All, I just helped a friend rebuild a 48-475. It sounds great now but it has a couple of minor issues. When first turned on it sounds like a Jet aircraft engine starting up. When turned on while cold you hear a low frequency whine that keeps increasing with frequency until you can no longer hear it. This only happens during the first minute or two of startup. My friend played it all day and he says there is also a mechanical hum coming from chassis and not the speaker. Could these problems be related?
Thanks,
Keith
You could be having shielding problems. Make sure your RF shield cans are properly grounded. Also, a small capacitance mica cap in the RF section may be going defective.
That mechanical hum could be either the power transformer is not tightened down onto the chassis or the laminations from within the power transformer are becoming loose. I've solved the latter problem by heating the transformer up and in doing so this warms up the varnish to a point that it gets soft enough for the laminations to "restick" themselves together. Makes sure that the binding bolts are good and tight.
Keith, I don't see the schematic listed on N/A, does the radio have FM and if so is the problem only on FM?
Jerry
(11-07-2016, 04:48 PM)jerryhawthorne Wrote: [ -> ]Keith, I don't see the schematic listed on N/A, does the radio have FM and if so is the problem only on FM?
Jerry
Yes, the radio has FM and the startup whine can be heard in both the FM and AM bands. All of the paper and electrolytic caps were replaced.
Keith
I had my friend power the radio up on a variac slowly and the noise went away. So, I'm thinking I could have a component breaking down under startup conditions. Would this be a good application for a CL-90 inrush current limiter?
Thanks,
Keith