EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

I am trying to help a friend who is working on a Philharmonic AM FM model. There are two bleeder resistors shown on the schematic of the tuner chassis. One has +250V on one end and multiple taps. The other has 17V on one end and multiple taps. Both have one end grounded. One of them may be the R1 listed as being a 125 ohm 10W resistor. The other one does not appear to be listed anywhere. Have any of you measured the resistances of the sections of both resistors and their total end-to-end values?

In the set being worked on, the defective resistor is in the circuitry where one end is connected to +250VDC. This resistor has no continuity to any of its connections end-to-end or either end to any of the taps. My friend says that he is afraid to dig into the insulation of the resistor as it is so tough and brittle he is afraid that he will shatter it if he forces scraping or probing into the tap area. The way he described the part, it sounds as if the connections are depressed or sunken into the surface of the ceramic body in such a way as to defy attempts to expose wire, scrape clean and resolder (if it is indeed a wire-wound type as we suspect). He said that just looking at the part it appears it could be a 20Watt resistor.

Joe

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The Clarostat B+ divider is 5K 50Watt. Most folks use a Ohmite adjustable with 3 slide taps. Each tap is adjusted to provide taps of 150, 125, and 100 Volts. The originals in all of the sets I have restored were ether open or leaking to ground making a racket on the output audio. If you look at my photos on this site I have many under chassis pics on restored scott's using this resistor.  Jeff

My AM/FM Philly had 2 taps not 3.  I took hundreds of underside photos so if you need them let me know.

Jeffrey;

Thanks for this information!  Do you know if the 50Watt resistor is considered to be R1?  On the schematic from the Information Archive it has no R designation at all. Or is the resistor shown with multiple taps connected to +17VDC considered to be R1 since it is a 10Watt resistor instead of a 50Watt resistor?

In the sets that have a bleeder resistor with only two taps is there a separate resistor that handles power to the distribution line(s) of the schematic showing a resistor with three taps? Or, which voltage line was deleted?

I note that there is not a separate schematic in the Information Archive for the LW/AM/SW only models. I imagine that Scott did produce such a schematic for model versions without FM, but they did not survive to posterity.

I have started modifying the existing schematic from the Information Archive to simplify details in it and show vacuum tubes in more modern format to make work easier. This is a considerable task. Removal of the arc jumpers across other lines being crossed at right angles reduces a lot of clutter in the schematic. I am surprised that Riders did not modernize the schematics from Scott when they produced the versions in their documentation. There is a tendency to avoid extra work - we are often guilty of that.

The schematic that I have shows 2 taps on the B+ divider, 125V and 100VDC. The other clarostat divider is the bias divider. The top of the bias divider has -27VDC and is mounted adjacent to the B+ divider. I have not found it necessary to replace one so I can't help you there. I have an original scott schematic and there are no component numbers on it.

Jeff;

Thanks for that additional information about the bias divider. On the schematic from the Information Archive the hand written voltages had the maximum voltage looking like positive 17V, since the negative symbol was shown following the voltage notation instead of preceding it. The version schematic I am looking at is dated April 4, 1939. If the bias resistor is R1 then its value end to end is 125 ohms and the schematic parts list says it is a 10W resistor.

The total current through the 5K bleeder resistor should be .050A which would indicate that the resistor dissipates 12.5 watts of power as heat. A 20W resistor would probably be OK, but a 50W resistor should be operating pretty cool and not be stressed at all.

Joe

This is from the Riders Manual regarding the Bias Divider.

Green -28VDC 260 Ohms to GND

Green Black Tracer -15VDC 148 Ohms to GND

Green Red Tracer -6VDC 60 Ohms to GND

Red Yellow Tracer -4.5VDC 50 Ohms to GND

Black Green Tracer -3VDC  40 Ohms to GND

The resistors were encased in Bakelite before being installed in the metal jacket.  Ohms law would put the power dissipated by the bias resistor at 3 watts that's why so few failures in that puppy.

Jeff;

Thanks for the information on the bias divider! It makes sense that it would last so well with so little dissipation of heat. Heat tends to accelerate corrosion of metals and the ones used on power resistors fall subject to that. The lower the heat, the less problems develop.

For two days I have been trying to call my friend and pass on information, but am not getting any answers to my calls. He is 92 years old now and I hope he is not lying on his floor. I am trying to find out the phone number of his son so I can get a wellness check done for him.

Joe

I have another question. What is the value of the resistor that goes to pin 4 of the 6B8G RF AGC tube? Is it a 1K ohm? Or is it an 800 ohm resistor? The schematic notation is so blurry at that I cannot tell if it says R8 or R9.

Thanks in advance.

My friend turned out to be OK. It just happened that he was unable to answer both times I called. The last time he was in his dentist's chair and could not speak.

Joe

Pin 4 and pin 5 should be tied together, These are the diode sections that generate the -DC for control bias for the IF stages. The diodes are fed through a tunable coil and exits the can through a 10K resistor. The screen has a 1K and the Cathode a 600 Ohm. The IF Bias should be -11 Volts no signal and up to -30 Volts with strong local stations. I did spread sheets on both the RF and IF on 3 sets and that these voltages seem typical. It has been my experience that before a set can be considered reliable all paper caps including the multi section in the rectangular  metal can has to be replaced with plastic. The B+ divider is usually bad and smoked because the set was run with original leaky paper caps. The IF B+ feeds passing through the chassis 1K are sometimes smoked for the same reason.  Took me about 1 week to 10 days to restore a set and be reliable with out rumbles and cracking coming out of that Big old Jensen.

Joseph W Strickland said:

I have another question. What is the value of the resistor that goes to pin 4 of the 6B8G RF AGC tube? Is it a 1K ohm? Or is it an 800 ohm resistor? The schematic notation is so blurry at that I cannot tell if it says R8 or R9.

Thanks in advance.

My friend turned out to be OK. It just happened that he was unable to answer both times I called. The last time he was in his dentist's chair and could not speak.

Joe

Replacement for the B+ divider resistor is Ohmite D50K5K0E resistor with Ohmite 2121E taps and 9E-100 mounting tabs.  

Jeff;

I looked at your comment about pins 4 and 5 being tied together and realized while looking at the schematic that what I thought was a pin 4 numeral is actually not (or should not be) a 4. There is an internal lead of the tube shown that has an exit point to the right of the cathode. There is a character enclosed in a circle just outside the glass envelope that looks just like a number 4 with the center of the top part filled in and I interpreted it as a number 4 pin, but that cannot be true since the real pin 4 is shown on the left side of the tube diagram as drawn. There on the left is clearly seen pin 4 tied to pin 5. There is a grid entering the top right of the diagram of the tube that has no number associated with it, but it has to be pin 6 which is the screen grid.  There are two grids shown on either side of the screen grid in the diagram which exit the diagram as I described to the right of the cathode. Those would have to be the suppressor grid and the RCA tube manual I have, RC-15, shows that those are connected to the cathode internally and do not exit separate from the cathode. The diagram as drawn on the schematic is just wrong! It shows an external capacitor of type C8 between the suppressor grid and the cathode which is illustrated as connected to pin 8. It would make no sense to have a capacitor being shorted out if it actually had an external pin. The only pin not having an internal connection on the 6B8G is pin 1 and it is not shown on the schematic diagram for this RF AGC tube. The diagram of the tube should have been drawn with the internal connection shown between the cathode and the suppressor grid and no lead from the suppressor grid coming out separately with a character in a circle near it. Whoever drew that tube diagram in the schematic must have been drunk the night before.

Joe

Attachments:

R9 is a 1K.. The diagram has 2 pin 4's. My factory Schematic has no pin numbers or component designations.

It does have component values.

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