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I took a break from working on the Lowery C500 and started up on my rescued from the landfill Elegante..So far other than general cleaning of the cabinet i have went through all the drawbars and cleaned the contacts with deoxit and lubed the plastic side with some silicone, re-capped the demux board, re-capped the power supply, serviced the leslie. Everything seems to be working and in pretty decent shape the only real problem i have noticed thus far is some annoying white noise, hiss from the upper manual drawbars and presets .. the tab sounds are quiet and everything on the lower manual is pretty quiet, strings are working ect.. Could it be an aging transistor in the power supply? I have an old Tektronix 465B analog scope in good working calibrated condition that i can find my way around pretty good though my troubleshooting skills with it are a bit lacking, DVM and my old Sampson 260 .. Here are some photos..
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Usually a power supply problem would be global. Since it’s only on the upper flutes, I would look at the sine filter board. In fact, there are two, one for upper and one for lower. They are also identical. I would swap the two boards and check it again. If the problem stays on the upper, it’s not the filter. If the problem moves to the lower, then it is the filter.
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Excellent call, i swapped the boards and indeed the problem switched to the other manual.. How would you go about troubleshooting the board itself if i may ask?Originally posted by geoelectro View PostComment
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Since you have one known good one and one known bad one, and a good oscilloscope, alls ya gotsta do is compare them. :)Comment
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I think it's just that I lack confidence and worry that I will inadvertently blow up something irreplaceable probing around. That keeps me from diving in deeper many times although I do have a good basic understanding.Comment
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Well an oscilloscope is a high impedance device. So the only way you're going to let the magic smoke out of something is if you accidentally create a short circuit. As long as you're carefully using the pinpoint probe tip, that's easy to avoid.Comment
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I poked around with the scope this weekend on the sine board to see if i could pinpoint the noise but couldn't seem to find anything but normal looking sine waves.. I do understand part of what is going on with this board. It looks from the schematic it is in 7 sections each for an octave of frequencies.. i will post the schematics here, any advice would be appreciated..
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My advice is, don't specifically look for noise. Rather compare the two boards in operation and see where they differ. Start at the output, and check all round that amplifier circuit, and then work your way back to the filters.
Are those printed filter boards socketed, or soldered in? If they're socketed, I'd clean them and swap them with the other filter board.Comment
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You"re showing two versions of this board. You should only have one or the other.Originally posted by PaulStahl View Post
A quick test is to locate the coupling capacitor for each "channel". (C1, C2, C3 in the first pic) (The junction of R54-C37 on second version for example) Take a probe and momentarily ground the left side of the cap (1st bd) or that junction (2nd bd) while listening to the noise. When you hit the noisy channel, the sound will stop.
On the older type board there is only the "printed network" that could be the fault and...not too likely. On the second version there are several caps and the transistor that could be the culprit.
GeoComment
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Thank you for the responses.. You are right, my bad on the 2 different versions though maybe it will help someone with the older style.. I have 124-000653 Issue B, the newer one which would make sense as i have the later model Elegante 340207 with the main and echo switches. I will post a pic below of my board.. Also for anyone else who may see this the sine board that is closest to the top edge of the fold down board is the one for the upper manual.. I will try and explore some more with the new info..
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I had a chance this evening to do some more exploration and have found that by grounding junction R41-C28 the noise stops.. So would it be the coupling capacitor there that would be suspect? It seems some of these tiny non-polarized caps are glass and some look like poly film..Comment
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Considering that it's that kind of capacitor, it's likely. But it could be anything at or before the coupling capacitor.Comment
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Looking around again today with the probe i was able to see the noise which is what i really wanted to do. Turning the scope down on DC i could see the noise at the output but needed to shift over to AC to probe around the circuit at that low of a setting.. I could see the noise around Q3 which an electronics tech friend told me he thought was the problem and that i should swap Q3 transistor out with another channel to see if the problem shifts to the other since establishing that was the problem area just by grounding the coupling cap junction. I just really wanted to conquer actually seeing the problem although i couldn't specifically identify Q3 in that way. Swapping it out to another channel indeed did cause the problem to shift to the new channel (Q3 with Q4) I will post a few pics below.. The scope just showed a mild hazy disturbance with a good channel and a definite sawtooth looking wave noise out of the bad area.. Does anyone out there know a cross-referenced modern part to replace the bad transistor with? I haven't been able to find anything with the numbers i have. The schematic says all transistors are 001-021-261 but the number on the transistors is 21-261-348..
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Those are likely Hammond part numbers which will not be cross-referenced.Originally posted by PaulStahl View PostComment
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