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On Friday, July 27, 2001 at 2:34:43 PM UTC-5, MitchSF wrote:
Hi,
I'm almost finished restoring a KLH Model 21. I replaced three transistors
on the RF board and it now works!

The only problem left is a hum from the audio board. I disconnected it from
the input to the board so I'm sure it's not coming from anywhere else. I
replaced the filter capacitor and all electrolytics on the audio board and
it's a little quieter, but still there. The level of the hum doesn't change
with the volume control.

Any hints would be appreciated!

Thanks,
Mitch

Hi Mitch. The power supply filters, either of the black color are/were a common failure item. They should be replaced. However, note that the positive terminal was marked with the stripe as we would now associate with the negative terminal. It is also important that you do not increase the value of these capacitors as the power supply can't handle the extra in-rush and operational paramete5rs. Mark - Former National Service Manager, KLH Cambridge..