hopjohn
Silver Face
Replacing the "center voltage" trimmers on the KA-9100.
Bias is fixed on the 9100 so there is no adjustment required for it. The trimmers on these amp boards are stated to adjust the "center voltage" in the service manual, but are really nothing more than an offset adjustment. By measuring at the speaker terminals, this should be set as close as possible to 0V.
The offset was checked and set for each channel before any work on this amplifier took place. That's standard procedure anytime I receive a new item. The OEM trimmers VRe1 and VRe2 are Alps 470 Ohm single turn common in late 70's amps, receivers, tuners, etc. These are replaced for increased reliability and precision by sealed 500ohm multi-turn trimmers. (Replacement 470ohm trimmers are typically only available as single turn parts.)
Photographing or marking the position of the original trimmer once set is helpful in keeping the setting during the removal process. This is important because the setting will be duplicated by the replacement part to help keep the offset close to an ideal setting. This is even more critical when a bias trimmer is being replaced in models where that is relevant.
The center pin of the trimmer should be identified.
Using a multi-meter the resistance should be measured across the center pin and either the left or right pin and the value recorded.
The replacement trimmer should then be adjusted to match the resistance of the original trimmer measuring across the same pins as before.
The pins of the new trimmer need to be formed for proper fit.
An alligator clip is helpful for holding the trimmer in place for soldering.
With the new trimmers installed, and the amps / heatsinks reinstalled the mini grabber leads of the mult-imeter should be placed across the speaker terminals of the appropriate channel (It's easiest to do this from the interior). Volume should be set to minimum, with no speakers connected, the tone controls defeated, and the proper speaker selection set. After powering on, a plastic adjustment tool should be used to set the trimmer for an offset of 0 mV in each channel.
Bias is fixed on the 9100 so there is no adjustment required for it. The trimmers on these amp boards are stated to adjust the "center voltage" in the service manual, but are really nothing more than an offset adjustment. By measuring at the speaker terminals, this should be set as close as possible to 0V.
The offset was checked and set for each channel before any work on this amplifier took place. That's standard procedure anytime I receive a new item. The OEM trimmers VRe1 and VRe2 are Alps 470 Ohm single turn common in late 70's amps, receivers, tuners, etc. These are replaced for increased reliability and precision by sealed 500ohm multi-turn trimmers. (Replacement 470ohm trimmers are typically only available as single turn parts.)
Photographing or marking the position of the original trimmer once set is helpful in keeping the setting during the removal process. This is important because the setting will be duplicated by the replacement part to help keep the offset close to an ideal setting. This is even more critical when a bias trimmer is being replaced in models where that is relevant.
The center pin of the trimmer should be identified.
Using a multi-meter the resistance should be measured across the center pin and either the left or right pin and the value recorded.
The replacement trimmer should then be adjusted to match the resistance of the original trimmer measuring across the same pins as before.
The pins of the new trimmer need to be formed for proper fit.
An alligator clip is helpful for holding the trimmer in place for soldering.
With the new trimmers installed, and the amps / heatsinks reinstalled the mini grabber leads of the mult-imeter should be placed across the speaker terminals of the appropriate channel (It's easiest to do this from the interior). Volume should be set to minimum, with no speakers connected, the tone controls defeated, and the proper speaker selection set. After powering on, a plastic adjustment tool should be used to set the trimmer for an offset of 0 mV in each channel.
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