHT .1
RED/RED 26.5
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Post by sumgai on Aug 2, 2007 15:03:38 GMT -5
Hero, Those look like good numbers to me. The 0.1Ω on the filament windings look low at first, to the inattentive eye, but if that were really a short circuit, the main fuse would blow in a heartbeat. The deal is, that's the number you get if you do the math in calculating a desired voltage from a known input: Given: 120vAC input, and 3Ω impedance for that input winding; To get a lower voltage, there must be fewer windings. For a ratio of 120:6, we'll need 1/20th the number of windings. That gives us: 3 ÷ 20 = 0.15 From that, I deduce that either you abbreviated your results, or your meter wasn't set to display that fine a resolution. Doesn't matter, the number is good, thus the transformer is probably good. But for various reasons, it is slightly possible that the winding isn't open, but nonetheless it still fails to deliver the anticipated voltage. Which is why one more test is advisable. Hook the transformer up to the 120vAC source, and measure the actual voltage found on the Green/White pair. Be most careful to avoid contact with the Red pair of leads!!!If you see 6.3vAC here (and I think you will), then it's almost a fact of life that the transformer is good. But if you don't see it, or you see something way too low (wavering a bit above zero volts), then it's time to get out the crying towel, because you don't wanna get into the repair aspects of this thing. That's for the obsessed, and you've already got one of those maladjustments in your life, right? ;D (I'm talking about obsession with tone here.  Time to start taking resistance checks with the rest of the circuitry. Don't put the transformer back in just yet, it's only a dead weight when you want/need to shift things around. Trace out the run(s) from the header connector to the power tube sockets, specifically pins 2 & 7. If you can't find continuity here, then you're onto the real cause of the problem. Use your eyes! Look for the cause, it can't be hidden away from visual observation. (Although sometimes you have to go through bloody hell and highwater to disassemble things, just so you can actually see everything.  ) It might be disguised as a bad solder joint, or a wire that looks hooked up, but is actually hanging on by the insulation alone (tug on it a bit), a minute fracture in the printed circuit trace, those kinds of things. Subtle, but emminently findable. That's my final answer, Regis! ;D HTH sumgai |
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Post by pollyshero on Aug 2, 2007 22:24:20 GMT -5
7.2VAC on the heaters.
Looks like I'm gonna have to upgrade my readin' glasses. I'm glad that the tranny is (apparently) GTG - That's the most expensive part to replace!
I'M GOIN' IN AGAIN - & no need to worry about my beer - I'm keeping it over there, just out of reach - that way I won't spill it on the amp guts & fry myself at the same time.
+5 for your help sum, & a tip o' the bottle neck in your direction
PH
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Post by pollyshero on Sept 1, 2007 12:38:01 GMT -5
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Post by sumgai on Sept 1, 2007 15:04:57 GMT -5
PH, Yes, there certainly should be something, probably on the order of a few KΩs, but it could be as low as several hundred Ωs. But that's just the Reverb driver. If this is dead, as in open, then all that's gonna happen is that you get no reverb. If it were shorted, then you'd still get no reverb, but you'd also get a tube that glows cherry red, or worse, goes KerBlooey. But neither condition would stop the rest of the amp from working. Back to the grindstone! HTH sumgai |
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Post by pollyshero on Jan 14, 2008 0:42:48 GMT -5
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They are wired in either series (for higher input voltages) or in parallel (as shown, for domestic use). For that reason, the colors of the wires are a bit complex. Black and White is as good as anything to call the Primary colors. 
With the exception of the filaments (aka the heaters), all voltages on a tube are DC. (And even then, some manufacturers will put in the extra effort to put DC on the filaments, to make the tubes a bit quieter.)
) It might be disguised as a bad solder joint, or a wire that looks hooked up, but is actually hanging on by the insulation alone (tug on it a bit), a minute fracture in the printed circuit trace, those kinds of things. Subtle, but emminently findable.