800 B Speaker Question "Missing 9K Resistor"

[Joe Miller]

I'm restoring an 800 B for a friend and found an issue with the speaker wiring.

The schematic shows two resistors mounted on the speaker and connected to pins 2, 4, and 6 of Plug P4. Here's the schematic (see Scott page 39-40):

http://www.nostalgiaair.org/PagesByMode
... 016596.pdf

 

My speaker has the 675 ohm resistor mounted but no 9000 ohm resistor is there. The 9000 ohm resistor connects to pin 6. There is no connection on pin 6 and there never has been (its clean) The purpose of the resistor is to provide a shunt path to ground for B+ for some reason. I assume this set worked in the past and I assume this is the original coaxial speaker with the tweeter installed on the inside of the bass speaker. (It's a Jensen, PM speaker)

Can anyone solve this mystery? Was there a mod that Scott made to these sets that I can't seem to locate?

Any insights would
be appreciated.

[David C. Poland]

Hi Joe.
Yes -I noticed that on the higher ser # 800-B with PM speaker I restored a couple years ago. Radio worked well and sounded ver good after extensive recap, etc ... restoration.

A dark blue-green color 15 inch Jensen coax (PM or field) is the most frequently seen speaker, but there was a Stephens speaker sometimes, and maybe other variations. I speculate the 9000 ohm delete was a running change, perhaps when Scott Labs changed to the PM l version 15 inch coax speaker with the power resistor under the bell cover. I also believe the change was backwards compatible. The early and later Jensen coax speakers look identical but the later PM speaker had a plastic bell cover, painted the same color as the earlier metal bell cover.

Really complicated circuit diagram. I ended up using fine point markers to color code the B+, and 115 volt, and several circuits to better understand the radio. Be aware that certain control positions are required in order for the AM/FM relay switch to function. And that the tweeter switches off on AM and SW.

Have fun.

[Joe Miller]

Hi David,

Thanks for the info.  Given what you say, I don't plan to install the missing 9k resistor.  With it installed, the voltage to the 6L6 screen grids will be reduced (by about 5%), but it will add some heat to the chassis.

Do you agree with leaving the 9K out of the circuit?

Joe
 

[David C. Poland]

Joe,

I saw you put your question out on ARF too.

I left that circuit as I found it. Like you, I found no evidence the Jones plug pin #6 had ever been used, in spite of the diagram info.
I am accustomed to encountering small revisions during Scott production runs. After you have the radio running nicely, you could revisit the issue and set voltages and then decide what to do. Perhaps Scot Labs decided the radio performed better without it, or decided to save a fews cents.

What with today's higher line voltages, you could also use an inrush limiter (thermistor) to reduce the line voltage to the radio ahead of the power transformer by a couple volts and give it a softer start. This little device gets hot and needs some space for heat dissipation. Recently, I have been using the CL-90 (2 amp) and CL-80 (3 Amp) from Mouser in restorations and will eventually add them to older Scott and other restorations in my collection.

[Joe Miller]

David,

I think I will leave the circuit as-is until I get  it recapped, etc.  I'll power it up to see what I have as you suggest.  My thought is similar to yours--since pin #6 on the speaker plug has never been used, it appears Scott intentionally deleted the 9000 ohm resistor.  If someone later uses a speaker with a split field coil, that speaker's plug will have the 9000 ohm portion of the tapped FC connected to pin 6 and the 9000 ohm voltage divider will be back in the circuit, so it appears there shouldn't be a problem?

Joe