fini Posted January 24, 2005 Share Posted January 24, 2005 Regarding this Fisher console. It's a great little amp! No problems at all with the power amp section, but the tuner/pre is still giving me minor fits. After giving everything a very thorough cleaning and lubricating (including DeOxIt in the appropriate places), it worked fine. After about an hour or so, one of the channels dropped out to very low volume, and (with no input signal) had a low, rumbling (kind of like someone rubbing a mic over stubble, or the sound of a needle in a runout groove). Changing the preamp around showed the problem to be in the tuner/pre. I tried replacing the 4 12AX7s (with some used Baldwin/Sylvanias), and the problem disappeared! That was last night. This morning, I fired it up, and the same old trouble began. OK, so something is eating the 12AX7s. Of course, no internal parts have been changed yet, and I plan on doing that. Hopefully I can do it myself. Another symptom (if you can call it that) is about 10 seconds after switching it on, I get a hum that grows, then subsides (to an acceptable level) in volume. This takes about 5 seconds, or so. Craig has already pointed out the coupling caps in the power amp, but are there any likely culprits in the tuner/pre, especially those testable (and replaceable) by myself? I have a multimeter, a soldering iron, and a good attitude. Oh, more detailed photos can be had as well. Quote Link to comment Share on other sites More sharing options...
Timmikid Posted January 24, 2005 Share Posted January 24, 2005 ---------------- On 1/24/2005 12:40:14 PM fini wrote: Another symptom (if you can call it that) is about 10 seconds after switching it on, I get a hum that grows, then subsides (to an acceptable level) in volume. This takes about 5 seconds, or so. I've had this in my 300B PP amp and it turned out to be a faulty tube. So this hum IMHO could be caused by the error we are looking for (maybe faulty coupling caps), but it could also be caused by the result of the error we are looking for (faulty tube(s)). Did this hum also occur when you just switched tubes, if not second option sounds logical. Let me make clear that I know hardly anything of electronics, I just had this fit of sudden logics. Tim. Quote Link to comment Share on other sites More sharing options...
Clipped and Shorn Posted January 24, 2005 Share Posted January 24, 2005 Simply put, you want to know the location of the likely caps in the the preamp that , when bad, can erode away at the tube(s). It would probably help here to know which tubes in the sequence of the cirucuit are the ones being "eaten". Can you tell anything about the sequence of tubes starting at the input end and going to the output end? (Ok, now I see you photos) Perhaps you can get a clue by studying schematics which might be available for other close or similar Fisher preamps or preamp sections. Oh, I didn't see the photos at first. Good. What the heck is that large black thing in the lower left bottom clipped in under the chassis? Is that some large power conditioning cap? I would try more tube rolling, you have an abundance of questionable tubes, could be you just put in a couple that were on the verge anyway. Short of tube rolling, you (as I would be) are mostly out of your element for further fixing. Do any caps look obviously burnt up or punctured or otherwise damaged? C&S Quote Link to comment Share on other sites More sharing options...
fini Posted January 24, 2005 Author Share Posted January 24, 2005 ---------------- On 1/24/2005 2:01:28 PM Clipped and Shorn wrote: What the heck is that large black thing in the lower left bottom clipped in under the chassis? Is that some large power conditioning cap? Short of tube rolling, you (as I would be) are mostly out of your element for further fixing. Do any caps look obviously burnt up or punctured or otherwise damaged? C&S ---------------- Beats me what that dang thing is: I am a little concerned about pulling it out of the clip. No, nothing obviously leaky or burned. Maybe I should just count on burning through a quad of 12AX7s per hour. It did sound sweet! Hey, for checking resistor values with the VOM, is there a + and- ? Anyone recommend a forum/website where folks in the know can help me troubleshoot this? Quote Link to comment Share on other sites More sharing options...
3dzapper Posted January 24, 2005 Share Posted January 24, 2005 That thing would get you arrested at any airport in the world. Throw it out!!!! Don't touch it! That's a power supply capacitor and may hold high voltage long after the amp is turned off. No there is no polarity when doing resistance readings. just make sure you do them with the unit unplugged. We've got to find a schematic for that thing. What model is it? Rick Quote Link to comment Share on other sites More sharing options...
D-MAN Posted January 24, 2005 Share Posted January 24, 2005 Like Rick said, that's a PS cap, drain it first before playing around... DM Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted January 24, 2005 Share Posted January 24, 2005 This sound you describe, which might be motorboating, could also be the result of an open grid circuit -- associated externally with the grid leak resistor, or within the tube itself. As Rick and others mention, watch out for that power supply cap. Those things can shock hard. From the picture, it looks like there may be a bleeder already installed on it, but Mark's advice is important! Erik Quote Link to comment Share on other sites More sharing options...
Duke Spinner Posted January 24, 2005 Share Posted January 24, 2005 looks like a bleeder resistor, but in any event, you'll only want to get shocked offa that once ! Quote Link to comment Share on other sites More sharing options...
3dzapper Posted January 24, 2005 Share Posted January 24, 2005 Greg, Does that noise happen with the selector only in phono or any position? If in any position concentrate on the circuit from the selector switch to the 2 12AX7s near the output connectors. With the amp unplugged, check the connections of the leads etc, give 'em a little tug.Look for cold or broken/weak solder joints (use a magnifier). Since your problem occurs with heat, measure the voltages Mark asked for both when cold and after about 1/2 hour. Use a test speaker. When it is making it's noises, give it as my old lab instructor called it the "tap test". Using a well insulated small screwdriver CAREFULLY, so as not to cause a short, and lightly tap each component (cap, resistor tube, pin, junction block etc) and listen. If you hit the bad one it'll let you know. CAUTION DEADLY VOLTAGES!!! ALL LIVE MEASUREMENTS AND TAPPING ARE TO BE DONE WITH ONE HAND IN YOUR BACK POCKET!!!!! Rick Quote Link to comment Share on other sites More sharing options...
Champagne taste beer budget Posted January 24, 2005 Share Posted January 24, 2005 Even better than a screwdriver, use a wooden dowel. I bet you have one or two of those lying around. No other help as to the electronics aspect, but I at least know a for sure insulator when I grab one. Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted January 24, 2005 Share Posted January 24, 2005 fini: Something very easy to check out, which can also cause this 'scratchy' noise you're hearing. I had a pair of amps here with the same thing, and found that both the tube pins and socket had a small amount of green oxidation. You might pull the tubes and check with a magnifying glass for a good look (if you have one). Oxidation can be cleaned off easily. Also check the tension on the pins that grip the the tube. They may be loose, and can be re-tensioned with a small screwdriver or dental pick. IN ALL CASES, MAKE SURE YOU DON'T HAVE HIGH VOLTAGE PRESENT. It's not just the capacitor that can shock, but other sources of stored B+. It's possible to manually bleed caps, and you can calculate the value for a temporary bleeder from the output of the power supply to ground. If the filter capacitor is leaky, you may not be getting good B+, which might account for some of the loss in volume. Do you have any hum in addition to the scraching sound? Also, do you happen to have a spare rectifier on hand? Quote Link to comment Share on other sites More sharing options...
fini Posted January 24, 2005 Author Share Posted January 24, 2005 Thanks for all the advice, gentlemen! I am going to wait until tomorrow to do this testing (everyone else will be gone, no distractions, I won't be embarrased if I electrocute myself ). When testing the pin voltages, is there a particular range I should be working in with on my Radio Shack meter (i.e. what am I lookin' for, ballpark-wise)? I promise not to electrocute myself, and practice good safety procedures. And not to blow up my Klipschorns. Quote Link to comment Share on other sites More sharing options...
Marvel Posted January 24, 2005 Share Posted January 24, 2005 You could wear rubber gloves too! That would really make the wife wonder what you were up to. Marvel Quote Link to comment Share on other sites More sharing options...
fini Posted January 24, 2005 Author Share Posted January 24, 2005 I'm going with a whole-body latex suit. Quote Link to comment Share on other sites More sharing options...
3dzapper Posted January 24, 2005 Share Posted January 24, 2005 Put a hank of (black) WHOOPS, I forgot your from California, any color electrical tape around the tip of your probe leaving just the very point exposed. This will lesson the chances of shock or shorting anything. Start with the highest scale, You can always go down without burning out your movement. Rick Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted January 24, 2005 Share Posted January 24, 2005 Fini: Please don't take this the wrong way, but if you're not accustomed to doing this sort of work, it might not be a bad idea to consult, in person, with someone who can work with you. Whatever you decide, make sure that you unplug the amp from the wall outlet when any testing is done. It's something that's easy to forget. Don't probe around in an energized circuit with both hands -- keep one in your pocket in order to help prevent possible cardiac arrest by providing a path across your chest from each hand -- should you get shocked. Yeah, this sounds scary, but I'm really serious. If you do this, don't be alone at home when you do. I always tell my wife when I'm doing electrical testing, and she comes into the room from time to time just to see how things are going. If there is an electronics technician you know near you, maybe you could watch him/her do the work to see how it's done. Or get a book that describes how to do voltage and resistance tests, and then practice with the amp drained of any charge. Erik Quote Link to comment Share on other sites More sharing options...
fini Posted January 24, 2005 Author Share Posted January 24, 2005 Oh, to answer a few specific questions: Rick, It's a Fisher Model E44 console. The tuner/pre chassis is a 440T, and the amp is a 440A. Multiple Google searches have come up empty. I have found others looking for the schematic, though! The scratchy noise is mostly when I have it in "Aux Stereo", but is in other positions (although it sounds a little different). Extremely low volume in the B channel in all input settings. Erik, I will check the pins again, but I cleaned and "Deoxit"-ed them pretty well, and they went in & out a lot of times. Also, these 12AX7s were behaving quite nicely at first. They've gone south after that first hour, though. Regarding the hum, yes, about 10 seconds after switching it on I get a hum that grows, then subsides (but does not go away completely). It is not loud enough to be ojectionable, I think. No rectifier tube! It's SS: I will take your warnings completely serious, Erik. I already was probing around under there the last few days, biassing the EL84s, etc. If I think I can't do something by myself, I will definitely seek help. Maybe Clipped can drop by! Quote Link to comment Share on other sites More sharing options...
Marvel Posted January 24, 2005 Share Posted January 24, 2005 Rick, Somehting that works better than electrical tape is to use some small diameter heat shrink. Nice and tidy. I have a small giveaway screwdriver that has a piece of heat shrink over the metal shank, leaving only the last 1/8" exposed. It comes in handy. Marvel Quote Link to comment Share on other sites More sharing options...
3dzapper Posted January 24, 2005 Share Posted January 24, 2005 Marvel, I'm old school, 1975, they taught tube circuits! There was no such thing as shrink wrap back "in the day". But it is a good idea. I may steal it! fini, I tried a Google search myself and came up blank. You may have to trace the circuits and draw one yourself. You have one good side correct? Measure all the resistances and voltages on that output tube and compare them to the bad side. that will point you in the right direction. Rick Quote Link to comment Share on other sites More sharing options...
Marvel Posted January 24, 2005 Share Posted January 24, 2005 You tryin' to make me feel old now? You aren't far ahead of me. OT, but my older brother (age 58) went to his highschool reunion this past Fall. Couldn't believe all the old people there. '75 was not long after I got my ST-70 ($50), and ST-120 ($75). I kept the 70 and sold the SS 120. It is probably dead by now. fini, That latex will make you smell bad! Quote Link to comment Share on other sites More sharing options...
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