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42-321 reception - Printable Version

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42-321 reception - willydcamaro - 04-14-2019

Good Morning everyone ! Working on a 42-321. I've replaced the field coil speaker with a PM, adding a 500 Ohm resistor and increasing electrolytics with a 47 uf and a 100 uf . Reception is very bad but when turning off power to the radio on the power strip, as I do this the reception comes in loud and clear, as power is draining.. Any idea why? Thanks,Bill

RE: 42-321 reception - Radioroslyn - 04-14-2019

Sounds like you are getting noise from the ac line. Can be caused by many of these modern electronic devises.

RE: 42-321 reception - willydcamaro - 04-14-2019

Thanks Terry !!!

RE: 42-321 reception - Jake Blake - 04-14-2019

Terry,

I remember you saying something to me about changing filter cap values to adjust B+ voltages a while back...... would his changes to higher values increased his B+ , maybe to the point of affecting reception?. . .then upon his removing power, and the levels slowly dropping , he hears good reception briefly while they are within spec.?

RE: 42-321 reception - willydcamaro - 04-14-2019

Makes sense Jake. Thank you !

RE: 42-321 reception - Radioroslyn - 04-14-2019

Hi Jake,
It goes something like this. On set that uses a lot of inductance and uses a relatively small value input cap (like 1 or 2 mfds) if the increase the value of the input cap to 4 or 8mfds it will raise the output voltage of the power supply.Conversely you can lower the size in the input cap and it lower the output voltage. Now we are talking something in the order of 20% or so and you may have to increase the value of the output cap to eliminate any residual hum. You'll see these types of p/s circuits in set from the late '20's- the early '30's. Back then inductance was cheap and the technology was quite there to make caps more than a few mfd's (foil paper) with out them being very large physically speaking. By abt '31 or so electrolytic cap where coming on the seen which for there size packed a lot of mfd's.

Once you've increased the input cap to abt 15mfd there is not much of a voltage advantage by increasing the value short of reducing hum. It's also important that the input cap's value doesn't exceeded the listed rating that can be used for a given rectifier tube. Generally this abt 40 mfds. The reason for this is that the filter cap looks like a dead short to the rectifier tube till it starts to charge up from the voltage it's receiving from the rectifier tube filament/cathode. So for a second or two there is a lot of current flow between the two. The more mfd's the long it takes to charge so longer this higher current is flowing and it can damage the cathode/filament of the rectifier tube.

Hope this answers your questions.