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Topic: Standel Custom 15, hum problem |
Ken Fox
From:
Nashville GA USA
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Posted 19 May 2001 7:04 pm |
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| I'm working on a 1967 Custom 15. It had a pretty bad 120 cycle hum. I changed the filters and revised the power cord to a grounded cord as well as rewiring to have AC hot thru the fuse at all times. The hum didn't get much better. I added another 2200 to the +35 volt rail and the hum reduced quite a bit. Adding any more filtering to the negative rail makes no difference. Standby current from the 120VAC side is about .21 amp. My question is, are these amps normally noisy and would a power supply choke help? Any other suggestions appreciated. I've got a schematic on the way, courtesy of Danny McKinney, new owner at Standel (a really great guy!!). Another note, one of the preamp modules(on the reverb channel) has been replaced with a homebrew circuit board. That channels a little hotter than the other, can't turn the volume all the way down either. |
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Keith Hilton
From:
248 Laurel Road Ozark, Missouri 65721
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Posted 19 May 2001 8:33 pm
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| Some of the parts may be defective because of age. The bypass capacitors can cause just as much hum as the main filter capacitors. Check the the grounding of the homebrew addition. Different ground potentials can exist on the board unless everything is run to a single point. Single point grounding, or so called "Star" grounding is the best, in my opinion. Different ground potentials on a circuit board causes ground loops on the board. This is a wild guess, but I would say it is probably in old parts, bypass capacitors, or faulty grounding on the board in relation to case or power supply ground. Sometimes the worst thing you can do is ground the circuit to the power or earth ground. Check on relation of case and circuit board ground, to power ground. The "most" frustrating thing in the world is to get a ground loop in the wiring of a circuit itself. Be advised that is very easy to do, especially if there has been changes or additions to the circuit! Just my opinion. |
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Blake Hawkins
From:
Florida
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Posted 20 May 2001 9:16 am
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Ken, Check your rectifiers. If they are solid state, it is possible one of them has developed some leakage and could be passing some AC.
I'd be really careful about that "homebrew" addition. A poorly designed modification could introduce hum. I've seen that several times.
Blake[This message was edited by Blake Hawkins on 20 May 2001 at 10:16 AM.] |
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Ken Fox
From:
Nashville GA USA
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Posted 20 May 2001 10:55 am
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| Thanks for the suggestions. I re-routed some of the audio groud wires for home-brew addition. That didn't change anything, but it was a good thing to do. I went all over to makes sure all grounds were at the same potenetial. After checking the .1 volt P-P ripple at the speakers with my scope, I noticed that it was in fact from the -35 VDC supply! I located a filter (250mf) that was after a dropping resistor. I temporally added 350 mf to that point. The hum went way down. I replaced it with a total of 700 mf of filtering. Its down to about .05 P-P now. I previously replaced the bridge with a 25 amp version. It seem to handle the extra caps OK, in-rush is about .32 amps on my Fluke 77. Looks like the original 1.5 amp fuse will still suffice. It's not perfect, but a whole lot better now! By the way, the output of the preamp stages to the power amp looked really clean on the scope. Thanks again for the support! |
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Dana Duplan
From:
Ramona, CA
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Posted 21 May 2001 5:33 am
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I have 3 Standels (2 early 60's models and a new reissue), and my friend has a later 60's model. They are all studio quiet--something's going on with yours.
Good luck
DD |
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Ken Fox
From:
Nashville GA USA
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Posted 21 May 2001 5:13 pm
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| Your early 60's would be tube amps? Is the later 60's a solid state amp? The remaining noise seems to be in the power amp section of the amp. It's really a clean signal from the preamp stages (on my scope). If it has a problem that crops up, I'll have a schematic by then and we'll go another round! Thanks for all of the helpful comments from all of you. |
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Dana Duplan
From:
Ramona, CA
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Posted 22 May 2001 3:48 pm
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The early sixties amps I have are hybrids--solid state preamp (but with one preamp tube), and tube power amp. They were built ca '61, '62. Shortly thereafter went to all solid state. I have played several all solid state models and they were quiet. Could maybe be a faulty component in one of the once replaceable modules?
DD |
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Ken Fox
From:
Nashville GA USA
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Posted 22 May 2001 7:06 pm
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| The out put from all the preamp sections lookes really clean on the scope. The amp is back with the owner now. I just got a schematic today!! I'll probably take another look at it soon. The amount of noise is relatively small at this point. There is some background white noise at a lower level from the preamp stage (the #1 input module that was "homebrewed is the source". I talked to Danny MacKinney, at Stadel. He referred me to an "Old pro" that works on these Standel amps in Nashville. I wish I could remember his name! He is going to start building replacement modules for these old amps!! Anybody interested could call Standel, I'm sure Danny will give his number to you. Danny is very interested in developing a steel guitar amp. I've encouraged him to visit some of the steel guitar shows. Look for him to show up in St Louis this year! |
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