View Full Version : Motorola 7" dropping resistors - what value?


kramden66
03-01-2017, 01:28 PM
Replaced the seleniums with 1n4007 and was wondering where the resistors or resistor should be placed , what value and wattage
Thanks
Mike

timmy
03-01-2017, 02:12 PM
Don't make much difference voltage only goes up a few anyway all my sets don't have any resistors just 1n4007, don't need them. I'm working on one now without the resistors.

Electronic M
03-01-2017, 02:23 PM
I'd compare B+ to schematic. If there is less than a 20% increase the droppers won't make an appreciable difference in longevity or performance. I skipped droppers in my TS-4 Moto too.

If you still decide that you need to add a dropper take that B+ measurement and figure how many volts you need to shave off, measure the B+ current off the highest rail past the rectifiers and before the divider resistors for the lower B+ rails (the best place to put such a resistor). Use ohm's law (R=E/I) to calculate the resistance and multiply voltage and current to get your wattage (P=IE ...Mmm pie :D ) rating (it is wise to ~double that number so your new dropper resistor is not too hot to touch in operation).

timmy
03-01-2017, 03:13 PM
I found after warm up with diodes in these sets the voltage was up around 4-5 volts from what the spec is, 150 b+ 250 b++ without resistors.

EdKozk2
03-02-2017, 12:16 AM
Mike,
Are you using a NOS ballast? For the B+ right after the rectifier I calculated a 9 to 10 volt drop was needed. The household AC gets as high as 124 volts. I did this for a TS-4C chassis.
Ed

dieseljeep
03-02-2017, 11:11 AM
I found after warm up with diodes in these sets the voltage was up around 4-5 volts from what the spec is, 150 b+ 250 b++ without resistors.
I just stuck diodes in mine. It's my opinion, that set can use all the help it can get. :scratch2: