12-31-2014, 06:08 PM
(12-31-2014, 05:19 PM)John R Wrote:(12-31-2014, 09:03 AM)Diverted Wrote: Hi again. This is an edit to a previous response I had about the filament not having continuity. Well it does; I pulled the cap off the rectifier and jiggled the socket and got ohms! So a little remedial soldering is in order.
MOVING ON to other things, my next question has to do with candohms and one particular resistor that appears to have been swapped out.
On the schematic, resistor 82 is listed as a 5K wirewound. On the chassis, it's a candohm-style resistor bolted to the chassis. it runs between the field coil and the 3mfd electroytic to ground. Looking last night, I noticed that the field coil side of the resistor was disconnected. ?? Ohmed it out and it read many megs. OPEN. So I looked around and the only other likely replacement resistor I could find was an aftermarket Ohmite 15K, 10W tied in on one end to the middle lug on bakelite capacitor number 58. The other end goes to ground. That middle is used as a tie in post to the field coil, so I'm thinking this resistor is a replacement and whoever put it in just found that to be a convenient spot to tie it in.
But it's a 15K! Three times what's called for on the schematic. Obviously I'm going to replace it with a five. But how many watts should I use?
Similar question for the five section candohm (part 82). Several sections are quite high so I'm going to replace it. But I don't know what wattages to use, and space is at a premium. Anyone have any idea what wattages I should use for each section? Thank you!
http://www.nostalgiaair.org/Resources/222/M0013222.htm
Ted,
Believe the 4 section candohm is 196+18+22+22 = 258 ohms total used for the (-) negative control grid biasing of the audio tubes, etc. Use a 200 ohm to replace the 196 ohn section. Will make very little difference. The schematic and associated info. from Nastalgia Air shows about -33VDC developed on the C.T. of the high voltage transfomer feeding the candohm at 115VAC in. So say at 120VAC may be worst case -35VDC. Since this is a series string resistor the current to ground is common thru the four sections. Once you know the I. you can calculate the power rating of the individual resistors...P=IxIxR. I=E/R. I = 35/(200+18+22+22)= ~.134 amps. or 134 mamps.
For the 200 ohm section...P= (.134)(.134)(200)= ~3.6 watts. I would use a 200 ohm, 10 watt resistor. You can calculate the power rating needed for the other three sections. The result would indicate that 1 watt resistors should
be sufficient.
R82 is a 5600 ohm bleeder resistor in parallel with the 3 ufd filter cap. on the B+ buss to the driver tubes, 6J5's. The
schematic data shows about 235vdc at 115VAC into the set. I would think that the value of this resistor is not critical as it looks to me to mainly be used for discharging the B+, and to minimize pops, at turn on and turn off of the set.
The B+ is primarily determined by the filter choke, capacitors, field coil in the power supply circuit.
Say you get 245vdc on this B+ buss at 120VAC. The power dissipated by R82 would be P=ExE/R = (245x245)/5600= ~10.7 watts. Holy Toledo no wonder in burnt out . A 5600 ohm resistor here would require at least a 20 watt rating. I can see why someone put in a 15K resistor which I feel would have essentially no effect on the circuit operation. A 15K would limit the current rating needed to 10 watts. If you wish to stay with original values then a 20-25 watt resistor is needed. I seems that R82 accounts for slightly less than 10% of the power dissipated by the entire set if my calculations are correct. Good luck, John
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