SCOTT 16 HUM PROBLEM

[Dave McClellan]

I started a new thread for this problem since a previous thread *SCOTT 16 CAPACITOR QUESTION" had drifted off of the original question, which was answered. The radio works, but there is a strong 60hZ hum. I discovered that the B+ at the receiver chassis measures 231.9 VDC but also 1.09VAC at 60Hz. At the power supply chassis B+ measures only 0.08 VAC. The DC resistance between the power and receiver chassis measures 0-0.1 ohm, but when operating there is an AC drop of 1.118 VAC at 60Hz between the two chassis. The power cable is warm. I realize that it is carrying the filament current so that is probably normal. The 1.09 VAC appears at the phase inverter plate, and is thus amplified - thus the hum. The power cable shield connections look secure on both ends, but both depend on a rivet to the chassis. I am not sure where to go from here.

[David C. Poland]

Some hum problems trace back to the 1st audio in some models, including the 6K7-G in the Sixteen.

Illusive hum problems are a trial for sure.

The low 60 cycle ac appears only at the inverter plate and no other tube plates?

Cathode to filament leak in that tube under load?

Is the hum present regardless of bass control setting?

Hum disappears with the inverter tube pulled out?

Hum disappear if either driver 6J5 is pulled?

You have probably already tried different tubes in the inverter socket and the 1st audio socket.

Try bridging the several riveted ground connections to ground with heavier gage short patch cords.

Reflow the first B+ cable connection in the receiver.  Reflow the first ground wire  connection in the receiver. 

[Dave McClellan]

Thanks for your response.

Some hum problems trace back to the 1st audio in some models, including the 6K7-G in the Sixteen.

The hum is there with the 6K7 removed, or its output disconnected from the inverter, or even if the inverter grid is grounded.

The low 60 cycle ac appears only at the inverter plate and no other tube plates?

It appears at the top of the voltage divider resistor (direct B+ to the power supply) The inverter plate supply is the ONLY audio tube that gets the direct B+ The 6K7 first AF gets B+ through a filter, 50K and 0.5mfd

Is the hum present regardless of bass control setting?

The hum is present at ZERO volume. The tone controls have no effect.
Hum disappears with the inverter tube pulled out? Yes
Hum disappear if either driver 6J5 is pulled?

It disappears only when the 6J5 driver that is connected to the inverter plate is pulled. If the 6J5 drive connected to the inverter cathode is pulled, no effect on hum. The inverter plate and B+ feed have a 1+ volt AC component.

You have probably already tried different tubes in the inverter socket and the 1st audio socket.

All three 6J5s have been swapped and substituted. No effect.
As you suggested, I will next try bridging the B+ and ground leads between the radio and power chassis.
Thanks for your input. I have NO experience with Scott radios.

[David C. Poland]

There should be 5K resistor between the top of the voltage divider and the inverter plate.

Also poke around the bass resonator circuit ahead of the inverter.

Using 6J5-G tubes? or metal or GT.

Anyone?

[Dave McClellan]

Well, I made a rookie mistake. I apologize to the Forum for wasting your time. When I removed the tuner shield to inspect for any original capacitors left (there was one), the shield pinched a pilot lamp lead when reinstalled. In this radio, the pilot lamp leads were connected to the back of the eye tube socket. The eye tube cable had been replaced by a relatively thin multi-conductor shielded cable. The result was a low resistance short (but not a dead short) of one pilot lamp lead to the chassis. The resulting high current draw resulted in the receiver chassis being elevated above ground. Removing the short solved all problems. The radio works well with no hum. I still need to replace the circa 1985 filter capacitors and align the set. Again, sorry for the trouble.

All of the tubes are G type currently. But someone did use metal tubes in the past, since several tube shield bases had been wrecked. I did change the inverter plate and cathode resistors so that they were close in value. I will also check the match of the 6J5 drivers and output tubes. Currently the hum balance pot is sitting on one end of the  element.

[Norman S Braithwaite]

Dave, you are not the first to encounter the dial light short problem!  At least in my case it was a full short and quickly found.

Norman

[Alex R. Whitaker]

Yep, a dial light or heater string full short can get "interesting" really quickly!