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Im currently working on a custom build based off a princeton reverb but with 7591's. I finished the layout and had a chance to fire it up yesterday. Most everything went smoothly, except I had a lot of noise that sounded like dirty DC. I was probing around the PI output and noticed I had 170VDC at the output of one of the PI caps. I suspected a leaky cap(M150), but it wasnt that. After trying everything, I still could not isolate the problem. I completely removed the PI circuit, and after doing so I noticed there is voltage being applied to several naked turrets that are not connected to anything. What I noticed is that the turrets located closest to the PI B+ node had upwards of 200VDC, and the few turrets further away had less and less with about 50VDC at the furthest one closest to a ground connection. When I disconnect the B+ the voltage goes away. I checked for any sort of continuity and found no trace that could lead to this problem. Any ideas where to look?
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What is the base material for your eyelet board? Moisture absorbson and soldering flux can cause problems like this. I would suggest scrubbing the board with an alcohol based flux cleaner and an old tooth brush. Then drying the board with a hair dryer.
You won't see leakage like this with a 2 Meg ohm meter range. I found a DVM at Radio Shack that has a 2000 Megohm range. It finds most leakage. A guy that used to work for me used an old Triplett 630-NA 20,000 ohms/volt VOM. It used a 30V battery for the ohm meter. If you saw any meter deflection at all on the Ohms X100K range, there was leakage.WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
REMEMBER: Everybody knows that smokin' ain't allowed in school ! -
As LT said. But if it's one of those old or retro black fiber things just throw it out. There's no spec for the carbon content on that stuff and I've seen too many black fiber boards that were conductive. I wouldn't take a lifetime supply for free. Too much work replacing them."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." HelmholtzComment
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Well I figured out the problem. The turret strip in question was definitely conductive/leaking. There was about 2.8-3M resistance between various turrets, which was resulting in the voltage bleed. Although it was frustrating, I was able to desolder the component leads on that side and install a new strip. Everything is functioning perfectly as I initially expected. I had never encountered something like that, and Im glad to have gained that experience!Comment
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I'm glad it was only a terminal strip and not an entire board. Just for posterity, what was the nature of the insulator?"Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." HelmholtzComment
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