Granco Radio

[William]

I have a Granco radio that a friend gave me a couple of years ago. It was not working so I went thru it replacing all the caps and the resistors that needed replacing. It plays great on radio, but the phono input is another matter. I hoping one of you might have an answer for me.

If I plug a record changer into the phono input it plays great, no problem. If I plug my laptop in, it has a hum and clicking sound? The laptop works just fine in other things I have plugged it into so I know that is ok. I was going to use the Granco input to have better sound while I'm watching the laptop while I walk on my treadmill. Nope, it does not work for some reason? Why would it work with a record changer but not the laptop? Any ideas?

Bill

It looks like this:


https://www.pinterest.com/pin/387098530443776102/

[TC Chris]

Is it an AC-DC model, or does it use a power transformer? If AC-DC, try reversing the plug? Or maybe find an isolation transformer for the AC line, or for the aux input.

Chris Campbell

[electra225]

What Chris said. You have AC hum getting into the audio. Run the radio on your isolation transformer when you have your device plugged into it. Granco was the Muntz of radios. Minimalist parts.

[William]

It does not have a PT and is not AC-DC. This was one of their more deluxe models so built more like a normal AA5 radio. This one has six tubes and is actually a robust performer.

To be clear, is the lap-top putting AC hum into the radio? If so, why does it not do that to everything else I own that I have plugged it into? Thinking about what I just said, they may have all had PT's.

Bill

[electra225]

If it doesn't have a power transformer, it is what is called a "Hot chassis" set. The chassis is connected directly across the power line. There may be positive voltage on the input jack on the radio chassis. The hum is created because your laptop isn't happy eating positive voltage. You have a Variac, which has a transformer in it. Use your Variac as an isolation transformer to power the radio. See then if your hum disappears. A Variac is a variable voltage power transformer.

[TC Chris]

But is a Variac an autotransformer, and if so, does it really isolate?

Chris Campbell

[William]

Thanks guys for setting me straight. I never thought about a none PT powered device doing that. I plugged the laptop into my RCA 1R81 (PT powered) radio and it worked fine.

Bill

[Firedome]

A Variac (General Radio's trade name) is a variable autotransformer but is not an isolation transformer... beware.
Hot, ground and neutral must be totally isolated between input and output.
There are some variable autotransformers that do provide this extra feature, but most do not.

[electra225]

My iso/Variac/dim bulb contraption is home built by my second elmer, Lloyd. Mr Test Equipment himself. My Variac runs thru the iso, so mine does isolate. I was under the impression that Bill's variac was similarly connect, which is not the case. My bad. Sorry for any confusion.