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Bias adjustment on an old hohner Orgaphon
Bias adjustment on an old hohner Orgaphon
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Old 7th November 2019, 08:42 PM   #1
Rose yeld is offline Rose yeld
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Join Date: Nov 2019
Default Bias adjustment on an old hohner Orgaphon

Hello guys
I do appreciate this forum and i m looking for some help on an Orgaphon 33. My amp is in a very nice condition. But there is an hum which increase with the volume. I also catch a radio chanel on the third Chanel.
Some one told me i have to make some bias adjustment.
What do you think ?
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Old 7th November 2019, 09:01 PM   #2
Enzo is online now Enzo  United States
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Join Date: Sep 2004
Location: Lansing, Michigan
Hum increase with volume, and radio have nothing to do with bias.

Radio stations usually mean a poor ground at the input. Hum can also mean the same thing. Loosen the jack nuts, move them around a little in the hole, especially move any star washers to a slightly different position, then snug up the nuts again. ANy help? We were trying to refresh the surface contact between the jack and the chassis.
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Old 9th November 2019, 06:09 PM   #3
Rose yeld is offline Rose yeld
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Join Date: Nov 2019
Hello Enzo
Thanks very much for that i ve cleaned my ground with a contact spray (because it is impossible to access without dismount Numbers of components). Now it is great but at hight volume i ve got an ugly sound of cracking. Particularly in the low frequency with a guitar.
The chassis is non screwed on the amp as i m making tests.
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Old 10th November 2019, 10:06 AM   #4
Thomasha is offline Thomasha
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Join Date: Jun 2012
Hi, I worked on an concertophon some time ago, and it also had a loud hum. Is yours one of those Hohner amps with two chassis, one for preamp and one for the power amp? Mine hada a bad connection between chassis.

Another thing is that some of those came with that plastic DIN input connectors. The first thing one would do is change them to normal jacks, but then you have ground loops between inputs. In this case,you just want one of the jacks to touch the chassis.

The cracking is probably a bad coupling capacitor or resistor. Check if you have any paper capacitor or the brown WIMAs. They go bad after some years. The bad resistors are hard to spot. Unless they used some really bad ones, was what not the case in my amp.
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Old 10th November 2019, 12:18 PM   #5
Rose yeld is offline Rose yeld
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Join Date: Nov 2019
Hello
Yes exactly i ve got Two ch‚ssis.
I send you pictures
Orgaphon 33 - Google Photos
Tell me if it works.
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